Pressure Relief Valves For Air Compressors

Pressure relief valves, or safety valves, are a simple but critical part of any air compressor system. Pressure relief valves control and limit the pressure build-up in a system. If the pressure exceeds the amount allowed by the pressure relief valve, the pressure relief valve will automatically open and release air until the pressure reduces.

Why Do Pressure Relief Valves Matter?


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Pressure relief valves keep everyone safe. If the pressure within an air compressor system or air receiver tank gets too high, one or more components could explode, and pressure relief valves prevent this from occurring.

The rated pressure of the pressure relief valve must not exceed the maximum allowable working pressure (MAWP) marked on the air pressure vessel. This requirement ensures the pressure relief valve will always open before the vessel reaches its maximum pressure tolerance.

In addition, the minimum relieving capacity of the pressure relief device must meet the requirements of the American Society of Mechanical Engineers (ASME) Code. ASME is the international leader in developing and revising codes associated with mechanical engineering.

How Do Pressure Relief Valves Work?

Pressure relief valves for air compressor systems are simple, spring-loaded mechanisms. When the inlet pressure force is greater than the spring load, the safety valve opens in proportion to the increase in pressure and allows air to “leak out” as needed.

Pressure relief valves for compressed air applications are direct-acting, automatically reacting if the pressure gets too high. If excessive pressurization occurs, a disc seal moves up due to system pressure against a spring, which holds the valve closed. If the compressed air force exceeds the force exerted by the spring, the valve disc lifts off the valve seat, and the valve discharges the compressed air to the atmosphere.

Under the requirements of the ASME relief valve standard, the full discharge capacity of the valve typically will be achieved when the system pressure climbs to no more than 10% above the set pressure of the valve. Full shutoff must be achieved if the system pressure falls below the set pressure value, usually set at 90%.

How To Test Pressure Relief Valves

A set pressure function test should be carried out at least once per year to maintain pressure relief valve effectiveness over time. At VMAC, we recommend inspecting pressure relief valves annually for signs of corrosion or loss of functionality.

The pressure relief valve tests are the responsibility of the user or operator, while the air compressor manufacturer should supply test instructions. For example, VMAC’s manuals include test instructions to perform a pressure relief valve inspection:

“To test the pressure relief valve functionality, turn the system on and bring it up to operating pressure. Pull the ring on the pressure relief valve to depressurize the system. Turn the system off, and ensure the system comes back to operating pressure when the system is restarted. If the pressure relief valve is showing loss of functionality, contact your local, authorized VMAC dealer for a replacement part. Relief valve failure can result in air/oil tank over-pressurization leading to system failure or rupture.”

If you are operating a system that is not a VMAC air compressor, check your manual or contact the manufacturer for detailed instructions on how to test your pressure relief valve.

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What Is An FRL – Filter Regulator Lubricator

Filter Regulator Lubricator (FRL) – 70 CFMAir leaving a compressor is hot, dirty, and wet—which can damage and shorten the life of downstream equipment, including valves, cylinders, and air tools. So before compressed air exits the system, it needs to be cleaned and lubricated. That’s where an FRL comes in! An FRL combines a filter, regulator, and lubricator into one component to keep air compressor systems in optimal working condition.

FRL Components

An FRL has three primary components:

  • Airline filter
  • Pressure regulator
  • Lubricator

Each of these individual components has its role, supporting the larger air compressor system. We will explain more about these roles in the following sections.

Airline Filter

An airline filter cleans compressed air. It strains the air, traps solid particles (dust, dirt, rust), and separates any liquids (water, oil) in the compressed air. Filters are installed in the airline upstream of regulators, lubricators, directional control valves, and air-driven devices such as cylinders and air motors.

Filter Regulator Lubricator
FRL System

Because airline filters remove contaminants from pneumatic systems, they prevent damage to equipment and reduce production losses due to contaminant-related downtime. Downtime is expensive, and it is often the result of a contaminated and poorly maintained compressed air system.

Selecting the proper filter size for any application should be done by determining the maximum allowable pressure drop, which the filter can cause. The pressure drop can be determined by referring to flow curves provided by the manufacturer.

Types of Air Filters

There are three common types of air filters:

  • General-purpose
  • Coalescing (oil removal)
  • Vapor removal

General-purpose filters are used to remove water and particles while coalescing filters remove oil, and vapor removal filters remove oil vapor and odor.

Pressure regulators

Pressure regulators reduce and control air pressure in compressed air systems, including rotary screw air compressors. Regulators are also frequently referred to as PRVs (pressure-reducing valves).

Optimally, a pressure regulator maintains a constant output pressure regardless of variations in the input pressure and downstream flow requirements. In practice, the output pressure is influenced by variations in primary pressure and flow.

Pressure regulators are used to control pressure to:

  • Air tools
  • Blowguns
  • Air gauging equipment
  • Air cylinders
  • Air bearings
  • Air motors
  • Spraying devices
  • Fluidic systems
  • Air logic valves
  • Aerosol lubrication system
  • Most other fluid power applications

General-purpose regulators are available in relieving or non-relieving types. Relieving regulators can be adjusted from high pressure to low pressure. Even in a dead-end situation, relieving regulators will allow the excess downstream pressure to be exhausted. This pressure relief causes a loud hissing sound which is perfectly normal.

Non-relieving regulators that are similarly adjusted will not allow the downstream pressure to escape. Instead, the trapped air will need to be released in some other way—for example, by operating a downstream valve.

Downstream equipment flow and pressure requirements must be determined to size the correct regulator for the application properly.


A lubricator adds controlled quantities of tool oil into a compressed air system to reduce the friction of moving components. Most air tools, cylinders, valves, air motors, and other air-driven equipment require lubrication to extend their useful life.

Using an airline lubricator solves the problems of too much or too little lubrication that arise with conventional lubrication methods such as a grease gun or oil. Airline lubricators also supply the right kind of lubricant for the tools used.

Once the lubricator is adjusted, the air-operated equipment is supplied with an accurately metered quantity of lubricant. The only maintenance required is a periodic refill of the lubricator reservoir.

Adding lubrication to a system also “washes away” compressor oils that travel through the system in vapor form. Mineral oils added to the system prevent synthetic compressor oil build-up on system components. When lubricators are not used in a system, a coalescing filter should be installed to remove compressor oil aerosols.

Downstream flow requirements determine the size of lubricators. Therefore, an analysis of airflow use must be made. A lubricator can be chosen after deciding how much airflow is needed.

Types of Airline Lubricators

Airline lubricators come in one of two types:

  • Oil-Fog
  • Micro-Fog

Oil-Fog airline lubricators are used in simple, heavy-duty applications, such as single tools, cylinders, and valves. Micro-Fog lubricators are used for applications with more than one lubrication point, or several cylinders or valves.

In oil-fog lubricators, all the oil droplets visible in the sight dome are added directly into the airflow, which results in relatively large oil droplets passing downstream. In micro-fog lubricators, the oil droplets visible in the sight dome are atomized and collected in the area above the oil in the bowl. The smaller, lighter particles are drawn into the airflow and pass downstream. As a result, typically, only 10% of the visible oil drops in the sight dome are passed downstream.

Choosing The Right FRL For Compressed Air Systems

Compressed air is clean, readily available, and simple to use, but it can be the most expensive form of energy in your application if it is wasted. Unregulated or improper pressure settings can result in increased compressed air demand, which results in increased energy consumption.

Excessive pressure can also increase equipment wear, resulting in higher maintenance costs and shorter tool life. A rule of thumb states that every 2-psig increase in operating pressure adds 1% to compression energy cost.

Point-of-use FRLs (filter, regulator, and lubricators) are needed to ensure that every tool or process receives a clean, lubricated supply of compressed air at the proper pressure to provide peak performance.

Choosing A Filter For Compressed Air

Reliability is one of the most vital reasons to use compressed air, and proper filtration is the key to maximizing reliability and longevity. Unfortunately, compressed air can carry condensed water, oil carryover from compressors, solid impurities (pipe scale and rust) generated within the airlines, and other wear particles from the ambient air. These contaminants can cause problems at every point of use and should be removed by installing suitable filters.

Contaminant particle size is measured in micrometers (µm), representing one-millionth of a meter or 0.000039 of an inch. Filters are rated according to the minimum particle size that their elements will trap. For example, although filters rated at 40 to 60 µm are adequate for protecting most industrial applications, many point-of-use filters are rated at 5 µm. Note that finer ratings increase the pressure drop through the filter, equating to higher energy costs to compress the air.

In addition, finer filters clog more rapidly, also increasing pressure drop. (In other words, while filters finer than necessary do not harm downstream components, they will negatively impact air system operating cost.)

 point-of-use filters

Point of use filter

Many filter manufacturers will define the expected pressure loss and dirt holding capacity using curves related to pressure and flow. Therefore, particle-removal filters should be selected based on acceptable pressure drop and pipe-connection size.

A typical pressure drop through such filters would be between 1 and 5 psig. Therefore, a larger body size filter will produce less initial pressure loss and provide longer operating life than a smaller filter with the same removal ratings.

Most point-of-use filters claim to remove condensed water, typically via a cyclone separator at their inlet end. However, the water-removal efficiency of such filters is very dependent on the incoming air velocity. Therefore, these filters must match the intended airflow, rather than the acceptable pressure drop.

If the filter is intended to remove moisture, an automatic float-type drain should be provided to remove accumulated liquids from the filter bowl periodically. Generally, such filters have transparent polycarbonate bowls, which allow easy visual inspection of the sump level.

Numerous chemicals can attack this plastic material, and it only performs well at pressures below 150 psig and temperatures between 40° and 120° F. If the filter may be subjected to conditions beyond those limits, a metal bowl is required. A metal bowl is also needed if the filter is used with synthetic compressor lubricants, which often contain harmful chemicals to polycarbonate.

Coalescing-type filters,

Coalescing filter

Coalescing Filters

Most of the oil and some condensed water in a compressed airstream will be in the form of mists or aerosols that can pass through the openings in standard airline filters. Air for instruments, spray painting, and bulk-material conveying frequently requires removing such droplets, and coalescing-type filters will accomplish this job.

Aerosol carryover through such filters is commonly stated as parts per million (ppm) of oil vs. air by weight and ranges from 1 to as little as 0.01 ppm. Coalescing filters are often rated to remove aerosols that are substantially smaller than the nominal size of the smallest solid particle that would be captured. Some models offer dual-stage filtration; the first removes solid particulates to protect the coalescing element in the second stage.

Because all coalescing filters create a more significant restriction to the airflow, pressure losses will be higher than those of conventional compressed air filters. Coalescing filters have an initial (or dry) pressure drop and working (or saturated) pressure drop based on pressure and flow rate. Therefore, the effective removal efficiency of such filters depends significantly on the air velocity passing through the filter assembly.

Choose a coalescing filter based on acceptable oil carryover, expected airflow rate, and pipe-connection size. For example, a coalescing filter rated at 0.1 ppm will typically have a clean, wetted pressure drop between 2 and 5 psig. A high-efficiency filter rated at 0.01 ppm can cause a reduction up to 10 psig once it becomes wet or fully saturated during service.

Flow in scfm

High-efficiency filter pressure by flow

Selecting The Right Pressure Regulators

Once a minimum suitable operating pressure has been determined for any compressed air application, it is essential to supply the air at a constant pressure, regardless of upstream flow and pressure fluctuations. Thus, installing the proper regulator or pressure-reducing valve in the airline is critical.

Air regulators are special valves that reduce supply pressure to the level required for efficient operation of downstream pneumatic equipment. A filter to protect the regulator’s internal passages from damage should always be installed upstream from it.

Poppet-Style Valves

There are several types of air regulators, and the simplest type uses an unbalanced-poppet-style valve. This design incorporates an adjustment spring, does not have a separate diaphragm chamber, and is non-relieving. Turning the adjustment screw compresses the spring, which forces the diaphragm to move, thus pushing a poppet to uncover an orifice.

As pressure rises downstream, it acts on the underside of the diaphragm, balancing against the force of the spring. The poppet throttles the orifice opening to restrict flow – and produce the desired downstream pressure. A spring under the poppet assures that the valve closes completely when no flow exists. This is the least expensive type air regulator.

Pressure regulators

Regulator with diaphragm chamber

Diaphragm Chambers

Larger, more expensive regulators incorporate a separate diaphragm chamber with an aspirator tube exposed to the output pressure. Segregating the diaphragm from the primary airflow minimizes its abrasive effects and extends the valve’s life.

As flow through this regulator increases, the aspirator tube creates a slightly lower pressure in the diaphragm chamber. As a result, the diaphragm deflects downward and opens the orifice without significantly reducing the output pressure. The effect is the same as increasing the adjustment setting. Thus, this style regulator has a minimal drop (output pressure decay) as supply pressure varies. The table below compares how that variance occurs with a small and a large diaphragm.

The larger diaphragms in these regulators improve response and sensitivity. However, as discharge flow through the regulator increases over its entire range, output pressure drops. Thus, setting the regulator’s desired output pressure must be done under typical flow conditions.
Supply Pressure in PSI
Supply pressure in small vs. large diaphragms

Flow in scfm
Flow in scfm of small vs. large diaphragms

Balanced Poppet & Precision Regulators

Another type of regulator incorporates a balanced poppet, but otherwise has the same general construction as the separate diaphragm version. It has a significantly larger orifice to allow for greater airflow. The poppet is pressure-balanced to maintain good stability. Thus, the effects of output pressure fluctuations cancel out, which improves sensitivity and response, and reduces droop.

Finally, precision regulators often employ several isolated diaphragms acting against flapper valves and nozzles in a balancing principle and are normally manufactured in limited flow capacities with smaller connection ports.

Considerations When Selecting Regulators

Selecting the best type of regulator for a specific application first requires choosing among these styles. Mini-regulators are commonly the direct-acting, non-relieving type, while most standard regulators fall within the self-relieving, separate-diaphragm-chamber style.

The next consideration becomes primary (unregulated supply) pressure versus desired secondary (output) pressure.

Finally, the desired airflow rate must be selected. Adjusting screws are available in two styles: a tamper-resistant, locking Tee type or a push-lock, plastic knob type. The first is best when a fixed operating pressure will be set once and left alone. However, the adjustable knob style (quite common on modular FRLs) is the correct choice for general use, where operating pressure can be adjusted without tools. Regulators also are defined by body size (orifice flow rating) and connection size.

Although several models may appear acceptable for any given airflow and pressure, a larger body size regulator will produce better setting sensitivity and less droop than a smaller body model under the same set of operating conditions.

An output pressure gauge is essential, although many manufacturers frequently offer it only as an option. Mounting brackets are another useful option.

Choosing The Best Airline Lubricator

Airline Lubricators

Airline lubricator

Many pneumatic system components and almost all pneumatic tools perform better when lubricated with oil. Injecting an oil mist into the airstream can continuously lubricate valves, cylinders, and air motors for proper operation and long service life.

Locating the lubricator last in the pipeline is important to ensure that the correct amount of lubrication reaches each device. Too little oil can allow excessive wear and cause premature failure. On the other hand, excessive oil in the pipeline is wasteful and can contaminate the surrounding area when the air exhaust carries oil out of the tools and valves.

Intermittent lubrication may be the worst condition of all because the oil film can dry out and form sludge or varnish on the internal surfaces of the equipment.

Airline lubricators meter oil from a reservoir into the moving airstream. As high-velocity air passes through a venturi, it draws the oil up and through a capillary, then drips it into the airstream.

The moving air breaks up the oil into a mist (small droplets) or fog (larger droplets) and carries it downstream into the air-powered device. In a typical lubricator, all the air passes through the venturi during low-flow conditions.

Under higher flow conditions, a spring-loaded bypass valve opens to direct the excess flow around the venturi to a point downstream where it rejoins the lubricated flow. A manual adjusting valve sets the oil drip rate, and a sight glass enables the operator to monitor the output. A fill plug provides access to refill the reservoir, often made from polycarbonate. The same precautions about polycarbonate apply to lubricators as they do to airline filters.

Pressure drop in PSI

Lubricator pressure and flow

Lubricators typically have a larger flow range than an equivalent size regulator or filter, but their pressure drop increases quite rapidly as flow increases.

The standard acceptable pressure loss for a lubricator is 3 to 7 psig. Lubricators are generally selected based on pipe connection size, oil reservoir capacity, and allowable pressure loss versus flow rate. Many manufacturers publish a minimum flow rate at which the venturi will function properly.

Remember to account for this added downstream pressure loss when setting the pressure regulator. Set it at desired use pressure plus lubricator loss (drop).

Modular & Combination FRL Units

Modular or combination units

Combination FRL unit

Manufacturers frequently preassemble filters, regulators, and lubricators to form combination units. They are packaged together as standard body sizes with common connection port sizes. Interconnections may be via threaded nipples or modular face connectors.

The modular connectors allow easy removal of components for servicing or cleaning. In addition, some manufacturers combine filters and regulators in stacked assemblies where the filter head becomes the regulator body. The components share common inlet and outlet connections, making the assembly very compact.

Individual pressure regulator

Packaged combination units are practical for most industrial applications, whether FR only or complete FRL. The selection criteria are similar to the individual components, except that only the combined pressure and flow performance are considered.

When critical requirements dictate the use of specialty filters or precision regulators, the assembly must be made up of individual selections and connected with pipe nipples.

VMAC Air Innovated

How To Size An Air Receiver Tank

Many air compressor applications can benefit from installing an air receiver tank. An air receiver tank increases the amount of air available on-demand, allowing for higher duty cycles and more air power.

Air receiver tanks are sized in gallons and range from small 5- and 10-gallon tanks to massive tanks that hold thousands of gallons of air. The ideal size of an air receiver tank will depend on the air compressor and the application.

Air Receiver Tanks For Portable Air Compressors

Tanks for Reciprocating Air Compressors

Reciprocating air compressors use an air receiver tank to store air and eliminate pulsation. Once the tank fills with enough air, the tool or equipment can run. Using the tool will drain the tank in many applications, and operators will need to wait for it to fill back up before more compressed air can be used. Properly sizing the air receiver tank with the reciprocating air compressor can help reduce interruptions and time wasted waiting for the tank to refill.

A straightforward rule for sizing an air receiver tank for a reciprocating air compressor is to take the tool with the highest CFM requirement at the required PSI, multiply that CFM requirement by 1.25 or 1.5, then round up to the closest gallon size.

Air Receiver Tank Size Per CFM Requirement

CFM x 1.25 → round up = minimum tank size in gallons

CFM x 1.5 → round up = recommended tank size in gallons

CFM Requirement1.25 Multiply1.5 MultiplySuggested Tank Size
20253030 gallon
40506050-60 gallon
6581.2597.590-100 gallon
80100120100-120 gallon

While these calculations may not completely eliminate the waiting time between tank fills, they will help minimize them.

Tanks for Rotary Screw Air Compressors

For a rotary screw air compressor, many applications don’t require an air tank at all. Rotary screw air compressors supply a continuous air stream without interruption and pulsation. An air receiver tank isn’t required if your tool requires less CFM than the air compressor produces.

However, some operators may choose to use an air receiver tank to give their compressor a little boost for higher CFM tools. For example, if an operator routinely uses a 1″ impact wrench that requires 40 CFM but only has a 30 CFM air compressor, he may choose to add a 12-gallon air receiver tank to compensate for the difference. By the time the air receiver tank is empty, the task will be complete. Using this strategy, operators can sometimes save money or “make do” with a smaller system.

Air Receiver Tanks For Stationary Air Compressors

Properly sizing an air receiver tank for custom stationary applications is complex and should be done by a qualified engineer. These air receiver tanks need to be sized according to the volume and pressure variations in air consumption (i.e., demand), air compressor size, pipe or hose size and length, and the control system strategy (i.e., modulation or on-off control.)

There is a commonly used formula to find the ideal air receiver tank size for a stationary air compressor system:

t = V (p1 – p2) / C pa


  • V = volume of the receiver tank (cu ft)
  • t = time for the receiver to go from upper to lower pressure limits (min)
  • C = free air needed (SCFM)
  • pa= atmosphere pressure (14.7 PSIA*)
  • p1 = maximum tank pressure (PSIA)
  • p2 = minimum tank pressure (PSIA)

*PSIA = Pounds Per Square Inch Absolute; pressure relative to a vacuum.

Example: Stationary Air Compressor Tank Sizing

Let’s look at an example, using an air compressor system with the following specifications:

  • mean air consumption = 20 CFM
  • maximum tank pressure = 175 PSI
  • minimum tank pressure = 90 PSI
  • time the tool will run = 1 minute

The approximate ideal volume of the receiver tank can be calculated by modifying the sizing formula to:

V = t C pa / (p1 – p2)
= (1 minute) (20 CFM) (14.7 PSI) / ((175 PSI) – (90 PSI))
= 3.46 ft3
= 25.9 gallons

However, this formula works best for large reciprocating air compressor systems with variable airflow. Stationary rotary screw air compressor systems run at 100% duty cycle, which eliminates or reduces the air receiver tank size requirement if the air compressor is properly sized for the application.

Calculating Maximum Air Consumption

Identifying the maximum consumption of an air compressor system is critical when sizing an air receiver tank for a stationary compressor. Ideally, the air receiver tank will provide enough air to meet or exceed maximum consumption.

In the t = V (p1 – p2) / C pa formula, maximum air consumption is measured in SCFM and represented by “C.”

To calculate the maximum consumption in the system, summarize the air demand of each air tool or consumer that will be used simultaneously. The summarized consumption must then be multiplied with a utilization factor for each consuming item.

The Utilization Factor

The utilization factor is the way a tool is used and how that use affects airflow.

Let’s say you have an air tool like an impact wrench, which the manufacturer rates for a consumption of 20 CFM at 100 PSIG**. This wrench may be turned on for only 20 seconds at a time to tighten an individual lug nut to its required torque value.

Initially, the tool will consume the full rated 20 CFM as it tightens the lug nut against almost no resistance, but as the torque rises on the nut, the tool consumes less air until the final torque is achieved. The tool also won’t consume air when it isn’t used in between lug nuts.

The tool’s air consumption under load is not uniform throughout the process of torquing the lug nut, and the interval between applying the tool between individual lug nuts varies. This difference in CFM load and time interval becomes the utilization factor.

In other words, just because the tool is rated at 20 CFM, it does not mean that the tool requires the full rated CFM for each full minute nor the full minute to complete the job.

Because of this utilization factor, some air receiver tanks can meet heavy, short time demands of certain equipment at volumes exceeding the installed compressor’s supply capabilities. The minimum receiver capacity for certain applications may also be calculated, but experience and judgment are important at this point.

**PSIG = Pounds Per Square Inch in Gauge; pressure within the ambient atmospheric, measure with a gauge.

Pressure Band / Differential

The pressure band (differential) should also be considered when calculating the ideal air receiver tank size.

If the consumption process requires 100 PSIG and the compressor is set to deliver 100 PSIG, there is no storage and no buffer. Any increase in demand will result in a tank pressure drop below 100 PSIG until the compressor responds by increasing the air volume compressed to refill the tank and restore the 100 PSIG.

If the compressor is set to deliver 110 PSIG, the difference between 110 PSIG and 100 PSIG accounts for the air stored in the receiver.

If the 100 PSIG demand increases, the tank pressure can drop 10 PSIG before the minimum set pressure requirement is met. Keep in mind that the discharge piping and hoses also form part of the storage volume.

Pressure and flow controllers can be used after the receiver tank to stabilize downstream pressure to 100 PSIG and flatten demand peaks.

When Does Exact Sizing Matter?

Even with the knowledge above, properly sizing an air receiver tank is a complicated and time-consuming process. Operators who use straightforward tools and air compressors can default to simple CFM recommendations and choose a receiver tank using a 1 CFM to 1.25-1.5 gallon ratio. However, engineers developing complex and custom systems will need to determine more exact sizing requirements and need to put in the work.

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Compressed Air Aftercoolers

Air AfterCooler – 70 CFM

What are compressed air aftercoolers, and why are they required? This article will tell you everything you need to know about aftercoolers; discover the two most common types, find out why air aftercoolers are important, and learn their role within an air compressor system.

What Is An Aftercooler?

An aftercooler is a mechanical heat exchanger designed to remove the heat and moisture of compression from a compressed air stream so the air is cool and dry enough for use in air-operated equipment.

A compressed air aftercooler has three primary functions:

  • Cool the air discharged from the air compressor
  • Reduce the moisture in compressed air
  • Protect downstream equipment from excessive heat and moisture

Why Are Air Aftercoolers Required?

Compressed air discharged from an air compressor is always going to be hot. However, the exact temperature will vary according to the type of compressor used.

Typical Compressor Average Outlet Temperatures:

Oil-Injected Rotary Screw        200°F
Oil-Free Rotary Screw              350°F
2-Stage Reciprocating              300°F
Centrifugal                                 225°F

High-temperature air is typically not usable in air-operated equipment, as heat detrimentally impacts equipment lubrication and sealing materials. Hot air also contains large quantities of moisture vapor which, as it condenses, contributes to rust, scale build-up, washing out of lubricant, and possible freezing issues.

Water exists in a vapor state in all atmospheric air. As the air is drawn into a compressor and pressurized, this moisture becomes more concentrated. Due to the high temperature of the air after compression, the moisture remains in a vapor state, above the dew point temperature.

The dewpoint is the temperature at which air becomes saturated at 100% of its capacity to hold water in a vapor state. A general rule is that for every 20°F rise in temperature, the air can hold twice the moisture load in a vapor state above the dewpoint.

large-Compressed Air Aftercoolers Chart-01

However, as the compressed air cools, the dewpoint lowers, and water vapor condenses. When the amount of moisture in the air exceeds the dewpoint, the water vapor turns into water droplets, which must leave the compressed air.

For example, a 200 CFM air compressor operating at 100 PSIG can result in as much as 45 gallons of water in the compressed air system each day without an aftercooler.

Types Of Aftercoolers

The two most common types of aftercoolers are air-cooled and water-cooled.


Air-Cooled Aftercoolers

Air-cooled aftercoolers use ambient air to cool the hot compressed air. Compressed air enters the air-cooled aftercooler and travels through either a spiral finned tube coil or a plate-fin coil design, while ambient air is forced over the cooler by a motor-driven fan. The cooler ambient air removes heat from the compressed air.

Most air-cooled aftercoolers are sized to cool the compressed air to within 15°F to 20°F of ambient cooling air temperature, also called approach temperature. As the compressed air cools, up to 75% of the water vapor condenses into a liquid that should be removed.

A moisture separator installed at the discharge of the aftercooler mechanically removes most of the liquid moisture and solids from the compressed air.

The moisture separator utilizes centrifugal force, and in some cases baffle plates, to collect moisture and solids at the bottom of the separator. An automatic drain should then be used to remove the moisture and solids.

Belt Guard Air-Cooled Aftercooler


Source: AKG /

A belt guard air-cooled aftercooler mounts to the compressor’s v-belt guard. The compressor’s belt pulley has fins designed to force ambient air over the compressor and air-cooled aftercooler. The air passing over the aftercooler facilitates the heat transfer. The pulley also sends air over the compressor helping maintain proper operating temperature.

Water-Cooled Aftercooler

Water-cooled aftercoolers are similar to air-cooled aftercoolers but are often used in stationary compressor installations, where cooling water is available. There are a few advantages to using water as the cooling media:

  • Water has minimal seasonal fluctuation in temperature
  • Cold water is cost-effective and available in large volumes
  • Cold water can efficiently approach the ambient air temperature, which eliminates condensation downstream

Water-cooled aftercoolers typically have an approach temperature between 10°F and 15°F, which is also beneficial when cooler air is required.


Source: Southwest Thermal Technology

Water-cooled aftercoolers come in several different styles. The most common style is a Shell and Tube aftercooler. This aftercooler consists of a shell with a bundle of tubes fitted inside. The compressed air flows through the tubes in one direction as water flows through the shell in the opposite direction.

During this process, heat from the compressed air is transferred to the water. As the compressed air cools, liquid water forms within the tubes. The moisture is then removed by a moisture separator and drain valve, just like with air-cooled aftercoolers.

How To Size An Aftercooler

Aftercoolers require three metrics for proper sizing: CFM, PSI, and temperature. The CFM and PSI are straightforward, as they are determined by the air compressor’s output for the application. Meanwhile, the temperature metric requires additional considerations.

Coolers are usually sized with Cold Temperature Difference (CTD) of 10°F, 15°F or 20°F. This means that the compressed air temperature at the aftercooler outlet will be equal to the cooling medium temperature plus the CTD when sized for the specified inlet air temperature and flow. The lower that temperature needs to be, the larger the aftercooler needs to be.

The required compressed air temperature is often determined by other components downstream in the system. Each component’s maximum temperature must be determined and considered to discover the true temperature requirements for the aftercooler—this step is critical.

large-Average Operating Temp of Air Comp Components-01

Disclaimer: Individual air compressor components may vary from the chart above; always check with the component manufacturer for specs and tolerances, including typical and maximum operating temperatures.

Once the aftercooler requirements are understood, choose an aftercooler with specs that meet or exceed these requirements. Compressor manufacturers may already consider common aftercooler requirements and include aftercoolers as a component of the compressor system. Otherwise, a stand-alone or freestanding aftercooler will need to be purchased and installed downstream of the compressor.

VMAC’s Air Aftercoolers

Air AfterCooler – 185 CFMVMAC offers 70 CFM and 185 CFM air aftercooler options for mobile air compressors. Both aftercooler models were designed to improve air performance and extend the life of air tools by removing up to 80% of the moisture from compressed air. Follow the links below to learn more:

Want to learn more about air compressors? Check out our other air compressor articles!

The VMAC Diesel Driven Air Compressor’s Helpful Control Box Features & Functions

D60 control box

D60 Control Box

The control box is essentially the brain of the VMAC diesel driven air compressor system. It tells the system what to do and makes decisions based on the programmed settings. The control box is also the communication hub for the air compressor, relaying important information to the operator on an “as needed” basis.
Continue reading “The VMAC Diesel Driven Air Compressor’s Helpful Control Box Features & Functions”

What Is An Air Receiver Tank & What Does It Do?

A300047-A 10 gallon air receiver tank

10-Gallon Air Receiver Tank w/ Mounting Fee

An air receiver tank is a metal tank that stores compressed air until it is required. Air receiver tanks can vary in shape and in size, depending on the application. Air receiver tanks on service vehicles are typically 30 gallons, but still range from 6 to 50 gallons.

Some air compressors, such as reciprocating air compressors, require an air receiver tank to operate. But even air compressors that don’t need an air receiver tank can benefit from the installation of one.

What Does An Air Receiver Tank Do?

Air receiver tanks serve many important functions and roles within an air compressor system. An air receiver tank:

  • Dampens pulsations from the discharge line of a reciprocating compressor, resulting in an essentially steady flow of air in the system.
  • Serves as an air reservoir to take care of sudden or unusually heavy demands for air that’s in excess of the compressor’s designed capacity.
  • Prevents the excessive cycling of a compressor.
  • Knocks out solid dirt and particulate matter that may have passed through the compressor inlet filter or may be the result of compressor wear.
  • Precipitates out contaminants and oil carry-over from the compressor oil that might get into the compressor discharge
  • Helps cool the compressed air and precipitates out moisture that inevitably results from air compression

Pulsation Damping

During the compression of air in a reciprocating compressor, air is delivered in pulses through the discharge line. This pulsation is caused by the alternating suction and compression stroke of these types of compressors.

Adding an air receiver tank dampens these pulses and allows a smoother delivery of air to the tool or piece of equipment being used. Pulsation dampening also helps protect hoses and components from damage caused by pressure spikes.

Air Reservoir

Air receiver tanks provide a reservoir of compressed air. An air receiver tank is capable of storing compressed air to be used once the compressor has been turned off, and to offset sudden, heavy demands of air.

If sized appropriately, an air receiver tank may also allow the use of a smaller compressor in low duty-cycle applications requiring higher air flow than the compressor can continuously deliver. For example, a low CFM rotary screw air compressor that can’t quite power a 1” impact gun continuously may be able to do so with a small air receiver tank

Reduce Compressor Cycling

35 Gallon Air Receiver Tank

35-gallon air receiver wing tank

As mentioned above, the tank can store air and act as a buffer.  This also allows the compressor to cycle on and off less frequently, which can reduce wear and tear and reduce energy consumption. Care must be taken to ensure that the compressor system is rated and able to run long enough to fill the size of tank chosen without overheating or causing accelerated wear.

Particulate and dirt removal

During everyday air compressor operation, dirt, wear particles, or other foreign matter may bypass the air filter or otherwise find their way into the compressed air stream. This debris in the air stream can then cause blockages, premature contamination of filters, and excessive wear on tools and equipment.

An air receiver tank helps remove some of these particulates and dirt from the compressed air system, as some of this material will naturally fall to the bottom of the air receiver tank before the air reaches the tool or equipment that’s in use.

Moisture Removal

Atmospheric air typically contains moisture that is drawn into the compressor during its intake cycle. The moisture holding ability of air naturally increases as the temperature and pressure of compressed air temperature rises. Hot, compressed air typically holds moisture in a vapor state, but that moisture will condense into liquid form as the air cools past its saturation point.

Air receiver tanks are typically uninsulated, which means that hot compressed air enters the receiver tank and immediately starts to cool by transferring heat to the environment through the walls of the tank.

Without an air receiver tank or aftercooler, this cooling happens in an airline and is commonly seen as liquid water coming out of the tools or equipment. When using an air receiver tank, much of this liquid is collects in the bottom of the tank, which can easily be drained at the end of each working day.

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Compressed Air Filtering Basics – Contaminants, Filters & More!

Take a deep breath. Breathe out. Breathe in… Now, let’s talk about all the contaminants in that air you’re breathing!

Atmospheric air is naturally contaminated. In addition to oxygen, air also contains water vapor, dust and dirt, and a medley of other filthy particles depending on the surrounding environment. The average metropolitan area, for example, contains approximately 4 million dirt particles per cubic foot of air.

When air is compressed, some of those contaminants need to be filtered out. Contaminated atmospheric air can cause damage to air compressors and air tools, and isn’t acceptable for certain medical and industrial applications.

But atmospheric air isn’t the only source of contamination for compressed air. Air compressors contribute their own share of pollution from wear particles and, if the compressor is oil-lubricated, carbonized compressor lubricant.

Between atmospheric air contamination and air compressor system contamination, there’s a lot of particles dirtying up our compressed air. Fortunately, we have compressed air filtering to save the day.

A Bit of Dirt Never Hurt Anyone…

Don’t worry about that dirty air you’re breathing. Your lungs act as your body’s filtration system, removing contaminants after they’re inhaled. Air compressor systems obviously don’t have lungs, but they do filter out air contaminants in other ways. Better still, a little contamination is usually okay.

Contaminated particles are measured in microns. The bigger the particle, the bigger the micron. Most fine particle tests use 0.3 micron as the standard to measure liquid or solid particle filtration. If a filter tested on this particle size proves to be 100 % efficient, then it’s fairly safe to say this filter can remove any particle above this size.

Many compressed air applications can handle contamination levels well above 0.3 microns. Tire service, construction, and most other mobile air compressor applications tend to tolerate quite a bit of contamination without problem. Operators with these types of jobs can get away with a more basic filtration system, such as an intake filter, which eliminates contaminants in the 30 to 40 micron range.

Industrial air compressor applications tend to be less tolerant of contamination than mobile applications and up to 80 % of industrial contamination is smaller than 2 microns. Therefore, many industrial air compressors need better filtration than mobile air compressors. Industrial air compressor systems often utilize advanced dry particulate and coalescing filters that can clean air down to 0.01 microns. For applications that require super-clean air for OSHA purposes, an additional charcoal activated filter may also be used.

However, these industrial applications are the exception. Outside of massive factories and industrial enterprises, most people use mobile-style air compressors that can tolerate quite a bit of contaminants and don’t need to think twice about their air compressor’s filtration capabilities.

What Contaminants Are In Compressed Air?

Now that you realize just how contaminated our air is, let’s talk about what it’s contaminated with.

Compressed air contains three types of contaminants:

  • Dry particulates
  • Vapors
  • Aerosols

Dry particulates are exactly what they sound like: dirt and other tiny solid particles. Vapors are the gas-forms of particles that condense into liquids at lower temperatures—for example, water. (The gas state is what allows these “liquids” to exist in air.) Meanwhile, aerosols are very fine solid particles that get trapped in air or gas, becoming suspended. Airborne dust is one familiar example of an aerosol.

Each type of contaminant has its own unique characteristics and properties, requiring their own filtration methods.

There are two primary types of filtration that are used in compressed air systems:

  1. Dry Particulate Filtration
  2. Vapor & Aerosol Filtration

In the next couple sections of this article, we’ll talk about these filtration types and the filters used to eliminate dry particulates, vapors, and aerosols.

The Principles of Dry Particulate Air Filtration

Here’s where we get technical. We already know that most contamination in a compressed air system can be removed simply by filtration. However, it’s important that your air compressor systems use the correct type of filtration for the particles being filtered.

Dry particulate filters rely on three principles to separate contaminants from the air:

  • Direct interception
  • Inertial impact
  • Diffusion & Brownian movement

Direct interception affects the larger particles in an air stream, which are literally sieved out through a filter.

Inertial impact occurs when a particle traveling in an air stream is eventually unable to negotiate the torturous path between the filter fibers and cannot change direction as quickly as the air stream. The contaminants then collide with a fiber and become attached to it.

Diffusion or Brownian movement affects fine particles. With diffusion, small particles merge with other gas particles and begin to move erratically. This erratic movement is called Brownian movement. As these particles move separately from the compressed air flow, they are more likely to become trapped in the filter.

Air Filtering though diffusion

All three of these principles work together in a dry particulate filter to capture and trap contaminants from the compressed air.

If you’re interested in reading more about the principles and physics behind air filtration, check out this “Mechanism of Filtration For High Efficiency Fibrous Filters” report by TSI.

Vapor & Aerosol Filtration for Compressed Air

Vapors and aerosols slip past dry particulate filters, which may require their own filtration systems. In this case, there are two options that may be utilized:

  • Coalescing
  • Adsorption

Coalescing filters trap moisture and oil. The compressed air enters through the inlet port and travels down into the filter, passing through a filter media before it leaves through the discharge port. Moisture and oil droplets bond together during this process, forming larger droplets that then drip into a moisture trap below.


Coalescing filters are commonly used in oil-injected air compressors, such as rotary screw air compressors. In VMAC systems, these filtration methods include a couple types of dry particulate filters, as well as a coalescing filter.

Adsorption filters help eliminate vapors and lubricants, using activated charcoal or similar chemicals to bond with the vapor molecules. Adsorption comes into play when vapors must also be eliminated from a system. Adsorption filters are typically only used in specific industrial applications.

Filtration Systems of Rotary Screw Air Compressors

Reciprocating air compressors that don’t use oil can often get away with just a dry particulate filter. That’s because the contaminants in atmospheric air are negligible for most construction and automotive applications, travelling through the air compressor without causing much problem.

However, oil-injected rotary screw air compressors require additional levels of filtration. The oil used to lubricate the rotors is necessary for this style of air compressor, but that same oil needs to be cleaned and separated from the air.

Therefore, rotary screw air compressors require two types of filtration systems:

  • Dry particulate filters
  • Coalescing filters

In a typical VMAC air compressor, you’ll find both types of filters throughout the system:

  1. Air filter: atmospheric air entering the system goes through a dry particulate air filter.
  2. Coalescing filter: Air that leaves the rotors, now mixed with oil, goes through a coalescing filter, which separates the oil from the clean air. The oil gets recirculated while the air exits the system.
  3. Oil filter: The separated oil then goes through the oil filter, which is another dry particulate filter that separates particles from the oil.filters

It’s these same filters that occasionally need to be replaced and will be included with VMAC’s service kits. Replacing the air filter, oil filter and coalescing filter ensures the air compressors continue to trap contaminants and keeps your air compressor in tip-top shape.

Why Does Air Hose Size Affect My Compressor Airflow?

Appropriate sizing is important for all air compressor system components, including hose sizes. If you invest in an air compressor system, restricting the flow anywhere in your system can make it significantly underperform or require unnecessary energy costs to run that compressor over its lifetime.

Air travels from the compressor head to your tool through components such as hoses, fittings, valves, and tanks. Each of these components will restrict the flow of air in some way, depending on the geometry of each component and the magnitude of the flow passing through it.

For example, a 100-foot, 1” hose delivering 100 CFM at 90 psi will result in a 3.35 psi pressure drop. If that same hose is tripled to 300 feet, the pressure drop is 10.1 psi, which means the air is now pressurized to only 80 psi.

As a result, your compressor is working harder and using more power than it should to keep up with your air demand, or—if it can’t keep up—your tool performance will suffer. In some cases, where power at the tool is essential, you may not be able to complete your work.

90° fittings like this may restrict airflow

90° fittings like this may restrict airflow

How Fittings Cause Pressure Drops

Pressure drop is due to the restriction created by the pipe or fitting. Anyone who has tried to breathe through a drinking straw can tell you that forcing a large flow of air through a small hole can be difficult. Why? The smaller the diameter, the higher the velocity is required for the air to travel through the hole.

Higher velocities result in more friction created due to boundary layers at the walls of the pipe or fitting creating more losses. With pipes and hoses, the loss is by the length of the pipe. Consider the reduced diameter of the pipe with the fitting when doing pressure drop calculations.

It can be surprising how small the flow diameter is in some fittings, and a quick-connect fitting is one of the worst culprits. Next time you are looking at a quick connect, look inside to see how small the actual flow area is.

Calculating Pressure Drop of Fittings

You can use quick calculators or charts, such as those found at Engineering Toolbox, to calculate the pressure drop in your pipes or components.

Note that this calculator is for hard pipe, which is a well-defined shape. Flexible hose typically contains many bends and loops, so creating an entirely accurate calculator is impossible. Although flexible hose will have more losses than a pipe with an identical inside diameter, we can still use the pipe loss calculation to get a decent estimate and see the influence diameter has on pressure loss.

When sizing your air compressor, consider each of the following components that can cause pressure loss:

  • Hose reels
  • System piping/tubing
  • Filters
  • Regulators
  • Lubricators
  • Fittings

Components such as filters will often have pressure, so check the documentation and specifications carefully to match components to the system.

When considering fittings and quick connects, work with your suppliers to ensure they are rated for the maximum pressure your compressor system is rated for and will not cause excessive pressure drop at the required flow rates.

Pressure Drop Sizing Examples

To illustrate the dramatic difference that pipe or hose diameter makes on pressure loss, we used the pressure drop calculator tool to compare 100 ft long pipes with internal diameters of ½”, ¾”, and 1”.

In this example, we have 70 CFM free air delivery (FAD) of compressed air, delivered at 100 psi gauge pressure (equivalent to 114.7 psi absolute pressure*) at the upstream hose entrance.

The approximate pressure loss from end to end for the three pipe sizes is:

  • 1″ x 100’ = ~1.4 PSI pressure loss
  • 3/4″ x 100’ = ~5.7 PSI pressure loss
  • 1/2″ x 100’ = ~44 PSI pressure loss

Your compressor would have to be operating at a constant ~134 psi gauge pressure† to maintain 100 psi at the tool with the ½” pipe.

Increasing Pressure vs. Increasing Supply Line Size

Increasing the pressure of your compressor to compensate for flow losses can dramatically impact on the amount of work your compressor system is doing. Meanwhile, increasing the size of the supply lines can provide the following benefits:

  • Less fuel/energy used by your compressor
  • Less heat generated by the compressor
  • Longer oil life and service interval
  • Lower noise output from the compressor
  • Improved safety due to lower operating pressures and lower temperatures
  • Lower load on drive system components
  • Less wear and longer life of your compressor

Each restrictive fitting, hose, accessory, and bend added to your system results in cumulative pressure drop and can negatively affect the performance of your tool or equipment. Recognizing this and planning and sourcing the right-sized components will enable your air system to perform better.

Keep in mind that formulas and calculators like the one used above are just a guide. Real-life scenarios depend on many factors, and each will affect your individual results. Using a larger diameter hose may cost more, but it can result in long-term savings and may even allow for a lower pressure or output compressor, saving money upfront.

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*Absolute pressure is the pressure relative to a perfect vacuum. Gauge pressure is relative to atmospheric pressure. Standard atmospheric pressure is 14.7 PSI so for a calculation such as this one, which requires absolute pressure, atmospheric pressure (14.7 psi) needs to be added to your gauge pressure.

† Note that it may seem that the entrance pressure would need to increase by the same pressure as the loss in the pipe from the first calculation. However, the increase is slightly reduced. As the pressure increases, the volume flow and flow velocity decreases (for the same air mass), and therefore the pressure drop decreases as well.

Auxiliary port driven air compressor system considerations

Today many industrial engines come equipped, or can be ordered with, an auxiliary port (pump drive) that is used to power accessory equipment.  Quite often you will find a hydraulic pump or a standard piston type air compressor mounted there; what isn’t seen much is a rotary screw compressor mounted there.  We’ve had many inquiries asking if there is a compressor available that would meet the heavier duty cycle and air flow requirements for specific jobs.  These range from spray foam, pump priming, air gouging, to air tools and dust collection.  While VMAC does have direct drive, port mounted rotary screw compressors, there are several factors to consider before choosing to mount a rotary screw compressor to that auxiliary port. Continue reading “Auxiliary port driven air compressor system considerations”

Why Is My Compressor Shutting Off? Temperature Sensors and Switches

Temperature Sensors and Switches

Temperature switches and sensors are used on applications which require a solution to a temperature control situation. Often less complicated than most electronic controls, temperature sensors and switches are relatively easy to set up.

Temperature sensors are often used for monitoring compressor coolant, oil and air inlet temperatures as well as discharge air temperatures and logging the variations.

Temperature switches are slightly more complex, and include a sensor plus the ability to send signals. A temperature switch senses temperature levels. When the temperature passes a set point the switch sends a signal to a controller to do something to the application, like cut the power, sound an alarm, turn on a light, or disengage the clutch.

Some immersion temperature switches are appropriate for applications that require an inexpensive solution to simple temperature control. These types of switches activate with a specific rise in temperature and are available with a wide range of temperature pre-set values as well as a set point range, set point tolerance, maximum temperature cut-out setting, and probe length.

On-off controllers

An on-off controller is the simplest form of temperature control device. The output from this type of device is either fully on or off, with no middle state. An on-off controller will cut power or disengage the clutch when the temperature passes the set-point. On-off control is usually used where a precise temperature control is not necessary.

A limit controller is an on-off controller used for alarm indication. This type of controller uses a latching relay, which must be manually reset, and is used to shut down a process when a certain temperature is reached.

VMAC temperature sensors, switches, and controls

VMAC air compressors are equipped with a switch which includes an oil temperature sensor. If the compressor oil gets too hot, the switch sends a signal to the on-off controller to disengage the air compressor’s clutch. This shuts down the system and prevents high temperature related damage.

Why is my compressor shutting off?

If your compressor trips on over temperature, it could be for any of the following reasons:

  • Ambient temperature too high or not enough ventilation
  • Too low oil level
  • Wrong type of oil being used
  • Dirty oil cooler
  • Thermostatic valve not working
  • Dirt / obstruction in oil lines
  • Plugged oil filter
  • Restricted air flow over the air to liquid cooler
  • Too high an engine liquid coolant supply temperature in a liquid to liquid cooler
  • Faulty temperature switch

Excessive oil temperatures can cause damage to your air compressor including premature lubricant degradation, high oil and moisture carryover, and varnishing of the compressor internals and system components (such as the oil filter, cooler, and separator filter). Lubricant flash points also present a fire hazard.

The costs related to rectifying these issues through the use of temperature switches and controls can result in significant cost-savings, as risks of down-time and injury are minimized.

Interested in learning more about air compressor components and accessories? Browse our collection of air compressor accessory blogs.


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Components of your VMAC mobile rotary screw air compressor system

You’ve decided that a rotary screw compressor system is the right choice for your application; maybe you are converting from using reciprocating compressors. When installing your new compressor you’ll need to locate a few different components and plumb hosing between them. In this article, we’ll briefly explain the functions of the components of a rotary screw compressor system. Continue reading “Components of your VMAC mobile rotary screw air compressor system”

The Purpose and Functions of Compressed Air Storage Tank

We’re often asked if a particular air compressor installation requires the use of an air receiver tank. Most applications will benefit from the use of air storage whether it’s a vertical or horizontal air tank. The choice of what style of tank is generally made by the installation location and the amount and type of space available. Vertical receiver tanks are readily available in sizes from 10 – 2560 Gallons, and horizontal receivers are available from 5 – 2560 Gallon capacities.
Continue reading “The Purpose and Functions of Compressed Air Storage Tank”

What are Dual Tower Regenerative Desiccant Air Dryers (and how do they work?)

Heaterless Type (Pressure Swing Dryers)

Dual tower desiccant air dryers are used to produce dewpoint temperatures below the freezing point of water, as well as reduce the moisture content of compressed air used in critical process applications. Typical dewpoints produced by these types of dryers are -40° F to -100° F, although lower dewpoints are possible. Continue reading “What are Dual Tower Regenerative Desiccant Air Dryers (and how do they work?)”

Why the fuel valve is important on small gas engines

Protect your engine, turn off your fuel

You just finished a job using your gas drive air compressor and you’re getting ready to drive to your next job. Did you remember to shut off your fuel valve?  In this article we’ll explain why you should.

Most small gas engines have a fuel valve that should be shut off by when the engine is not in use.  This can be easy to forget, especially when using remote controls.

Fuel shut-off becomes important when moving equipment as vibration can cause the carburetor needle valve to move allowing fuel to trickle into the carburetor, the float chamber and down the intake valve.  This can cause:

  1. Engine flood, causing downtime waiting for the flood to clear.
  2. Dilution, when fuel goes past piston rings and mixes with oil, causing engine damage.
  3. Hydraulic lock, when incompressible liquid causes engine damage or failure.
Best practice for small gas engines – ensure equipment is on level ground, and the fuel valve is shut off when re-fueling and when equipment is not in use.

Why does the engine flood?

Any time vibration causes the carburetor float to drop in the float chamber, pressure is reduced against the needle valve.  Reduced pressure against the needle valve allows pressurized fuel from the fuel tank to pass through the valve.

If this happens frequently, fuel will overfill the float chamber, flood down the throat of the carburetor, and flow into the cylinder through the open intake valve.

Fuel in the cylinder can flood the combustion chamber above the piston, creating hydraulic lock, preventing the engine from turning. This fuel will also slowly drain past the piston rings, diluting the oil in the crank case. If the engine manages to start with diluted oil, severe and premature engine damage will follow.

How does the float work?

The float chamber is located below the carburetor body. Through the operation of the float and the needle valve, the float chamber maintains a constant fuel level while the engine is working. The fuel flows from the tank into the float chamber through the needle valve. When the fuel rises to a specific level, the float rises. When the buoyancy of the float is balanced with the fuel pressure, the needle valve shuts off the fuel passage, thereby maintaining the fuel at the predetermined level.

Any other reasons?

Not only does shutting off the fuel valve prevent the engine from flooding while being transported, it prevents flooding because of contamination in the float valve, and extends the life of the float valve by decreasing pressure on it.

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Is this unique to Honda engines?

Most manufacturers of small gas engines have this same issue.  Like the Honda engines used in VMAC G30 Gas Drive Air Compressors, Subaru and Kohler engines used in other air compressor brands state in their literature that fuel valves should be shut off when not in use, including during transport.

VMAC G30 Gas Drive Air Compressors are powered by Honda’s GX390 air cooled 4-stroke engine.  The G30 is a Honda-approved application.  The engine includes electric start capability, is EPA and CARB-compliant, and comes with Honda’s 3-year warranty.

Do you have questions about VMAC’s G30 Gas Drive Air Compressor?  Please give us a call at 1 888 514 6656 or email us at [email protected].

If you have any questions about this article or anything mobile compressor related, please contact us.

Rotary Screw Air Compressor Oil – Answers To Your Top 8 FAQs!

What is air compressor oil?

High performance synthetic oil

Air compressor oil is standard or synthetic oil made specifically for air compressors. Unlike motor oil, air compressor oil does not contain detergent and typically has lower amounts of carbon, sulfur, and other contaminants that can cause build-up.

Generally, rotary screw air compressors use synthetic oil, while recreational compressors may use standard oil. Air compressor oil varies in composition and color but is often yellow or light brown and becomes darker as it becomes contaminated over time.

Why do rotary screw air compressors need oil?

Air compressor oil plays several important roles within a rotary screw air compressor system:

  • Lubricates moving parts
  • Creates a seal between rotors to trap air
  • Prevents premature wear on parts
  • Helps cool air during compression

Rotary screw air compressors are sometimes referred to as “oil flooded” or “oil injected,” which describes how oil is injected into the rotor housing. The oil mixes with the air during compression but is separated back out before the compressed air leaves the system.

Some rotary screw air compressors don’t require oil at all. However, these designs tend to be louder and more expensive. Oil-free rotary screw air compressors are typically only used in industrial applications in which oil contamination absolutely can’t occur, such as food processing or medical oxygen.

What kind of oil does an air compressor take?

Some air compressors require a particular type of oil, while others may not be as specific. Before deciding on a type of oil for your rotary screw air compressor, check your manual and warranty details to determine whether a specific oil is required.

If a manufacturer specifies a specific type of oil, always use that oil. For example, all VMAC air compressors require VMAC’s high-performance synthetic rotary screw compressor oil to perform correctly and maintain the limited lifetime warranty. We have tested all our systems with this oil and know it has the perfect chemical composition to lubricate and protect components, create proper seals between our two rotors, prevent overheating, and ensure optimal air output.

Manufacturers who don’t specify an exact brand of oil may still recommend a 20-weight or 30-weight non-detergent oil. 20-weight oil is typically used in colder environments, while 30-weight oil is better suited for warm climates. Some operators will switch between these oils for the summer and winter seasons.

When should I change air compressor oil?

Maintaining proper service intervals will keep your air compressor running in optimal condition and help maintain the warranty. When applicable, you should always check and follow your air compressor manufacturer’s service maintenance schedule.

For example, here is a breakdown of VMAC’s air compressor oil service intervals:

SystemCompressor TypeOil Service Intervals
G30Gas Powered200 Hours / 6 Months
D60Diesel Powered500 Hours / 6 Months
H60Hydraulic Driven500 Hours / 1 Year
DTM70PTO Driven200 Hours / 6 Months
UNDERHOOD™ 70Engine Mounted200 Hours / 6 Months

As you can see, the service hours and periods range significantly between different air compressors—even when designed by the same manufacturer. Therefore, you should always check your air compressor’s manual when determining when to change the compressor oil for your system. Contact the manufacturer directly if you can’t locate a manual for your system.

How do I check the air compressor oil level?

Check Oil-level

You should check your air compressor oil level daily, or every time you use the air compressor. To check the oil level, follow these simple steps:

  1. Ensure the vehicle is parked on level ground and that the compressor system is depressurized and cool to the touch.
  2. Check the oil level in the sight glass. On VMAC systems, ensure the oil is between the “MAX” arrow and the “ADD arrow.”
  3. If the level is below the required level, remove the fill cap on the tank and use a funnel to pour oil into the fill fitting. Use the sight glass to achieve the desired oil level and avoid overfilling.
  4. Replace the fill cap and tighten it securely.

If the air compressor does not have a sight glass, remove the oil fill cap and look inside for a dipstick. Pull out the dipstick and look for the “min” and “max” lines at the bottom tip of the stick, then use these lines to determine whether more oil is needed.

How to change air compressor oil?

When changing your air compressor’s oil, follow the instructions in your air compressor’s manual. Although the basics of an oil change are simple—drain the old oil and then pour in new oil—the individual steps can be significantly more detailed.

As an example, here are the steps required to change the oil in VMAC’s G30 gas driven air compressors:

  1. Clean the area around the air compressor’s WHASP tank and oil filter to prevent contamination.
  2. Remove the oil drain plug and drain the oil into a container with a capacity of at least 1 gallon (4L).
  3. Inspect the Viton O-ring on the oil drain plug for damage, hardness or defects and replace if necessary.
  4. Install and tighten the oil drain plug.
  5. Remove the oil filter.
  6. Ensure the threaded nipple did not unscrew with the oil filter.
    1. If the nipple came out with the oil filter, remove it from the filter, being careful to avoid damaging the threads.
    2. To reinstall the nipple, thoroughly clean the threads and apply Loctite 242 (blue) to the end with the short threads and replace it in the air oil separator tank.
  7. Clean the gasket sealing surface on the front of the tank and inspect it for damage. The surface must be free of old gasket material and smooth to ensure a good seal.
    change filter
  8. Apply a thin coat of compressor oil to the rubber gasket on the oil filter.
  9. Spin the filter onto the threaded nipple until the gasket contacts the sealing surface on the tank, then tighten the filter an additional 3/4 to 1 turn to seat the gasket.
  10. Remove the filler cap on the WHASP Tank. Fill the WHASP Tank with VMAC compressor oil until the oil in the sight glass reaches the “MAX” mark. The air compressor system holds approximately 1 gallon (4L) of oil.
    filler cap
  11. Check the oil level at the sight glass on the front of the WHASP Tank. Continue adding oil until the level is correct.
  12. Reinstall the fill cap.
  13. Start the engine and check for oil leaks.
  14. Allow the system to build to pressure (factory setting 145 psi) and for the engine speed to decrease to base idle.
  15. Turn off the engine.
  16. Once the system has sat for 5 minutes, check the oil level through the sight glass. The level must be between the “MIN” and “MAX” level indicators.
  17. Verify there are no oil leaks.

How much air compressor oil do I need?

There is a significant range in how much oil air compressors require, with no clear standard across manufacturers. VMAC air compressors hold approximately 1-2 gallons (4-9 liters) of compressor oil, depending on the VMAC system, but other systems may contain more or less oil. Read your air compressor’s manual or contact the manufacturer directly to find out how much oil your air compressor system will require.

Where can I buy air compressor oil?

You can buy air compressor oil from air compressor dealers, hardware stores, and some auto shops. VMAC air compressor oil can be purchased through any authorized VMAC dealer.

5 Ways To Reuse Your Air Tank (Now That You Don’t Need One)

One of the coolest benefits of rotary screw air compressors is that they don’t need an air receiver tank. Instead, these powerhouse air compressors operate at 100% duty cycle and provide instant air on demand.

But there’s one “problem” that many operators experience after they upgrade their recip to rotary screw: their old air receiver tank is suddenly a stationary, unnecessary hunk of metal.

Fortunately, we’re here to help. In this post, we’ve put together a list of five ways to reuse an air tank now that you don’t need one…
Continue reading “5 Ways To Reuse Your Air Tank (Now That You Don’t Need One)”