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Easy Guide to Rotary Screw Air Compressors (2019 Edition)

G30 Gas Driven Air Compressor

G30 Gas Driven Air Compressor

Updated: December 11, 2018.

Rotary screw air compressors operate by trapping air between two meshed rotors and reducing the volume of that trapped air as it moves down through the rotors. This reduction in volume results in compressed air, which can then be used to power air tools, inflate tires, or in numerous other applications.

In this guide to rotary screw air compressors, we tell you everything you need to know, including:

What is a Rotary Screw Air Compressor?
Oil-Injected vs. Oil-Free Rotary Screws
Air Compressor Components
Basic Functions & Operation
Power Sources For Mobile Air Compressors
CFM & PSI
Advantages of Rotary Screw Air Compressors
Disadvantages of Rotary Screw Air Compressors
Types of Rotary Screw Air Compressors
Manufacturing Rotary Screw Air Compressors

What is a Rotary Screw Air Compressor?

When most people think of air compressors, they picture reciprocating air compressors (also known as piston air compressors.) These air compressors physically push air into a small space, using pistons, and then collect that compressed air in an air receiver tank. Reciprocating air compressors are relatively inexpensive and have been around for decades.

reciprocating air compressor

Pistons compressing air

Rotary screw air compressors are a newer, improved type of air compressor. They are more expensive than traditional reciprocating models but have numerous benefits that are quickly making rotary screw air compressors the system of choice for service truck and van fleet managers around the world.

Rotary screw air compressors operate by trapping air between two meshed rotors and reducing the volume of that trapped air as it moves down through the rotors. This reduction in volume results in compressed air, which can then be used to power air tools, inflate tires, or in numerous other applications.

VMAC rotors

The 10,000th pair of rotary screws manufactured by VMAC, accomplished April 2014

You can learn more about rotary screw versus other types of air compressors in these articles:

Oil-Injected vs. Oil-Free Rotary Screws

Rotary screw air compressors used on trucks and vans are oil-injected, which means oil is used to lubricate and seal the rotors in the air-end. This allows the rotors to create high pressures quite quickly and compress air in a single stage. The oil then gets separated out from the air before the air exits the system and is recycled back into the air-end for reuse. Oil-injected rotary screws allow the compressor systems to provide high CFM and psi on a continuous basis.

Oil-free rotary screw models are used in industrial, production or medical applications when absolutely no oil can enter the airflow, like for food packaging or medical oxygen. Oil-free rotary air compressors are more expensive, as they require 2 stages of compression to reach the same pressures of an oil-injected rotary system. Most companies only use oil-free rotary screw air compressors when they absolutely need to so oil-free air compressors aren’t usually found on vehicles.

This guide is dedicated to rotary screw air compressors used on vehicles and therefore focuses on oil-injected rotary screws by default.

 

Air Compressor Components

Air-ends

Many components make up an air compressor, but the heart of the system is the “air-end.” The air-end is the part of a rotary screw air compressor system in which the air is compressed. An oil-injected air-end looks like this:

 

Internal view of VMAC air end

Internal view of VMAC air end

External view of VMAC air end

External view of VMAC air end

 

 

 

 

 

 

 

 

Air enters the air-end through an inlet valve, where it is mixed with oil while being compressed. The oil is then separated from the air, which exits the system. Learn more about how the air compressor system works in the Basic Functions & Operations section below.

While the air-end is where air compression actually happens, there are many additional components required to make a rotary screw air compressor work.

Other Common Components

Air filter

air filter

Air filter

The quality of air entering your compressor will affect the longevity of your system. Using an air filter suited for the environment and the air flow required

is a necessity.

That air filter can either be mounted directly to the compressor or remotely mounted and connected with an air intake hose. Cool, clean air should be considered when choosing the location of the filter.

Primary Separator Tank

Primary separation filter

Primary separation filter

When compressed air leaves the air-end it is mixed with compressor oil. This mixture enters a primary separator tank where the first steps in separating the oil from the air happens. The primary separator tank uses mechanical separation and centrifugal force to drive oil molecules together, which form droplets that eventually fall to the bottom of the tank. A large amount of oil is removed from the air in this stage. Often, the separator tank also acts as an oil reservoir.

Secondary Separation Filter

Secondary separation filter

Secondary separation filter

Once the compressed air leaves the primary separation tank it is close to being oil free. The remaining oil mist mixed with the air will enter the secondary separation filter, which is a coalescing filter. This filter includes a membrane material that gathers the remaining oil particles and circulates the oil back into the main oiling system. The air leaving the coalescing filter is considered oil free.

The secondary separation filter can be attached to the primary separation tank or mounted remotely on its own filter manifold. Figure 7 above includes both the primary separation filter (labeled Stage 1) and the secondary separation filter (Stage 2).

Oil filter

Oil filter

Oil filter

As a rotary screw compressor has a closed loop oiling system, the installation must include an oil filter. The oil filter’s job is to collect any loose particles that have collected in the oil and shouldn’t re-enter the air compressor system. The oil filter can be located on the compressor, the separator tank or mounted remotely on a filter manifold.

Oil Cooler

Oil cooler

Oil cooler

The process of compressing air generates heat and lots of it! This heat warms up the oil, which must pass through a cooler before it is circulated back to the compressor. A liquid to liquid cooler can be used in conjunction with an engine’s cooling system or a stand-alone air to liquid cooler can be used; each has its own benefits.

The liquid to liquid version will need sufficient cooling capacity to cool the compressor oil combined with the engine cooling. The air to liquid cooler will need clean, cool air to keep the oil within an optimal temperature range.

Hoses

Air compressor hose

Air compressor hose

Moving oil and air between various air compressor components requires the use of hoses. The hoses must meet the requirements of heat, pressure and the chemical components of the compressor oil. Incorrect hose selection will result in premature failure of those hoses, which can be expensive and dangerous.

Controls

VMAC hydraulic system control

VMAC hydraulic system control

Air compressors will use mechanical or electrical controls in the form of a button, switch, or lever. These controls will turn the air compressor off and on and may also provide important diagnostic or service information. The location and type of components used in the controls should reflect the environment the air compressor will be in.

Oil

High performance synthetic oil

High-performance synthetic oil

Oil is a critical component of an oil-injected rotary screw air compressor, performing many important jobs at once. Oil lubricates the system, protects components from wear, keeps the system cool, and helps trap and remove contaminants. Without the right type of synthetic oil, oil-injected rotary screw air compressors would not function.

The components listed above are necessary, in some form, for a rotary screw air compressor system. There are many more options for filter manifolds, coolers, oil separators, and so forth. Each system is a bit different, which means the required components are as well.

 

Encapsulated Air Ends

Encapsulated air end

Encapsulated air end

Some manufacturers use encapsulated air ends, which combine several components within the air end’s metal casing, including the rotors, intake valves, and separators in a seemingly convenient package. However, encapsulated air ends are bulky and restrictive, which can present design challenges for air compressor manufacturers, vehicle upfitters, and OEMs. The combined pieces result in an awkward, inflexible shape that must be accommodated.

When manufacturers choose not to combine multiple components within an encapsulated air end, they have the freedom to design smaller systems with more efficient designs, without compromising on power or efficacy.

 

Basic Functions & Operation

Rotary screw air compressors are pressurized systems, comprised of many interconnected components that work together to compress air.

Air and oil are both important aspects of a rotary screw air compressor system. Compressed air is the end goal, which requires the use of atmospheric air, but the oil is just as important. Oil is used to lubricate the system and is the not-so-secret weapon that makes rotary screws so efficient at a lower price-point.

But the use of oil also makes the air compressor systems more complex. In addition to requiring a mechanism for producing compressed air, rotary screw air compressors also need to circulate, filter, and recycle oil.

In a sense, there are two separate systems working together: one makes air and the other circulates oil. Combined, the systems look something like this:

Air compressor system flow

Air compressor system flow

A step-by-step guide to the system flow can help explain how the air and oil processes flow together within a single cohesive system:

System Flow Process

Step 1: Atmospheric air enters the inlet valve.
Step 2: Air flows through the system pressure line to a valve on the regulator, which sets the pressure for the entire system.
Step 3: Air is mixed with oil and compressed via the rotors in the air-end.
Step 4: Air mixed with oil exits the air-end via the air discharge hose.
Step 5: Air mixed with oil enters the primary oil separator tank, which separates most of the oil from the air.
Step 6: Air enters the second separation filters, which catches remaining oil mist in the air.
Step 7 – A: Oil-free air exists the system; if an air receiver tank is used, the air collects in the tank.
Step 7 – B: Oil is moved into the oil cooler, cooled down, and then sent to an oil filter.
Step 8: The oil filter catches any debris remaining in the oil.
Step 9: The recycled oil is returned to the air-end, via the scavenge line.

Note that visualizing the process as steps makes it easier to understand the overall flow, but these steps operate concurrently. As soon as the air compressor is turned on, all the steps outlined above occur simultaneously and continuously.

 

Power Sources For Mobile Air Compressors

Air compressors need a power source to run. Fortunately, vehicle mounted air compressors have several options: a separate air compressor engine, using a truck’s existing engine, or tapping into an auxiliary power source like a PTO or hydraulic port.

For example, here’s a breakdown of VMAC’s rotary screw air compressors and their power sources:

VMAC Air Compressor/Multi-Power SystemPower Source
G30 Gas DrivenStandalone Gas Engine
D60 Diesel DrivenStandalone Diesel Engine
Multifunction 6 in 1Standalone Diesel Engine
H40/H60 Hydraulic DrivenHydraulic Port
Direct-Transmission Mounted™PTO
UNDERHOOD™Gas or Diesel Vehicle Engine

 

In summary, there are a lot of power options for mobile air compressors. The right one depends on a truck’s existing power options and whether they will work with an air compressor that provides enough CFM and psi. Vehicle engine mounted and hydraulic air compressors are convenient, while standalone engines are efficient and cost-effective.

 

CFM & PSI

Air power is typically measured in CFM and psi. CFM or “Cubic Feet per Minute”, is the amount of air that’s being delivered. Meanwhile, psi or “Pounds per Square Inch” is the amount of force behind that air. Together, CFM and psi determine how much air is being delivered and at what pressure. Air tools require the right psi and the right CFM to operate efficiently.

Rotary screw air compressors tend to produce lower psi than reciprocating air compressors, but this isn’t a problem for most mobile air applications. Most air tools run at 80 to 110 psi, which is well within the capabilities of rotary screw air compressors.

Find out more about CFM and psi in these two articles:

Advantages of Rotary Screw Air Compressors

Rotary screw air compressors have many benefits that make them ideal for both mobile and standalone applications. As a result, you’ll find rotary screw air compressors on vehicles and trailers, as well as in industrial, production and medical facilities, where high quality equipment is required.

The benefits of rotary screw air compressors over other compressor types include:

  • Continuous airflow/100% duty cycle
  • Larger quantities of air
  • Higher CFM per hp
  • Longer lifespan
  • Better warranties
  • Quieter
  • Energy efficient

Most people appreciate the longevity, reliability, and easy access to instant air. Because rotary screw air compressors can run continuously, there’s no need to wait for an air receiver tank to fill before you can start using the air. Those tend to be the most popular benefits of rotary screw air compressors.

Many operators also appreciate the limited lifetime warranties that come with some rotary screw air compressors, like VMAC’s. These types of warranties are available because the rotors and air-ends are proven to stand the test of time—or, more specifically, the life of a modern service truck. This longevity makes rotary screw compressors a drama-free option.

For example, the very first VMAC rotary screw air-end outlived the truck it was installed on. The rotors were returned to us and now have a permanent home in our trophy case.

VMAC's first rotors

VMAC’s first rotors, which outlived the truck

Rotary screw air compressors’ lengthy lifespan is a huge advantage over reciprocating air compressors, which are jokingly referred to as “throw-away air compressors” because they’re expected to fail after 3 to 5 years.

 

Disadvantages of Rotary Screw Air Compressors

Rotary screw air compressors have a lot of advantages, but they aren’t the right choice for everyone. Common rotary screw air compressor disadvantages are:

  • Upfront cost
  • Skilled maintenance required

The number one reason people choose a reciprocating air compressor over a rotary screw is the initial cost. Rotary screw air compressors often cost twice as much as reciprocating air compressors, but it’s a situation where you genuinely get what you pay for.

Rotary screw air compressors may be 2x the cost, but they last at least 4x longer, on average, and produce more air. That makes them a solid investment for any business that plans to use compressed air over the long term.

The other disadvantage to rotary screw air compressors is that they require skilled maintenance. Every type of air compressor needs regular servicing, but the complex nature of rotary screw systems requires a higher level of expertise.

 

Types of Rotary Screw Air Compressors

There are many types of rotary screw air compressors used on vehicles in mobile applications. VMAC’s air compressor systems demonstrate the range of rotary screw compressor options for vehicles:

UNDERHOOD™ Air Compressors (VR40, VR70, VR70-G, VR150)

UNDERHOOD air compressor systems are one of the coolest options out there, as the air compressor components are integrated with existing truck or van components. The air-end is mounted under the hood and the compressor system is powered by the vehicle’s existing engine. UNDERHOOD air compressors are super light, weighing in at 62 to 200 lbs (depending on the system), which saves precious GVW and cargo space.

Installed UNDERHOOD air compressor

Installed UNDERHOOD air compressor

Direct-Transmission™ Mounted Air Compressors & Multi-Power Systems (DTM)

The Direct-Transmission Mounted air compressor uses a vehicle’s PTO to drive the air compressor. Like the UNDERHOOD, the DTM is a lightweight “out of sight” option that integrates with a vehicle’s existing components.

DTM70-H

DTM70-H

Gas Driven Air Compressor (G30)

The G30 gas driven air compressor has a separate Honda GX390 gas engine, like you’d find in a ride-on lawn mower or other small vehicles. This style of air compressor is mounted to a truck or van and can be easily accessed by any operator with a key.

G30 Gas Driven Air Compressor

G30 Gas Driven Air Compressor

Diesel Driven Air Compressor (D60)

Similar to the G30, VMAC’s D60 is a diesel driven air compressor with its own Kubota diesel engine. These air compressors are also easily mounted to a work vehicle.

D60 Diesel Driven Air Compressor

D60 Diesel Driven Air Compressor

Hydraulic Driven Air Compressor
(H40, H60)

Vehicles that have existing hydraulic systems can tap into those systems to power their air compressor. The VMAC hydraulic driven air compressor makes sense for operators with hydraulic cranes and other hydraulic-powered equipment.

VMAC Hydraulic Air Compressor

VMAC Hydraulic Air Compressor

Multifunction Power Systems

Multipower or multifunction systems are a popular option now being offered by some manufacturers. Multipower systems combine air compressors with other vehicle-mounted equipment, such as welders, generators, and boosters, in one convenient system. VMAC’s Multifunction Power systems are powered by a Kubota diesel engine.

 

Internal View of VMAC Multifunction System

Internal View of VMAC Multifunction System

 

Manufacturing Rotary Screw Air Compressors

Rotary screw air compressors are made up of numerous parts that need to be assembled into a highly efficient system. True air compressor manufacturers will create these parts within their facilities, using an in-house foundry, CNC machines, and other specialized equipment to make the required components. Individual components are then assembled on-site into an air compressor system.

Crucible & furnace in VMAC's foundry

Crucible & furnace in VMAC’s foundry

One of VMAC's CNC machines

One of VMAC’s CNC machines

Design Tolerances

When engineering and manufacturing parts for any system or machine, there is an acceptable margin of error, called tolerance.

VMAC's coordinate measuring machine

VMAC’s coordinate measuring machine

The required dimensions for air ends in rotary air compressor systems are so precise that these tolerances are incredibly low. Computer Numerical Control machines—CNC machines, for short—allow manufacturers to meet the exact tolerances needed for rotary screw air compressors.

For example, these are the maximum tolerances VMAC allows for parts made with CNC machines in comparison to the parts made using manual machines:

 Maximum ToleranceSuggested Tolerance
CNC Mills±0.0003±0.005
CNC Lathes±0.0003±0.005
Manual Mills±0.001±0.005
Manual Lathes±0.001±0.005

 

A stringent quality control process ensures the required tolerances for each machine are met. If a machine fails to produce parts within the acceptable tolerance, those parts are melted back down or otherwise recycled. The designs and machines will also be evaluated and adjusted, as needed.

Fortunately, manufacturers using modern quality control technology can usually predict when a CNC machine will need to be adjusted well in advance, and plan for it accordingly.

True Manufacturers vs. Assembly Companies

The major advantage to making parts on-site is that the parts can be modified at will. Instead of being restricted by existing component shapes and sizes, true manufacturers have the freedom to innovate on individual parts on their own desired schedule, creating more efficient, powerful and compact air compressor systems.

A more common type of air compressor manufacturer will purchase existing parts from suppliers or, less often, contract their creation out to external companies. If a custom part is required, these air compressor companies have to book a space with third-party foundries, machinists, and so forth, and then rely on their general expertise for modifications. This style of air compressor manufacturer essentially assembles pre-purchased pieces into air compressors before shipping them out to customers.

VMAC is one of the only true rotary screw air compressor manufacturers in North America.