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 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. The tool or equipment can run once the tank fills with enough air. 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 Requirement||1.25 Multiply||1.5 Multiply||Suggested Tank Size|
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. The task will be complete by the time the air receiver tank is empty. Operators can sometimes save money or "make do" with a smaller system using this strategy.
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, eliminating or reducing the air receiver tank 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 compressed air volume 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 complicated and time-consuming. 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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