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The Truth About Air Compressors

Air compressors, they are a machine that everyone has interacted with in their life time. Whether it is in the garage, at the gas station, or in a manufacturing environment compressed air is everywhere. Compressed air is one of the most widely used utilities out there, but also one of the most expensive utilities out there. The key to is to make sure that you have a compressor that isn't going to be constantly running, and the best way to do that is to model an air compressor using AFT Arrow or AFT xStream.

As of writing this blog, there is currently no specific junction that will model a compressor and tank combo, so we must get a little creative with how we model one. To start, we must understand how an air compressor works; most air compressor systems are what you would refer to as a tanked compressor. In other words, a compressor (either centrifugal compressor or a positive displacement compressor) supplies air directly to a tank. Air compressors use a control system to manage the pressure inside the tank, so the compressor will not run all the time, and instead only when the tank needs it.

Now that we have an idea of how air compressors work the question remains, how do you model an Air Compressor? In standard operation of an Air Compressor there are typically 3 different Scenarios that you will want to look at:

  • Scenario #1Sufficient Supply: The air compressor has a partial to full tank
  • Scenario #2 Insufficient Supply: The tank is has emptied to a point that the compressor can't keep up with proper operation
  • Scenario #3 Normal Operation: Transient Operation analyzing the tank draining/refilling

For Scenario #1 in Arrow the best way to model this is to just use a tank junction with a given pressure inside. Since Arrow only looks at a steady state operation we don't need to worry about looking at the compressor. In standard operation of an air compressor the pressure in the tank is what supplies the pressure, this means that the tank is all we need to look at.

For scenarios #2 and #3, an assigned pressure set to atmospheric pressure (1 atm) connected to a User defined pipe that is defined as frictionless pipe with an ID of 36" or larger. This is to mimic the atmosphere as best as possible.

Scenario #2 which is insufficient supply from the compressor models a tank that is now empty. Meaning we can eliminate the tank junction from the scenario since it will no longer influence the system with a source of pressure. This is because we know that the air compressor will turn on the compressor motor when the pressure in the tank is low enough and there is no pressure being supplied by the tank. This means that the only junction you will need for an analysis is the compressor itself. For cases like this you will typically see lower pressures in the system then in normal operation for the air compressor.

Lastly Scenario #3 which is modeling normal operation; in-order to completely analyze the transient around how the tank and compressor motor interact with your system, you will need AFT Arrow XTS or AFT xStream. This is because the system will need to be set-up as a transient around the compressor knowing the pressure in the tank and adjusting comparatively. First, to set-up the compressor, under the Transient tab of the Compressor Properties window you want to select 'Dual Event Cyclic' for the Initiation of Transient and for Event Type select 'Tank Pressure.' Now, for the transient information on the compressor there are two parameters: 

1.Maximum pressure in the tank – If the pressure in the tank reaches or exceeds this pressure the motor will shut off and stop filling the tank. In the transient data you should enter the flow that the compressor will supply over the set amount of time eventually ending up at 0 for the mass flow. The ending time can be an arbitrarily large amount of time such as 24 hrs.

2. Minimum pressure in the tank – If the pressure in the tank reaches or drops below this pressure the motor will turn on until it reaches the max pressure. In the transient data you should enter the flow that the compressor will supply over the set amount of time, eventually ending at the max flow and continuing at the max capacity that the compressor can supply. The ending time can be an arbitrarily large amount of time such as 24 hrs. 

The Dual Event Cyclic transient will allow the compressor junction to oscillate between the maximum and minimum pressure that the tank is set to. After the compressor is set-up, use another frictionless pipe to connect the compressor to the tank; then ensure that the tank is set as a finite and the volume is entered. Once the air compressor is set-up you can design the rest of the system. Once you finish with the rest of the system, run the model. Once the model converges you can begin your analysis

For further information please check out the help site for AFT Arrow and AFT xStream. If you need any other support questions or questions on any of our fluid flow analysis programs, please contact our support team at This email address is being protected from spambots. You need JavaScript enabled to view it.

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Thursday, 30 March 2023
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