The basics…Pressure, Flow, Tank (Receiver) Size, and Available Electrical Supply
Pressure: Force applied uniformly over a surface, measured as force per unit area.
For our discussion we’ll use pounds per square inch, psi. My apologies to our friends across the pond. In my years of experience in manufacturing and the compressed air industry I have found that most equipment is designed to operate in the 80-90 psi range. Though I don’t know the exact history of industry settling on this specific range, it appears to be a happy medium for the amount of work that can be done for the size of standard actuators and tools available.
My preferred starting point for sizing compressors is to know the pressure required for all devices consuming air from the system. If you have a single device requiring a higher pressure you may need a separate, dedicated system or other solution. Let’s save that scenario for another discussion.
Don’t be fooled by box stores and some sites that promote 175 psi compressors. There appears to be some fascination with high pressure air even though it is not needed. Higher pressures have some advantages for storing more volume but providing 175 psi to air tools rated for 90 psi is only good for the person selling tools or tool rebuild kits…not to mention it could be dangerous. Excessive pressure requires regulators which is just another expense.
Once you have verified pressure requirements for your air tools, CNC’s, t-shirt presses, etc., let’s move on to flow requirements.
Flow: Unit volume per time
We’ll use cubic feet per minute, cfm. Tool and equipment manufacturers will publish or provide the flow requirements for their products. Usually they will provide flow requirements in the form of cfm at a specific psi. For example, 10cfm @ 90psi.
Compile a list of all equipment flow requirements. Next, estimate which devices will be consuming air simultaneously. Once you have decided which devices will be used together, add the flow requirements to get a total cfm demand for the system. I add 25% to the total when sizing compressors for customers. This covers leaks, occasional higher demand, and some misuse that always occurs. You can add 10% to save a little on cost but could see pressure drop below the 80 psi minimum at times.
Tank (Receiver) Size
No definition needed for this one but we will specify tank volume in gallons. Standard receivers are available in vertical or horizontal configurations. Most box store compressors and light commercial compressors are tank mounted from the factory. Heavy commercial grade or industrial compressor will be available in floor mount or tank mount versions with other options available as well.
The rule of thumb for compressed air receiver sizing is four times the cfm in gallons. In other words, multiply the cfm required for your system by four and this will yield the recommended receiver size in gallons. Depending on the compressor size required to meet your pressure and flow requirements, some tank mounted compressor might not have the recommended tank volume. For compressors in the 5 to 10hp range it just isn’t worth worrying about the receiver size. On 15hp and above you should consider additional storage in your compressed air system. Reasons for sizing receivers properly on larger systems is beyond the scope of this article.
Once you have established a minimum system pressure and the maximum flow requirements it’s time to pick the compressor. Don’t fall for the 175 psi, 80 gallon compressor advertisements. You want a compressor with sufficient horsepower and pressure rating to cover the demand of your system. Excess pressure is just money down the drain in electrical consumption and downstream equipment maintenance or damage.
Let’s say for example that your equipment requires 90 psi minimum pressure and all devices in the system running at the same time requires 25 cfm (including the 25% fudge factor). Compressors in the range of our discussion typically produce 4 cfm per horsepower at 125 psi. 25 cfm would mean a compressor with approximately 6.25hp. Well…the problem is we can’t get a 6.25hp compressor. It will have to be a 5hp or 7.5hp, since those are standard motor sizes. Dropping to the 5hp is going to cut into our fudge factor so I recommend the 7.5hp. That will give us a compressor capable of 28 cfm at 125 psi.
A 7.5hp compressor that is tank mounted will likely have a volume of 80 to 120 gallons. Our 4 x cfm number is 112 so we’ll call that acceptable for this size compressor.
Available Electrical Supply
Compressors in the 3hp+ range are going to require at least 208-230-volt supply. For units up to 7.5hp a 230-volt, single phase motor option is usually available. 10hp units and above will be 208-230/460-volt, 3-phase. Some premium compressors are often available in tri-voltage versions. This means they can be field configured to operate on 208, 230, or 460-volts, 3-phase supply.
Sizing of the electrical supply should always be done by a licensed electrician. Please contact a local electrician to assist with the sizing and installation of the electrical portion of your compressed air system.
I hope this info has been useful in your quest to size and pick a compressor. I realize that it is a very brief summary of the methods and information available regarding compressed air. If you have questions or would like to request article on other topics, please comment or drop me a line.