Air Compressors: Selection
Things to know:
1. Your compressed air requirements, application and future plans
If you haven’t yet determined your total CFM requirements are, and at what pressure you need that compressed air delivered, you can’t be expected to select a compressor that suits. You really must do this first.
Remember, when you’re sizing your compressor, that 1 HP of compressor output generates about 4 CFM of compressed air at 90 PSI.
This is the accepted wisdom for air compressors over 10 HP. For DIY compressors under 10 HP, you must rely on the figures provided by the manufacturer or “guestimate” that you’re getting around 2CFM of compressed air per HP @ 90 PSI
Don’t forget where you are planning to be in 5 years regarding compressed air-related demand. Allow for expected growth. If you have yet to purchase tools, or are expecting to add some more as your plant grows, this chart will help in determining your additional air capacity required, providing the info necessary for your decision: upgrading your present air supply, or acquire a brand new air compressor.
Consider “factoring in” the highest air consumption figures you find for the tools you expect to use. You may oversize your air compressor capacity by doing so, but under sizing the compressor you purchase will bring greater long term costs to your organization.
2. At what air pressure do you need that flow:
Note that a compressor will always show a higher discharge flow rate at a lower pressure than it will at a higher pressure and compressor manufacturers will often highlight a high flow rate and lowlight the CFM. Remember, it will be air flow at higher pressure that you will require for your application!
3. What are you using compressed air for?
Have you determined what you’ll be using compressed air for? In order to correctly size your compressor, you really must do some homework towards figuring out the volume of compressed air you will require.
You should determine which of those you are or are planning to have, and that will be consuming compressed air in your plant.
4. All at once, or once in a while?
If it is necessary in your plans to run all of those compressed air using appliances at the same time, and for long periods of time, then you are going to require a much bigger compressor than if they are used intermittently and sporadically.
5. Other compressed air use:
Air tools are only one of many applications in a plant that consume air. It’s necessary to give consideration to all of the ways that you will consume air in your plant and applications.
6. Continuous Run or Start & Stop Compressor?:
Whether your compressor should be a continuous run style, or one that starts and stops based on the pressure cycle, may be answered by your demand requirements. If it isn’t clear cut, talk to the manufacturers about their benefits for a compressor that fills a reservoir and stops until low pressure signals a start, or one that runs continuously, but only compresses air as the demand is there.
Field portable units, which often run on gasoline or diesel, are usually continuous run, as the need to start and stop a fossil fuel motor every few seconds is very negative to the motor.
7. Portable or Fixed Installation?:
Are you planning to take it with you to various locations or job sites, or is this air compressor heading for a fixed installation?
While most plants rely on fixed installations, there will be many applications for portable air compressors too. If that’s your choice, you need to discuss with the vendors the methods of portability; is the air compressor carried, come already installed on a wheelbarrow or cart, is the compressor itself wheeled, trailer-ed, etc., etc.? You discuss this with the compressor sales outlet based on your expected needs.
8. Available Power Supply?:
Regardless of the type of compressor you purchase, it will have to have a power supply. If it’s to be installed in a plant, what voltages or amperage’s do you have available? To get the volume of air you need, will you have to upgrade your electrical system?
Since the amount of power available in a typical home 120 VAC circuit limits the size and therefore the volume of air from the compressor, if you are planning on running high demand air tools or multiple tool locations, you will want to look at 220 VAC systems.
If it’s a portable style of compressor, do you want it powered by gasoline, diesel or perhaps propane? Are there any other power options available or necessary for you, depending on where you are in the world?
9. Operating Costs?:
Your compressed air supply and pressure requirements will drive you to select compressor manufacturers that actually make compressors of the capacity and flow that you require.
Then, the available power supply may further narrow the list of vendors to those that can provide a unit that runs on the power you have available or that you choose to use.
Regarding electrically driven compressors, higher voltage units sometimes generate compressed air at lower cost. Ask about the voltages available from that vendor, and your benefit as it pertains to lower operating costs depending on that voltage.
Perhaps you might find that a propane unit offers lower operating costs than gasoline if you need a portable unit.
What is required regarding maintenance for each compressor you are looking at? What are the mean times between maintenance functions; what are the cost of the various consumables each compressor may require, for regular and long term maintenance?
If oil lubed, how often between oil changes, and what is the type of oil recommended? Some manufacturers recommend synthetic oils which come at higher cost. If so, what are the benefits you can expect for that greater expense?
11. Life expectancy:
How long will this particular model last when used at the recommended levels?
If you’ve considered under sizing your air compressor to save some capital expense up front, and end up pushing it past its acceptable duty cycle, how will that affect its life expectancy? Will you be buying another compressor much sooner than expected?
Not all warranties or guarantees are made equal. To get your business, some manufacturers may improve on their warranty. If that is the case, make sure you get any such promises in writing.
You might save a lot of money on a compressor that offers a poor warranty, yet find yourself replacing it completely, long before a more expensive unit, backed by a secure warranty, would have expired.
13. Who fixes?:
Do you have someone in your shop that’s capable of fixing your compressor? If not, who will, where are their offices, what’s the response time, and how much will it cost?
If the repair is out-of-warranty, and you don’t have a person in the shop that is qualified to fix compressors, there are many firms that specialize in doing so. We’re sure you can find a compressor repair company in most cities, as compressed air is used in almost 70% of all manufacturing firms.
14. Repair costs – parts:
Once you know who will fix your compressor, ask for a parts breakdown, determine from the vendor which parts are likely to fail, and ask what the purchase price is for those parts.
There will be significant pricing disparities for similar parts from different vendors.
15. Capital cost:
How much does it cost to purchase each type of compressor you’ve narrowed your list down to?
Some might quote delivered your plant, others not, so add delivery / installation costs to the capital cost of each if the price quoted doesn’t include that, so that all vendor’s prices are quoted the same.
Are there any creative financing options available from the various compressor firms to help entice you to purchase from them?
Some may “carry the paper” of the loan, and if they will, and you decide not to take that benefit, perhaps refusing it might lower the cost of the unit somewhat?