Determining what size of compressor to buy is a very difficult problem. This is especially true if the compressor will be for a new system that hasn’t been built yet. If you answer this question wrong, you might end up with pressure problems if the compressor does not have enough output to feed peak air flows. And if the compressor is too big, it will cost you a bundle in wasted electrical and maintenance costs.
But help is out there. There are quite a number of resources available on the internet to help you correctly estimate the compressed air load for various hand tools. Try searching on “Free Air Requirements of Tools” and most often the suppliers of the compressed air powered production equipment can assist you in estimating the compressed air demand. You will need to carefully estimate the percentage of time the tools will be operating during peak production—that is the most difficult part!
Note that calculations such as this will estimate the peak requirements during the busiest time in the plant—but take into account the diversity of the load. Also included are estimated plant leaks and a design factor to guard against under-sizing due to unanticipated flows that have been left out of the calculation. Calculations like this often result in the purchase of a compressor that is too large. Why? Because very few people ever get fired for purchasing a compressor that is too large, but purchase one that is too small and watch out!
As a result of this risk, design factors are added to the estimates. We can see in the table above that a factor of 15% has been applied for leaks and 30% as a design factor for future loads. This results in quite a large compressor, about 100 to 125 hp for this small assembly plant.
The overall average flow is most often quite a bit lower than the total estimate, especially for shift-oriented facilities that operate with different shifts. You should take this into account when choosing compressors. Using one large compressor in this case might cost a significant amount of money to operate due to oversizing—and may even cause the compressor to require additional maintenance. Two or even three smaller units would likely be better, especially if they are controlled efficiently.
For existing systems, a direct measurement of flow is a good step. Over the years, flow measuring instrumentation has come down in price, so this is perfect for do-it-yourselfers. A properly installed flow meter connected to a data logger can get you familiar with your flow profile. After measuring for a period of time, it is fairly easy to correctly size air compressors to feed the existing characteristic flows.
If you want this measurement done for you, most compressed air vendors offer system assessments that could help you in correctly choosing your compressor size. In addition to this, the study might also identify problems with your existing system and areas of waste. The savings gained if these items are corrected might pay for your new compressor, especially if your power utility has energy efficiency incentives.