Often, when it comes to choosing an air compressor for a service shop, there is a choice between types and sizes of compressors. Do you choose a standard reciprocating compressor or do you go with a more modern lubricated screw? But depending on the way you use your compressed air, and how the compressor is installed, the operating costs may be quite different for the two choices—and not in the way you might think.
Reciprocating compressors use pistons to compress the air to the line pressure you desire. They are simple units, often mounted on a small receiver tank, and they usually run in start/stop mode on a mechanical pressure switch. You can certainly tell when they are running, because the pistons create a lot of noise. The cooling systems of these units are typically just fins across which a pulley mounted fan blows a stream of cooling air. The air produced by these units is usually very hot and oily—difficult to clean and dry unless some sort of auxiliary cooling and filtering method is used. If one looks up the efficiency of these units, you may find that at full load they consume more power per unit output than a screw compressor (usually rated in kilowatts per 100 cfm, called specific power). But these units are not designed to run continuously at full load, average loads of over about 60% of the compressor capacity with overheat and damage the internal components, mainly due to the poor cooling method.
Screw compressors, on the other hand, are more complex and expensive units and can often be mounted on the same sized storage tank. These units most often run in a different mode, load/unload which means they run continuously; alternately producing air at full load or running unloaded with zero compressed air output. Screw compressors are much quieter than reciprocating units and have very good cooling systems, similar to automotive radiators across which passes cooling air generated by a fan. These units are designed to run at full load and will produce much cooler and cleaner air, which can be sent directly to an air dryer to remove water vapor. These units usually consume less power per unit output at full load than reciprocating units, making them more efficient in that condition.
But compressors running in a service shop rarely run constantly at full load. Experience has shown that compressors in this type of duty often run at average flows of 10 percent or less. And when one tracks the energy consumption of both types of compressors at this low load quite a surprising picture emerges.
Screw compressors running in load/unload with small storage receivers rarely turn off completely to limit the number of motor starts, which could prematurely burn out the motor; therefore, they will run continuously, consuming power even when they are unloaded and producing no air. This unloaded power consumption typically consumes about 35% of the compressor full load value.
To compare the two types of compressors, the math works like this:
For a 10 cfm load, a 100 cfm 25 hp (21.3 kW at full load) screw compressor consumes 21.3 kW for 10% of the time (2.1 kW) and 7.4 kW for 90% of the time (6.7 kW). This means the total average power consumption is 8.8 kW to produce 10 cfm, a specific power of 88 kW per 100 cfm.
A similar sized 25 hp reciprocating compressor that, say, can produce only 90 cfm at 21 kW, runs 11% of the time to feed the same load (2.3 kW) but consumes no power when it is not producing air. This works to a total average power of 2.3 kW or a specific power of 23 kW per 100 cfm—almost 4 times more energy efficient!
Want to get a screw compressor running like a reciprocating compressor? Add much more storage (ten time normal) and widen the load/unload pressure band. And purchase a compressor with auto/dual mode, which will turn off the compressor between cycles. Your compressor supplier can help you with this.