Efficiency of air compressors

If you are talking of efficiency purely in terms of power consumed per cubic foot of air delivered then it could be true that screw compressors are less efficient than piston (reciprocating) compressors.

Smalls screws' of your size are certainly single stage, that requires a little more power than a 2-stage, plus the fact the screw' also pumps (large amounts) of oil. The oil is not just for lubrication but also essential to make an air-tight seal on the screw. This requires more power. Having said that, the cooling effect of oil in the compression process reduces the power needed. Overall, it could all balance out.

Your problem lies with the off-load power. All compressors consume (a lot of) power when running off-load (running but not pumping) and screws' are worse in this respect (bad to be honest). This can be reduced by running at a lower max pressure.

It comes down to the method of off-load control. Stop the compressor. Fine, but motors overheat on frequent start-ups, so this might not be a good idea if your use is frequent but intermittent. You can reduce start-ups by an off-loading device that allows the compressor to run on awhile before shutting down, and/or a larger receiver (tank).

Increasing off-load efficiency thus adds to the complexity of the control devices that must be built into the original screw compressore package (it is not a DIY retro-fit option) - which adds to the purchase cost.

Once up and running, apart from power, you are left with other running costs, and that as you guess involves filter changing. This is critical for a screw compressor. Do not neglect it. Find the filter changing schedule and cost. And make sure the compressor is in a well ventilated area to keep it running cool.

All in all, on-load power consumption is much the same. Off-load power makes a big difference for a screw' - so make sure adequate control devices are included. This is most important if you buy a screw bigger than you immediately need - as you might have to - because for your size there is not a lot of choice available.

Finally, the oil carry-over from a screw' is very low in bulk (when the internal oil-separation filters are properly maintained) - but what oil mist there is will be in submicron particle sizes. You will need proper high effciency coalesing filters in the air-line.

I hope this helps.

The most important thing in selecting a rotary screw compressor is proper sizing. As the previous post mentioned, the compressor's motor should not be started and stopped. The compressor will be most efficient when it is operating as close to 100% full load as possible; say around 90-95%. As it is sometimes difficult to determine the needed size of a compressor, then it is a good idea to rent what you think you will need and see how it performs as far as loaded-unloaded time. If the sizing is not optimum, then go up or down a bit. And as the rotary screw machine ages it will become less efficient due to blowby, so it needs to have just a tiny bit of reserve.

The reciprocating compressor are efficient only because of their large flywheels that maintain motion while the compressor is unloaded. These machines operate ideally at a load-unload time of 50:50, as the cooling systems of these machines is never that good and the valves become extremely hot.

While testing compressors for proper size it is a good thing to install a quick plug-in for both the electrical power and discharge for the rental compressor. This is in case you ever need a rental unit while your compressor is being serviced.

Ing. Robert Forbus

70 to 80%

Thanks guys That does help! Dick

You will need proper high effciency coalesing filters in the air-line, but "KEEP in mind that even using the run-on of the compressor you will have oil carry over on short cycle. Even with the oil-less compressors! When you are replacing the coalesing filters media, it is not cheep with the down time and the material cost. I would not over size a compressor if the air has to be "clean" I would put my money into a larger reciver tank, and a compressor after cooler.

As a millwright in the pulp and paper industry for over 43 years I have had a of lot maintenance experience with Air Compressors, both piston and screw. A small shop using intermintent needs for air, a small piston compressor is best as motor does not run continously. The screw compressor is great for a shop tha uses a large continous volumes of air and remember the motor/rotor system runs continously. Shutting down the screw compressor when no air is required or no personell are availble to do radom operating checks is very impotant in preventing failures, Most Screw Compressor require water cooling as well as the large piston compressors. I have seen several screw compresors, after an internal failure, which the damage almost always require a total expensive Screw/Rotor rebuild. One large screw compressor failure caused a large explosion and fire when the superheated lube oil got into the compressed air system and ignited. This was several years ago. (Bellingham,Wa) The maintenace on a small piston compressor is mainly replacing the inlet and outlet valves, changing oil and filters. Screw Compressors need less mechanical maintenance, however, they require more daily routine operational inspections for filter plugging, oil levels and needed tempeture recording with other data that can track any operational preformances exceeding operational limits, which might require immediate or needed/planned maintenance. DonnieH