Choosing a piston compressor.....

Piston Compressors Vs Screw Compressors

For stop start running a piston is always a better purchase over a screw, even with the add ons of an aftercooler and dryer, (the latter often found on a screw) the piston will be cheaper to purchase and run over its working life.

For clean dry air a stand alone aftercooler should always be installed before the dryer, unless an oversized dryer is budgeted for.
Depending on the dew point required up to three times the compressor capacity. Some cheaper units mostly from China have an aftercooler bolted to the belt guard, this cools the air, however it also blows heated air over the cooling fins of the compressor, heating up the cylinders and in turn increasing the outlet tempreture.

A properly installed aftercooler will remove 70-80% of the water condensate, leaving the dryer to strip out the balance. A quality air dryer should have a hot gas by pass as standard with good pre and after filtration to remove oil and other condensates found in compressed air.

The following are important factors in choosing which piston air compressor to purchase....

1, Single or two stage compression, single stage units compress the air to pressure in one stroke and have a inlet filter on each cylinder, two stage units conpress the sir over two stages with an intercooler between stages producing cooler air and have a single inlet filter.

2, Compressor construction, a quality compressor will be made from cast iron and not die cast aluminiium, will have valves made from quality metal and a heavy gauge steel air receiver that doesn't ring like a bell when taped.

3, Compressor speed, the slower the RPM the better. Cooler running, generates less condensate, and promotes longer life.

4, Compressed air performance data claims, beware of marketing companies claims, we have seen the same compressor marketed through different companies each claiming different data. The only data one can rely on is the compressor manufactures and only then if it is backed by an international standard, ISO 1217 or CAGi PNEUROP PN 2 CPTC 2/3. A good dryer standard is DIN ISO 7183.

5, Electric motor, all electric motors sold new in NZ must be MEPS2 )Eff 1) compliant. For better cooling of the motor body, end shields should be made from cast iron. Motors made to this standard should also carry a two year or more warranty.

In summary, if truly industrial components are used eg.a slow revving 400-900 RPM, an all cast iron piston air compressor is used, a 3-1 life can be expected over a rotary screw unit. We have piston compressors in service that are over 50 years old.

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