A medical air compressor system does not necessarily make a good laboratorycompressor system. Medical air compressor systems are designed to deliver clean, 50 psi, breathing air as specified by a CSA Standard. Laboratories may not be best served by a medical air compressor. Here are some things to think about when procuring a laboratory compressor system
OIL-LESS VERSUS LUBRICATED
Oil-less air compressors are expensive relative to lubricated compressors. Today’s tight budgeting has required the installation of lubricated compressors in laboratories where the low risk of compressor lubricant in the compressed air is acceptable.
The 50 psi pressure supplied by a medical air compressor is often not enough for a laboratory. Laboratory equipment may require 80 to 120 psi pressure. Determine from the user what pressure is required. A compressor running start/stop will need to shut off at a pressure 20 to 40 psi above the required pressure to allow for pressure switch differential, purification pressure loss, pressure regulation and pipeline pressure drop.
CUBIC FEET FREE AIR PER MINUTE VS. CUBIC FEET COMPRESSED AIR PER MINUTE
Compressors are usually rated in CFM free air. This should be the quantity of air delivered referenced to the compressor inlet conditions. Cubic feet compressed air equates to free air as follows
Hence, at 80 PSIG, 35 CFM free air equals 5.4 CFM compressed air.
Years ago Peerless Engineering was requested by a consultant to supply a 10 HP, 35 cubic feet free air per minute compressor to a laboratory facility. It was determined after installation that 35 cubic feet compressed air per minute was required. To quote Homer Simpson “Do’h!”
The solution was to install a second 50 HP, 175 cubic feet free air per minute compressor. However, the building was plumbed with only ½ inch compressed air lines. To quote Homer Simpson again “Do’h!”.
DELIVERED CUBIC FEET VS. INLET CUBIC FEET
Some compressor manufacturers rate their compressor by inlet cubic feet per minute which equals delivered air divided by the volumetric efficiency. Given that the volumetric efficiency of a compressor can be 70%, rating a compressor by inlet cubic feet per minute makes that compressor look much better to the naïve.
Compressed air dried to a pressure dew point of -40°C by a desiccant air dryer costs more to make than compressed air dried to a pressure dew point of +4°C by a refrigerated air dryer. Many laboratories only require compressed air with an acceptable relative humidity at maximum pipeline pressure and minimum pipeline temperature. This can often be accomplished by a refrigerated air dryer at a lower cost.
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