Running a timed pump-up test is a relatively accurate way to check your existing air compressor’s capacity or output. This will confirm that your shortage of compressed air is not due to a worn unit or a malfunction. Check the receiver volume in cubic feet. Check the pipe volume between the compressor and receiver in cubic feet. Operate the compressor at load. Close the air valve between the receiver and plant air system. Drain the receiver down to 70 PSIG. Close the drain valve quickly. Record in seconds the required time to pump to 100 PSIG.

• COUPLINGS, HOSES, TUBES AND FITTINGS – Tubes and push-to-lock fittings are common problems.
• DISCONNECTS – O-rings required to complete the seal may be missing.
• FILTERS, REGULATORS AND LUBRICATORS (FRL’S) – Low first-cost improperly installed FRL’s often leak.
• OPEN CONDENSATE TRAPS – Improperly operating solenoids and dirty seals are often problem areas.
• PIPE JOINTS – Missed welds are a common problem.
• CONTROL AND SHUT-OFF VALVES – Worn packing through the stem can cause leaks.
• POINT OF USE DEVICES – Old or poorly maintained tools can have internal leaks.
• FLANGES – Missed welds are a common problem.
• CYLINDER ROD PACKING – Worn packing materials can cause leaks.
• THREAD SEALANTS – Incorrect and/or improperly applied thread sealants cause leaks.

• COMPRESSORS – These are machines which compress air or gases from atmospheric pressure to a higher discharge pressure.
• BOOSTER COMPRESSORS – These are machines which compress air or gases from a pressure higher than atmospheric to a still higher discharge pressure.
• VACUUM PUMPS – These are machines designed for compressing air or gases from an initial pressure which is below atmospheric to a pressure which is at or close to atmospheric pressure.
• RECIPROCATING COMPRESSORS – These are positive displacement machines used to increase the pressure of a definite volume of gas by volume reduction. The compressing element is a simple piston which reciprocates back and forth in a cylinder.

Once the actual existing compressed air capacity is known, it is relatively easy to mathematically determine the air required to bring the air system up to 100 PSIG:

• CFM P2 | P1 | 653 (114.7) = 884 CFM | 84.7

Therefore, the total air capacity required to hold 100 PSIG is 884 CFM and the additional then is 884 – 653 or 231 cubic feet per minute. Additional compressed air is required to meet the current demand. Depending on the type of system and type of air supply, a “leakage” or “unload factor” should be added to any requirement. This is generally from 20% to 30% depending on the condition. Using a 20% extra capacity factor, the total air requirement would then be 884 x 1.20 = 1060 CFM

A shortage in capacity is often due to or certainly partially due to system leakage. Air system leaks are a continuing source of lost power and should always be minimized. A number of small leaks to that of a 1/4” orifice would at 100 PSIG pass CFM of compressed air. This is a 25HP air compressor to you. A .04 per KWH operating 8,000 hours per year, (3 shifts) this would cost you $6,600 in power cost to do no work. Defective tools, shut-off valves, packings, fit-ups, drain cocks, etc., should be continually checked. Most plants can always afford the maintenance labor and parts to correct leaks. Total system leakage can be identified by measuring time in seconds for the system (receiver) pressure to drop from 100 to 90 PSIG with no air supply or usage.

For example, assume the total receiver and piping of the system is 120 cubic feet. If the plant has a 90 second bleed down rate is 90 PSIG when no production air is being used, this is leakage. The calculated leakage capacity is as follows – (120) (114.7 – 104.7) (60) = 54 CFM | (90) 14.7

• TOTAL COMPRESSED AIR LEAKAGE = 54 CFM X 1.15 = 62 CFM – Add 15% to adjust for the higher leakage rate at the 120 PSIG to 90
• PSIG (30 PSIG x .5) – Any leakage rate beyond 5% of the total system should be corrected.

If you have an air-cooled rotary screw compressor, follow these basic guidelines to help prevent major damage:

• KEEP YOUR OIL COOLERS AND AFTER-COOLERS CLEAN!
• USE THE PROPER COOLANT!
• MAINTAIN THE APPROPRIATE COOLANT LEVEL!
• MAKE SURE THERE ARE NO AIR RESTRICTIONS ON THE INTAKE COOLING AIR!
• HAVE YOUR COOLERS CLEANED AT LEAST TWICE A YEAR. TRY TO SCHEDULE IT SO ONE OF THE CLEANINGS IS DONE RIGHT BEFORE SUMMER!
• MAKE SURE DISCHARGE TEMPERATURE IS WITHIN NORMAL RANGE (175° F TO 200° F)!