In the previous post, we outlined some of the considerations for selecting either a reciprocating or rotary scroll air compressor. One of the major considerations will be the routine maintenance, which in turn will affect the “Cost of Ownership over a specified period of time”.
Maintaining Oil-less Air Compressors, Reciprocating and Rotary Scroll
- Air Intake Filter: It is a basic concept that in order to compress air, you must be able to intake air to the compression chamber. Air intake filters protect the compressor from ingesting contaminants that may cause cylinder and piston ring wear or damage, or cause intake and discharge valves to malfunction. As the filter becomes restricted by contaminants, the compressor will lose capacity. Depending on the cleanliness of the location, and the amount of run time for the compressor, filter cleaning or change-out intervals may vary considerably. Generally speaking, filters should be checked every 3 to 6 months and replaced as required.
- Vee Belts: Most reciprocating and rotary scroll compressor use a vee belt drive. Smaller compressors, such as the APPL JP-series wobble piston compressors have an integral motor which drives the pistons at motor speed, so no belts are used. On other, however, vee belts will represent a major part of the maintenance requirements. Vee belt tension should be checked every 3 to 6 months. Belt wear will accelerate if proper tension is ignored. As checking and tension normally require the removal of a belt guard, more time will be spent on belts than on most other maintenance issues.
- Noise and Vibration: Personnel who are responsible for the compressor, should know what the normal noise level and vibration levels are for the unit. Changes in either should result in an investigation. Reasons for abnormal vibration could be the result of a loose or shifted motor pulley, or compressor flywheel, or an accumulation of dirt or contaminants on the flywheel. Excessive noise and vibration could indicate worn wrist pins, bearings, or connecting rod bearings or main bearings.
- Unloader Valve/Discharge Check Valve: When a compressor starts up, it is essential that there be no pressure at the pump, as this will increase the power required to bring the compressor up to speed. In many cases, this extra load may prevent startup and overload the motor. Most reciprocating compressors with auto start/stop controls use a discharge check valve with a pressure switch unloader valve to vent head pressure each time the compressor shuts down. The unloader valve vents air from the compressor discharge line to the atmosphere, while the check valve prevents additional air from returning to the discharge line from the receiver. With rotary scroll compressors, the discharge line air (at shut down) is allowed to vent back through the compressor to its inlet, and the discharge line check valve prevents air from returning from the receiver. With most scroll compressors, this will result in a brief reversing of the pump at shutdown as air is vented back through the pump. In each case, it is necessary to ensure that the discharge check valve is sealing compressor shut down, and for reciprocating compressors, that the unloader valve is venting to atmosphere. Constant bleeding of air during shutdown through the unloader valve on reciprocating compressors, or constant venting of air through the inlet on scroll compressors, will indicate a failed check valve which must be serviced. A visual/audible check should be made weekly to ensure compressor unloading is taking place.
- Cleaning of Components: Cleanliness is not simply a matter of good housekeeping. There are many areas on both reciprocating and rotary scroll compressors that rely on clean surfaces to allow for proper heat dissipation. On reciprocating compressors, the flywheel, cylinder fins, heads and air-cooled aftercooler (if supplied) should all be kept clean to prevent dirt from insulating the components and preventing heat build-up. On rotary scroll compressors, a cooling fan enclosed in a shrouded duct directs cooling air over the finned compression chamber for cooling. This air flow canal, compression chamber fins and cooling fan, and air-cooled aftercooler (if supplied) must all be maintained in clean condition. If heat cannot be properly dissipated from these surfaces, operating temperature may increase, resulting in premature grease and bearing failure.
- Checking for Loose Fasteners: A visual inspection should be made of all fasteners. As compressors are a vibrating piece of equipment, and some components are subject to being heated and cooled on a regular basis, fasteners may loosen over time. An inspection and re-torquing may be required periodically.
Condensate Drainage: Compressing of air generates heat. Most compressors have air aftercoolers which cool the air. As the air cools, it will reach a saturation level (dew point), where further cooling must result in condensate forming in the line. Some compressors discharge from the aftercooler into the receiver, while some may include a condensate separator immediately after the after cooler. In either case, condensate is normally discharged using an automatic drain. This may be a float-type drain valve or an electronically timed solenoid-type drain valve. These devices must be checked, and operation verified. If neither is used, and a manual drain is located on the bottom of the air receiver, and it must be opened on a regular basis to drain condensate from the receiver. If this is not done, the receiver will eventually fill with water, and the compressor will cycle on and off with much greater frequency as the water in the receiver takes up space which should be used for compressed air storage.
- Bearing Greasing: The greasing of bearings is required only at major runtime intervals. It is important for these maintenance issues, to have an hour meter on the equipment in order to know when maintenance is due. For rotary scroll compressors, bearing greasing is usually required between 5000 to 10,000 hours, depending upon operating pressure. For reciprocating compressors, re-greasing of the wrist pin bearings is normally required between 5000-8000 hours.
- Piston Ring/Tip Seal Replacement: Piston rings on reciprocating compressors will normally require replacement between 6000-8,000 hours. Tip seals on rotary scroll compressors will require replacement between 8000 to 10,000 hours.
The above checks are all general requirements, and for the specific model and manufacturer maintenance requirements, refer to your compressor maintenance manual.
How to Determine Compressor Health
Often, compressors will be neglected, and it is not until complaints of low air pressure or excessive compressor operation are realized, that a problem comes to light. The best way to determine if your compressor is operating at its normal efficiency is to do a pump up test. Although it is easy to calculate the approximate average air delivery from your compressor based on a pump up test, it is best to perform a test when the compressor is new and known to be in good operating condition. To do this, turn the compressor off, drain all air (and condensate) from the air receiver, close the receiver service valve, and start the compressor up. Monitor the time that is required to pump the air receiver from 0 PSIG up to 100 PSIG, and document it. You can compare this pump up the time to future tests you perform to see if the mechanical efficiency has deteriorated. If the future times are considerably longer, it may be time to perform some seal, ring or valve maintenance.
Air Power Products Ltd. is one of the most well-known and experienced suppliers of high-quality air compressors in Canada (since 1980). We specialize in oil-less compressors and design and build systems to suit most applications.