Air Compressors

General

Compressed air is utilized in many commercial and industrial facilities for a variety of purposes and is often considered as a utility which is essential to production. All too frequently, however, little thought is given to the cost of compressed air and the potential for reducing these costs. In fact, compressed air is an expensive utility and users should make every possible effort to use this utility wisely and efficiently. This fact sheet is intended to give the user of compressed air some general ideas about improving the efficiency of a compressed air system. The user is encouraged to seek specific advise concerning the compressed air system in their facility.

Compressed air is delivered to the end by a system, consisting of a compressor, various types of conditioning equipment ( such as filters, dryers, oil separators, pressure regulators, etc.), and a distribution system. Each of these components is unique to each system and each has an impact on the total cost of compressed air. Efficiency improvements should consider all these components.

Compressed Air Leaks

All compressed air systems have leaks. Generally, leakage rates are considered acceptable as long as they don't exceed 5% to 10% of total compressed air usage. Leaks should be corrected as they are found or observed in the normal course of maintenance, however, a test can be made of the system during periods of no production to determine the overall leakage rate. It is recommended that this be done annually to determine the need for corrective action. Leakage in a compressed air system can be very costly. For example, a 1/8" diameter hole in a 100 PSIG system can cost $1,240 per year.

Reduce System Pressure

The operating of a compressed air system greatly affects the cost of compressed air. Operating a compressor at 120 PSIG instead of 100 PSIG, for instance, requires 10% more energy as well as increasing the leakage rate. Every effort should be made to reduce the system and compressor pressure to the lowest possible setting.

Typically, there should be no more than a 5 to 10 PSIG pressure drop between the compressor discharge and the equipment requiring the highest pressure. Each component in the compressed air system should be evaluated for excessive pressure drops. All too often, system pressure is raised to a high level or new compressors are purchased to satisfy a deficiency at a particular piece of equipment when the real problem lies in undersized piping or clogged filters, dryers, or separators.

Pressure regulators are recommended to isolate high pressure needs from low pressure needs in order to insure air usage is not excessive. This also reduces the leakage rate in the low pressure zones. If one particular piece of equipment requires high pressure air with the balance needing only low pressure, it may be cost effective to provide totally independent compressions for this purpose.

Heat Recovery

Only 20% of the energy input into an air compressor is converted into compressed air energy. The remaining 80% is converted into heat. This heat is available to supplement space or water heating requirements.

For air cooled compressors, heat recovery simply involves using ductwork to divert the 120°F to 200°F discharge air to the space to be heated with a bypass to the outdoors when heat is not required. Approximately 50,000 BTU/hr per 100 CFM can be recovered and most manufacturer's fans will tolerate the additional static pressure. For water cooled compressors, heat recovery is less effective and more costly but should be evaluated for compressors over 125 HP.

High Efficiency Motors

Many air compressors use standard motors as the prime driver. High efficiency motors in standard frame sizes are available that can increase efficiency by up to 5%. These should be considered both as a retrofit option and as an option for new compressors.

Inlet Air

Reducing the intake air temperature by 5°F will result in a 1% increase in compressor efficiency as a result of colder air being denser and already partially compressed. Extending the compressor intake air to the exterior is recommended and can be very cost effective.

Dryers

Most compressed air systems require that the delivered air be relatively dry and free of moisture in order to prevent damage to the end use. Several methods of providing dry air are available and the energy consumption for these dryers can vary enormously.

The first step in providing dry air is the after-cooler which lowers the compressed air temperature and condenses water in the process. Specifying or installing an oversized after-cooler is recommended as this is typically the least energy intensive method of water removal. Air will typically leave the after-cooler at 100% relative humidity however and further drying is generally required.

The most efficient method for additional drying is the deliquescent dryer which uses an absorbent material such as salt or potassium carbonate to absorb water. This type of dryer uses no external energy but does require regular replacement of the absorbent.

The most efficient type of dryer is the refrigerant dryer which is usually specified to supply air at a 35°F dew-point. The energy used for refrigeration is typically 6% of the compressor energy. These dryers are typically arranged to run continuously, using hot gas bypass to control capacity, and should not be oversized. Some refrigerant dryers are designed with multiple compressors for capacity control and these are recommended to increase operating efficiency.

The most intensive type of dryer is the desiccant dryer which can provide air at a dew-point of -40°F. These are similar to the deliquescent dryer but use electrical heat or air purging to regenerate the absorbent. This level of drying is rarely required in general industry applications. If only a small portion of the compressed air must be dried to this level, a separate dryer for this portion only is recommended.

A relatively new concept is the use of a re-heater after the after-cooler to raise the temperature and lower the relative humidity of the compressed air with "free" heat from the compressor. These are very efficient, with low maintenance, but are suitable only for systems capable of using warm air.

Compressor Controls

Most compressed air systems operate at part load for over 90% of the time. Many methods, with widely varying energy requirements, are available to provide this part load capability depending on the type and number of compressors installed. Since any method which reduces capacity while allowing the compressor to operate, uses more energy per CFM than a compressor operating at full load, compressor controls (as well as compressor size, quantity installed, and general system arrangement) should be evaluated to minimize energy consumption and operating costs.

The most efficient method for operating any compressed air system at other then full load is to turn the compressor(s) on and off. This is limited by the need for consistent air pressure and the need to limit short cycling. A properly sized receiver (with pressure regulator) in the compressed air system can help reduce or eliminate pressure variation but in most cases multiple compressors are also required to effect this optimum strategy. Each compressor is then staged to turn on or off based on the pressure in the receiver.

For reciprocating compressors, the next most efficient choice is to have multiple steps of unloading within each compressor using either valve lifters or clearance pockets. A fully unloaded reciprocating air compressor uses approximately 10% of its full load energy.

For rotary type air compressors, there are several types of part load control. The least efficient is full modulation using an inlet butterfly valve to reduce capacity. With this type of control, a rotary compressor will consume approximately 60% of its full load power at 0 output. The most efficient type of control for rotary compressors is to blowdown (also known as on line/off line operation) when capacity is not required. With this type of control, a rotary compressor will use approximately 20% of its full load power at 0 output but will cause pressure pulsing unless a properly sized receiver is installed and may also increase the stress on the air end of the compressor. A combination of these methods, known as modified throttling, may be used. This allows throttling down to approximately 60% of capacity before the compressor runs on line/off line. Yet another method of part load control for rotary screw compressors is the use of a spiral valve to vary the displacement. This is more efficient than the use of inlet valves and may be used in conjunction with the other control methods.

Air Compressors

As the heart of the compressed air system, air compressor efficiency is a prime factor in the overall system efficiency, however a particular compressor's efficiency is related to the overall system and should not be evaluated alone. Many efficiency improvements are related to the compressor selection and are most cost effective when purchasing a new compressor, therefore careful consideration to the entire system and various compressor options should be evaluated at that time.

One of the primary factors in the operating efficiency of a compressor is its capacity and pressure rating in relation to the system requirements. Oversized compressors should be avoided due to their poor performance at part load. A realistic appraisal must be made of existing and future compressed air requirements as well as the extent of compressed air leaks. The same is true of the operating pressure. In general, system deficiencies such as undersized piping and excessive pressure drop should be corrected prior to selecting a particular air compressor.

For pressures over 100 PSIG, two stage air compressors with inter-cooling between stages should be evaluated. These can be 12% to 15% more efficient than single stage compressors at both full and part load.

In general, reciprocating compressors are more efficient than rotary compressors with double acting reciprocating compressors offering the best efficiency. For large capacities (over 400 HP), centrifugal machines may be more efficient. Also, in general, multiple smaller compressors with appropriate controls offer a more efficient system arrangement than one large compressor.

When comparing the performance of two or more compressors, it is imperative that the performance be based on similar test standards as performance may vary by 20% or more depending on the test conditions. Unfortunately, there is no single standard in the industry and each manufacturer is free to quote performance at whatever standard he desires. The purchaser may have to state the standards himself and require the manufacturer to submit guaranteed test results for those conditions in order to make valid comparisons. The American Society of Mechanical Engineers (ASME) does publish a standard for testing (PTC 9 for positive displacement compressors and PTC 10 for centrifugals) but does not specify test conditions.

Maintenance

Maintenance practices for a compressed air system can significantly affect the operating costs and efficiency. Delaying or ignoring maintenance can end up costing far more than the cost of labor or parts.

Manufacturer's recommendations and service intervals should be followed to maintain maximum performance. Deterioration of valves and seals is especially significant in terms of CFM delivered vs. energy input.

Regular leak detection (and system leak testing), filter replacement, and overall pressure drop evaluation should be incorporated into the maintenance program. Checking and adjusting control setpoints should also be performed on a regular basis. Defeating or modifying compressor operating controls can be particularly costly.