Technology Primer

Definitions

ACFM	Actual Airflow deliverd after compressor losses
ACFM-FAD	Airflow before filter (Free Air Delivery)
CFM	Cubic Feet per Minute
ICFM	Airflow at inlet flange
Modulation Control	Method of reducing airflow to match part load requirements, includes throttle, turn, spiral, or poppet valve.
Psi	Pressure in pounds per square inch
Psig	Pressure gauge, referenced to atmospheric pressure
Psia	Pressure absolute, which is 14.7 psi at sea level
SCFM
Unloading and Unload  Point	Pressure at which compressor unloads

Equipment

Compressors	Reciprocating and Screw compressors are the most common types of air compressors in idustrial applications.  Air compressors usually require ancillary equipment to dry the air, remove oil and to buffer pressure fluctuations. Reciprocating compressors Reciprocating compressors, use pistons to compress air in cylinders. Their operation is similar to automotive engines.  Small reciprocating compressors use a single action compression stroke, while larger ones use double acting pistons.  Double acting pistons compress air on both the up and the down stroke.  Reciprocating compressors offer good efficiencies over a wide range of operating conditions and are quite efficient for low-pressure applications.  However, reciprocating compressors require more maintenance than screw compressors.  In the northwest reciprocating compressors are less common than screw compressors. Screw compressors Screw compressors are the most prevalent types of compressor in the northwest.  Screw compressors use two mated screws.  These turn, forcing air between them. As air progresses through the screws the volume of the gap between the screws decreases, thereby compressing the air.
Air Dryer	Air dryers are included in most compressed air systems. Many industrial applications require air with low moisture content. For example, pneumatic controls typically require dry air. In colder climates moisture-laden air can lead to ice which blocks or breaks the lines. High moisture content can also lead to corrosion in any compressed air system. Drying requirements and the volume of the air required dictate the type and size of air dryer required. Typical air dryers either use refrigeration or desiccant to remove moisture from the air. Aftercoolers Aftercoolers provide initial cooling of hot compressed air from over 150 F.  Water cooling can be more effective by using cooling tower water near the wet bulb temperature. Air from the compressor room is often warmer than outside air.  Aftercoolers condense some water vapor in the compressed air, but usually not adequately for air tools or pneumatic controls.  Consider uses for the waste heat.  Avoid city water cooling flowing down the drain. Refrigerated Air Dryers Refrigerated air dryers are able to drop the dew point (the temperature at which the air becomes completely saturated and moisture in the air begins to liquefy) of compressed air to 35-50 oF.  Often these are all that is required and are typically the most economical means of drying air.  Refrigerated air dryers work by cooling the compressed air with a refrigeration system.  Moisture condenses out of the cooled air and is captured at the dryer.  Operating costs are roughly $5.00 to $8.00 per million cubic feet of air.  This is associated with 130 kWh per million cubic feet of air.   Desiccant Air Dryers Desiccant air dryers can reach a lower dew point than refrigerated dryers; down to –150o F.  Desiccant air dryers consist of two desiccant beds through which the compressed air flows.  A control system channels all of the air through one bed while the other is regenerated.  The regeneration process varies among desiccant dryers and has a large impact on operating costs.  All systems blow dry compressed air across the desiccant bed to regenerate it and then purge the air and moisture to the atmosphere.  Some models heat the dried air first to increase its capacity to absorb moisture.  Heated types have better efficiencies because the energy required to heat the air to 300o F is less than the energy required to compress and dry the additional air that would otherwise be necessary to dry the bed.  Heated air dryers require only about 1-7% of the total compressed air to purge the desiccant bed, while non-heated dryers require 15% or more.  Desiccant dryers are more expensive to purchase, maintain and operate than refrigerated air dryers, but to achieve low dew points these are often the only option.  Operating costs range from $15 to $30 per million cubic feet of compressed air. This is associated with 300 to 500 kWh per million cubic feet of air.  This cost includes the energy used by the dryer, but it does not include the cost of the compressed air used to regenerate the dryer.  The extra compressed air required can be a huge cost.   Membrane Dryers Membrane dryers are relatively new to the market place.  They use a semi-permeable membrane that allows dry air to pass through while holding back the water vapor.  These dryers are easy to maintain and achieve dew points as low as 35o F, however, they cause a 9-10% drop in system capacity as much compressed air is lost along with the water vapor in the membrane system.  The operating costs for this type of air-drying are minimal, as the membranes need to be replaced infrequently.
Receiver Tank	Compressors with unloading or on-off controls require a receiver tank to store a volume of compressed air for later use.  Receiver tanks should be large enough to limit the amount of compressor cycling to a minimum.  A common rule-of-thumb is a gallon of receiver space per SCFM of output, however, like in Texas, bigger is better since it lengthens the compressor on-off cycles.
Oil Separator	In screw compressors the two screws that mesh to compress air would wear quickly without oil.  The oiling procedure employed by many screw compressors introduces oil into the compressed air.  Oil separators remove this oil after the compression process.  An oil separator is essentially a coalescing filter mated with a metal baffle.  A coalescing filter traps microscopic oil particles in its consumable elements.  As air moves through the filter the entrained oil coalesces and drops out.  Any oil remaining in the air would shorten tool life so it pays to keep the separator working. A poorly maintained oil separator often introduces an excessive pressure drop into the compressed air system.  The pressure drop should not be more than 5 psi across the separator.  When the pressure drop is greater than 5 psi maintenance is required.
Air Reheater	Some air compressors are equipped with an air reheater to heat the air after it is dried.  The heated air expands, increasing the pressure and reducing the work the compressor has to do to achieve a given pressure.  The degree of the heating and the effectiveness of this system varies from one situation to another.  A reheater does not help if heat is lost in the distribution system, which is why the strategy is not used or recommended often.

Performance Relationships

Common Opportunities

Reduce Pressure

Seperate Compressed Air Grids

Replace Throttle Compressor

Additional Information

Walkthrough Checklist

 Are large compressors serving minimal system needs during off-hours such as maintaining the minimum pressure requirements for a Dry Fire Suppression System?

Does the facility have more than one compressor feeding into a common header? Are these compressors operating at less than full output (cfm) capacity?

Are compressors operating at zero capacity for extended periods of time?

Is the discharge pressure of the compressors higher than 110 psig?

Is high pressure air being used for tasks that do not require high pressure air?

Does the compressed air system have significant air leaks?

Is the compressor cooling water discharged to the sewer?

Is the pressure drop across auxiliary equipment such as dryers, oil separators, or filters excessive?

Is compressed air the best utility for the given application?

Does the facility utilize any air nozzles? Have the air nozzles been designed for maximum efficiency?

Is the Air Compressor on a Regular Maintenance Schedule? Review maintenance logs to verify if the following is being done on a scheduled basis?

Is throttle control used to modulate the compressor output capacity?

Is Turn Valve control used?

Is Poppet Valve control used?

Does the facility have rotary vane air compressors?

Does the facility have desiccant air dryers?

Are large compressors serving minimal system needs during off-hours such as maintaining the minimum pressure requirements for a Dry Fire Suppression System?

Does the facility have more than one compressor feeding into a common header? Are these compressors operating at less than full output (cfm) capacity?

Are compressors operating at zero capacity for extended periods of time?

Is the discharge pressure of the compressors higher than 110 psig?

Is high pressure air being used for tasks that do not require high pressure air?

Does the compressed air system have significant air leaks?

Is the compressor cooling water discharged to the sewer?

Is the pressure drop across auxiliary equipment such as dryers, oil separators, or filters excessive?

Is compressed air the best utility for the given application?

Does the facility utilize any air nozzles? Have the air nozzles been designed for maximum efficiency?

Is the Air Compressor on a Regular Maintenance Schedule? Review maintenance logs to verify if the following is being done on a scheduled basis?

Is throttle control used to modulate the compressor output capacity?

Is Turn Valve control used?

Is Poppet Valve control used?

Does the facility have rotary vane air compressors?

Does the facility have desiccant air dryers?

Assessment Tools and Data Collection

Analysis Tools and Methodology

Additional Resources

• U.S. DOE Compressed Air Challenge
• U.S. DOE Industrial Technologies Program
• Houston Vacuum Pump & Compressor