Energy Efficiency Reference/Industrial/Compressed Air/Technology Primer

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Compressed Air: Technology Primer[edit]

  • The goal of this section is to familiarize you with the equipment and terms you will usually encounter in compressed air systems. The section is divided into three sub-sections:

Compressed Air Equipment

  • How to identify key components of industrial compressed air systems and associated equipment.

Compressor Performance Relationships

  • This section provides performance relationships for the different types of system controls. Performance profiles are included which graphically show the relationship between airflow and power.
  • Definitions:
    • ACFM: Actual airflow delivered after compressor losses
    • ACFM-FAD: Airflow before filter (Free Air Delivery)
    • CFM: Cubic Feet per Minute of airflow
    • 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: Psi gauge, referenced to atmospheric pressure
    • Psia: Psi absolute, which is 14.7 psi at sea level
    • SCFM: Equivalent airflow at Standard Conditions (2 different standards, CAGI and ASME, use this same term)

Standard Conditions

  • CAGI (Compressed Air & Gas Institute0: 14.7 psiz, 60 F, 0% rh (relative hunidity)
  • ASME: 14.7 psia, 68 F, 36% rh
  • Unloading and Unload Point: Pressure at which compressor unloads

Compressed Air Equipment

  • To evaluate equipment air systems it is important to be able to identify the type of equipment in use, and understand generally how that equipment works. The following section presents common equipment you are likely to encounter in a compressed air system along with important details.


  • Reciprocating and Screw compressor are the most common types of air compressors. Other types of compressors are described in the appendix. 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. A more complete description of a reciprocating compressor is given in the appendix.

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 examples, 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 dictate the type and size of air dryer required. Typical air dryers either use refrigeration or desiccant remove moisture from the air.
    • Aftercoolers provide initial cooling of hot compressed air from over 150 degrees 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 are able to drop the dew point (the temperature at which the air becomes completely saturated and moisture in the air beings to liquefy) of compressed air to 35-50 degrees F. 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 associated with 130 kWh per million cubic feet of air.
    • Desiccant Air Dryers can reach a lower dew point than refrigerated dryers; down to -150 degrees F. Desiccant air dryers consist of two desiccant bends through which the compressed air flows. A control system 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. Heat types have better efficiencies because the energy required to heat the air to 300 degrees 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 lower dew points. These are often the only option. Operating costs range from $15 to $30 million cubic feet of compressed air. This is associated with 300 to 500kWh per million cubic feet of air. This cost of the compressed air used to regenerate the dryer. The extra compressed air required can be huge cost.
    • Membrane Dryers are relatively new to the marketplace. They use a semi-permeable membrane that allows dry air to pass through holding back the water vapor. These dryers are easy to maintain the achieve dew points as low as 35 degrees 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 to later use. Receiver tanks should be large enough to limit the amount of compressor cycling to 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 essential 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 indtroduces 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 drops 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 exapnds, increasing the pressure and reducing the work the compressor had to do to achieve a given pressure. The degree of the heating and effectiveness of the 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.

Compressor Performance Relationships

  • Following is an introduction to the relationship between power and airflow. This relationship is central to all the calculations used in analyzing potential savings.
    • Power (%P) refers to compressor power expressed as a percentage of full load power (FLP). For example, if a compressor's full load power is 100 kW and at a particular load the compressor uses 74 kW then its percent power for that load is 74% (74/100).
    • Airflow (%C) refers to air delivered by the compressor expressed as a percent of capacity. For example if a compressor's capacity is 200 acfm and at a particular lad it delivers 120 acfm its percent capacity is 60% (120/200)>
  • Power and airflow can be compared to create a compressor performance curve. The relationship between power and airflow depends on the strategy used for matching compressor output with the load. The following section presents power and airflow relationships for different control strategies.
  • Information on each tyep of control includes:
    • A brief explanation of the control strategy.
    • Equations relating power (%P) and airflow (%C).
    • Advantages and disadvantages
    • Compressor performance charts showing the relationship between power and airflow.

1)Throttle Control

  • Compressor Type: Screw
  • Operation
    • Throttle control works by using a slide or butterfly valve to create a partial vacuum at the compressor inlet. The partial vacuum limits the air mass that enters the compression chamber, lowering the amount of air that is compressed.
  • Power and airflow relationships:
    • Power: %P = (Full load %P - No load %P) x Load %C + NO Load %P
    • Airflow: %C = (Load %P - No load %P)/(Full Load %P - No load %P)
  • Advantages:
    • Constant Discharge Pressure
    • Good high load efficiency
  • Disadvantages:
    • Poor Low Load Efficiency
  • Typical No Load Power Consumption: 60-72% of Full Load Power

2) Turn Valve or Poppet Valve Control

  • Compressor Type: Screw
  • Operation:
    • Turn valve controls vary the amount of air that is compressed by varying the effective length of the compression chamber. A threaded shaft rotates to open ports in the compressor housing. Air compression cannot begin in rotor sectors with open ports. This changes the chamber length experienced by the air and changes the volumetric compression ratio. Poppet valves work similarly to turn the valves. Instead of a rotating valve, a series of valves, similar to automotive engine valves, at discrete positions along the length of the screws allow the air to exit without further compression. In both cases, the valves are operated to match compressor output with plant air demand.
  • Power and Airflow Relationships
    • Power: %P = (Full Load %P - No Load %P) x Load %C^2 + No Load %P
    • Airflow: %C = ((Load %P -No load %P)/(Full load %p - No Load %p))1/2
  • Advantages:
    • Good High Load Efficiency
    • Low Receiver Capacity Requirement
    • Constant Discharge Pressure
  • Disadvantages:
    • Power Low Load Efficiency
  • Power Consumption: 55-60% of Full Load Power

3) On-Off Control

  • Compressor type: Screw, Reciprocating
  • Operation:
    • On-Off controls turn the compressor on and off as needed. The compressor works at full lad until it reaches maximum pressure, then turns off. Starting and stopping larger compressors can be hard on both the compressor and the motor, and therefore is more common on smaller compressors. On-Off controls can also be coupled with other modulation strategies to reduce or eliminate energy use at low or no air demand.
  • Power and Airflow relationships:
    • Power = Airflow
  • Advantages:
    • Maximum Efficiency at all loads
  • Disadvantages:
    • High receiver capacity requirement
  • Typical No Load Power Consumption: 0% of Full Load Power

4) Unloading Controls

  • A compressor will typically be modulated by one of the above controls, however, unloading controls may be added to a compressor to increase part load efficiencies. Unloading controls allow the compressor to spend unloaded time at reduced power. There are two types of unloading strategies, Load-Unload and Low-Unload
  • A. Load-Unload Control
    • Compressor Type: Screw, Reciprocating
    • Operation:
      • Load-Unload controls operate in a manner similar to On-Off controls. Instead of turning off the compressor unloads. When the compressor reaches maximum pressure a solenoid valve opens, reducing the discharge to atmospheric or at least lower pressure. The compressor then with lower pressure difference from the intake to the discharge side. A check valve prevents back-flow of air at system pressure. The compressor uses less energy while unloaded.
    • Power and Airflow relationships:
      • Power: %P = (Full Load %P -No Load %P) x Load %C + No Load %P
      • Airflow: %C = ((Load %P - No Load %P)/(Full Load %P - No Load %P))
    • Advantages:
    • Good Efficiency at all Loads
    • Disadvantages:
      • High receiver capacity requirement
      • Small air loss during unloading
    • Typical No Load Power Consumption:20% of Full Load Power
  • B. Low - Unload Control
    • Compressor Type: Screw
    • Operation:
      • Low-Unload controls are a combination of load-unload and modulation controls. They modulate the compressor at higher loads and unload the compressor at lower loads. Low-Unload controls have a set unload point about which the compressor modulates and below the compressor unloads.
    • Power and Airflow relationships:
      • a) Above the Unload Point
        • The modulating power and airflow formulas depend on the type of control the compressor employs. (see earlier descriptions)
      • b) Below the Unload Point
        • Power: %P = (Unloading %P - No Load %P) x Load %C + No load %P
        • Airflow: %C = ((Load %P - No Load %P)/(Full Load %P - No Load %P))
    • Advantages:
      • Good High Load Efficiency
      • Requires less receiver capacity & pressure range for reasonable operation
    • Disadvantages:
      • Not as efficient as load unload
      • Small air loss during unloading
    • Typically No Load Power Consumption: 20% of Full Load Power