Compressed Air Terminology
There are a huge number of terms involved in the compressed air industry; this leads to confusion even amongst those familiar with the field, to a newcomer it's a potential minefield.
Below is a list of terms that might come in useful
Absolute Pressure Total pressure measured from absolute zero
ACFM Actual cubic feet per minute Flow rate of air measured at some reference point and based on conditions at that reference point
ADP Atmospheric Dewpoint
Aftercooler Heat exchanger for cooling air discharged from air compressors
ATM Atmospheres
Atmosphere(s) Pounds-force per square inch (lbf/in2) (psi) 14 696 psi
Atmospheric Pressure Pressure above absolute zero at a specific location and altitude See PSIA
Bar Pounds-force per square inch (lbf/in2) (psi) 14 504 psi
BARG Bar gauge 14 504 psig
Barometric Pressure (see Atmospheric Pressure) Must be referenced when rating air compressors for altitude at which they will be operated See PSIA
BHP Brake horsepower Horsepower delivered to the output shaft of the drive motor Unit of comparison between motors Total package bhp is the sum of all motor shaft outputs, including compressor and cooling fans
Blow-Off Valve Valve assembly mounted from the air compressor discharge line or from a port adjacent to the air discharge port of the air compressor casing High-performance wafer style butterfly valve or stainless-steel ball valve Consists of positioner, actuator, and valve May be referred to as "bleed" valve or "anti-surge" valve Provides for reduced demand modulating and for "Surge" relief to atmosphere May be used as a "re-circulation" valve with nitrogen and other service applications or compressor packages Constant blow-off modulating may signal a need to "down-size" your air compressor capacity to eliminate wasted energy dollars
Capacity The amount of air flow delivered or required under some specific conditions
CFM Cubic feet of air per minute Volume rate of air flow
CFM, Free Air Cubic feet of air per minute, free air Cfm of air delivered to some specific point and converted back to ambient (free air) conditions
Check Valve During un-load or shut-down, prevents reversal of air flow from an air system (and loss of system pressure!) through a centrifugal air compressor See Surge
Cold Start Starting a compressor from a state of total shutdown Usually done with "local" control at the compressor May be done with "remote" control, but only advised with "heavy" instrumentation and monitoring accessories
CTD Approach temperature Usually the difference between cooling water temperature in to compressed air temperature out of an inter-cooler or after-cooler Sometimes used to define oil cooler efficiency (cooling water temperature in to oil temperature out)
DELTA (D) P Pressure drop Loss of pressure in a compressed air system due to friction or restriction Also, the water pressure drop across coolers
DELTA (D) T Temperature drop Rise (or decrease) of temperature between two points
Demand Flow of air under specific conditions required at a particular point
Discharge Pressure Rated air pressure produced at a rated reference point At the discharge flange of an air compressor
Discharge Pressure Required Air pressure required at point of entry to the system
Displacement Amount of air (cfm) displaced by a reciprocating compressor piston under no load, discharging directly to the atmosphere
FAD Free Air Delivered
Free Air Air at ambient conditions of temperature, humidity, and atmospheric pressure at any specific location
Fouling Accumulation of foreign matter, such as mud or debris, in a cooler, pipe, or valve In a cooler, H2O DP and DT will be seen to increase, as well as CTD ( H2O T in to Air T out)
Hot Start The compressor is started automatically, depending on demand Control panel is energized with no "pre-start" cycle required, as pre-lubrication pump and buffer (seal) air are always "on" A state of pre-start exists Steam turbine compressors are "slow-rolling" to maintain "pre-start" turbine temperatures at an adequate, recommended level "Heavy" instrumentation and monitoring accessories are recommended
ICFM Inlet cubic feet per minute Cfm flowing through the compressor inlet filter or inlet valve under rated conditions Most air compressor flows are rated using icfm rather than air flow delivered at discharge
Inlet Conditions The combination of temperature, pressure, and humidity at the inlet to the compressor after inlet filtration At sea level, inlet pressure is usually 14.4 psia, after filtration
Inlet Pressure The air pressure at the inlet flange of the compressor Should always be expressed in psia
Inlet Valve Valve assembly at the air inlet to an air compressor Butterfly (wafer style, a k a damper) or IGV (inlet guide-vane valve) Consists of positioner, actuator, and valve
IGV Inlet guide-vane valve Valve assembly at the air inlet of a "blower" (single stage, low pressure, centrifugal air compressor) Usually advised to be mounted in very close proximity to the "blower" impeller Provides "pre-swirl" of air flow in same rotational direction as "blower" impeller Proven to improve efficiency (reduced bhp) during throttled-down modulation of "blowers" Effectiveness, when used with multi-stage centrifugal air compressors, is much less than with single stage "blowers"
Application with multi-stage centrifugal air compressors is paradoxical, i e , "Centrifugals are most efficient (kW/scfm) when fully loaded" The "savings range" for multi-stage I-R Centac* "standard" or Centac II centrifugal air compressors is very narrow Unlike Joy, Elliott, and others, the "unique" casing design of Centac* does not allow mounting an IGV in close proximity to the first stage of compression Beware of offers for IGV "improved efficiency" if it is offered for sale alone "Re-rate" of impeller(s) to improve compressor efficiency is much more viable
Load Factor Ratio of the average compressor load to the maximum rated compressor load during a given period of time
Modulating Control Compressor controls to run an air compressor at constant pressure at
varying loads to accommodate flow demand variations
Nm3/hr - Normal cubic metres per hour - A psuedo-massflow, defining the density of the air as 0oC, 100mbar, 0%RH
Pressure Force per unit area
PSI Pounds per square inch Force per unit area exerted by compressed air
PSIA Pounds per square inch absolute Pressure above absolute vacuum Atmospheric pressure is stated in psia
PSIG Pounds per square inch gauge Pressure at some reference point as measured with a gauge and dependent on atmospheric pressure
PSID Pounds per square inch differential Pressure difference between two points
PDP Pressure dew point Temperature at which water will begin to condense out of air at a given pressure
Receiver Tank used for storage of air discharged from a compressor
Scaling Build-up of foreign matter on the interior (H2O) surface of coolers and pipe Often caused by the precipitating-out of calcium carbonates due to high temperatures at the "hot" end of a cooler With a cooler, seen as an increase of CTD (high air temperature) and lower DT Unfiltered, untreated, and oxygenated water are the most frequent causes in pipe
SCFM Standard cubic feet per minute (scfm) Flow of free air measured at some reference point and converted to a standard set of reference conditions (e g , 14 4 psia, 15oC, and 60% relative humidity )
Sea Level Where absolute air pressure is 14 7 psia, before inlet air filtration
Setpoint Desired pressure at which compressor pressure controllers or switches are set to operate the compressor Running a compressor at higher than design pressure setpoint results in a drop in compressor discharge flow and thus an increase in operating costs per cfm Some system demands are better met by operating below compressor design setpoint
SPC Specific Power Consumption
Surge The rapid reversal of air flow through a centrifugal air compressor High temperatures are generated and gross instability occurs in the air compressor! If it continues unabated; the compressor "buys the farm"! Surge may be preceeded by fouling, scaling, or by-passing of inter-coolers, thus allowing high temperatures (reduction in air mass density) Surge may occur due to lack of adequate cooling water flow Surge may also occur because of high air intake DELTA P (fouled/clogged air intake filter elements, under-sized air intake pipe, malfunctioning or poorly "stroked" inlet valve, and/or poorly calibrated LLR)
Two-Step Control Load/unload control system that tries to maximizes compressor efficiency by matching air delivery and air demand Compressor is operated at full load or idle See Dual Control