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Compressed Air Glossary of Terms :: D ::

Glossary Index

Dalton's law
States that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the constituent gases. The partial pressure is the pressure each gas would exert if it alone occupied the volume of the mixture.

Direct current. A continuous, one directional flow of electricity

Dead end pressure
Is the suction pressure attained by an ejector or positive displacement vacuum pump at zero capacity with the suction absolutely blanked off.

Degrees Celsius (°C)
An absolute temperature scale. ((°F - 32)x 5/9).

Degrees Fahrenheit (°F)
An absolute temperature scale. ((°C x 9/5) + 32).

Degrees Kelvin (°K)
An absolute temperature scale. The kelvin unit of thermodynamic temperature, is the fraction 1/273,16 of the thermodynamic temperature of the triple point of water. The triple point of water is the equilibrium temperature (0,01 °C or 273,16 K) between pure ice, air free water and water vapour.

Degree Rankine (°R)
An absolute temperature scale. (°F + 459,67).

Degree Réaumur (°Ré)
An absolute temperature scale. ((°F - 32) x 4/9).

Degree of intercooling
Difference in air or gas temperature between the outlet of the intercooler and the inlet of the compressor.

Degree of saturation
Is the ratio of weight of vapor existing in a given space to the weight that would be present if the space were saturated at the space temperature.

Melting and becoming a liquid by absorbing moisture.

A solid absorption agent used in deliquescent type dryers.

Delta P
Describes the pressure drop through a component and is the difference in pressure between two points.

Delta T
A term indicating a temperature relationship between two temperatures or temperature variation between two points.

Flow of air under specific conditions required at a particular point.

Demand side management (DSM)
The planning and implementation of strategies designed to encourage consumers to improve energy efficiency, reduce energy costs, change the time of usage, or promote the use of different energy source.

The ability of a fluid that is insoluble in water to separate from water with which it may be mixed in the form of an emulsion.

Is the weight of a given volume of gas, usually expressed in lb / cu ft at SPT condition.

Depth filter
a filter medium that retains contaminants primarily within tortuous passages.

An adsorption type material used in compressed air dryers. Industry standards are activated alumina, silica gel and molecular sieves.

Design pressure
The maximum continuous operating pressure as designed by the manufacturer.

Opposite of absorption or adsorption. In filtration, it relates to the downstream release of particles previously retained by the filter.

Dew point
Of a gas is the temperature at which the vapor in a space (at a given pressure) will start to condense (form dew). Dew point of a gas mixture is the temperature at which the highest boiling point constituent will start to condense.

Dew point cup
An apparatus consisting of a small, polished, stainless steel cup placed in a container into which is passed the sample gas. The temperature of the polished surface is lowered by immersing dry ice (solid carbon dioxide) in an acetone solution contained in the cup. The temperature at which fog appears on the cup is the dew point of the sample.

A stationary element between stages of a multistage centrifugal compressor. It may include guide vanes for directing the flowing medium to the impeller of the succeeding stage. in conjunction with an adjacent diaphragm, it forms the diffuser surrounding the impeller.

Diaphragm compressor
Is a positive displacement reciprocating compressor using a flexible membrane or diaphragm in place of a piston.

Diaphragm cooling
A method of removing heat from the flowing medium by circulation of a coolant in passages built into the diaphragm.

Differential pressure
The difference in pressure between any two points of a system or component.

Differential pressure indicator
an indicator which signals the difference in pressure between any two points of a system or a component.

A stationary passage surrounding an impeller, in which velocity pressure imparted to the flow medium by the impeller is converted into static pressure.

Direct current
DC. A continuous, one directional flow of electricity.

Directional control valve
A valve to control the flow of air in a certain direction.

Dirt holding capacity
The quantity of contaminant a filter element can trap and hold before the maximum allowable back pressure or delta P level is reached.

The movable seating surface in a valve.

Discharge piping
Is the piping between the compressor and the aftercooler, the aftercooler separator and the air receiver.

Discharge pressure
Is the total gas pressure (static plus velocity) at the discharge port of the compressor. Velocity pressure is considered only with dynamic compressors.

Discharge temperature
Is the temperature existing at the discharge port of the compressor.

Displacement compressor
A machine where a static pressure rise is obtained by allowing successive volumes of gas to be aspirated into and exhausted out of a closed space by means of the displacement of a moving member.

Displacement of a compressor
The volume displaced by the compressing element of the first stage per unit of time.

Disposable filter
a filter element intended to be discarded and replaced after one service cycle.

The U.S. Department of Energy.

Dioctylphalate aerosol (Efficiency Test Material).

Double acting compressor
A positive displacement type compressor.

The portion of the flow stream which has already passed through the system or the portion of the system located after a filter or separator/filter.

Occurs when a valve does not close completely after popping and remains partly open until the pressure is further reduced.

Drain valve
A device designed to remove surplus liquid from the compressed air system. Manual units range from petcock to a ball, gate or globe valve. Mechanical types consist of ball float. Electrical drains include solenoid type that is energized by a timer signal, or electric motor driven units. Also pneumatically activated drains.

Is a pipe extending downward from the bottom of the airline to collect any condensation flow in the pipe.

A coupling between the compressor and the engine or motor. The three types of drives most common are; flange mounted motor, V belt drive or direct coupling.

Is a pipe coming from the top of the airline to feed air to an outlet for tools or air operated devices, so that condensation does not easily flow into the dropleg.

Dry adiabatic lapse rate
Rate at which unsaturated air cools as it travels vertically, provided that all temperature change is adiabatic (without heat exchange), and no condensation occurs.

Dry bulb temperature
Is the ambient gas temperature as indicated by a standard thermometer.

Dry gas
Is any gas or gas mixture that contains no water vapor and/or in which all of the constituents are substantially above their respective saturated vapor pressures at the existing temperature.

Dry unit (oil free)
Is one in which there is no liquid injection and/or liquid circulation for evaporative cooling or sealing.

Dynamic losses
Friction against duct walls, internal friction in the air mass and direction variations will cause a speed reduction and are therefore called dynamic losses.

Dynamic type compressors
Machines in which air or gas is compressed by the mechanical action of rotating vanes or impellers imparting velocity and pressure to the flowing medium. (Raise the pressure of the air by converting the energy from the velocity of the air to pressure.)

Dynamic viscosity ( Dynamic )
Is the force in newton required to move a fluid layer of one square meter area and a thickness of one meter with a velocity of one meter per second.

Dual control
Load/unload control system that tries to maximize compressor efficiency by matching air delivery and air demand. Compressor is operated at full load or idle.

A pipe, tube or channel that conveys a substance (such as air throughout a building).

This term refers to the hardness or softness of gaskets.

Dust cake
A layer of dust built up on an air filter.

Dust holding capacity
The amount of atmospheric dust which a filter will capture.

Duty cycle
Percentage of time a compressor unit can operate at full load over a thirty minute period.

Glossary Index

Thanks to Impact RM for permission to reprint the glossary.

Impact RM

Compressed Air Glossary © Impact RM

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