D-E-F-G

بدضعیفمتوسطخوبعالی (بدون رتبه)
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D

Deliquescent dryers:
A dryer which employs a chemical in the form of a tablet to absorb moisture. The chemical degrades into a liquid which is drained off periodically. Similarly the dryer has to be ‘topped up’ with new tablets to continue working. See dryers.

Desiccant dryers:
Also called pressure swing dryers, these use a chemical which can be repeatedly dried out. See dryers.

Demand:
The flow of air required to satisfy usage at specified temperature and pressure conditions.

Dewpoint:
When air is cooled it eventually reaches a point where the moisture contained within it condenses into liquid. This temperature is called the dewpoint. See pressure dewpoint.

Diffuser:
An item within an aerodynamic compressor which converts the velocity head into a static head. The diffuser usually consists of a set of vanes at specific angles so that the discharged air is efficiently slowed down.

Discharge Pressure:
The actual pressure that is available from the air compressor. This is not the same as the system pressure or the required pressure. It is important to ensure that a new compressor has a discharge pressure which is great enough to overcome losses through pipes and any air treatment, so that it can meet the required system pressure.

Discharge Regulations:
See Water Resources Act – 1991.

Displacement:
Otherwise referred to as swept volume. All air compressors have losses due to slippage, valve losses and clearances. These cause in-efficiencies and reduce the actual amount of air that the compressor will deliver. The displacement or swept volume is the theoretical amount of air that the compressor could deliver if these in-efficiencies did not exist.

Dryers (Driers):
A separate section of its own. Briefly, a device which lowers the pressure dewpoint of the compressed air in a system.

Dual Control:
An Americanism which refers to the method upon which the compressor is controlled. This control system is sometimes called on-line/off-line control and means that the compressor either produces air at full capacity, or it runs off-load. An example would be a butterfly valve being either fully open, or fully closed.

F
Flow Rate:
See CFM and Volume Delivered.

Free Air Delivery:
See CFM.

G
Global Warming Potential
Not to be confused with Ozone Depletion Potential. This is the ability of a ‘lower emitting’ green-house gas to trap heat in the atmosphere. This is expressed as a number based upon the GWP of carbon dioxide being 1. The GWP is calculated over a 100 year time horizon.

For example:

Carbon Dioxide = 1

Nitrous oxide = 310 (ie Traps 310 x as much heat).
See ozone depletion potential for a comparison.

Guide Vanes (Inlet):
These are fitted on the inlet throats of aerodynamic compressors. The air approaching the compressor is spun in the same direction as the rotation of the impeller, prior to reaching the impeller. This makes the machine slightly more efficient. However as the IGV’s begin to close, the approaching air is throttled which not only reduces flow rate, it reduces overall compressor efficiency.

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