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Sizing Compressed Air Systems

Compressed Air Demand

Demand for compressed air can be extremely variable. The designer has to consider normaloperation, as well as the maximum and minimum air demands. These figures will be built up

from the air consumption figures of individual users. These are usually quoted in either “normal

cubic metres per hour” (Nm3/h) or “standard cubic feet per minute” (scfm).  Normal cubic metres

are usually taken at 0oC and 1 atmosphere (1.01325 Bar), whilst standard cubic feet are usually

taken at 60oF (15.5

oC) and 1 atmosphere (1.01325 Bar). To avoid error, the figures should

always make clear the conditions at which the volume refers to.

Air consumption figures can be taken as:

  Hand tools –  air consumption depends on the size of hand tool –  typical air consumption

is between 20Nm3

/h and 100 Nm3

/h.  Instruments  –  air consumption is up to 3 Nm

3/h per control loop –  a figure of 1.5 Nm

3/h

 per control loop is typical.

  Other air consumptions figures (such as equipment purges) need to be estimated on a case

 by case basis.

Was this information useful? In addition to this article, we offer a process engineering design service, sc

   Advice on assessing maximum air demand  

   Hydraulic check to confirm pipe sizes 

   Process specification of the air compressor to obtain quotes from compressor vendors. 

Air Receiver

The method of sizing the air receiver depends on whether it is being used as a dampener to

moderate the compressor control or as buffer to supply air in the event of a compressor failure.

1. Moderate Compressor Control

Typically the capacity of the air receiver can be calculated using the following equation (thisshould be confirmed by the compressor vendor):

Air Receiver Capacity (in litres) = 6 x FAD 

Where:

FAD –  Free Air Delivery –  The volume of air delivered by air compressor at ambient

temperature and pressure in litres

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2. Provision of Air Buffer Capacity

This results in a larger air receiver than one sized just for compressor control. The size of theair receiver can be calculated using the following formula:

Air Buffer Receiver Volume, V (in m

3

) = TCP / (P1 – 

 P2) 

Where:

T Time required to maintain air supply, in hours. Typically, 0.25 hours (i.e. 15 minutes).

C Air demand, in Nm /h. Usually the air demand for essential users (i.e. process instruments).

P Atmospheric pressure (= 1.013 bara)

P1Initial Pressure, in barg –  pressure at which the shut off valve closes to prioritise flow to

essential users (typically 6 barg)

P2Final pressure, in barg - minimum pressure at which the process instruments work

effectively (4 –  4.5 barg is a typical value).