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SIZINGRECEIVER TANKS

Receiver tanks are found in most modern compressed air systems. Hence, one of themost common design tasks in sizing of a compressor system is the sizing of receiver

tanks. To size the receiver tanks appropriately, we need to identify the key function ofthe receiver tank. In the modern compressed air plant, receiver tanks are mainly usedfor only 2 purposes:

a) as a buffer to supply air during short demand peaks or power failure.

b) as a protective element to prevent too frequent a cycling of the compressor/s.This can be to prevent mechanical failure of the inlet valve due to too frequentswitching of onload/offload state or electrical failure of motors due to too frequentstart/stop for small piston compressors.

A) Sizing as a Buffer

The formula used to determine the appropriate size of the receiver tank whenused as a buffer is:

VR =

pFAD x t

where VR - Size of receiver (m3)

FAD - Free air delivery (m3/min)T - time (min.) the free air is required

p - Pressure difference (bar)

For example, suppose a plant requires 5 m3/min of free air during any powerfailure to operate some pneumatic valves. The minimum pressure at the receivertank during plant operations is 7 bar and the minimum pressure to operate thepneumatic valve is 5 bar. If the free air is required for 1 minute, then theappropriate size of the receiver tank is:

VR =(7 bar 5 bar)

5 m3/min x 1 min

= 5 / 2

= 2.5 m3

Hence the minimum size of the receiver tank required is 2.5 m3.

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B) Sizing as a Protective Element

The formula used to determine the appropriate size of the receiver tank whenused as a protective element is:

VR =p x Z

FAD x (A-A2)

where VR - Size of receiver (m3)

FAD - Free air delivery (m3/min)A - Utility ratio

i.e. utilized capacity / available capacityA-A2 - Utility factor

p - Pressure difference (bar)Z - Maximum cycling frequency (times/min)

In the above formula, it must be understood that, A, the utility ratio can varythrough out the time the compressed air plant is in operation due to the varyingconsumption of compressed air. This will result in A-A2, the utility factor alsovarying. The graph below shows the possible values of the utility factor, A-A2 inthe above equation when the Utility ratio, A varies from zero to full utilization.

Hence for conservative designs, the worst case value of 0.25 is used.

Variation of Utility Factor, A-A2

0

0.05

0.1

0.15

0.2

0.25

0.3

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 1A

A-A2

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For example, suppose a compressor is operating in onload/off load mode with afree air delivery of 5 m3/min. The cut-in pressure is set at 7.0 bar and the cut-outpressure is set at 7.5 bar. If the maximum switching frequency allowed foronload/offload operation is 1 time/min., then the conservative size of the requiredreceiver tank is:

VR =(7.5 bar 7.0 bar) x 1 time/min5 m3/min x 0.25

=0.5 x 15 x 0.25

= 2.5 m3

Hence the conservative minimum size of the appropriate receiver tank is 2.5 m3.

As receiver tanks are rather costly items, less conservative sizing using a less

conservative utility factor, A-A2 (such as 0.16), a bigger pressure differential, p(such as 1 bar) and a higher maximum cycling frequency (such as 2 times/min)have been used. This can be rather dangerous and may cause unforeseen breakdown of the compressors. As such, for the optimum safety, the conservativedesign factors are always recommended.

Beyond the size issue, other requirements for receiver tanks to be used in Malaysia are:

a) Design approval from the Factories and Machinery Department, Malaysia.b) Tested to the satisfaction of Factories and Machinery Department, Malaysia.

The receivers should also be equipped with Safety Valves of the appropriate flow andpressure ratings, Pressure Gauges of the appropriate size, accuracy and range anddrain mechanism. For further details, please refer to the Factory and Machinery (SteamBoiler and Unfired Pressure Vessel) Regulations, 1970.