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By pass Factor in Air conditioning

For ventilation purposes, fresh air is drawn from outside and mixed with room air (return air) in the usual A/c system. This mixture ( Room air + Fresh air ) is then passed through cooling coil or chilled water washer. Thereby, the mixture is treated and this is sent to the room again.

All the air passing through the cooling coil cannot come into contact with it. This is because of the gaps between fins and tubes. Thus, this portion of air which has no contact with cooling coil actually 'by passes' it. It comes out of the cooling coil at the same condition at which it entered and it is called 'By pass air'. 




The state of  the contacted air is that of  the saturated air at the temperature of  the surface. The uncontacted (bypassed) air remains at the entering state. 

The By pass Factor (BF) is defined as the 'ratio of  the  amount of  uncontacted (bypassed) air to the total amount of air passing through the surface'.

i . e, By Pass factor, 
B.F = Amount of Bypass Air / Amount of  total air passing.  
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