Bypass Factor & its significance

Bypass factor is the function of the physical & operating characteristic of the conditioning apparatus. It represents that portion of the air which is considered to pass through the conditioning apparatus completely unaltered.

The physical & operating characteristic affecting the bypass factor are as follows:

1. A decreasing amount of available heat transfer surface (i.e. less rows of coil, less coil surface area, wide spacing of coil tubes etc.) results in increase in bypass factor.
2. A decrease in the velocity of air through the conditioning apparatus results in decrease in bypass factor. i.e. more time for the air to contact the heat transfer surface.

Decreasing or increasing the amount of heat transfer surface area has greater effect on bypass factor than varying the velocity of air through the apparatus.

Psychrometric Relation

There is a psychrometric relationship of bypass factor to GSHF & RSHF. Under specified room, outdoor design condition & quantity of outdoor air, RSHF & GSHF are fixed. The position of RSHF is also fixed, but the relative position of GSHF may vary as the supply air quantity & the supply air condition change.

• To properly maintain the room design condition, the air must be supplied to the space at some point along the RSHF line.
• Therefore as the bypass factor varies the relative position of GSHF to RSHF changes as shown by the dotted line.
• As the position of GSHF changes the condition of air entering & leaving the apparatus, the required air quantity, bypass factor & the apparatus dew point also changes.

The effect of varying the bypass factor, on the conditioning equipment is as follows.

1.Smaller By pass factor

a. Higher ADP: DX equipment to be selected for higher refrigerant temperature & chilled water equipment would be selected for higher temperature chilled water & lesser flow rate. Possibly smaller refrigerant machine.

b. Less air requirement : Smaller fan & fan motor.

c. More heat transfer surface: More rows of coil or more coil surface available.

d. Smaller piping if less chilled water is used.

2. Larger Bypass Factor

a. Lower ADP: Lower refrigerant temperature for DX equipment & more water or lower temperature for chilled water equipment. Possibly larger refrigeration machine.

b. More Air: Large fan & motor

c. Less heat transfer surface: less coil row or coil surface available.

d. Larger piping if more chilled water is used.

As previously indicated the entering & leaving air conditions at the conditioning apparatus and the apparatus dew points are related psychometrically to the bypass factor. It can be accurately evaluated mathematically from below equation.

• The quantity (1-BF) is frequently called contact factor and is considered to be that portion of the air leaving the apparatus at the ADP.

• When the by-pass factor is zero, the supply air temperature is equal to coil ADP.

Reference: Carrier Handbook of air-conditioning system design.