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Define bypass factor of a cooling coil. What are the factors affecting it?

Subject:- Refrigeration and Air Conditioning

Topic:- Design of air conditioning systems

Difficulty:- Low

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By-Pass factor: The inability of a coil to cool or heat the air to its temperature is indicated by a factor called by-pass factor (BPF) or Coil Bypass Factor. This inability is due to the coil inefficiency and some amount of air just bypassing the coil without getting affected by it. A coil with low BPF has better performance. BPF can also be defined as the ratio of air which is unaffected by the coil to the total quantity of air passing over the coil.

BPF =$\frac{t_{s}-t_{d2}}{t_{s}-t_{d1}}$ For heating coil

BPF =$\frac{t_{d2}-t_{s}}{t_{d1}-t_{s}}$ For cooling coil

Factors affecting BPF:

1. The number of fins provided in a unit length i.e. the pitch of the coil fins. The smaller the pitch, greater will be the number of fins per unit length coming in direct contact with the air and hence BPF decreases.

2. The number of rows in a coil in the direction of flow. Higher the number of rows, higher will be the surface area exposed to the air flow. Hence BPF decreases.

3. The velocity of flow of air. It should not be too high, otherwise huge amount of air will get past the coil without having enough heat transfer due to the short contact time