Psychrometrics – Heating coil bypass factor (BPF) Atmospheric air at a dry-bulb temperature of 15 °C enters a heating coil whose surface is maintained at 40 °C. The leaving-air dry-bulb temperature is 25 °C. Compute the by-pass factor (BPF) of the heating coil.

Introduction / Context:
Heating and cooling coils in HVAC analysis are often evaluated using the by-pass factor (BPF). For a sensible heating process, BPF quantifies the fraction of air that effectively “bypasses” the coil surface temperature and leaves warmer than the coil surface would otherwise impose.

Given Data / Assumptions:

• Entering air dry-bulb temperature, t_in = 15 °C.
• Coil surface (apparatus) temperature, t_coil = 40 °C (assumed uniform).
• Leaving air dry-bulb temperature, t_out = 25 °C.
• Process is sensible heating (moisture change neglected for BPF relation).

Concept / Approach:
For heating coils, the standard relation is BPF = (t_coil − t_out) / (t_coil − t_in). This mirrors the cooling-coil form with temperatures appropriately referenced to the coil surface.

Step-by-Step Solution:
t_coil − t_out = 40 − 25 = 15.t_coil − t_in = 40 − 15 = 25.BPF = 15 / 25 = 0.60.Therefore, the by-pass factor is 0.60.

Verification / Alternative check:
A higher BPF indicates poorer coil contact (more “bypassing”). Here, leaving air (25 °C) is much cooler than the coil (40 °C), consistent with a relatively high BPF of 0.60.

Why Other Options Are Wrong:

• 0.376, 0.40, 0.24, 0.67: These do not satisfy the simple ratio with the given numbers; only 0.60 matches the formula.

Common Pitfalls:
Using the cooling-coil form without flipping the temperature order; forgetting temperatures must be absolute only when using thermodynamic ratios (here a simple difference suffices in °C).

Final Answer:
0.60