Cooling coils and air-side design: How does the by-pass factor (BPF) of a cooling coil change with fin spacing and the number of tube rows?

Introduction / Context:
The by-pass factor (BPF) of a coil is the fraction of air that effectively “misses” perfect contact with cold surface and leaves closer to its entering condition. Designers aim for a low BPF so that supply air approaches the apparatus dew-point for better dehumidification and cooling.

Given Data / Assumptions:

• BPF depends on surface area, face velocity, fin geometry, and number of rows.
• Other variables (airflow rate, coil cleanliness) are assumed constant for comparison.

Concept / Approach:
More surface area and longer air path improve heat and mass transfer. Decreasing fin spacing (more fins per metre) increases area and surface contact. Increasing the number of rows extends contact time and path length. Both measures reduce BPF by allowing more of the airstream to approach coil surface conditions.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer coil performance data show BPF trending downward with higher fins per metre and additional rows at the same face velocity.

Why Other Options Are Wrong:

Increasing fin spacing reduces area, raising BPF.Reducing rows shortens air contact, also raising BPF.Independence from geometry is incorrect; geometry is a primary driver.

Common Pitfalls:
Forgetting that extremely tight fin spacing can increase air-side pressure drop and risk frosting; practical design balances BPF against fan power and maintenance.

Final Answer:

It decreases when fin spacing is decreased and the number of rows is increased