Cooling Coil ADP and Bypass Factor Calculator
Enter the dry-bulb and wet-bulb temperatures before and after a cooling coil to find its apparatus dew point and how much of the air effectively bypasses it.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Apparatus dew point (ADP)
- 52.5°F
- Bypass factor
- 0.091
ADP 52.5 °F with a bypass factor of 0.091 (9.1 % of the air behaves as if it missed the coil).
- Contact factor (1 − BF)
- 0.909
- Entering humidity ratio
- 78.2gr/lb
- Leaving humidity ratio
- 60.4gr/lb
- Leaving relative humidity
- 94%
Show the working
- Humidity ratios from dry-bulb and wet-bulb (ASHRAE equations)78.16 → 60.42 gr/lb
- Extend the straight line from entering through leaving air until it meets saturation (100 % RH)ADP = 52.49 °F
- Bypass factor BF = (T_leaving − ADP) ÷ (T_entering − ADP)(55 − 52.49) ÷ (80 − 52.49) = 0.0913
What ADP and bypass factor mean
The bypass-factor model treats the air leaving a wet coil as a mixture of two streams: air that touched the coil and left saturated at the coil surface temperature, and air that passed through unchanged. That effective surface temperature is the apparatus dew point (ADP). The bypass factor is the fraction that behaves as if it missed the coil; the contact factor is 1 minus the bypass factor.
How it is calculated
On a psychrometric chart, the entering state, the leaving state and the ADP lie on one straight line. The tool finds the humidity ratios from the dry-bulb and wet-bulb readings, extends the line through the two states until it meets the saturation curve, and reads the ADP there. Then:
- Bypass factor = (leaving dry-bulb − ADP) ÷ (entering dry-bulb − ADP)
The same ratio holds for humidity ratio and enthalpy.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Entering air dry-bulb
- 80 °F
- Entering air wet-bulb
- 67 °F
- Leaving air dry-bulb
- 55 °F
- Leaving air wet-bulb
- 54 °F
- Site elevation
- 0 ft
Result
- Apparatus dew point (ADP)
- 52.5 °F
- Bypass factor
- 0.091
- Contact factor (1 − BF)
- 0.909
- Entering humidity ratio
- 78.2 gr/lb
- Leaving humidity ratio
- 60.4 gr/lb
- Leaving relative humidity
- 94 %
ADP 52.5 °F with a bypass factor of 0.091 (9.1 % of the air behaves as if it missed the coil).
Assumptions
- Bypass-factor model: leaving air behaves as a mix of untreated entering air and air saturated at the coil surface temperature (the ADP); the same ratio holds for temperature and humidity ratio, so the ADP lies where the straight process line meets saturation.
Limitations
- Valid for a wet (dehumidifying) coil at steady state with well-mixed readings; a 0.5 °F wet-bulb error moves the ADP noticeably.
- The process line is drawn in temperature–humidity-ratio coordinates, as on a psychrometric chart.
Frequently asked questions
What sets a coil’s bypass factor?
The coil’s construction and the air velocity through it. Use measured readings or the coil maker’s data rather than a rule of thumb.
Why does a small wet-bulb error change the answer so much?
The ADP comes from extending a line, so small errors in the two points are magnified at the saturation curve. Use accurate, well-mixed readings.
What if the coil is not removing moisture?
For a dry coil the line is horizontal and the ADP equals the entering dew point. The bypass factor is still defined from the temperatures.
Sources
- Multiple model approach and experimental validation of a residential air to air heat pump (Garde, Boyer, Pignolet, Lucas, Brau; CLIMA 2000, 1997) — arXiv:1212.5665 (open access). Coil bypass-factor model: leaving air is a mix of air unaffected by the coil and air at the coil surface temperature (apparatus dew point); BF = (T_out − T_adp)/(T_ent − T_adp) = (h_out − h_adp)/(h_ent − h_adp) = (w_out − w_adp)/(w_ent − w_adp).
- PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
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