What is the relationship between Contact Factor and Bypass Factor?

The contact factor (β) and bypass factor (BPF) are complementary indicators of cooling coil performance. Their sum is equal to 1 (BPF = 1 - β). Contact factor represents the fraction of air coming into direct contact with the coil surface, while bypass factor represents the uncontacted fraction.

How does coil depth/rows affect the bypass factor?

Increasing the number of row depths or fin density increases the contact time and surface area, which raises the contact factor and reduces the bypass factor. A typical 4-row coil has a BPF around 0.10 to 0.15, whereas an 8-row coil can achieve a BPF below 0.03.

Why is coil face velocity kept below 2.5 m/s (500 fpm)?

Maintaining air velocity across the coil face below 2.5 m/s prevents condensed water droplets from being stripped off the fin surfaces and carried downstream into ductwork, preventing mold and water damage.

Air cooling : Contact factor and Bypass factor of air cooling coils

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1. Cooling humid air without condensation
2. Cooling humid air with condensation : dehumidification

Air handling units require the cooling of humid air to condition it thanks to a cooling coil. The cooling coil is never 100% efficient. The actual efficiency of a cooling coil can then be expressed with a contact factor or a bypass factor (both are related).

The case considered here is a cooling coil that allows to cool down air from a condition A to a condition B, while the cooling coil is at the temperature of point C (part of the water in the air is condensed here, known as the Apparatus Dew Point or ADP).

Psychrometric chart : cooling humid air with condensation in AHU

More information on humid air cooling with condensation can be found at this page.

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided exclusively for preliminary estimation and educational purposes. It is not intended for detailed design or equipment procurement without certified vendor rating. No warranty, expressed or implied, is provided, and no liability is assumed.

⚡ Interactive AHU Cooling Coil Performance Calculator

Unit System: SI Metric (°C, kg/s, kg/kg, kW) US Customary (°F, lb/min, lb/lb, BTU/h)

Inlet Air Temp (\(T_A\)) [°C]:

Outlet Air Temp (\(T_B\)) [°C]:

Coil Temp / ADP (\(T_C\)) [°C]:

Inlet Humidity Ratio (\(x_A\)) [kg/kg]:

Outlet Humidity Ratio (\(x_B\)) [kg/kg]:

Coil Surface Humidity (\(x_C\)) [kg/kg]:

Air Mass Flow Rate (\(\dot{m}_{air}\)) [kg/s]:

Calculation Results

Contact Factor (\(\beta_{temp}\))

0.400

Bypass Factor (\(BPF_{temp}\))

0.600

Contact Factor (\(\beta_{hum}\))

0.400

Bypass Factor (\(BPF_{hum}\))

0.600

Sensible Cooling Load

20.12 kW

Latent Cooling Load

10.01 kW

Total Cooling Capacity

30.13 kW (8.57 Tons)

Sensible Heat Factor (SHF)

0.668

Dehumidification Rate

14.40 kg/h

Enthalpies (\(h_A, h_B, h_C\))

60.83 kJ/kg

45.53 kJ/kg

22.66 kJ/kg

1. Contact factor

The efficiency can be represented with a contact factor.

Calculation of the contact factor of a cooling coil

The contact factor can be calculated with the following formula :

\[ \beta = \frac{x_A - x_B}{x_A - x_C} = \frac{h_A - h_B}{h_A - h_C} \]

With :

β = contact factor
\(h_A\) = enthalpy of humid air at condition A, inlet of the cooling coil, (J/kg_dry_air)
\(h_B\) = enthalpy of humid air at condition B, outlet of the cooling coil, (J/kg_dry_air)
\(h_C\) = enthalpy of humid air at condition C, in contact with the cooling coil, (J/kg_dry_air)
\(x_A\) = humidity ratio (=Absolute humidity of air) at condition A (-)
\(x_B\) = humidity ratio (=Absolute humidity of air) at condition B (-)
\(x_C\) = humidity ratio (=Absolute humidity of air) at condition C (-)

In practice, the contact factor can be approximated thanks to a ratio of the temperatures of points A, B and C :

\[ \beta \approx \frac{T_A - T_B}{T_A - T_C} \]

With :

\(T_A\) = temperature of humid air at condition A, inlet of the cooling coil, (K)
\(T_B\) = temperature of humid air at condition B, outlet of the cooling coil, (K)
\(T_C\) = temperature of humid air at condition C, in contact with the cooling coil, (K)

Example of contact factor calculation : Step by Step calculation

Air at 35°C and 30% RH is to be cooled down until 25°C with a cooling coil at 10°C, what is the contact factor value ?

STEP 1 : approximate the bypass factor with temperatures

The contact factor can be approximated as :
\[ \beta \approx \frac{T_A - T_B}{T_A - T_C} = \frac{35 - 25}{35 - 10} = \frac{10}{25} = 0.4 \]

STEP 2 : calculate with a psychrometric chart

The psychrometric chart given at the beginning of the page can be used to calculate the contact factor with the humidity ratio for example :
\[ \beta = \frac{x_A - x_B}{x_A - x_C} = \frac{0.010 - 0.008}{0.010 - 0.005} = \frac{0.002}{0.005} = 0.4 \]

💡 Industrial Plant Engineering Rules of Thumb & Design Margins

Coil Row Depth vs. Bypass Factor: BPF is heavily dictated by coil depth. 2-row coils generally have BPF ≈ 0.20-0.30; 4-row coils BPF ≈ 0.10-0.15; 6-row coils BPF ≈ 0.05-0.08; 8-row coils BPF < 0.03.
Face Velocity Limit: Air velocity across the coil face must be limited to 2.0 - 2.5 m/s (400 - 500 fpm). Exceeding 2.5 m/s risks stripping condensed water droplets off fins, causing water carryover into ductwork.
Apparatus Dew Point (ADP): Choose coil coolant supply temperature such that ADP (\(T_C\)) is 2 to 3°C below the required outlet dew point for efficient moisture condensation.
Safety Margins: Apply a +10% to +15% design capacity margin over total calculated load (\(Q_{total}\)) to account for fin fouling, pipe scale, and peak latent loads.

2. Bypass factor

The efficiency can be represented with a bypass factor.

Calculation of the bypass factor of a cooling coil

The bypass factor can be calculated with the following formula :

\[ BPF = \frac{x_B - x_C}{x_A - x_C} = \frac{h_B - h_C}{h_A - h_C} \approx \frac{T_B - T_C}{T_A - T_C} = 1 - \beta \]

With :

BPF = bypass factor
β = contact factor
\(h_A\) = enthalpy of humid air at condition A, inlet of the cooling coil, (J/kg_dry_air)
\(h_B\) = enthalpy of humid air at condition B, outlet of the cooling coil, (J/kg_dry_air)
\(h_C\) = enthalpy of humid air at condition C, in contact with the cooling coil, (J/kg_dry_air)
\(x_A\) = humidity ratio (=Absolute humidity of air) at condition A (-)
\(x_B\) = humidity ratio (=Absolute humidity of air) at condition B (-)
\(x_C\) = humidity ratio (=Absolute humidity of air) at condition C (-)
\(T_A\) = temperature of humid air at condition A, inlet of the cooling coil, (K)
\(T_B\) = temperature of humid air at condition B, outlet of the cooling coil, (K)
\(T_C\) = temperature of humid air at condition C, in contact with the cooling coil, (K)

3. Cooling requirements

It is possible to calculate the heat flow required in the cooling coil thanks to the flow of humid air to cool down. The heat flow can then be expressed as :

\[ Q_{cooling} = \dot{m}_{air} \cdot (h_A - h_B) = \dot{m}_{air} \cdot Cp_{air} \cdot (T_A - T_B) + \dot{m}_{air} \cdot h_{vap} \cdot (x_A - x_B) \]

(\(Q_{cooling}\) is expressed here as positive, please adjust the sign according to the convention used in your calculations)

With :

\(Q_{cooling}\) = heat flow rate or power to bring the air from condition A to B (kW)
\(\dot{m}_{air}\) = mass flow rate of air (kg/s)
\(h_A\) = enthalpy of humid air at condition A, inlet of the cooling coil, (kJ/kg_dry_air)
\(h_B\) = enthalpy of humid air at condition B, outlet of the cooling coil, (kJ/kg_dry_air)
\(Cp_{air}\) = specific heat capacity of air (kJ/kg.°C) = 1.006 kJ/kg.°C
\(h_{vap}\) = enthalpy of vaporization of water (kJ/kg) = 2501 kJ/kg