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Apparatus dew point: the key to cooling coil selection
The apparatus dew point (ADP) is the temperature at which the coil process line on a psychrometric chart intersects the saturation curve (100% RH line). It represents the effective surface temperature of the cooling coil from a psychrometric perspective. If the coil had perfect fin contact with all the airflow and zero bypass, the leaving air would be fully saturated at the ADP. In practice, some air passes through the coil without touching a fin surface, so the actual leaving condition falls on the process line between the entering state and the ADP, determined by the bypass factor.
To find the ADP, extend the straight process line from the entering air state through the leaving air state until it hits the saturation curve. This graphical construction is automated here. The slope of that line equals the sensible heat ratio (SHR), and the same ADP applies whether the coil is 30% or 90% effective, as long as the SHR and entering conditions don't change. This is why ADP is a fundamental coil selection parameter independent of coil size or bypass factor, and it underlies the standard rating procedures in AHRI Standard 410 for forced-circulation air-cooling coils. Use the SHR calculator first if you only have total and sensible loads rather than full state point data.
Bypass factor, contact factor, and coil rows
Bypass factor (BF) quantifies how much air bypasses the coil without heat and mass transfer: BF = (Tleaving − TADP) / (Tentering − TADP). Contact factor (CF = 1 − BF) is the fraction of air that reaches the ADP condition. More coil rows lower the bypass factor by forcing more air to contact the fin surfaces. A 6-row coil might achieve BF of 0.08 to 0.12, while a 4-row coil might reach 0.14 to 0.20 at the same face velocity. For a given leaving dry-bulb setpoint, a lower ADP (colder coil) with a higher BF achieves the same result as a higher ADP with a lower BF. The trade-off is refrigerant or chilled water supply temperature versus coil size, and it matters most in high-latent applications such as a commercial kitchen makeup air unit, where the coil has to condition a large, humid outdoor airflow. Use the bypass factor calculator to analyze this trade-off for a specific coil design.
Chilled water temperature and ADP in Canadian systems
For chilled water coils, the ADP must be at or below the chilled water supply temperature for dehumidification to occur. Canadian central plants typically supply chilled water at 6 to 8°C, which supports ADP values of 8 to 12°C for standard comfort cooling, and loop temperature reset requirements for these systems are addressed directly in the National Energy Code of Canada for Buildings. Where the load analysis requires ADP below 6°C (high-latent applications like commercial kitchens, laboratory fume hoods, or indoor aquatic centres), chilled water at 4 to 5°C or a DX system is needed. In Calgary and Edmonton, the dry climate often allows ADP in the 12 to 16°C range, enabling higher chilled water temperatures and better chiller efficiency. Use the coil load calculator to translate ADP and bypass factor into equipment capacity specifications for the project.
Frequently Asked Questions
The apparatus dew point (ADP) is the effective coil surface temperature from a psychrometric standpoint. It is the point on the saturation curve where the coil process line (drawn from entering to leaving air state) intersects when extended. If all the airflow had perfect contact with the coil surface, air would leave at the ADP condition, fully saturated. Since some air bypasses the fins, actual leaving conditions fall between the entering state and the ADP based on the bypass factor. The ADP sets a lower limit on the required chilled water supply temperature or DX suction temperature. Use the SHR calculator alongside this tool to get the full picture of coil psychrometric performance, and verify moisture removal with the moisture content calculator.
The chilled water supply temperature sets a floor on the achievable ADP. For dehumidification to occur, the ADP must be below the dew point of the entering air. The chilled water supply temperature must be at or below the ADP. Canadian central plants typically supply at 6 to 8°C, supporting ADP from 8 to 12°C for standard comfort cooling; the chiller plant calculator can size the plant around that supply temperature. High-latent applications requiring ADP below 6°C need chilled water at 4 to 5°C or a DX system. In dry cities like Calgary (1045 m elevation), the entering air dew point is often low enough that ADP can be 12 to 16°C even in summer, allowing higher chilled water temperatures and better chiller coefficient of performance. See the bypass factor calculator to determine how many coil rows are needed to reach the leaving condition from the ADP.