In HVAC systems, the cooling coil plays an important role in controlling both temperature and humidity. Understanding its operating characteristics is essential for proper sizing and for addressing potential issues that may occur. This post explains the cooling coil characteristic on Psychrometric chart as the air flow through it and how a cooling coil functions under part-load conditions.
The cooling coil is designed to meet the peak cooling load. During part-load conditions, the thermostat sends a signal to the control valve to reduce the chilled water flow through the coil according to the temperature setpoint (sensible heat dependent). As a result, both the sensible and latent cooling capacities of the coil decrease. When the chilled water flow is reduced to a certain flow (e.g., 40% of the design flow rate as you can see in the illustrated note), the coil stop to condense moisture. So only sensible heat is removed.
Related notes:
Left-hand and Righ-hand coil in HVAC system
Flow characteristic of control valve
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