A basic air-conditioning schematic for summer mode is shown below:

Here, return air from the conditioned space (2) mixes with required amount of outdoor air (1) at point (3) for acceptable indoor air quality and energy saving. The mixture is then cooled and dehumidified to ADP (4) at the cooling coil. The actual off-coil condition is represented as (5) due to inefficiency (bypass) of cooling coil. The conditioned air is supplied to the room through the supply fan, supply duct, and ceiling diffuser (6). Supply air then absorbs the sensible and latent load from the space, becoming the space air (2). Room air is returned back to the cooling unit again and forms a closed cycle. Part of the return air is exhausted to balance the outdoor air intake and infiltration.

The psychrometric process is illustrated below for summer cycle.

You may notice that the off-coil condition is represented by node 5 away from ADP point 4. This is because the cooling coil is not 100% efficient and some of the air going through a cooling coil does not come into contact with the tubes or fins of the cooling coil and is therefore not cooled to the ADP temperature.

This may be looked upon as an inefficiency of the coil and is usually given as the cooling coil contact factor.

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