A secondary fluid cooling (SFC) process is also shown in Figure 10, where (he air-water vapor mixture is being cooled from the same initial condition, condition 1. The SFC fluid process decreases the enthalpy (and temperature) of the air-water vapor mixture as the mixture is cooled. The water vapor content (the humidity ratio) remains constant as the mixture decreases in temperature to approximately 63 F (17.2 C), shown as condition 4. According to the performance of the cooling coils, continued cooling of the air-water vapor mixture forces the mixture to decrease in temperature along a cooling coil performance curve so that the coil exit temperature is at a slightly warmer temperature than the saturation condition (see, e.g., Trane 1965).

For example, water vapor will begin to condense before the entire airflow reaches the 100% relative humidity condition at approximately 70 F (21.1 C), shown as condition 4a. Continual cooling will follow the cooling coil performance curve, which approaches, but does not reach, saturation. If cooled to 40 F (4.4 C), the condition is shown as condition 5. Condition 5 represents a mixture where the relative humidity of the air is nearly 100% with decreased humidity ratio, forcing part of the water vapor to condense out from the mixture.

An SFC system can be utilized with or without chilled water or ice thermal storage. The SFC system may have a higher capital cost and use more energy than a direct refrigeration system because of the secondary fluid loop, the pumping required, and the increased...