When selecting a commercial split DX system with a constant volume draw-thru AHU and matching condensing unit for low supply air temperature (SAT) applications, the saturated suction temperature (SST) is a concern. The SST is the temperature of the refrigerant inside the cooling coil, so we assume the cooling coil will be the same temperature. Ice begins to form on the cooling coil with a SST of 38° F; any lower temperatures are not advisable. To prevent the coil from icing and to have some leniency for fluctuation in the system and/or design, some manufacturers recommend a SST no less than 41° F.
A DX AHU selection for the design CFM may yield a SST less than 41° F when the LAT off the cooling coil will be 49° F with a SAT of 52° F; these are low temperatures and run the risk of freezing the coil. One solution to increase the SST to 41 ° F can be achieved by increasing the air flow, preventing the coil from freezing, and the extra air can be distributed to non-vital areas. This is not the most energy efficient method, but it will keep the initial cost down when price is an issue. Utilizing energy recovery equipment or desiccant wheels to reduce the cooling load on the DX coil is also an option.
Refrigerant capacity control is important to maintain adequate SST. Capacity control allows the unit to operate, maintain space temperature and humidity by avoiding short cycling during low load conditions and prevent icing of the coil. Hot gas by-pass or variable volume compressors are appropriate control strategies.
