AB-10-C004 -- Optimizing Supply Air Temperature with ASHRAE 62.1 PDF

ASHRAE 62 in its 2004 and 2007 has introduced among others two new concepts which are that the ventilation air rate now has two parts (one based on occupants and one based on floor area) and that this rate is now directly linked to the supply airflow to the critical space.

On the other hand, design engineers have long been trying to optimize the supply air temperature and move away from the standard 55 °F. Some have opted for cold air supply while others have opted for higher supply temperature.

If supply air temperature is reduced, this will surely decrease the supply fan horsepower and duct material. However, this now decreases the supply air to the critical space yielding an increase in the required ventilation rates at the system level for multiple zones VAV systems and subsequently a higher cooling load and energy consumption.

The paper derives individual equations linking the ventilation rates to the supply air rates to the cooling capacities. Moreover, the paper will present the result obtained by using MATHEMATICA to differentiate these expressions to obtain the optimum supply temperature for either minimum power or minimum life cycle cost.

The resulting equations are complicated in nature. However, they were programmed into a spreadsheet that will able to indicate to the designer the optimum supply temperature based on all of the above inputs.