Supply Air Temperature Cooling

When the outdoor dry bulb temperature odb is at or below 15 6c the setpoint is 18c.

Supply air temperature cooling.

First let s start with 55 degrees. So even though there s no single ideal temperature you do want a 16 22 f difference from the supply air and return air. The cold air systems distribute air at a temperature much lower than 55 f. Mixing systems generally supply air such that the supply air mixes with the room air so that the mixed air is at the room design temperature and humidity.

Since the lowest supply air temperature is already achieved without any mechanical cooling supply air temperature optimization is not necessary in this outside air condition. Professionals call this temperature difference the evaporator delta t. In between the setpoint is varied linearly. For generations large commercial air conditioning systems have been designed around a constant supply air temperature setpoint of 55 degrees.

The temperature your ac puts out is relative to the temperature you set on your thermostat. The supply air temperature can be always maintained at its design setpoint 55 f by modulating the outside airflow using the outside air and return air dampers. Your cooling schedule that sets cooling setpoint temperature changes values between 0 and 24 on weekdays and the summer design day which is the design day used for cooling sizing. When the odb is at or above 26 7c the setpoint is the off coil air temperature set point.

Why use cold air distribution. The well known fact that 55 degrees is the preferred supply air temperature comes to us from the commercial world. The supply air temperature is 20 or more degrees above the supply air temp alarm for cooling set point for 10 minutes. The coldest practical air temperature is about 38 f with most cold air designs using 42 48 f.

The unit controller compares the supply air temperature to the supply air temp alarm for cooling and initiates the alarm based on the following criteria. For a given air conditioning load as the supply air temperature is reduced the supply air volume is reduced proportionally. For example if the air supply system is designed for a constant air supply temperature of 55 f the valve controlling the supply of hot water to the heating coil will close as the supply air temperature approaches 55 and will open wider if the supply air temperature begins to drop below 55 f.