7 Basic Steps in DOAS Design

Design Steps

In our continuing series of DOAS topics, I'll share some approaches on designing a dedicated outdoor air system.
In general, three factors will determine the DOAS unit cooling coil load, and these are: (a)Ventilation airflow; (b) Outside air enthalpy condition; and (c) Leaving air enthalpy condition.
Normally the ventilation airflow is a constant. Enthalpy difference determines load as given by the formula as

4.5 * ventilation cfm * enthalpy difference between outside air and coil leaving air.

Determining leaving air design state is the key. In determining the required coil leaving air temperature for the DOAS:

• Design for coil leaving ventilation air that is drier than the space humidity target.
• Design to handle the ventilation air sensible and latent load plus the space latent load.

Look for the worst humidity case for the outside air:

• This will likely not be at the peak DB temperature for the location.
• The engineer must look at the peak dew-point and the mean coincident peak WB for the highest enthalpy condition.
  

With that said, there are seven basic steps to determining the DOAS coil load and ventilation air requirement: 

1. Determine outside air enthalpy conditions based on the project location.
2. Determine the maximum allowable space humidity by asking the end-user or using ASHRAE recommended levels.
3. Determine the space latent loads by determining the number of occupants and applying the ASHRAE latent load factors based upon their activity levels. Add any unusual latent machine contributions, if any.
4. Determine the required ventilation cfm by using the formulas in ASHRAE Standard 62.1.
5. Determine the required dewpoint and enthalpy of the supply air.
6. Calculate the coil load as described previously.
7. Determine the required leaving air temperature by deciding if this will be a “neutral” or “cold air” system.

In our next post, I shall discuss these steps in more detail and provide an illustrated example to show how to calculate the required load for the DOAS.