Designing a Dedicated Outdoor Air System (DOAS) - Part 2

This is a continuing series in topics relating to DOAS. If you missed "Part 1", because I didn't title the previous post as such please check it out: Part 1 - DOAS design. As I mentioned previously there are 7 steps to follow in designing a DOAS, and step four requires an understanding of ASHRAE 62.1.

What is ASHRAE 62.1 anyway?

No, no ASHRAE is not where you put cigarette butts in. For HVAC people (and I assume you are), ASHRAE 62.1 is the standard for Ventilation for Acceptable Indoor Air Quality. As of this posting, I am using the 2010 edition. Hence for "step four" of our design, to determine the amount of outside air ventilation we have:

V_bz = (R_p * P_z + R_a * A_z ) / E_z

where:
A_z = zone floor area: the net occupiable floor area of the ventilation zone ft² (m²)
P_z = zone population: the number of people in the ventilation zone during typical usage
R_p = outdoor airflow rate required per person as determined from Table 6-1 of Ashrae 62.1
R_a = outdoor airflow rate required per unit area as determined from Table 6-1 
E_z = zone air distribution effectiveness factor from Table 6-2

 Important Note: The Equation above accounts for people-related sources and area-related sources independently in the determination of the outdoor air rate required at the breathing zone. 

The ventilation effectiveness factor E_z is typically 1.0 for cooling systems that use ceiling diffusers and 0.8 for heating systems that use ceiling diffusers. For a complete list of the factors used for varying applications refer to Table 6-2 of Ashrae 62.1.

The rate factors for calculating the amount of outside ventilation air can be found in Table 6-1 of Ashrae 62.1-2010 but some of the more common factors are shown below:

The most important aspect, and should not confuse you, for determining the outside air requirement is to consider the two most important factors, the people and the area they would occupy in the breathing zone.

In the next post, I shall discuss further the other steps in the design process for DOAS. And in the subsequent posts illustrative examples shall be shown to get a better grasps of the principles. Don't forget to subscribe.