How to design a DOAS unit as sole source heating and cooling
When possible, it makes life easy to have a separate DOAS system from the space HVAC system. In this case, each system has one specific job:
-The DOAS’s job is to bring in 100% Outside Air, when the space is occupied, and delivers neutral temperature air.
-The main heating & cooling system can be (in most cases) 100% recirculation, and is only designed to maintain space temperature (and possibly an Rh component).
But how do you design a system when one unit is performing both functions?
More specifically, how do you know if you have enough capacity to take care of both components of the load? And how do you know if you have the correct leaving air temperature to satisfy both the outside air load AND the skin + internal loads?
Since the heating side is a little easier, we will start here. All we need to do is make sure we have enough extra heating capacity for the skin load (on top of the makeup air load). The “effective skin capacity” of the DOAS unit is the difference between the LAT (leaving air temperature) and the space temperature. For example, if the space temperature will be kept at 72, and the LAT of the DOAS unit is 95, you have 23 degrees of “skin capacity”. Assuming your unit is 10,000 cfm of supply air, here is how you calculate the skin capacity in btuh:
Qs = 1.085 * cfm * DT
Qs = 1.085 * 10,000 * 23 = 249,550 btuh
As long as your skin load is 249,550 or less, you’re good! If not, you need more capacity.
Cooling is a bit trickier because you are dealing with both sensible and latent capacities. It’s also a little tougher because you might need a psychrometric chart (or you can just call your cfm sales engineer or territory manager).
First, let’s look at SENSIBLE cooling capacity:
To check cooling sensible capacity, the same rule applies for the heating.
So if your space temperature will be kept at 75 degF, and the LAT of the DOAS unit is 55 degF, than your “effective cooling sensible capacity” is:
Qs = 1.085 * cfm * DT
Qs = 1.085 * 10,000 * 20 = 217,000 btuh
If this number is bigger than your sensible load, so far so good!
At this point, you are probably good to go, since almost all of your latent load will be taken care of in the getting the air down to 55. But it never hurts to double check the latent capacity.
First, calculate total capacity:
Qt = 4.5 * cfm * DH
DH is the difference in enthalpy from your desired space condition and the LAT of the DOAS.
(this is where you might need your psychrometric chart)
Enthalpy for a space temp of 75 & 50% RH is 28.3. Enthalpy for a LAT of 56/55 is 23.3. So DH = 28.3-23.3 = 5.0
Qt = 4.5 * 10,000 * 5.0 = 225,000
Finally, Ql = Qt- Qs = 225,000 – 217,000 = 8,000 btuh latent capacity.
If your latent capacity (not including the outside air) is 8,000 or less, then you are good to go! If it is more, you have two options:
#1. Change the design indoor space condition: If you allow the indoor condition to be 75 and 55%, you would increase latent capacity to 54,000 btuh.
#2. Increase capacity of DOAS: If you can get the DOAS LAT to 55/54, you would increase latent capacity to 30,650 btuh
Once again, check to make sure you have enough sensible and latent capacity. Once you have these covered you are good to go!
If you have any questions please give us a call or comment below!
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