Cooling load due to 100% outside air and no return air

[aumechengr]

I am new to HVAC design and am looking for a little a guidance. I need to determine the cooling load for a unit that will supply a room with 100% outside air and 0% return air. The room will be exhausted at a rate of 6 ACH. This gives me a minimum air flow rate through the unit. I understand that the cooling apparatus will need to be sized to handle the sensible and latent load from the outside air plus the additional room load. My question is does the room load need to include both sensible and latent loads or the sensible load only? Any help would be much appreciated.

aumechengr

 

[newmep]

As you mentioned, the outside air has latent and sensible load components on the unit, and also the interior space could have latent and sensible load components. You have to add all loads of same type together to size the unit.
hope this helps.
Regards,

 

[KiwiMace]

The largest portion of the latent load is removing humidity from the outdoor air at the coil. Coils are designed with an increasing # of rows of tubes to address larger latent loads. Coil area is a function of the airflow.

The space load is dominated by sensible heat, the latent load in the space is normally managed indirectly by considering the final outcome of mixing cool, low absolute humidity air from the coil with warm space air. This is worked out on a psychrometric chart.

So [if you supply the space with air at the right temperature and absolute humidity based on your effort with the psych chart and coil design] your calculation of how much air to supply the space is based on sensible heat only [but really the latent load in the space is also indirectly being taken care of].

 

[SAK9]

I would suggest the following:

a.Calculate the airflow based on 6 ACH/hr.Select a coil leaving temperature a depending on the moisture load in the fresh air and relative humidity to be maintained in the conditioned space.This could be between 12~15C.Plot this on a psychro chart to get a better handle on the process.

b.Calculate airflow based on room sensible heat.Room air flow is governed by sensible heat only.Moisture load comes into play while selecting coil leaving air temperature only.

C.Select the greater of a or b as the design air flow.

d.If a is greater and is used as the design air flow,use reheat to achieve room conditions

 

[aumechengr]

All,

Thanks for such a quick response to this question. I am still a little confused based on the responses. I would have agreed with NEWMEP that you include the latent heat gains from the room in your calcs. But, KiwiMace and SAK9 are in agreement that you need to condsider sensible heat gains only. Based on that view, how do you account for a high latent heat gain in the space? (By the way, I do not have a high latent heat gain)

But, by adding the latent heat gain to the calculation it stands to reason that the total room heat gain is increased, therefore, the amount of cooling required is increased. I am leaning towards this approach. It may provide a slighty oversized system, but if needed the extra capacity would be available. Thoughts?

aumechengr

 

[KiwiMace]

I would suggest stepping through the process of plotting a cooling and dehumidifying process onto a psych chart.

Your room latent load will change the sensible heat ratio, and the slope of the space condition line that you plot onto the psych chart.

You are not wrong in saying that the equipment will need to be sized for the total load in sensible and latent. But you still calc the airflow based on the sensible heat Qs.

 

[MintJulep]

This is one of those cases that really lends itself to an old fashioned graphical solution on a psychrometric chart.

Your original post doesn't provide enough information to solve the problem.

Most importantly, you need to decide what conditions you want to maintain in the space (which can be assumed to be the conditions of the exhaust air).

Using the exhaust condition and the space SHR, draw a line on the chart.

Your supply air condition has to be on that line, and difference in total enthalpy from supply to exhaust has to equal the space total load. Now you have defined your supply condition.

You know your outside condition.

Draw a line between outside and supply.

Size a coil accordingly.