Air changes per hour for room 1

[HawksHockey]

Ok, I've struggled with this one. I'm new at this, so here goes: I'm trying to determine a volumetric flow rate of air for a room which houses electrical equipment. I have the dimensions of the room at 15 x 27 x 20 and I have determined the cooling load at 250,000 BTU/hr. The room is underground, and will draw air at 90F and 90% RH. Here is the question, if I have accounted for 8 ACH (in my sensible and latent heat loads) but have 250,000 BTU/hr, therefore what is governing? From AHSRAE, Flow=Total Heat Load/(density of air x specific heat of air x 60 x temperature difference). With density at 0.07 and specific heat @ 0.24, and a temperature difference of 15 F, my flow rate comes up to 16,500 CFM? Could this be the volumteric flow rate through the 21 ton unit in order to cool the air and not the flow rate into the room (which I thought was 8 times the volume of the room)? I'm lost.

 

[KiwiMace]

I think the confusion is between the following:

1. Using 100% outdoor air to reject heat by mechanical ventilation only.
2. Using Air Conditioning.

The 8-10 ACH number is a normal figure for plantrooms with no air conditioning (1).

When you provide air conditioning (2), the amount of outdoor air you need to provide drops considerably, to the point where it supports only the human occupants, or provides pressurisation.

If you need 80degF in the space and the outdoor air can be 90degF, option 1 above is not viable. You have to use mechanical cooling and therefore you will only need in the order of a few hundred cfm of outdoor air per ASHRAE 62.
I consider it unrealistic to say that no occupancy would be envisioned for this space. Perhaps only 2 pp tho.

 

[HawksHockey]

I think after all this time, that I've finally gotten the basics under wraps. My problem was exactly what you stated, I was mixing ACH (air changes per hour) with total volumetric flow rate. If my total volumetric flow rate was equal to my ACH (100% outdoor air) my Latent load would have been enormous! Once I finally understood that ACH is part of the total volumetric flow rate - it all made sense. So now I am supplying a minimum number of ACH based on a minimum occupancy (because there will be people in the electrical room), and the rest of the air is being recirculated through the system. Thank you all for the help.