Air changes per hour for room
Air changes per hour for room
(OP)
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.





RE: Air changes per hour for room
Just bring in the minimum of outside air for ventilation requirements and recirculate all (almost) the air inside the room. If your 250,000 btu/hr includes the latent and sensible loads to bring the room down to whatever the design ambient in the room will be (I usually size 85 deg. F for worst case in an electrical room) then you will need a lot less then a 21 ton unit.
RE: Air changes per hour for room
It is much more pratical to consider mechanical cooling. At which point, you only need to bring in the minimum O/A and add that load to your 250,000 Btu/hr sensible load.
Is your minimum outdoor air 8 ACH? That seems high for an electrical room.
Given your high RH evaporative cooling doesn't look like an option.
RE: Air changes per hour for room
RE: Air changes per hour for room
You definitely want to keep the RH lower than 90% in an electrical room. For ventilation only cooling, if you have high heat loads and high ambient air it requires a very high air flow.
If the sensible load in the room is 210,000 btu/hr you need to size for it. The heat load seems a little high for an electrical room. What is in it? Transformers and VFD's have relatively high heat loads for electrical equipment. FVNR starters, switchboards, etc. have relatively low heat loads.
RE: Air changes per hour for room
RE: Air changes per hour for room
RE: Air changes per hour for room
At any rate, as suggested, if you use non-mechanical cooling methods (all 90 deg air) and your upper limit is 104 deg F (40C), then the 14 deg temp difference will force the air flow full volume to be around 16,000 CFM as you calculated, assuming 250 MBH is correct. Is there a higher upper limit allowed by the electrical equipment, perhaps 50C? If so, your CFM without mechanical cooling will drop down to around 7,000 (temp diff of 32 deg F). WIth mechanical cooling, the delta T will go up and the air flow rate requirement will go down as well.
RE: Air changes per hour for room
RE: Air changes per hour for room
HH, you have two loads:
1) Electrical equipment load
2) Outside air load
If you use mechanical cooling you will need to have a fan coil or similar device to cool the 250 MBH load you have from your electrical equipment.
If you introduce O/A (at 90F) you will need to cool this air to the acceptable room temperature.
If 90F is lower than the acceptable room temperature, you can use O/A to provide some, or all, of your cooling. The factor that will determine how much O/A you are using is what is an acceptabe air volume rate.
Where does the 6-10ACH come from? What is the acceptable room temp?
RE: Air changes per hour for room
RE: Air changes per hour for room
RE: Air changes per hour for room
Now, you have the volumetric flowrate with you. If this is less than the no. of ACPH you require, you can increase the flowrate. However, you may have to either reheat the conditioned air or treat part of the air and then mix with untreated air. In this case, volume flowrate through coil and volume flowrate through room can differ.
If volumetric flowrate by cooling load calculation is more than volumetric flowrate obtained by no. of ACPH, then the flowrate through coil and that of room should be equal.
RE: Air changes per hour for room
RE: Air changes per hour for room
RE: Air changes per hour for room
RE: Air changes per hour for room
Since you want to keep the space at 80 or something, we've established you can't do it with ventilation alone, therefore you need mechanical cooling. Since it's an unoccupied space, you don't require ANY ouside air. Therefore, you can recirculate all of the air and cool it to satsify the load and the CFM requirement will come from this analysis alone, not ACH. Now, as I suggested, if you want to bring in 10% or so of outside air, this will help pressurize the space and may even provide (coincidentally) around 6 to 10 ACH of fresh air.
RE: Air changes per hour for room
If their is a recommended ACH (from some source) then it is definetly outside air only.
RE: Air changes per hour for room
RE: Air changes per hour for room
I agree with ChrisConley that ASHRAE requirements for electrical rooms don't require 8-10 ACH of outside air.
Obviously minimizing the outside air will minimize your cooling load since the outside air is warmer than the temperature setpoint of the room.
RE: Air changes per hour for room
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.
RE: Air changes per hour for room