Room Temperature Rise???

[mech5]

I have a question regarding the temperature rise of a pump and VFD room. I used some software to calculate the heat load of the space. The software gave me this output.

Sensible gain = 28270 btuh
Latent gain = 9980 btuh
Supply Air = 1336 cfm
Coil LAT = 61 F

Now the equipment I specified can only supply 900 CFM and is capable of 36000 btuh. My question is what will be the expected room temperature?
I get around 90 F. I used q=(CFM)*1.08*(Tr-Tc), with q= 28270 btu, CFM= 900 and Tc=61 F.

Is this right? Or am I using the wrong approach?
Thanks for your help.

 

[Drazen]

what software are you using? i am somewhat curious that you have leaving air temperature as an output result, for the reason that software cannot know shr ability of coil itself, it is customary that you specify lat, and receive supply air volume flow as an output. there are software packaged where you can specify all coil characteristics in advance, but i would assume that you did not follow that path.

if your equipment works properly, is not too much oversized, your dry bulb room temperature will be the one set by controls.

the question is how much of latent capacity can be handled by that equipment. you should have some spec for latent capacity.ž

calculating indoor air temperature for design load only means a little. your system has to maintain design temperature at all conditions. how that will be achieved depends on system and controls configuration.

 

[urgross]

Is this an open loop system? How is the equipment generating a latent load?

 

[MintJulep]

No, it's not right.
Yes, you are using the wrong approach - if randomly throwing unrelated numbers at equations can be considered an approach.

There is not enough coherent information in your post to allow anyone to offer any better guidance.

 

[SAK9]

I have a sneaky suspicion you do not know hwat you are doing.Mechanical ventilation is sufficient for most pump rooms unless you are in extreme weather zone.Even if you decide to air condition it,there should not be any latent loads(unless ofcourse you have leaking seals on the pumps!)

 

[mech5]

This was initially calculated and specified with a conventional 5 ton system. I had a 56 mbh load with 250 cfm of OA. The client was putting in a 3 ton ductless split instead of the 5 ton unit. So for the above example, I removed the OA from the calculations and lowered the heat output from the equipment. They were requesting to find out what the temperature rise was gonna be with the smaller unit. For the initial 5 ton load, I had the room setpoint at 75 F. Is it possible to find out the room temperature rise. Like I stated above, the ductless split only has 36 mbh of cooling at 900 cfm with no OA. What would I need to provide to find that out?

 

[mech5]

The latent load is there because I was positively pressurizing the building to avoid dust infiltration. I'm bringing in OA with a seperate duct. This is of course in the initial calculations with the 5 ton load. The client decided against the conventional split system and went with a 3 ton ductless split. The ductless splits have no OA capability. I was just trying to find out what the temperature rise of the room was gonna be because there are some VFD's in the space. The VFD's cannot run properly if the space temperature is above 104 F.

 

[317069]

you have removed the OA to reduce the load, let say it is ok.
But how did you lower the heat output from the equipment?
if you can do this, why didn't you do it in the first place when you've calculated the load as a 5 tons.

 

[SAK9]

Most VSDs should run allright up to 115F.VSDs are normally located in plant rooms where temperature is usually high.There is not any need to provide dust ingression protection in plant rooms except for electrical rooms where the dust can settle on contacts leading to contact failure.By trying to pressurise the space you are bringing outside into the airconditioner which will require more frequent cleaning becuase of the dust that comes with outside air.What happens when the air conditioner breaks down?
The other issue when using split units or any DX unit for that matter for high temperature application( ie 90F or higher)is that it results in high compressor oil temperatures and eventual compressor breakdowns

 

[mech5]

Well most of the designers I work with pressurize the spaces we cool. It's required by IMC to bring in a certain degree of OA. Not sure of the actual number. I think it's something like .18 CFM per SF. for that type of space. The only time we can neglect putting in OA is when the openable area to the outdoors (windows, doors or OH doors) is at least 4 percent of the floor area.
If we bring in the OA, we can control it better. We can put a sensor across the filter to notify when it is dirty.

 

[ElSid1]

Pump rooms/non-occupied rooms can get away without OA. Once the door is opened, there is your ACH. If it's a large enough room, it can be prop'd open. I did a similar room, but it was for elevator equipment instead of pumps. Run your calcs for the extreme hot. If you are still below 104 ... you should be good to go. BTW, the elevator control room had an electrical outlet in case a swamp cooler needed to be brought in (extended work hours)

 

[mech5]

Thanks for the input everyone.