PDA

View Full Version : Powerplant HVAC Failure - UPS Room Temperature Rise

InDucingPanic
12-14-2011, 09:40 PM
Good Evening,

My name is Mark, I am a Mechanical Engineer and I could use your help. I am not an expert on HVAC, or alot of things, but my father taught me that when I do not know something to just say so. To humble myself and ask the folks who actually do the work for help. So here I am.

I am working a problem for a power plant that has (2) UPS systems crammed in to an extremely small room. They are pumping 300 CFM air at 55F in to the room. So, in order to look at the pure cooling capacity of that air I used the following information found in the post I referenced above. But I substituted in my values which were the same but for 300CFM. So, did doing this actually give me the cooling potential of the 55F air moving at 300CFM. The potential cooling capacity of that air in BTU/hr??

CFM = 400
Density of air at 55*F = 0.07711 lbs/ft3
Enthalpy of dry air at 55*F = 13.2 BTUs/lb

Calculations:
400 CFM x 0.07711 lbs/ft3 = 30.844 lbs/minute of air moved
30.844 lbs/minute x 60 minutes/hour = 1850.64 lbs/hour of air moved
1850.64 lbs/hour x 13.2 BTUs/lb = 24428.45 BTUs/hour moved

motoguy128
12-14-2011, 09:52 PM
You over complicating it. CFM*1.08*temp drop or rise above ambient. If the target temperature is lets say 80F, that's only 8100 BTU's/hr... sensible capacity... which for cooling equipment is all you care about.

It depends on your indoor temp if the 55F air is constant.

What's the approx heat load of the room including lighting. What is the insulation/room construction and size? What is the heat load from the batteries? Can you measure the power consumption of the circuits that feed the UPS'?

Most room like this would benefit from a mini split AC units although we just use regular split systems in our MCC's, I/O rooms, UPS rooms ad such at my workplace. We're looking to switch over to minisplits for better reliability in dusty condition where condenser get clogged easily.... like they need cleaning weekly... or more.

InDucingPanic
12-14-2011, 10:04 PM
Thank you sir for your response. And yup, I have been complicating the obvious my whole life. That is why I became an engineer.

I will go in to the room construction in a minute, but it is very important for me to know. Did I arrive at the total cooling potential of that 55F flow? If I consider the heat in the room from the systems (24000BTU) separately - disregarding the temperature rise for the moment, did I in fact calculate the total available cooling in BTU/hr of that flow correctly...please dear God in heaven tell me I am close enough for government work? I am only looking at comparing the BTU of heat vs. the BTU of available cooling for now.

The room is 895cuft, made entirely of concrete and located in a basement surrounded by dirt on 3 of the 4 walls. Minimal lighting and no occupants in the room. They are pumping 300 CFM in at 55F and 290CFM out. They are getting 85F at 3' above the ground and 92 at 9ft. The roof is 10.5FT.

genduct
12-15-2011, 07:03 AM
My Son is an application Engineer for Carrier, email me via profile and I'll introduce you to the "kid" who I am happy to say takes after his mother and is probably 3x smarter than his "old man"

InDucingPanic
12-15-2011, 08:21 AM
Email sent. Still needing some fast help.

genduct
12-16-2011, 07:41 PM
Email sent. Still needing some fast help.

USE the Cell # if need quicker

ACFIXR
12-16-2011, 10:20 PM
Thank you sir for your response. And yup, I have been complicating the obvious my whole life. That is why I became an engineer.

I will go in to the room construction in a minute, but it is very important for me to know. Did I arrive at the total cooling potential of that 55F flow? If I consider the heat in the room from the systems (24000BTU) separately - disregarding the temperature rise for the moment, did I in fact calculate the total available cooling in BTU/hr of that flow correctly...please dear God in heaven tell me I am close enough for government work? I am only looking at comparing the BTU of heat vs. the BTU of available cooling for now.

The room is 895cuft, made entirely of concrete and located in a basement surrounded by dirt on 3 of the 4 walls. Minimal lighting and no occupants in the room. They are pumping 300 CFM in at 55F and 290CFM out. They are getting 85F at 3' above the ground and 92 at 9ft. The roof is 10.5FT.

Measure the air temp above the fans on the top of the ups. My guess is that it is 100 plus degrees. You have no way of measuring the air volume to calculate the heat gain, but you have indicated that your measured wattages are equal 24,000 btuh. How did you measure the 300 cfm at the register?What is the return air temp?

ACFIXR
12-16-2011, 10:37 PM
Anyway, in the small ups rooms that I have done:
One has a 10 ton package unit :eek2:and the other has a 2 ton mini split.
Both on 1200 cu ft rooms +/- a few cubes.