Com Room Heat Loss 1

[Senselessticker]

Can anyone point me to a standard for estimating heat loss (or heat added to a room) in say Watts/ft^2 for a communications room in a research building? This information will be used by the mechanical folk to determine HVAC requirements.

Thanks!

 

[IRstuff]

??? Heat loss should be a number that they can figure out.

Heat gain is simply the heat load from your equipment plus body heat.

Additional heat load from solar or whatever, they can figure out.

TTFN

 

[Senselessticker]

Sorry, I failed thermo a couple times...and I never got the terminology down. In any case, I've never been approached by the mech guys (or gals in this case), for this type of info. Is there a reference I can point him/her to determine the HVAC requirements for a Communications Room which will contain telecom board, com cabinet, etc...

 

[IRstuff]

The heat gain from the equipment should be something that you can come up with by looking at the data tags on the equipment as well as performing some measurements to see if the printed data is even remotely close to what actually being dissipated.

If these people are actually qualified to do A/C, they should only be asking for the heat dissipation values for your equipment.

TTFN

 

[sathanasiou]

You can use the following figures as a guideline:

People: 74 W/person sensible and 58 W/person latent
Lights: 2.5 W/ft2
Equipment: 1.0 W/ft2

Hope this is usefull.

 

[tombmech]

Guidelines are unreliable in many of these cases. Even direct catalog data for heat rejectection may be suspect. There is a long tradition in the computer/electronics industry of low-balling heat rejection values. "It doesn't require any additional cooling. You can just place it in a normal office environment." is a refrain I've heard from salesmen for decades. Trouble is, a customer would take that statement, purchase 2 or 3 dozen, and shove them all into one tiny room. I have seen loads - especially in research applications - go to as much as 5-7 W/sq.ft.

The task of determining the load in such a situation is the heart of the problem. If possible, obtain cut sheets of catalog data on every piece of equipment. Always compare the electrical load data with the heat rejection values - if listed. If they don't make sense, find out why. If you can't resolve the discrepancy, then count on the higher number. If none of that is available, then tally the nameplate data yourself: 1W = 3.41 Btuh - burn that into your memory. If needed, I have actually taken amprobe readings on the power lines to get a better idea on the load.

There are a lot of pitfalls with sizing the A/C equipment, too.

 

[Senselessticker]

Thanks for all the feedback folks. This particular problem has been resolved, however, I've learned something very interesting from an HVAC engineer. I suppose it is par for course for mech engineers to think of "a watt is a watt" when is comes to determining heat gain from equipment. This theory of designing A/C watt for watt seems to conservative. I've learned that one engineer will design to cool space based on the rating of a transformer. (If a transformer is rated to 75kVA....he assumes75 kW of heat from that transformer spead out in the area (inside his space he is trying to cool). I find this approach to be way too conservative and seems it will lead to oversized A/C equipment.

Not all of the 75kVA turns into heat added to an area. Of course the transformer has heat losses associated with operation, but 100% of its rating certainly not transfered into heat. Furthermore....small transformers often operate at 20-40% of the rating.

Is it common practice for HVAC designers to assume full rating heat gain from electrical equipment when sizing A/C units? If so...its a good thing I'm not a mech engineer, as the idea behind such practice makes no sense to me.

Thanks,

 

[tombmech]

The rejected heat from a transformer is directly related to its efficiency (losses). Nothing more is rejected as heat. However, if the transformer is placed in the same space with the rest of the equipment it's serving, then using the transformer's rating to size the A/C for that equipment is legitimate. There may be some safety factors and room for expansion in the load sizes, but even that has a parallel with the A/C sizing.

I would always recommend tallying the specific load, but in some cases (perhaps yours?) the customer knowledge is of little help. Resorting to this ultimate case may be useful in that type of environment.

No offense, but I can understand why some of these things make no sense to you. That's why there are "mech engineers."

 

[Senselessticker]

So, hypothetically, if a 5 hp motor (let's say located in a mech room), drives a fan, which serves some duct...Are 5 x 746 Watts assumed as heat added to that room from the motor? It seems the me the only heat added to the room should be the heat loss as a product of % eff. of the motor...not the entire rating of the motor. I know we are way off the original subject now, but I enjoy gaining a better understanding of this topic. Thanks

 

[tombmech]

Senselessticker,
Technically speaking, your example of the fan is correct. Practically speaking, fan motors are usually inside the air handler and in the airstream, which means it's a "coil" load, not a "room" load. You are right to say that's off topic, and it's not useful to digress further.

Back to the x-fmr analogy, you were also correct, but you weren't seeing the full scope of the problem. A transformer - minus some judgment on safety factor - may be a reasonable estimate of connected load in the absence of other information. If someone was simply trying to cool the transformer, then the load would simply be the heat rejected due to the winding losses. I was offering a very real alternative to your conclusion that the ME didn't know what he/she was doing.

Back on topic, you said "Com" for communications, aka electronics, digital devices, computerized switching, etc., etc. Ever see a communications device convert electrical energy to mechanical energy? Ever hear of an electric heater or load bank? Except for the trivial amount of mechanical energy spent on disk drives or printers shuffling paper, the product of all that connected load is 100% HEAT.