Computer Heat Gain
Computer Heat Gain
(OP)
My boss argues that ALL the power drawn by a computer will end up as heat gain in a room. I know that energy needs to be conserved (1st law of thermo.)
My questions:
1-How does ALL the power drawn to rotate a hard disk or an inserted CD (Rotational power=Torque X rotational speed) end up as heat gain to the room? (This is not the only operation a computer performs, I know)
2-ASHRAE Fundamentals 2005 section 30.8: "Actual power consumption of office equipment is assumed to equal total (radiant plus convective) heat gain,.."(is this line proving what my boss is talking about?) How do we argue that ALL the radiant heat gain will end up as heat gain to the room? (Convective is understandable)
3-What are you guys using as heat gain from a computer nowadays? (ASHRAE handbook talks about a conservative 155W together with a monitor.)My boss wants me to use a 500W! Checking Dell site regular office computers have 300-400W NAMEPLATE (which is the max?) inputs.
Thanks in advance for answers.
My questions:
1-How does ALL the power drawn to rotate a hard disk or an inserted CD (Rotational power=Torque X rotational speed) end up as heat gain to the room? (This is not the only operation a computer performs, I know)
2-ASHRAE Fundamentals 2005 section 30.8: "Actual power consumption of office equipment is assumed to equal total (radiant plus convective) heat gain,.."(is this line proving what my boss is talking about?) How do we argue that ALL the radiant heat gain will end up as heat gain to the room? (Convective is understandable)
3-What are you guys using as heat gain from a computer nowadays? (ASHRAE handbook talks about a conservative 155W together with a monitor.)My boss wants me to use a 500W! Checking Dell site regular office computers have 300-400W NAMEPLATE (which is the max?) inputs.
Thanks in advance for answers.





RE: Computer Heat Gain
2-Why do you think that radiant energy is not conserved? If the equipment is hotter than the room, then it will have a net radiance into the room.
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RE: Computer Heat Gain
RE: Computer Heat Gain
Your Dell computer has a max nameplate power consumption of 400 Watts. Due to safety reasons, most equipment never draw more than 80% of the rated power even during peak demand. With this in mind, lets assume 320W power consumption.
1st law of thermo states...engery in = engery out. The pc draws 320W therefore 320W must be dissipated in the form of heat, sound and friction.
So if you were to design a cooling system for this computer, design it for 320W. NOT so fast....
But why does ashrae say 155W?
This is because both you and I both know the computer will not be operating at 100% CPU speed all the time. (Even high density data centers do not operate at 100% nameplate capacity all the time.) The actual time your CPU is running at 100% is about 1-10% of the time. ASHRAE takes this diversity factor into account and recommends a heat dissipation for computers at 155W. This implies that during normal operation, the computer will draw 155W from your power outlet.
To comment on your boss's rule of thumb of 500W per computer....this will result in oversizing the HVAC system. Oversized system equals wasted money.
Imagine a room with 50 computers. According to your boss, you will need an HVAC system capable of cooling 25,000 W.
According to ASHRAE, you will need to cool 7,750 W.
The difference between the two is 17,250W and can amount to an extra $15,000 of operating expenses each year. (assuming 24 hour operation). You might want to spring this on your boss and enlighten him.
Hope I didnt lose you with the long explanation.
----
A green thought..."We don't inherit the earth from our ancestors, we borrow it from our children." (unknown)
RE: Computer Heat Gain
But you do have to look at the application and environment. Yes, I use heavy computer loads for computers in general but then I'm using that to cover miscellaneous office equipment that no one can tell me about when I'm running loads (like copiers fax machines and scanners etc.). The other item to take into account is the type of computers they plan to install. Higher end Cad machines and workstation for fairly intense engineering or the low end terminals that often show up in call centers.
Its a judgement call. If you pick the low end be careful to account for all the equipment. If you pick the high end make sure your equipment can do part load efficiently.
Mike
RE: Computer Heat Gain
TADiep: Thanks for your explanation. "Black box" meaning I will simply need to consider a computer nothing different than a dummy resistance from now on I think!
RE: Computer Heat Gain
Don't understand your last comment. "Radiation" refers to blackbody radiation, which is photonic and will be absorbed by walls, etc. What little there is that's RF in nature is miniscule.
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RE: Computer Heat Gain
Typically we use 15 or 20W/m2 for small power gains in a typical offices based on 10m2/person, ie 150 to 200 W/person to allow for PC's, photocopiers etc.
I have seen some papers that say the small power usage is often overestimated in AC assumptions (but not always!)
RE: Computer Heat Gain
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RE: Computer Heat Gain
We've installed CRAC units for a computer room that became jam-packed with additional racks with 2 years, the two 5-ton units we had originally were no longer sufficient, while at the project commissioning, one unit was sufficient.
Sizing for 500W/SF as your boss says makes sense for computer room, I am told that as computers gain in speed, they also use more power (to be verified though-I'd think more efficient).
Use multiple units, one unit sized for the ASHRAE recommneded 155W/SF so no one says you're not within industry standards. Let the owner know of your rational, and write a report about your rational in your calculations file, should another engineer take over for you while you're not around for whatever reason.
RE: Computer Heat Gain
Second, look at the ANTEC site; some power supplies coming out now are E-80 rated; at least 80% efficient at load values between 20% and 95%.
The Intel Core 2 Duo E6700 draws 21 watts at idle......
RE: Computer Heat Gain
Computer users generate heat, too. An average person just sitting burns maybe 550 calories over 8 hours. And, 1 food calorie = 1000 thermodynamic calories = 4184 Joules.
Since 1 Watt = 1 joule/sec, (550 x 4184 J)/28,800 seconds = 80 Watts.
But, maybe users absorb photons of knowledge from their computers, package them and transmit as product to customers, maybe over the bundle of tubes known as the Internet?
TGIF.
RE: Computer Heat Gain
RE: Computer Heat Gain
INPUT: 100-127V @ ~6.0A (or 200-240V ~3.0A,
OUTPUT: 5 V @ 25A*, 3.3V @ 16A*, 12V @ 4A.
* (5V + 3.3V combined total cannot exceed 25A).
So total max output is 173 W. Divided by total input 720 W, gives efficiency 24%. It was called a 160 Watt unit, and still is: http://www.impactcomputers.com/0950-3751.html
So, a P/S is rated by the delivered power.
RE: Computer Heat Gain