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  1. #1
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    Rate of temperature rise



    Can someone help me out here please.

    I'm trying to determine what the temperature rise will be after 1 minute for a computer room in the event of a power cut to the mechanical plant (pumps, chiller & fans etc.)

    The cooling load is 3000kW, the supply temp is 16deg C & the computer room temp is 26 deg C.

    Is there a formula I can use for this task?

    Any help will be appreciated.



  2. #2
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    Re: Rate of temperature rise

    3 MWatts are 3 MJ/s.
    1 Joule is 4.186 cal
    1 Watt is then able to increase 4.186 grams of water by 1°C in 1 second.
    1 KW is able to increase 4.186 Kg of water by 1°C in 1 second.
    3000kW increase 12'558 Kg of water by 1°C in 1 second.

    If the supply temp is 16°, I assume the return temp is 21°C, therefore the water flow is 143 Kg/s (1 liter = 1 Kg).

    It will take 12558/143=87 seconds for the water to increase the temperature from 16 to 17°, and the room temperature will increase proportionally.



    Hope I did my homework right.

  3. #3
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    Re: Rate of temperature rise

    No,

    In this case I would say that you are wrong...

    Does it really matter what temp the water rises to?
    If you have a total system failure including the fans, then the heat will not even reach the water.
    As soon as the fans stop you will start experiencing hotspots in the room.
    The rate of temperature raise will then depend on the thermal weight of the room and the kit inside it.
    As the normal kit would require too much power for an UPS to run what we have done at some (well, OK, ONE) site is to install a full fresh air system as a backup. In case of a power failure (or if the A/C system fails) this fresh air system will kick in and prevent the room from melting (OK, won't work well for a week or so in the summer, I didn't say it was perfect). And as it's only 1 fan (extract) and 3 dampers (supply) it is easily fed by the UPS.

  4. #4
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    Re: Rate of temperature rise

    Quote Originally Posted by The Viking View Post
    As the normal kit would require too much power for an UPS to run what we have done at some (well, OK, ONE) site is to install a full fresh air system as a backup. In case of a power failure (or if the A/C system fails) this fresh air system will kick in and prevent the room from melting (OK, won't work well for a week or so in the summer, I didn't say it was perfect). And as it's only 1 fan (extract) and 3 dampers (supply) it is easily fed by the UPS.
    Has it ever been tested and proven to work?

  5. #5
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    Re: Rate of temperature rise

    Quote Originally Posted by frank View Post
    Has it ever been tested and proven to work?
    The answer to the first part of the question is Yes.
    The answer to the second part is Yes and No.

    First, you have to bear in mind that the idea of this system came from the end user, this means that any test we carry out shows that it works (at least in his mind, because he can't be wrong).

    The system is tested for 10 minutes every 3 months but always at 3 AM on a Sunday morning.

    During these tests the temperature in front of their server racks rise with between 2 and 5 degrees, depending on ambient temp. (The system is thermostat controlled so by default the temp has to rise with 2 degrees before it kicks in).
    The UPS is running it OK.

    If the system is turned off during the generator test then the temperature rises much quicker.

    BUT......
    What happens when the ambient is 35 C on a nice sunny summer afternoon???

    Also, in this case we are helped by the fact that most of the comms room is allowed to shut down in case of power failure.

  6. #6
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    Re: Rate of temperature rise

    Quote Originally Posted by The Viking View Post
    No,

    In this case I would say that you are wrong...

    Does it really matter what temp the water rises to?

    I assumed that at least pumps were running.

    Quote Originally Posted by The Viking View Post
    If you have a total system failure including the fans, then the heat will not even reach the water.
    Ok, in that case the calculation is still valid, only replace the specific heat of water, with the specific heat of air (1.2 cal/m3).

    Quote Originally Posted by The Viking View Post
    In case of a power failure (or if the A/C system fails) this fresh air system will kick in and prevent the room from melting
    This is not working in all countries. It is mainly used for shelters (telecom containers).

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