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  1. #1
    jak_juk's Avatar
    jak_juk Guest

    Cold Room Infiltration by Air Exchange



    Hello everyone,

    I'm now working on programming a software to calculate Refrigeration Load for a Cold Room , we know that there are several types of load that need to be included in the calculation such as, solar transmission load, internal load and product load, but when it comes to Infiltration Air Load, there are several parameters that made me confuse, i refer to ASHRAE Handbook REFRIGERATION 1998 chap 12 (Refrigeration Load), in sub-chapter of Infiltration Air Load, Heat gain through doorays from air exchange is as follows:

    qt = q(Dt)(Df)(1-E)

    where
    qt = average heat gain for the 24-h or other period,kW
    q = Sensible and latent refrigeration load for fully established flow, kW
    Dt = Doorway open-time factor
    Df= Doorway flow factor
    E = effectiveness of doorway protective device

    my question is, how do we obtain the value of Dt, Df and E since it is not clearly stated in this book, is there any table that i have to refer to?? i appreciate any help from anyone of you, thnx



  2. #2
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    Re: Cold Room Infiltration by Air Exchange

    I donīt have the 1998 issue in hand but Dt and Df are correctly explained in the 2002 and later editions. The following should be in this edition too:

    Dt is the portion of time the door is open (<1), something you have to measure in the field or assume a value and depends on the number of doors, door open/close time, time the door is forced open, etc.

    It would be much better if your program explained the user how important it is that he estimates this value to make him think and plan to minimize these losses from the beginning.

    Df is the doorway flow factor = air exchange/fully established flow. Fully established flow occurs when the door is open for a while.
    The air exchange can be estimated adding the flow due to density difference of the air in the conditions on both sides of the door and the direct flow through the doorway due to a pressure differential.

    These flows are implicit in the infiltration loads explained in the same chapter and depend heavily in on site data. Capturing this data may be difficult and your program could point out the important of field measurement.

    You can research typical industry literature. For example number of air changes depending on room size and temperature and you can fit this equation to their data and get default values.

    It is better to calculate each door independently if the conditions where they open to vary (prevailing winds, different temperature differences through the door, types of traffic, etc).

    The problem with all these calculations is that in the end your program will not get quality data from the user and you will be forced to introduce safety factors, but if you convince one user to get up his chair and start measuring, your program will be a success!

  3. #3
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    Re: Cold Room Infiltration by Air Exchange

    You should make two options in your software .

    Quick calc . For a rough ball park figure so you can get a good idea how much the unit will cost and what size it will be . Also people who want to use this software and dont have all the extra information on them.

    Then advanced . To confirm That the unit is large enough . And to get an exact calc .


    ----
    The coolrooms at bottle shops seem to be open alot . , Also some coolrooms are open for awhile when loading stock in.
    Should they get air curtains they go on when the door is open or is this just silly ?

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