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  1. #1
    Join Date
    Mar 2012
    Fort worth
    Rep Power

    How to calculate pull-down time


    I work for a company that designs air conditioners for aircraft, and we are setting up a psychometric testing facility to improve our future systems and better characterize the existing ones. We want to be able to determine the steady state cooling capacity of the systems, as well as the pull down times for varying sized air conditioned volumes. The steady state capacity (fixed inlet temperature and humidity) is easy enough, but we are trying to determine now if it is necessary to have an adjustable volume to determine pull down time. The plan is to heat the room to a heat soaked condition (ex. 130 F), and then have a set heat rate added (ex. 22,000 Btu/hr.) while the A/C system is ran to determine the minimum temperature achievable, and pull down time to a given temperature. I was wanting to know, if I can well characterize the system by plotting the heat removal rate vs. the incoming air temperature and also by the moisture removal vs. incoming relative humidity or humidity ratio, can I integrate both of these over the temperature range and get the final pull down time. For example, if I know the enthalpy of the air in the room from the starting point, say 130 F at 50% humidity, and I integrate the heat removal (Btu/hr.) vs. temperature to a final temp of 80 F to get the time and integrate the moisture removal vs. incoming humidity for the same amount of time that I got from the previous calculation to determine the final humidity, I should be able to set determine the theoretical pull down time and room air state. Is this correct?

    Hope this makes sense.


  2. #2
    Join Date
    Aug 2009
    New Zealand
    Rep Power

    Re: How to calculate pull-down time

    The system is dynamic.
    If you are using a fairly standard A/C system, then your net cooling effect "duty", changes overtime.
    So effectively you have break down the cooling process into time segments, The more the segments the greater the accuracy.
    For example you may 130% net cooling duty (of a nominal number) at the start or the process and say 70% at the end (of the same nominal duty)
    You also have to consider that the SHR (sensible heat ratio will also change), "amount of moisture removal"

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