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Homework Help: Simplified Air-Conditioning Calculations

  1. Feb 25, 2017 #1
    Hello everyone!
    I am currently having some real issues estimating the power requirements for a concept vehicle's Air-Conditioning System, and I mean I've been going over this on and off for the better part of a fortnight.

    At this stage I feel like I have fried my brain and have no idea if I am even on the right track, so really I am just looking for some external input.

    Any guidance would be deeply appreciated <3

    1. The problem statement, all variables and given/known data
    Ideally, I need to find the energy required to lower the cabin's internal air temperature from X to Y, and subsequently find the required Mass Flow Rate and the required theoretical Heat Transfer Coefficient/Thermal Conductivity of the coolant.

    Doing a fair amount of research, it seems to be a fairly complex subject, with entire Research Papers dedicated to the development of new models, so I stress that this needs to be rough, I'm not taking into account the variation of solar input due to date, time and location for instance. I've found a few sources depicting calculations for heating and cooling in Civil Engineering (Offices etc) situations but I'm not entirely sure how relevant they are, and they do not reference the coolant aspect.

    2. The attempt at a solution
    - Assume the cabin to be a cuboid
    - Q = m c dT to find the total energy required to cool the cabin assuming zero losses
    - Q = h A dT to find the losses due to Convection
    - Grey Body Radiator equation to find losses due to Radiation
    - I also tried accounting for Conduction but this gave a MASSIVE number

    - The equations found here: http://www.watlow.com/en/resources-a...ools/Equations [Broken]

    - The equations found here: http://www.arca53.dsl.pipex.com/
    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Feb 25, 2017 #2
    In the end I have tried to manipulate building models to suit my needs but I've not been able to find any examples to reference my findings against. All my research, including the Library and my University's access to the SAE Database has just brought up discussion on how systems work and possible improvements as opposed to detailed calculations

    Ultimately I have found this:
    - Mass Flow Rate required to maintain 16C (interior) under 50C (ambient) of 0.06 kg/s
    - Mass Flow Rate required to cool the interior from 50C to 16C based on a upper power limit of 4 kW of 0.117 kg/s
    - Required Fan Power based on a 10" fan of about 200 W

    But as I say, I have been unable to find anything to reference my findings against so I have no idea if that is even nearly right other than finding the velocity of the Mass Flow as 0.062 m/s and thinking, well, you can't feel the air in the car moving like you do the wind (a few m/s) so maybe it is realistic? =P
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