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Heat Gain Formula

  • 11-03-2013 2:25pm
    #1
    WhatNowForUs?
    Closed Accounts Posts: 687 ✭✭✭


    Hi,

    I'm looking for a formula that would calculate how long it would take to heat 1m3 of air to 10 Deg C if it was 0 Deg C when starting.

    It is surrounded by 10 Deg C of soil. Equal sides.


Comments

  • #2
    godtabh
    Moderators, Science, Health & Environment Moderators Posts: 23,243 Mod ✭✭✭✭godtabh


    is this for a college project? Have you tried text books/lectures/other students?


  • #3
    WhatNowForUs?
    Closed Accounts Posts: 687 ✭✭✭WhatNowForUs?


    Yep. Its for college. I'm looking for a quick fix answer.
    Can't get exactly what I want from the NET. My fellow students would not know as I'd be closest linked in this class to this particular field (mature students different backgrounds of engineering).
    I'm going to be looking through my old books but I have a feeling I'm going to have to go manipulating a couple of formulas to get my answer which will be time consuming as my head is not functioning to capacity.


  • #4
    mawk
    Registered Users, Registered Users 2 Posts: 2,738 ✭✭✭mawk


    im too stupid right now to make up integration, so there are going to be some educated guesses and approximations

    logically, the heat transfer has to be q=heat in
    heat in is the sum of the convective and radiant heating. the convective being the greatest by far. so thats where ill be doing numbers.

    convection is calculated by way of the cooling equation:

    q = hc A dT

    plug in the values of 0.7Wm^-2K, 6m^2 and 10deg gives us 42W

    this assume that the dt is always 10 degrees which it isnt and probably reduces close to exponentially.so, the average cooling rate of the soil is (guesstimation).... 1/4 of that for most of the early transfer. so.... 10W ish
    1200j/kg/k ==> 120 sec/degree

    20 minutes altogether or there abouts..

    I was too lazy to integrate and take note of the changing dt or account for the emmisivity of the soil but you feel free. at the surface the emmisivity k is .38


    any help?


  • #5
    WhatNowForUs?
    Closed Accounts Posts: 687 ✭✭✭WhatNowForUs?


    mawk wrote: »
    im too stupid right now to make up integration, so there are going to be some educated guesses and approximations

    logically, the heat transfer has to be q=heat in
    heat in is the sum of the convective and radiant heating. the convective being the greatest by far. so thats where ill be doing numbers.

    convection is calculated by way of the cooling equation:

    q = hc A dT

    plug in the values of 0.7Wm^-2K, 6m^2 and 10deg gives us 42W

    this assume that the dt is always 10 degrees which it isnt and probably reduces close to exponentially.so, the average cooling rate of the soil is (guesstimation).... 1/4 of that for most of the early transfer. so.... 10W ish
    1200j/kg/k ==> 120 sec/degree

    20 minutes altogether or there abouts..

    I was too lazy to integrate and take note of the changing dt or account for the emmisivity of the soil but you feel free. at the surface the emmisivity k is .38


    any help?

    Thanks very much. Your stupidness beats my intelligence. Cheers


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