Thermal dynamics

Discussion in 'DIY Zone' started by slide, Feb 2, 2014.

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  1. slide

    slide Well-Known Member

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    Does anybody have info on thermal dynamics? I studied first year of engineering but cant recall a lot of the theory behind it.
    Im really looking for something along the lines of w volume of air at x temperature plus y watts of heat equals z final temperature. Obviously ventilation comes into play also.
    I know that racks are simple to build and have many self made racks that function just fine.
    Its just this stuff has been running through my head and I would like to trial some setups that use the minimum amount of wattage to obtain temps required. Also wouldnt mind testing the old grey matter too.

    Thanks in advance to anybody with helpful information.

    Aaron
     
  2. slide

    slide Well-Known Member

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    Going once...
     
  3. Snowman

    Snowman Guest

    Watts cant be used as a stable. As the same watts used in different methods produces different amount of heat.
    IE A 75W incandescent 75w ceramic, and 75W cord will all put out different amounts of heat.
    An efficient globe puts out more light and less heat. There are even different amounts of heat given from different types of 75W incandescent globes.
     
  4. Wing_Nut

    Wing_Nut Well-Known Member

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    The Basic Formula is

    Energy(in joules)=mc(deltaT)

    m = mass of air = volume x density
    c = specific heat capacity of air (at constant air pressure)
    deltaT = tempreture change

    it then follows that

    1 kwh = 1000watt power for 3600 seconds

    Energy = power x time

    so

    1kwh = 1000 x 3600 = 3,600,000

    so divide the Energy(joules) by 3,600,000 to get the energy required in kwh

    This is a very basic way to calculate what your looking for, without taking into consideration the myriad of different circumstances, and changes, which would be quite difficult to calculate.

    Hope this helps.

    WingNut
     
  5. Bluetongue1

    Bluetongue1 Guest

    There are some other factors that you may or may not want to consider in your thermodynamics exercise...

    The specific heat of air changes with pressure and temperature but this will have minimal effect for your purposes. The amount of moisture present, in particular suspended water droplets will have a significant effect due to latent heat in vaporisation. Ambient temperature is likely to vary, yet you need to nominate a starting temperature. If you intend to have a temperature gradient you will need to choose an appropriate mean temperature as your end pint – the PBT being most appropriate. Heat transfer from air to reptiles is much slower than via absorption of IR conductive transfer from a bottom heat source with good conductivity and high specific heat. Are you excluding all other object with which the air comes in contact as they will take on heat from the air once a differential in temperature is established.

    Irrespective of the amount of heat required to warm an enclosure, it is the rate of heat loss that determines the long term energy input required. Rate of loss is influenced by: IR escaping through transparent section of the enclosure; convection through vents and gaps; conduction through opaque walls of enclosure; evaporation of moisture, particularly from the water bowl; the temperature differential between ambient and internal; freedom of airflow around the enclosure; the nature of the substance supporting it underneath; and whether other nearby enclosures give off heat, how much and how close.

    Blue
     
  6. slide

    slide Well-Known Member

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    Thanks for your replies. It seems that there are a lot of variables affecting the equation. Will see if I can get my head around it.

    Cheers,
    Aaron
     
  7. andynic07

    andynic07 Very Well-Known Member

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    I guess you could apply the basic formula that you were given without the variables and then see the difference with the actual application and compensate. This will give you the rough wattage you will need which I think is what you are after. Also the wattage of lights is no that variable so unless you have a variable ac output you will have large jumps anyway. I just used a bit of trial and error with different size lights to find a good enough temperature to run my lights on timers only. I also on my large enclosures almost segmented off smaller areas to more effectively heat and have the rest of the area as ambient temperatures.
     
  8. slide

    slide Well-Known Member

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    I am mainly looking to apply this to my racks. Heating would be heat cord and Im looking to figure out the minimum amount of wattage/m2 for the hot end without overheating the cool end of the tub to dangerous levels on hot days if the thermostat fails, whilst still providing enough heat at the hot spot in winter.

    Aaron
     
  9. andynic07

    andynic07 Very Well-Known Member

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    I think most heat cords are roughly the same wattage per metre of length so the larger the wattage the longer it is and most people use two runs of cord for hatchling tubs but you may need to have more for larger tubs. I think trial and error may be your best option as you will maybe have two tries before you get it right.
     
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