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How to measure heating power

  1. Sep 20, 2010 #1
    Dear all,

    I have a piece of brass material at high temperature (around 300 Celsius) and I would like to know how can I measure the heating power stored by this piece. the piece is designed in a way that we can flow a fluid through it.

    For low temperatures ( below 100 Celsius) it is maybe possible to measure the heating power by circulating water and measuring the input and the output temperatures of it. but for high temperatures (above 100 Celsius boiling temperature of water) maybe the fluid should be changed to an oil or some other fluid. So what do you think of that.

    Another question:

    If the piece of brass is at 60 Celsius and I am using water to measure the heating power. Shall I flow water at around 60 degrees or is it ok to flow water at ambient temperature?


    Thank you in advance for your contributions
     
  2. jcsd
  3. Sep 20, 2010 #2

    diazona

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    Homework Helper

    What exactly do you mean by "heating power"?
     
  4. Sep 20, 2010 #3

    Drakkith

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    Staff: Mentor

    I believe there is something somewhere that will give you the amount of heat contained within your brass pipe if you know the dimensions, composition, ETC. I just dont know where to find it.
     
  5. Sep 20, 2010 #4
    Thank you diazona for your interest.

    I will try to explain you what I mean by heating power:

    Let's consider that the temperature of my piece is equal to 50 °C. So there is an energy stored in this piece. If we circulate chilled water at 10 °C through the piece this water will come out at higher temperature let's say 30 °C. and hence the temperature of the brass piece will decrease to reach its equilibrum temperature.

    The heating power in this case is the sensible heat recuperated by the water = water flow multiplied by the specific heat of water multiplied by the difference of temperature ( 20°C in our case).

    but my question is how to measure the "heating power" at piece's temperature above 100 °C.
     
  6. Sep 20, 2010 #5
    Thank you Drakkith for your answer.
    I liked very much " something somewhere" :smile:
    but seriously, the problem here is that the dimensions of my piece are not regular I only know its weight.
    I will try to look for this something :smile:

    Thank you again for your contribution
     
  7. Sep 21, 2010 #6

    diazona

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    Ah, so it sounds like what you're looking for is the amount of thermal energy lost by the metal as it comes to thermal equilibrium with a certain amount of water at a certain temperature. This quantity will, of course, depend on how much water you circulate and what temperature it enters at.

    You can calculate the amount of thermal energy that a certain amount of metal loses as it goes through a given temperature change using the formula
    [tex]\Delta Q = m C \Delta T[/tex]
    where m is the mass of the piece of metal, C is the metal's specific heat capacity (you can look up the value for copper), and ΔT is of course the temperature change. So if you know the final temperature of the metal (or equivalently, of the water), you can just plug numbers into that equation.

    If you don't know the final temperature, but you do know the initial temperature of the water, then it takes a bit of algebra. You can write
    [tex]m_\text{water} C_\text{water} (T_f - T_{i\text{,water}) + m_\text{metal} C_\text{metal} (T_f - T_{i\text{,metal}) = 0[/tex]
    which just says that the heat lost by the metal must equal the heat gained by the water. You can solve for [itex]T_f[/itex] given numerical values for all the other variables.

    If you would rather make a physical measurement, consider these points: you can use water as long as the water is not in contact with the metal long enough to raise its temperature up to its boiling point. Using a large amount of water will extend the time it takes for the water to boil; also, keeping the water at high pressure in a sealed system of pipes would raise its boiling point. Alternatively, you could choose a different fluid with a large specific heat capacity and/or a higher boiling point. I'm sure there are many possibilities out there - 300°C is not a particularly high temperature.

    There is no reason you would need to use water (or whatever fluid you choose) at any particular temperature like 60°C. It can start out at any temperature. In fact, if you are concerned about boiling, it's probably advantageous to start with the water as cold as possible without being frozen (so, just above 0°C), since that maximizes the amount of heat it can absorb without boiling.
     
    Last edited: Sep 21, 2010
  8. Sep 21, 2010 #7
    Thank you diazona for your reply.
    Actually I want to measure experimentally the thermal energy. So I think I will try to find a kind of oil which boiling temperature is higher than 300 °C. I should also think about how to pump this oil through the brass piece (I don't have now any pump).

    I used to measure the "cooling power" which is the heat supplied by an electric heater to the brass piece (which is at -40 °C) to increase its temperature to the thermal equilibrum temperature. but its my first time to measure the heating power.

    Thank you once more
     
  9. Sep 21, 2010 #8

    Lok

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    You can drop the brass in a 1KG of water, let the evaporation process to take it's place, wheigh the water and see how much of it has evaporated. So you get a value of energy by knowing how much energy was given by the brass to evaporate the grams of water plus the heat that the remaining water reached (usually below 100'C, as water boils locally around the brass and the rest just warms a bit)
     
  10. Sep 21, 2010 #9
    Hi Lok

    Thank you for your reply,
    it seems that your idea is very interesting but unfortunately, in my experiments it's impossible to realize because my piece of brass is a part of an experimental device and can't be removed.

    Thank you again for your idea
     
  11. Sep 22, 2010 #10

    Lok

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    Most oils don't boil at 300 C, even vegetable oils. They just degrade a very tiny amount, and considering that it won't stay at that temperature for long I recommend the cheapest motor oil you can find, or only in case of last choice saturated vegetable oil ( as it might be in liquid form only at 50 or above).

    Motor oil can be and has been used for temp of 800'C (and above) Steel quenching with ease so for a short duration 300'C it will be fine.
     
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