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Thermostatically controlled oil filled radiator than an electric fan h

  1. Apr 2, 2006 #1
    I was reading a discussion about heaters on another forum ages ago, one person claimed that there is a large difference in cost when running a thermostatically controlled oil filled radiator than an electric fan heater. Another person told him to contact the Institute of Physics and inform them that he had broken at least (according to him) two basic laws of thermodynamics, and that he would soon be very rich if they could prove that he had in fact broken these laws (being sarcastic obviously).

    I really don't know the answer as to who is correct, but I'm sure you lot will!
  2. jcsd
  3. Apr 2, 2006 #2
    The general idea is that heat is a low grade energy and electricity is a high grade energy and conversion of high grade energy to a low grade energy is a loss (though conversion of electricity to heat is 100%, almost).

    Just assume that you are producing electricity from burning coal and fuel oils etc. and again producing heat from the electricity. If you produce heat from burning the coal or fuel oil directly, then you are minimising the inefficiencies which result from the conversion of heat to electricity and viceversa.

    However, the economy may not always favor heat to heat. If yor requirement is very low, it is better to invest in an electric heater rather than much costlier models of fuel oil heaters.
  4. Apr 2, 2006 #3


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    Electric heat is 100% efficient regardless of how it is delivered, but it is possible that small differences in the usage will affect how much you need. Ie, if you blow the heat at a window, you'll eliminate the draftiness that windows cause due to convection, but you'll spend more because more of the energy will by absorbed through the window. That difference isn't all that big, though.
  5. Apr 3, 2006 #4
    I forgot to touch upon another issue. Lord Kelvin, long back, showed the thermodynamic inefficiency in the process of producing heat by burning fuels.

    If we produce heat from burning fuel and use it to run a heat engine, drive a refrigerator and exhaust heat from both the heat engine and refrigerator into the controlled space, we can get a COP as high as 6. But this may not be practical for all scales of magnitude. A similar kind of thing is a heat pump.

    In short, heat to heat or electricity to heat depends is subjective and varies from case to case.
  6. Apr 3, 2006 #5


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    The right answer is
    All the electricity used by any electric appliance is converted to heat which goes into the room. You could add heat to your room using an oil filled electric radiator or your television. Both convert 100% of the electric energy they consume into heat. The only advantage a thermostatically controlled electric heater might have is to shut down and not overheat a room as it works to control temperature.
  7. Apr 3, 2006 #6
    Thanks all, very helpful.
  8. Apr 3, 2006 #7
    The answer is the Oil filled radiator. I will explain why when I get home.
  9. Apr 3, 2006 #8


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    Just in case some pedant brings up heat pumps, I meant electric resistance heat....
  10. Apr 4, 2006 #9
    If we are strictly speaking of electrical resistance heater vis-a-vis a direct fired fluid heater, the outcome depends upon the fuel and electricity prices.

    A typical kind of fuel used for fired heaters in India (called as LDO, Light Diesel Oil) has a net calorific value of 10000 kcal/kg which is about 41800 kJ/kg. If you have a requirement where you have to burn 1kg of this fuel/hr then the heat output, considering a non condensing tupe furnace and efficiency of the heater based on the net calorific value, with an overall efficiency of 85% (in general case it is better than this), is (41800*0.85)/3600 = 9.87kW.

    If you have to carry this heat to the end point by a heat transfer fluid with a specific heat of mere 0.6 kcal/kg deg.C (or 2.5 kJ/kg deg.C), for one degree rise in temperautre, the flow rate will be about (9.87 kW)/(2.5 kJ/kg deg.C x 1 deg.C) = 3.948 kg/sec.

    If the circulating system is a closed loop circuit without being opened to atmosphere, 25m head is more than sufficient to take care of frictional losses in the circuit. So pump power for circulating this fluid, considering 90% motor efficiency, 70% pump efficiency and a direct coupled pump motor arrangement, will be 3.948 kg/sec x 25 m x 9.81/(0.9x0.7) = 1536.9 W or 1.5369 kW.

    The price of 1 kg of LDO is about Rs. 25 (1USD = Rs. 45) and power cost is Rs. 5.5 / (kW-hr) for an industrial setup.

    So total running cost of circulating oil heater system is 25 + (5.5x1.5369) = Rs. 33.45/- (with higher delta T of circulating fluid, the pumping cost will come down further)

    Total running cost of resistance heater for the similar application is 9.87 x 5.5 = Rs. 54.285/-

    Perhaps, prices of electricity and fuel oil are in the similar ratio for Cyrusabdollahi (in his geography) as well.

    PS: I focused only on the running cost here and not the total cost which should include the capital cost also.
  11. Apr 4, 2006 #10


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  12. Apr 4, 2006 #11
    Thanks for the post, quark. There's a lot for me to get my head around there (I don't know how you guys do it), but it's a great help.
  13. Apr 4, 2006 #12
    I take back what I said. Until more details on the internal workings of the oil radiator are given, there is no solution. The problem is illdefined.
    Last edited: Apr 4, 2006
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