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BASIC HEAT TRANSFER CALCULATIONS
The heat gained or lost (Sensible Heat), by a substance can be calculated using the following equation :
Q =m x c x t
Where: Q = Quantity of heat gained or lost in Joules m = Mass of material in kg c = Specific Heat of material in J/kg/°C t = Temperature change in °C
Examples 1 20 kg of water at 5 °C is mixed with 10 kg of water at 65 °C. The final temperature of the mixture is 25 °C.
What is : a). The heat gained by the 20 kg of water, and, b). The heat lost by the 10 kg of water ?
( S.H. of water = 4180 J/kg/°C ) a). Temp. change = ( 25  5 ) = 20 °C Heat gained = 20 kg x 4180 J/kg/°C x 20 °C (Note the cancelling) = 1,6720,00 J = 1672 kJ b). Temp. change = ( 65  25 ) = 40 °C Heat lost = 10 kg x 4180 J/kg/°C x 40 °C = 1,672,000 J = 1672 kJ The above calculations demonstrate the Law of Conservation of Energy in that , Heat Lost = Heat Gained.
Examples 2 Calculate the sensible heat needed to raise the temperature of 1 kg of Ice from 10 °C to 0 °C.
(The S.H. of Ice is 2.1 kJ/kg/°C)
Q = m x c x t = 1 x 2.1 x 10 = 21 kJ
Examples 3 Calculate the latent heat needed to change 10kg of Ice at 0 °C to water at 0 °C.
(The Latent Heat (L) of melting ice is 335 kJ/kg)
Q =m x L = 10 x 335 = 3350 kJ
