HEAT TRANSFER - PRINCIPLES & EQUIPMENT - BASIC HEAT TRANSFER CALCULATIONS
Written by Norrie
Wednesday, 17 February 2010 20:47
Page 3 of 9
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
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
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.
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
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
Last Updated on Wednesday, 24 February 2010 19:40