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By Grant Laidlaw
GRANT LAIDLAW
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy ( ACRA ) in Edenvale . He holds a Bachelor of Business Administration and an associate degree in educational administration . He has a National Technical Diploma and completed an apprenticeship with Transnet . He has dual-trades status : refrigeration and electrical . He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018 . Currently he is the SAIRAC national treasurer .
SENSIBLE HEAT VS LATENT HEAT : EXAMPLES
When an amount of heat is transferred to or from a mass of substance , which results in a change in temperature , the heat added is called sensible heat .
Stefan asks : Grant , Thank you for your contributions , We find that many people working in the field do not understand the basics , for example heat and temperature . Perhaps this would be of value in getting some basic understanding in place . Thanks .
Hi Stefan , yes what you say is correct . In the last issue of RACA Journal we looked at temperature , heat , latent and sensible heat . The resultant change in temperature , when considering sensible heat , can be calculated from : Q
= m cΔT where Q = The amount of heat transferred in kJ m
= The mass of the substance in kg c = The specific heat capacity in kJ / kg . K . ΔT = The temperature difference in degrees Kelvin .
Δ is the Greek symbol “ delta ” which is used to denote “ difference ”.
When the rate of heat transfer is required , the mass flow rate must be used instead .
Thus the formula then becomes : . . Q = m c Δ T ( the dot above the units means “ per second ”).
or , heat transfer rate = mass flow rate X specific heat capacity X temperature change . . Q = The amount of heat transferred in kJ / s which = kW . . m = The mass flow rate in kg / s . c = The specific heat capacity in kJ / kg . K . ΔT = The temperature difference or change in degrees Kelvin .
Quantity of latent heat . When an amount of heat is transferred to or from a mass or substance , which results in a change of phase , the heat added or removed is called latent heat .
The amount of heat required to change the phase of a mass or substance can be calculated from the formula .
Q = m . h f g
Where Q = The amount of heat required in kJ . m = The mass of the substance in kg .
h f g
= The latent heat required to change the phase of 1 kg of the substance in kJ / kg .
Where the heat transfer rate is required , the formula changes to : . . Q = m . h f g
. Where Q = The heat transfer rate in kJ / s or kW .
. m = The mass flow rate in kg / s
CALCULATIONS INVOLVING SENSIBLE AND LATENT HEAT Example 1 Question : Calculate the amount of sensible heat required to heat 5kg of water from 20 ° C to 100 ° C ? The specific heat of water is 4,2 kJ / kg .
Answer : The sensible heat formula is : Q = mc∆T The amount of sensible heat required is 5 X 4,2 X ( 100 – 20 ) = 1680 kJ .
www . refrigerationandaircon . co . za RACA Journal I September 2022 41