Technical
HEAT EXCHANGERS
PART 1
By Marius La Grange , general manager , Thermocoil
A basic introduction to fin and tube designs
“
Heat exchangers ( HEs ) transfer heat from one fluid to another without the fluids coming in direct contact ” – ( ASHRAE , 2008 , Chapter47 ). An exchange of heat energy taking place between at least two fluids . A fluid of course being a vapour or a liquid or a combination of both .
A refrigeration system would absorb heat energy from an enclosed environment and reject it in another environment . A compressor is used to drive this process drawing and compressing low pressure vapour from the low-pressure portion of the system and discharging it into the high-pressure portion of the system .
On the high pressure ( HP ) side of a vapour compression system , the flow of heat energy is from within the system at the air-cooled condenser into the ambient air – a phase change from vapour to liquid taking place . The ambient air drawn across the finned area gains heat as it passed towards the exit side . Hence the exchange of heat energy from within the system is rejected into the ambient air .
In the low-pressure portion of any system , the heat energy from an enclosed environment is absorbed by the refrigerant on the inside of the evaporator . The refrigerant changing phase , in this case , from a HP liquid to a vapour within the enclosed HP side of the system .
Another important rule to keep in mind is that heat energy is transferred or flows from one fluid to another when there is a difference in temperature between the two . Heat energy will flow from warm to cold and not the other way around and that is very much applies in each heat exchanger as well .
In the case of an evaporator the HE is commonly used to cool an airflow on the outside with a refrigerant expanding and absorbing heat resulting in a phase change from liquid to vapour . The transfer of heat energy of course from the warmer air on the outside driving the phase change of the liquid refrigerant on the inside .
The ideal exchange of heat takes place when the two fluids are perfectly counter flow . This applies to any heat exchanger type .
With two fluids flowing through a heat exchanger , the rate of heat transfer ( duty ) could be calculated as :
Q = Duty Watts ṁ = Mass flow kg / s Cp = Specific heat capacity J / ( K kg )
Δt =
Change in temperature of the fluid
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Discrepancies between the theoretical duty and the actual duty is often due the two fluids not being perfectly counter flow . A perfect counter flow is not always possible with a fin and tube coil .
This flow of heat could be in the form off conduction , convection or radiation within any head exchanger . The material separating the two mediums might conduct the flow of heat energy from the surface on the one side to the surface on the other side through the material .
The type of materials used is , for the greater part , dependant on the environment that the HE will be functioning in . How warm or how aggressive might the mediums experience that form part of the system .
FIN AND TUBE HEAT EXCHANGER IN THE HVAC & R INDUSTRY Early heat exchangers were commonly made up of tubes alone . This limited the surface area and potential heat transfer . Heat Exchangers ( HE ) are essential components used in every cooling , heating , air conditioning and refrigeration system .
Continued on page 49
Coil with no finned surface area .
Various heat exchanger products using fin and tube design .
www . hvacronline . co . za RACA Journal I April 2021 47