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HEAT EXCHANGERS : KEY IN MANAGING THERMAL FUNCTIONS
By Benjamin Brits
When managing or changing a fluid ’ s thermal state , the world of the heat exchanger is vast – with many solutions to fit .
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The basic rules in heat transfer theory is that heat will always be transferred from a hot medium to a cold medium , until equilibrium is reached . There must always be a temperature difference between the two mediums for heat transfer to take place and the heat lost by the hot medium is equal to the amount of heat gained by the cold medium , except for losses to the surroundings .
To continually transfer heat between two mediums , heat exchangers ( HE ) are used . Two main types of heat exchangers are the direct and indirect heat exchangers . In a direct heat exchanger , both mediums are in direct contact with each other and the indirect heat exchanger , both mediums are separated by a wall through which heat is then transferred .
Danfoss Alfa Laval
RACA Journal I February 2021
An illustration of the flow pattern in a gasketed plate heat exchanger .
A U-turn gasketed plate heat exchanger .
Outside of a HE , the predominant heat transfer means is air while internally it ranges from water , with or without additives , to natural and synthetic refrigerants , steam and even oils , to name the most common .
All air-conditioning and refrigeration applications make use of HEs and they extend to other industrial applications such as mining , power generation , military , transport and agricultural cooling .
To start , when determining the right HE per application , there are a number of factors to consider . “ Some of the most common factors include flow rates , maximum pressure inside of the HE , pressure drop , temperature parameters , system pressures , liquid viscosity and concentration , system upset conditions ( start up / shut down ), space availability , expansion plans , life cycle costs and maintenance requirements . Also consideration needs to be given to whether the application will endure continuous or cyclical conditions ,” says Zaur Kutelya , business development manager for the Danfoss Heating Team .
All of the various HE options typically have different detail requirements , as well as preferable refrigerants per application . Each HE type is therefore designed to perform under a particular application ’ s criteria .
HE sizing is also a function of this application , and this point particularly , affects every aspect of a HE coil . Other crucial considerations not already mentioned include tube sizing , fin spacing and the overall construction methodology .
“ As part of the initial design choices , different material types also suit different environments . Typical commercial applications will utilise copper tube and aluminium fins , while NH₃ and CO₂ plants will use stainless steel tubing with aluminium fins . Variations to these combinations are designed to enhance the life and / or performance of the unit in its particular environment . The impact of global warming too has focused design considerations around a refrigerant ’ s ozone depletion potential ( ODP ) and global warming potential ( GWP ) values . These are helpful metrics but we also believe in the value of further considerations such as total equivalent warming impact ( TEWI ) calculations , to ensure the
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