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Also , consideration needs to be given to whether the application will endure continuous or cyclical conditions . All of the various HEX options typically have different detail requirements , as well as preferable refrigerants per application . Each HEX type is therefore designed to perform under a particular application ’ s criteria .
HEX sizing is also a function of this application , and this point particularly affects every aspect of a HEX 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 .
Avoidance of dissimilar metals in the HEX unit also removes risks such as galvanic corrosion in applications like marine vessels . Offering the multiple material alternatives allows manufacturers to provide the market with a HEX suitable for a wide range of conditions and satisfying all specifications .
Some of the most important factors considered for an efficient heat exchanger solution are the overall heat transfer coefficient ; pressure drop across the plates and material of construction as mentioned . Overall heat transfer coefficient is a measure of resistance to heat flow . The resistance is caused by the plate material , fouling nature of fluids and type of HEX . Pressure drop ( ∆P ) is the price paid for high heat transfer . The higher ∆P , the higher turbulence and the thinner laminar film , resulting into an efficient heat transfer and a compact heat exchanger .
A balance between a compact unit with smaller surface area and electricity cost need to be worked out since the higher ∆P gives a higher pumping cost . It is also very important to select a compatible material for the application . Plates unlike tubes , are made of a thin material with no allowance for erosion or corrosion . The selection of heat exchanger can either be fusion bonded or semi-welded heat exchangers , depending on the type of a coolant or refrigerant , capacity and fatigue sensibility .
The biggest challenge of designing an efficient heat exchanger as noted by manufacturers is to improve the medium ’ s flow , at the same time to optimise the pressure drop utilisation and minimise the fouling .
PRESSURE DROP AND FLOW RATES Plant designers should know and use ‘ real ’ parameters , even if they will vary during the year ( or day / operation cycles ). Designing on the actual operating flow rate will ensure that the channel velocity and wall shear are kept high and that the pressure drop is fully utilised . Flow rate or temperature can be very different inside of the same unit . Designers should therefore include all of the different operating modes to enable suppliers to design the most appropriate unit to meet specifications .
© RACA Journal | Benjamin Brits HC Heat Exchangers Evapco
A plate heat exchanger installed at a manufacturing facility .
A larger process cooling heat exchanger .
A coil set for an ice storage installation is another example of the heat exchange design .
Refrigerant flows are most readily calculated from a pH diagram at the relevant system conditions . This method enables the mass flow required to be determined simply and quickly while pressure drops are highly design-dependent and require the use of dedicated correlations to suit each application .
System limits are also a factor and when suppliers know any such limits , they are able to include them in evaluations . There
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