Feature full system is the most sustainable solution possible ,” adds Gert van Rooyen , product engineer at HC Heat Exchangers .
Further , avoidance of dissimilar metals in the HE unit removes risks such as galvanic corrosion in applications like marine vessels . Offering the multiple material alternatives allows manufacturers to provide the market with a HE suitable for a wide range of conditions , 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 up by plate material , fouling nature of fluids and type of heat exchanger . 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 very important to select a compatible material for the application . Plates unlike the tube , are made of a thin material with no allowance for and 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 . It all depends on the plate design , and the biggest challenge of designing an efficient plate heat exchanger is to improve the mediums flow , at the same time as you optimise the pressure drop utilisation and minimise the fouling ,” explains Moses Modibela , heat exchanger product and application manager at Alfa Laval .
PRESSURE DROP AND FLOW RATES Plant designers should know and use ‘ real ’ parameters , even if they will vary during the year ( or day ). According to Kutelya , “ 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 include all of the different operating modes to enable the suppliers to design the most appropriate unit .”
Refrigerant flows are most readily calculated from a pH diagram at the relevant system conditions . Van Rooyen adds , “ This method enables the mass flow required to be determined simply and quickly while pressure drops are highly designdependent and require the use of dedicated correlations to suit each application .”
“ System limits are also a factor here and when we know any limits , we can work around them . There must be a system balance to ensure an efficient cooling or heating solution is delivered . Other factors in addition to pressure drop are the type of refrigerant , capacity and the use / application of refrigerant . Pump capability is also important and plays a role in pressure drop ,” notes Modibela .
HC Heat Exchangers
MAINTENANCE AND BEST PRACTICES Maintenance regimes are always site-specific , but every site will need some form of maintenance to ensure the proper continuing functioning of the plant . Proper maintenance guidelines would essentially fill several articles just like every topic we cover , but covering the basics the following can be considered .
“ In the case of coil HEs , monthly visual inspections of the units should be carried out to check that there are no drain line blockages and all defrost processes are working . Water build-up in drip trays can cause spillages and product damage as well as becoming a health and safety risk . If units do not defrost properly , performance will sharply degrade , and if ice build-up isn ’ t stopped , it will cause irreparable damage to a unit . Every three to six months the units should be cleaned , all electrical connections checked and tightened , motor integrity checked , fan blades inspected for abnormal wear patterns and insulation integrity confirmed . In the plant room , all electrical and mechanical equipment should be checked and all preventative maintenance carried out . Serious problems can often be avoided if routine checks are done and any abnormal system behaviours are identified , correctly diagnosed and timeously rectified ,” says van Rooyen .
Kutelya adds , “ On our plate heat exchangers , general cleaning can be carried out in two ways . Either by removing the plate pack and mechanically clean the plates , or through the clean in place ( CIP ) process which is the circulation of an appropriate chemical in the HE to remove the fouling . Both methods are effective when carried out correctly . Another way to clean an HE when you have a mechanical blockage is to dismount the HE and take out the plates to clean with water , re-assembling again .”
“ In these aspects , our first call would be to try and design a unit to minimise any need for maintenance . Our strategy is to
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A bespoke drycooler used for industrial power generation project cooling .
www . hvacronline . co . za RACA Journal I February 2021 25