Feature
A de-superheater ( front left ) and gas coolers installed at a cold storage facility . At this site heatreclaim heat exchangers are installed on the CO₂ refrigeration racks to generate hot water .
© Cold Link Africa | Benjamin Brits
is connected to a cooling tower where the heat is rejected to atmosphere . Temperature is controlled via a three-way bypass valve on the supply and return headers to the cooling to maintain a minimum return temperature to the oil cooling heat exchanger on the compressor . This maintains the oil in the optimum temperature range , protecting the compressor . The reclaim heat exchanger is installed in the discharge header out of the oil cooling circuit , before the bypass valve . The oil cooling circuit typically achieves heat recovery of lower temperatures – between 35 ° C and 55 ° C . If this temperature range is not high enough , the reclaimed energy can be fed into an augmenting device such as a heat pump that upcycles the product to higher temperatures . It is important to consult the equipment supplier in this process to ensure that the heat reclaim system will not be harmful to the compressor .”
The design parameters would also need to consider any leaks and how this would affect the overall operations and safety aspects . For example , risk of contamination may occur in a line feeding a food processing section , or the supply of hot water to a building may become compromised .
In addition to this , there are other associated aspects of legislation that come into the realm of heat recovery , particularly if one is going to tie into water system or if domestic hot water reticulation is involved . SANS10252 – related to hot water generation must be applied as well as various requirements of SANS10400 . With hot water systems , Legionnaires disease also becomes a consideration . 35 ° C to 40 ° C is the ideal temperature for legionella bacterial growth , so if the heat reclaim system that can only generate water at these temperatures , it needs to include an electric element , steam line , a heat pump or like device . This will then be used to periodically raise the temperature above 50 ° C to kill off any potential bacterial growth .
“ Control methodology is also extremely important when looking at heat recovery and you would need to understand the capacities of each heat exchanger type . Heat exchangers are typically designed and manufactured to accommodate various degrees of hot water , but if you reach boiling temperatures , steam arises , and this may cause damage to the product . Protection in this regard is therefore critical in the strategy and the placement of sensors too . One would also need to understand the differences between stand-alone heat recovery systems and systems directly integrated ,” says Alistair Bell , technical support engineer – refrigeration and air conditioning at Danfoss .
Further considerations would include the ability to bypass the heat recovery unit so as not to have a total system shutdown should something go wrong on that particular component , whether a closed or open circuit method will be used , and corrosion protection that can experience different outcomes based on temperatures . Further understanding a system ’ s total load profile and future expansion plans become important when looking at how heat recovery will affect the overall system in terms of generation capacity or pressure drops .
“ In addition , here , and depending on the application , if you are including water storage , you need to have a trustworthy tank . We have moved over to using mild steel tanks with a rust-proof coating applied on the inside . This particular tank is manufactured locally and has a five-year warranty . The biggest enemy , however , to any mild steel tank is rust that we have to deal with . We have tried fibreglass type tanks , but these prove to be too sensitive to pressure and temperature differences and are therefore limited . Also , any heat recovery or heat pump will always be more efficient than a direct element solution ,” says Cules Smit , general manager at Matador Refrigeration .
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RACA Journal I February 2022 www . hvacronline . co . za