www. refrigerationandaircon. co. za RACA Journal I September 2025 15
Events and Exhibitions
" Maintaining sufficient water volume in the system is absolutely essential for several reasons. Firstly, it ' s crucial for oil return to the compressors, which ensures proper lubrication and extends their lifespan. Secondly, especially for heat pumps, adequate water volume helps in limiting temperature drop during defrost cycles, significantly improving efficiency. Finally, and extremely important, it contributes to increasing chiller efficiency and longevity by preventing those damaging continuous start-stop cycles that degrade performance and shorten the equipment ' s life."
Rossettini provided a guideline for minimum water quantity: it should be calculated as the flow passing through the evaporator or condenser divided by 12( for a five-minute run time) or ideally by six( for a ten-minute run time). " Systems often have excessively high delta T( temperature difference) designs, leading to low actual delta T during commissioning. This indicates too much flow, requiring adjustments to reduce water flow for optimal efficiency during the commissioning phase. I ' d also caution that while secondary systems contribute to water volume, their contribution is typically limited to a maximum of 20 % of the total volume needed for the heat pump."
POLYVALENT SYSTEMS = THE PINNACLE OF EFFICIENCY Rossettini highly recommended polyvalent( four-pipe) systems as a reliable and highly efficient solution for simultaneous heating and cooling. These units can operate in:
• Cooling only mode: Standard chiller operation
• Heat recovery mode: Instead of discharging excess energy to the air, the system recovers it to produce hot water, significantly improving efficiency
• Heating only mode: The unit provides heating, with excess cooling discharged to the air is even more critical due to the complex circuits, necessitating substantial water volume."
STRATEGIC SIZING FOR LONG-TERM PERFORMANCE Rossettini emphasised the most critical aspect of designing with four-pipe systems: accurate sizing based on annual load profiles. A common and enormous mistake, he cautioned, is oversizing polyvalent units. For example, if a building has a maximum heating load of 200 kW and a maximum cooling load of 400 kW, selecting a 400 kW polyvalent unit is incorrect. Such a unit might produce 500 kW of heating, far exceeding the building ' s peak demand, leading to excessive short cycling and ultimately destroying the unit within four to five years.
" Instead, I propose two optimal design strategies. First, a combined system where you install a smaller polyvalent unit— for example, a 200 kW unit sized for the maximum heating load— alongside a standard chiller, also 200 kW, to meet a peak 400 kW cooling demand. In this setup, the polyvalent unit handles simultaneous heating and cooling needs, with the chiller supplementing cooling as required.
Even better, my second strategy involves average demand sizing. Here, you size the polyvalent unit based on the average heating demand, say 80 kW, and combine it with a heat pump that can flexibly provide additional cooling or heating to meet peak demands. This approach maximises the polyvalent unit ' s operational efficiency, minimising unnecessary start-stops and ensuring long-term system health."
Rossettini concluded with a call to action, urging attendees to design systems that not only meet immediate needs but also contribute positively to the environment and the well-being of future generations. RACA
" Double-circuit polyvalent units with multiple compressors and plate exchangers can further enhance versatility, working in cooling only, heating only, or simultaneous heating and cooling modes, all while drawing only one input power. The proper return of oil to the compressors in these units
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www. refrigerationandaircon. co. za RACA Journal I September 2025 15