ACR Journal

April | May 2021 ENERGY RECOVERY 20 Maintaining comfort and e ciency with centralised plant Volume 7 No.3 When we think of thermal comfort and climate control we tend to only consider air conditioning in the typical sense - through spilt systems or variable refrigerant flow designs. These product types have developed over time to o er high operating e ciencies, well controlled comfort levels and energy recovery systems through simultaneous heat recovery operation. Heat recovery, in basic terms, limits the amount of heat energy transfer into or from a system. This reduces the amount of power consumption either directly or indirectly, considering other systems that we might have to use to maintain conditions. When best discussing how a heat recovery VRF system works, the simplest way is that the heat energy Andrew Robinson, Founder of specialist HVAC supplier, Exi-tite, discusses how thermal comfort can be efficiently maintained using air handling units without sacrificing health. removed from the rooms operating in cooling mode is transferred via the refrigerant cycle to rooms operating in heating. Here it is rejected from the system, but rather than being wasted and rejected to ambient, it serves a purpose by heating the space. Decreasing comfort - increasing cost Everyone’s focus currently is on ventilation rates when designing building services, and with that comes the increased use in air handling units. The comfort processes available for air handling units are lesser known than those of typical air conditioning systems simply because they are not as commercialised or visually apparent. When we introduce an element of outside air into a building, which we mainly do to reduce CO 2 and other contaminants, we start to alter our building dynamic in terms of temperature. The removal of air through extract fans removes the heat energy that we already have, whilst adding fresh air from outside also introduces di ering thermal conditions to our space. To negate these e ects, we generally would consider increasing the capacities of existing air conditioning or heating systems or replacing this energy directly through direct expansion or hydronic heat exchange coils within the AHU. Neither of these processes is in line with our current design policies of driving down our carbon footprint, and neither would look prosperous in terms of operational costs. By having a ventilation process that is rejecting the energy of a building to the atmosphere through extract and adding load to other spot cooling systems such as VRF, through fresh air introduction, we are increasing the operating cost of the building and, in e ect, wasting energy that could otherwise be reused. Energy recovery A majority of air handling units now include methods of heat recovery. These include cross flow heat exchangers where energy is transferred between air streams across a membrane, and thermal wheels, which also act as an energy transfer membrane. Low temperature hot water or chilled water heat exchangers, served by external plant, have been a long-standing option for the tempering of fresh air or the conditioning of supply air within an AHU. Andrew Robinson, Exi-tite Run around coil showing energy transfer between the supply and extract air of two separate air handling units.

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