04 May 2021
Andrew Robinson, founder of specialist HVAC supplier Exi-tite, discusses how thermal comfort can be efficiently maintained using air handling units without sacrificing health.
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 offer high operating efficiencies, 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 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 CO2 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 differing thermal conditions to our space. To negate these effects, 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 effect, wasting energy that could otherwise be reused.
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. While purposeful in terms of designing systems to broad operational envelopes, these options add complexity by involving third-party integrations to the design and operation. Recent developments improving the low ambient operating ranges of direct expansion products (air conditioning) means that there is now a rise in cooling and heating solutions using DX heat exchangers with either third party or integrated heat pumps. Previous DX cycle logic and associated technology would restrict operation to a particular ambient range. More so in the UK, these restrictions in heating would be felt when the supply air ambient dropped below 10oC. Advances in technologies such as vapour injection, which maintains compression temperatures, and tighter refrigerant flow control through inverter technology; has allowed the reduction of costly electrical preheat elements usually used to raise inlet air temperatures.
Integrated heat pumps generally have a self-contained refrigeration circuit within the AHU chassis. A heat exchanger is positioned in the supply air stream and one in the extract air stream. The position of these heat exchangers is paramount to system operation and efficiency. To aid the heating process, the condenser, which is in the supply air stream, is placed downstream of the thermal wheel and the evaporator, located in the buildings extract, is before the thermal wheel. This means that heat energy removed from the building is transferred to the refrigeration cycle, raising refrigerant suction temperature/pressure and requiring less work from the compressor to raise the head pressure/temperature to, in turn, increase the air temperature. Further energy is recovered from the thermal wheel passing to the supply air stream, tempering the ambient air in really low outside temperatures before passing through the condenser.
Run around coil showing energy transfer between the supply and extract air of two separate air handling units
The current instruction from industry bodies to prevent transmission, infection and contamination is to turn off thermal wheels and remove air recirculation from the central plant. Whilst thermal wheels provide a very low mixing of air streams; further air sealing is present to reduce possibilities further for applications where hygiene is a priority. However, this still is not enough to satisfy the advice being provided. A further energy recovery system applied to air handling units is a run around system. This is a hydronic system of very similar principles to that of the DX, but rather than exchanging energy through refrigerant, it uses water as a medium and a circulation pump instead of a compressor. Run around systems are particularly useful for transferring energy between separate extract and supply systems as we see applied in clinical environments.
The ability to reuse energy in this manner improves the system, and in turn, building efficiency. It reduces the requirements for additional equipment to meet building demand and reduces operational costs. The application of DX cooling and heating within air handling units and smaller energy recovery ventilation systems is becoming the popular choice for maintaining broader climate control within a building as designers are pressing to use higher ventilation rates with no recirculation.