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Effectively applying energy efficient thermal wheels in ventilation systems

The thermal wheel is well established as an energy saving device for ventilation and air conditioning systems. This CPD article will summarise the aspects of the thermal wheel that make it an attractive option and consider how leakage and cross contamination is minimised and characterised and concludes with the current CIBSE advice on their use as the world emerges from the Covid-19 pandemic.
Heat recovery of some form is an important element of ventilation systems in buildings particularly when outdoor, ambient air is at a low or a high temperature or has a low or a high absolute humidity. The condition of an airstream entering the air handling unit may be moderated by directly or indirectly benefitting from the temperature and moisture content of the, already conditioned, air extracted from the controlled space. The Ecodesign(1) regulation requires that practically all bidirectional ventilation units (i.e. supply and extract systems)should include a suitably efficient heat recovery device. Thermal wheels are ‘regenerative’ devices able to provide high-efficiency heat recovery in a compact form and are listed, alongside plate heat exchangers, on the UK government’s Energy Technology List(2) of high performance energy-efficient products. Despite the relatively short time that the air takes to pass through the wheel and the moderate pressure drop across the wheel (typically less than 200Pa), the heat recovery efficiency can still be more than 80%(that exceeds the current Ecodesign minimum of 73%). Since thermal wheels can be designed to transfer latent energy between airflows, this can negate the need using a separate humidification device for incoming dry winter air and also eliminate the need for condensate drains.



The thermal wheel that, depending on its main function, may also be referred to as heat wheel, rotary heat exchanger, energy or enthalpy wheel, is based around a rotating cylinder that is packed with a material fabricated so that there are continuous channels passing from one side of the cylinder to the other, through which air passes. The packing material, that is typically aluminum, provides a large surface area to transfer heat between it and the passing airstream. The wheel is positioned in a casing, so that at any one time, half of the wheel is exposed to an airstream, that is typically outdoor air, while the other divided half is in a counter flowing airstream that is being extracted, typically from the conditioned space. The two halves are separated by a horizontal plate that has acritical, resilient seal against the wheel that prevents leakage between the two air paths. Depending on the manufacturer the seal is likely to either be a brush type contact seal that, although cheaper, can become more susceptible to leakage as the pressure differential increases; or a closely fitting shaped, non-contact wiper seal that employs air vortices to maintain a seal. An example wheel, shown in Figure 1, when operating for ‘winter’ operation would take air extracted from a warm and humid room, its temperature and absolute humidity being reduced as it passes through the cool packing. Then as the wheel rotates into the supply airstream, the heat is exchanged to the incoming cooler dryer outdoor air.

Simplified Schematic of Thermal Wheel
The wheel will typically rotate continuously, belt or chain driven by a low power electric motor. The rotational speed can normally be modulated, or the wheel stopped, to suit the particular conditions. The heat that is transferred via the regenerative wheel between the two airstreams may be sensible heat, leading to a change in the dry bulb temperature in the two streams or, more effectively, when the packing has surfaces that are hygroscopic, or that have a sorption coating, also include latent heat transfer, so affecting both the dry bulb temperature and the air moisture content as shown in Figure 2. Even where there is no specific coating to promote latent heat transfer, condensation will occur when the temperature of the packing material falls below the dew-point temperature of the warmer airstream and so release a proportion of the latent heat of vaporisation to the packing.

The psychrometry of a total thermal wheel

Referring to Figure 2the practical thermal effectiveness, ε, of the heat recovery devices may be described in a number of ways.

In terms of sensible heat exchange εS= m ̇A(θBθA)/m ̇R(θRθA), and for latent heat exchange, εL= m ̇A(gBgA)/m ̇R(gRgA), where the values of εS and εL are not necessarily equal for a device that has both sensible and latent heat exchange.

In terms of total heat exchange the enthalpies of the air streams may be used to give εT= m ̇A(hB–hA)/m ̇R(hR–hA). Modulation of output is commonly achieved either by adjusting the rotational speed of the wheel or by bypassing the supply air. Heat recovery efficiency increases with wheel speed but is ultimately limited by carry over(as discussed later).

The heat transfer properties are largely determined by the packing material that, in simple wheels, is often corrugated into sinusoidal shape channels. There is a surprising paucity of published work on the overall optimisation of thermal wheels. However, from tests of thermal wheels and validated models reported by D Antonellis et al(3) the most effective configurations are characterized by thin packing materials that provide a large accessible thermal mass across the face of the cylinder during the brief period as the cylinder rotates and air flows through the channels in each respective direction. The work also determined that a revolution speed, that is typically be between 10 and 15 revolutions per minute, has little impact on the wheel effectiveness but that at higher speeds there is an increased opportunity for the unwanted carryover of air between the two streams. If the heat wheel rotates slowly, the packing material average temperature becomes close to that of the air stream and so heat transfer decreases due to a reduced temperature difference. Antonellis notes that for thermal wheels that have a depth of 0.2m (as is common in commercial packaged air handling units) optimum characteristic channel sizes are in the order of 4mmwith channel aspect ratios being 1 or above. Deeper wheels will benefit from larger channels. Some manufacturers employ more sophisticated metal channel profiles, such as that shown in Figure 3 to increase heat transfer whilst maintaining moderate air flow resistance.

A proprietary design of a thermal wheel channel

The Ecodesign regulations(1) requires a thermal bypass for mechanical ventilation and heat recovery units. When employing a thermal wheel, the thermal bypass is achieved by simply stopping the rotation of the wheel. Dependent on the application, a small ‘ducted’ bypass around the wheel can provide reductions in fan energy for when the wheel is stopped.

Cross-contamination of the two airstreams can occur by carryover and leakage. This can be minimised by avoiding large pressure differences; providing effective seals and configuring the respective fans in a way that always promotes leakage into the exhaust airstream. Carryover refers to the air that is entrained within the wheel that is then transferred to the other, counter flowing, airstream as the wheel rotates. A purge section is typically installed.

Purge section

Any leakage around the periphery of the wheel will impact the overall thermal performance and can also contribute to the amount of air passing between the two air streams so it is important to employ good quality, maintained seals. There should be a positive pressure difference between the incoming, outdoor air, side of the wheel and the exhaust side, so driving any leakage from the supply to exhaust. This prevents adversely impacting supply air quality but, as the pressure difference rises, will reduce the overall energy efficiency of thermal wheel. Leakage from extract to supply on the room side of the wheel will depend on the pressure difference between the extract and the supply and if the fans are correctly positioned, this can be eliminated by automatically throttling the extract air so that the pressure difference is in the right direction. The extra pressure drop for the throttle must be catered for by the exhaust fan. It is important that the supply and extract systems are properly commissioned to ensure that appropriate pressurisation is maintained. As the supply fan speed modulates (in response to demands from the room) the relative pressures must be automatically monitored and extract fan speed appropriately modulated to main correct pressure differential. If the system has modulated to deliver low air flows, the rotary heat exchanger speed should be automatically reduced to ensure the correct purging air flow through the heat exchanger
as shown in Figure 4. Incoming fresh air(at the bottom right in the diagram)is used to ‘purge’ the air extracted from the room (shown in yellow) that has been entrained in the wheel and then propels it into the exhaust airstream(top right). Increasing the purge rate reduces the capacity of the thermal wheel since it will also transfer some of the heat into the discharge air. The extract fan drives this purging flow so it should be added to the required flow rate for the extract fan. An appropriately controlled wheel with a properly designed purging section will practically eliminate carryover.

Any leakage around the periphery of the wheel will impact the overall thermal performance and can also contribute to the amount of air passing between the two airstreams so it is important to employ good quality, maintained seals. There should be a positive pressure difference between the incoming, outdoor air, side of the wheel and the exhaust side, so driving any leakage from the supply to exhaust. This prevents adversely impacting supply air quality but, as the pressure difference rises, will reduce the overall energy efficiency of thermal wheel. Leakage from extract to supply on the room side of the wheel will depend on the pressure difference between the extract and the supply and if the fans are correctly positioned, this can be eliminated by automatically throttling the extract air so that the pressure difference is in the right direction. The extra pressure drop for the throttle must be catered for by the exhaust fan. It is important that the supply and extract systems are properly commissioned to ensure that appropriate pressurisation is maintained. As the
supply fan speed modulates (in response to demands from the room) the relative pressures must be automatically monitored and extract fan speed appropriately modulated to main correct pressure differential. If the system has modulated to deliver low air flows, the rotary heat exchanger speed should be automatically reduced to ensure the correct purging air
flow through the heat exchanger.

Packaged AHU

If the AHU is incorrectly configured, as in Figure 6 with both the extract an the supply fan placed on the room side of the wheel then leakage is likely to have a serious effect on the air quality as this arrangement will subject the seals to the maximum possible pressure
differential.

Bad practice positioning

To minimise the leakage of air between the airflows, the recommended arrangement of the fans is upstream of the wheel on both sides as shown in Figure 5 and Figure 7.

Good practice positioning

BS EN 16798-3:2017(4) employs two ratios to assist in the characterisation of leakage. Outdoor Air Correction Factor (OACF) is the ratio of the air mass flowrate of outdoor air entering heat recovery device to the mass flowrate leaving the device towards the room. An OACF of less than 1 indicates leakage from exhaust to supply.
Exhaust Air Transfer Ratio (EATR) is the percentage of exhaust air passing to the supply air through the seal around the wheel together with any carry-over leakage. The relevant test standard for heat exchangers, BS EN 308, that is currently under revision, will provide a more detailed interpretation of OACF and EATR. It is reported(5) that a soon to be published Eurovent REC 6/15-2020 will effectively limit EATR to 1% and OACF to between 0.9 and 1.1 however as OATF increases above 1 the fan energy use can increase significantly. In many cases, achieving an acceptable EATR requires that the extract air is throttled as previously discussed. The values of EATR and OACF depend on the actual pressure inside the unit and so requires the pressure drops along each of the 4 connected ducts for a particular system.

Wheels, particularly hygroscopic wheels, can carry water-soluble gases (such as cooking odours and tobacco smoke) between the two air streams. In many applications this is unlikely to be a problem but will be a consideration when selecting a heat exchanger for areas such as restaurants. If the discharge air stream is contaminated with dust, bacteria or viruses then there could be carry over from one stream to another. However, where the unit is operated appropriately this is thought to be a very small percentage and with correctly maintained ePM1 or ePM170 filtration upstream of the wheel, such as that shown in the unit in Figure 5, this should not be a practical problem. As the world looks forward to emerging from the Covid-19 pandemic the impact of ventilation systems on indoor air quality has never been more important. CIBSE’s current guidance on ventilation systems related to Covid-19 (abstracted in boxout) concludes that “the benefits of maintaining high outside air rates to dilute internal viral contaminants outweigh the risks of viral particles being transferred via a correctly configured thermal wheel”.

Abstracted notes from CIBSE Covid-19 ventilation guidance, version 4, 23 October 2020
Where thermal (or enthalpy) wheels are installed to recover heat, then a competent engineer/technician should check that the configuration and operating conditions are such that any leakage across the device is from the supply side to the extract side, to minimise the risk of transferring contaminated air into the supply. However, if adequate ventilation rates with suitable thermal comfort can be provided without use of the regenerative rotary heat exchanger then it is advisable to bypass the system if provision is available, or if no bypass is available then the rotor should be turned off.

The heat recovery function is usually integral to the system design in terms of simultaneously delivering adequate air flow and meeting heating or cooling demand. If the only way to provide adequate and safe outside air flows is by using the thermal wheel then it is advisable to turn the rotor on. The expected reduction in dilution of any potential indoor viral source with inadequate ventilation flow rates is considered to be a greater risk for viral transmission than the potential for viral transfer across the thermal wheel. Turning the rotor on will also improve thermal comfort conditions and has the added benefits of maintaining the energy efficiency of the system and helping to maintain appropriate humidity levels in the building.
 

(See the full document at https://www.cibse.org/coronavirus-covid-19/emerging-from-lockdown)

References:

1. The Ecodesign for Energy-Related Products Regulations 2010
2. https://etl.beis.gov.ukaccessed 6 Nov 2020
3. De Antonellis, S et al ‘Design Optimization of Heat Wheels for Energy Recovery in HVAC Systems’, Energies 2014, 7, 7348-7367; doi:10.3390/en7117348
4. Energy performance of buildings -Ventilation for buildings -Part 3: For non-residential buildings -Performance requirements for ventilation and room-conditioning systems
5. Lawrance W & Schreck T “Rotary heat exchangers save energy and prevent a need for recirculation which contributes to the decrease the risk of COVID-19 transfer” REHVA Journal, Vol 57, Issue 5, October 2020

This post originally appeared in the CIBSE Journal