Thermal Wheel of Fortune
A thermal wheel is also called a ‘rotary heat exchanger’. In more technical language the devices are sometimes known as a ‘rotary air-to-air enthalpy wheel. A thermal wheel is a kind of a heat exchanger located within the outflow stream of a system which already handles air. The thermal wheel performs an energy recovery function extracting the heat from the air as it passes. Variations on the thermal wheel include desiccant wheels, enthalpy wheels and Kyoto wheels, the latter of which is a variation on the device designed specifically for cooling purposes. See our explanation of ‘coolth’ below. A thermal wheel is constructed as a matrix honeycomb of materials which are chosen for their heat absorbing capacity. The thermal wheel rotates in the stream of warm air and absorbs heat from the warm air stream in the first half of the rotation before the thermal wheel goes round and dissipates the heat into the fresh air the other side. In doing so the thermal wheel transfers the heat from the warm waste air to the designated air stream. The thermal wheel delivers the heat in an amount proportional to the difference in temperature between the air streams. The thermal wheel relies on the ‘thermal gradient’ so the amount of heat transferred depends on the operating efficiency of the thermal wheel itself. Heat exchange works more efficiently when the air streams flow in opposite directions across the thermal wheel. The converse directional flows make for a sufficient temperature gradient across the thermal wheel. The principle can in fact operate in reverse across the thermal wheel. ’Coolth energy’ can be applied by the thermal wheel. ‘Coolth’ is however a somewhat clumsy concept which can be difficult to understand. In mechanical engineering, ventilation, and heating, ‘coolth’ basically means the ‘application of cold’ i.e. removal of heat or the addition of ‘coolth’. This idea in physics or any common parlance is nonsense but the point to be made is that thermal wheel can be used for the intended purpose of cooling the incoming air rather than for the specific purpose of transferring the heat in to the exhaust stream. In practice any such variation on the thermal wheel still means it’s performing the same heat transfer function as it always does but if cooling the incoming air flow is the priority and no particular use is being made of extracted heat, it may affect the design of the thermal wheel.
Wheel Construction for ‘Sensible’ Heat Exchange
The thermal wheel heat exchange matrix is usually made from aluminium but occasionally a thermal wheel will be made of synthetic fibers or plastic. In recent years there has been a return to aluminium for thermal wheel and all heat exchange appliance manufacture owing to the fact that even though aluminium is more expensive at the outset, costs over the lifetime of the aluminium thermal wheel work out lower that they do with synthetic materials. The thermal wheel rotates using a belt drive powered by a small electric motor. A more sophisticated thermal wheel might be powered by an inverter speed controlled motor to make control of the air temperature easier. If the thermal wheel is not required at any point due to temperature considerations the inverter speed motor switches off the thermal wheel
A thermal wheel should be much more efficient in its heat exchange operation than any other ‘air side’ heat recovery system. A thermal wheel transfers heat from one air stream to another without it having to pass through any exchange medium. The shallow depth of the thermal wheel heat exchange matrix compared with, for example, a plate heat exchanger ensures that the drop in pressure across the thermal wheel is lower than it would be in other systems. Usually a thermal wheel is chosen for face velocities of around 1.5 to 3 m/s. With a balanced air flow rate ‘sensible’ (as opposed to ‘latent’) heat efficiencies of 85% can be reasonably expected from a thermal wheel. For the uninitiated ‘sensible’ heat is the heat the thermal wheel exchanges which was present in the air solely owing to its temperature. ‘Latent’ heat is the heat available to the thermal wheel which can be extracted from the moisture in the air. The thermal wheel also requires a small amount of energy to rotate the thermal wheel device itself. Energy consumption by the motor however is very low as can be shown from the minimal seasonal efficiency variation in the thermal wheel. Depending on the materials it’s made of the thermal wheel may also be able to recover some further ‘latent heat’ from the air, If so, gross efficiencies can be improved by up to 15%.
Usually the heat transfer between the air streams is solely in the form of ‘sensible’ heat and has no effect on the moisture content of the two air streams. However if moisture and relative humidity in the returning air stream is sufficient to allow for condensation, the thermal wheel will exchange ‘latent’ heat as well. The heat transfer coating on the thermal wheel will develop a film of water with the effect of improving the thermal wheel’s heat transfer co-efficient and improving its efficiency. In addition to the change in the temperatures in the air streams the thermal wheel therefore changes the relative moisture content of the air on either side. The condensation film however slightly increases the pressure drop in the thermal wheel. This fact can increase resistance by as much as 30% and increase the fan’s consumption. The difference will be reflected in the seasonal efficiency of the thermal wheel. Some thermal wheel aluminium matrixes are available which have an applied hygroscopic surface coating. These coatings or alternatively porous synthetic matrixes assist in absorbtion and discharge of water vapor at humidity levels much lower than the levels at which latent heat transfer and condensation would otherwise take place and so allow for even higher thermal wheel efficiency. A thermal wheel of this construction however does dry the air, but sometimes dry air is beneficial.
Not Always the Best Solution
Despite its huge advantages however a thermal wheel is not always the best heat exchange device. There are some circumstances in which they are not suitable even when they fulfill their function well. They are not for example suitable if strict separation of the outgoing and supply air streams is required.The rotating thermal wheel passes between one air stream and the other so air will bypass the air stream interface at the boundary of the heat exchanger. This problem can be greatly reduced by fitting brush seals and by plating off a section of the wheel but a complete seal is unattainable on a practically usable and affordable device. This difficulty however may make a thermal wheel unsuitable where smells and even toxins may pass between the air streams. Another potential difficulty is that matrices made with hygroscopic coatings and fibrous materials designed for the purpose of transferring ‘latent’ heat are also almost impossible to clean and are liable to degradation and damage, so devices used for this purpose require filtration of both the fresh air side and the exhaust sides of the wheel.
KCS Advice will be returning to the subject of thermal wheels, their variations and other types of heat exchangers in the future. Heat exchange is a cornerstone of the technology required to achieve sustainable, renewable ands cheaper energy technology