Book - School of Science and Technology

508 Fans and air treatment equipmentOutside airTo plantTo exhaustReturn airMotor and driveFigure 17.32 Thermal wheel type heat exchangerThe media and the form of the heat transfer surface vary as between manufacturers anddetermine the characteristics of the energy transfer. Sensible heat transfer is obtainedfrom media formed by alternate flat and corrugated metal sheets of aluminium orstainless steel. As for plate exchangers, a protective coating may be applied for use inswimming pool applications and the like. To achieve both latent and sensible heat transfera corrugated inorganic hygroscopic material may be used; typically these would beproduced by an etching process or a lightweight coating of a hygroscopic salt.Cross-contamination between the two air streams is minimised by so arranging therespective fan positions that the supply air pressure at the recuperator is greater than thatof the exhaust stream. By using suitable labyrinth seals and incorporating a purge sectorwhich allows for the matrix to be scavanged before supply air passes to the building, it isclaimed that contamination is kept to less than 0.1%. Lithium bromide as used fortreatment of the hygroscopic type matrix is stated to be bacteriostatic, i.e. it inhibits thepropagation of bacteria. It should be noted, however, that the hygroscopic material mayabsorb toxic gases, or similar vapours, from the exhaust air which would not be completelyremoved by purging and hence present a riskof contaminating the incoming air.Wheels are available in sizes up to about 5.5 m diameter to handle air quantities in therange 300±30 000 litre/s but multiple units in the middle of the size range are often moreconvenient to handle large air quantities. Efficiency may be as high as 85% in sensibleheat reclaim and up to 88% is claimed by some manufacturers for transfer of total heat inhygroscopic types; however, efficiencies higher than 85% should be viewed with caution.Resistance to air flow at a face velocity of 3 m/s will be about 150 Pa. The power requiredto rotate the wheel is quite small being between 60 and 1100 W.`Heat pipe' heat exchangerAs in the case of the plate type, heat pipe units have no moving parts and are simple inconcept. A working fluid is however employed to effect heat transfer. Constructionconsists of a box enclosure having a dividing partition to separate the supply and exhaustair streams, through which an array of finned heat pipes is assembled.The `heat pipe' itself is a by-product of nuclear research developed further in connectionwith the space programme: in essence it is no more than a super-conductor of sensibleheat. Each individual conductor is a sealed tube, pressure and vacuum tight, provided