Book - School of Science and Technology

Air-to-air heat exchangers 5097060EvaporationVapourCondensationEfficiency, %50403020Heat inHeat out100–10Down0 +10Up+20Evaporator tilt, mmFigure 17.33 Heat pipe orientation and performance (in counterflow)with an internal wickof woven glass fibre normally as a concentric lining to the tube.During manufacture, a working fluid is introduced in sufficient quantity to saturate thewick. The actual fluid used is selected to suit the temperature range required and wouldtypically be one of the common refrigerants.In operation, heat applied to one end of the pipe will cause the liquid to evaporate andthe resultant vapour will travel to the `cool' end where it will condense, surrenderingenergy, and the liquid will return through the wickby capillary action to the `hot' end.Figure 17.33 illustrates this process and shows also how the heat transfer capacity of apipe may be adjusted by varying the angle to the horizontal at which it lies. Thischaracteristic may be used to match capacity to a given application or, by automation,to provide a means of control. Where the angle of tilt is used in this way, a facility must beprovided to reverse the action when the season changes, winter to summer.An alternative arrangement is where the pipes are installed vertically to transfer heatfrom a warm lower duct to a cool duct above. With this configuration, movement of theheat transfer fluid is by phase change; liquid in the lower section absorbs heat and changesto a gas, which condenses, releasing heat in the upper section, causing the liquid to drop tothe lower end. Vertical units will not function where the cool duct is below the warmer one.The capacity of a built-up heat exchanger of given overall dimensions will varyaccording to the number of rows of heat pipes, the fin spacing and the air velocity.Typically, a six row unit having 55 fins per 100 mm would, for equal supply and exhaustair quantities, have an efficiency for sensible heat exchange of up to 80% at a face velocityof 3 m/s and with a resistance to air flow of 200 Pa. Module sizes range from 150 to 36 000litre/s. Efficiency of heat exchange is dependent upon the relative direction of air flow inthe two ducts. Counterflow gives the higher performance, which is the basis for mostpublished data; parallel air flow will reduce efficiency by about one-fifth of the quotedpercentage. Subject to the effectiveness of the division plate and seals between the two airstreams there should be no cross-contamination between supply and extract air. Thismethod of heat exchange is seldom used due to the relatively high cost.Run-around coils (water circulation)This approach to the problem has the merit of extreme flexibility and is, moreover,founded upon a well understood technology. As shown in Figure 17.34, the basis of the