Book - School of Science and Technology

Air-to-air heat exchangers 505t 3 is the temperature of the supply air after it has been passed through the heat exchangerand is the efficiency (%). As will be noted, the expression makes use of dry bulbtemperatures and thus, strictly speaking, relates only to sensible heat recovery.Consider the case of a heat exchanger unit, having a quoted efficiency of 76%, which isapplied to a winter reclaim situation handling equal quantities of outside air at 1 C,saturated, and exhaust air at 22 C DB, 15.5 C WB.Thus,t 3 ˆ [0:76(22 ‡ 1)]1 ˆ 16:5 CFor use in summer, when the stated efficiency might have fallen to, say, 66%* takingthe same exhaust condition and the outside air supply at 28 C DB, 20 C WB, then:t 3 ˆ 28 [0:66(28 22)] ˆ 24 CIf these data are plotted on a psychrometric chart as in Figure 17.30(a), the varioussignificant properties may be read as summarised in Table 17.4.From the expression for efficiency given above and replacing temperatures by therespective enthalpy values the enthalpy efficiency may be established. Using the sameexample the expression may be rewritten in terms of enthalpy thus:Winter100 S 1 O 1 25:3 7:7ˆ 100 ˆ 49:2%R O 1 43:4 7:7Summer100 O 2 S 2 57:0 52:9ˆ 100 ˆ 30:0%O 2 R 57:0 43:4Table 17.4 Physical quantities related to state points indicated in Figure 17.30PointPosition from Figure 17.30StatementDBtemp.( C)Enthalpy(kJ/kg)Per kg dry airMoisture(kg/kg)O 1 Winter, outside 1 7.7 0.0035O 2 Summer, outside 28 57.0 0.0113R Room exhaust 22 43.4 0.0084S 1 Unit output, winter 16.5 25.3 0.0035S 2 Unit output, summer 24 52.9 0.0113S 3 Unit output, winter 16.5 34.8 0.0072S 4 Unit output, summer 23.5 46.7 0.0090* In winter, as may be seen from Figure 17.30(a), the exhaust side of the heat exchange surface will bewetted for part of the process. In the case of the hygroscopic thermal wheel, however, under the samewinter conditions as illustrated, there will be an increase in the moisture content of the incoming airstream, as indicated by S 3 in Figure 17.30(b).