Sponsored by COOLERS
24
Cooling below the wet-bulb
Can indirect evaporative coolers cool below the wet bulb? Sam Peli, general manager sales Europe, Middle East and Africa for Seeley answers.
N
ormally, indirect evaporative coolers aim to solve the problem of increased humidity by separating the air that is evaporatively cooled – called ‘the working air’ in this case – from the air supplied to the space, using a heat exchanger. Supply air is not directly cooled by the evaporative process, and so these are called indirect evaporative coolers. As with a direct evaporative cooler, supply air is cooled towards the wet bulb temperature of the outside air, but because neither the cooler nor the heat exchanger are 100% effi cient, indirect coolers cannot reach the wet bulb temperature, and in fact will deliver higher temperatures than direct coolers will. There is a specifi c category of indirect evaporative coolers with an integrated counter-fl ow heat exchanger that can actually overcome the limit of the wet bulb, and that are theoretically limited by the dew point.
An indirect evaporative cooler of this type – sometimes called ‘sub-wet bulb cooler’ – reduces the temperature of the air through a heat exchange surface, which is impervious to water. On one side of the heat exchange surface, the evaporation of water produces cool temperatures in water and air, allowing heat exchange to the incoming air on the other side of the heat exchange surface.
The primary air in the dry channel, is therefore cooled
without changing its moisture content. It should be noted that the cooling of air not only reduces the dry bulb temperature, but also its wet bulb temperature. In the confi guration used to construct the sub-wet bulb cooler, a proportion of the air which has been heat exchanged to a lower temperature is returned along the wet channel (Fig.1).
Since the air returned has a depressed wet bulb temperature, evaporation on the wet surfaces of the wet channel will produce temperatures approaching the now lower wet bulb temperature. This signifi cantly lower temperature then further intensifi es the heat exchange to the dry channel, further reducing the wet bulb temperature of the proportion returned to the wet channel. This process continues throughout the heat exchanger matrix continually intensifying the heat exchange and evaporation processes until the exit temperatures start to approach the dew point of the incoming air (Fig.2). Therefore, with sub-wet bulb evaporative coolers, instead of being limited by the wet bulb temperature, we are now limited by the dew point of the working air.
So, to answer our primary question, whether indirect evaporative coolers can cool below the wet bulb temperature, we can say: yes, they can!
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120
