There are four principal methods whereby moist air can be dehumidified:
(i) cooling to a temperature below the dew point,
3.4 Dehumidification 45
(iv) compression followed by cooling.
The first method forms the subject matter of this section.
Cooling to a temperature below the dew point is done by passing the moist air over a cooler coil or through an air washer provided with chilled water.
Figure 3.4 shows on a sketch of a psychrometric chart what happens when moist air is cooled and dehumidified in this fashion. Since dehumidification is the aim, some of the spray water or some part of the cooler coil must be at a temperature less than the dew point of the air entering the equipment. In the figure, td is the dew point of the moist air ‘on’ the coil or washer. The temperature tc, corresponding to the point C on the saturation curve, is termed the apparatus dew point. This term is in use for both coils and washers but, in the case of cooler coils alone, tc is also sometimes termed the mean coil surface temperature. The justification for this latter terminology is offered in chapter 10.
For purposes of carrying out air conditioning calculations, it is sufficient to know the
Tc tb td ta
Fig. 3.4 Cooling and dehumidification by a cooler coil or an air washer.
State A of the moist air entering the coil, the state B of the air leaving the coil, and the mass flow of the associated dry air. What happens to the state of the air as it passes between points A and B is seldom of more than academic interest. Consequently, it is quite usual to show the change of state between the ‘on’ and the ‘off’ conditions as occurring along a straight line. In fact, as will be seen in chapter 10, the change of state follows a curved path, the curvature of which is a consequence of the heat transfer characteristics of the process, not of the construction of the psychrometric chart.
It can be seen from Figure 3.4 that the moisture content of the air is reduced, as also is its enthalpy and dry-bulb temperature. The percentage saturation, of course, increases. It might be thought that the increase of humidity would be such that the ‘off’ state, represented by the point B, would lie on the saturation curve. This is not so for the very good reason that no air washer or cooler coil is a hundred per cent efficient. It is unusual to speak of the efficiency of a cooler coil. Instead, the alternative terms, contact factor and by-pass factor, are used. They are complementary values and contact factor, sometimes denoted by (3, is defined as
8a 8 b
K — hc
Similarly, by-pass factor is defined as
8b ~ 8c
8a 8 c
Hb — h,
It is sufficient, for all practical purposes, to assume that both these expressions can be rewritten in terms of dry-bulb temperature, namely that
O _ tq ~ h P ta — tc
(1 — P) = 7^ (3.4)
It is much less true to assume that they can also be written, without sensible error, in terms of wet-bulb temperature, since the scale of wet-bulb values on the psychrometric chart is not at all linear. The assumption is, however, sometimes made for convenience, provided the values involved are not very far apart and that some inaccuracy can be tolerated in the answer.
Typical values of p are 0.82 to 0.92 for practical coil selection in the UK. In hot, humid climates more heat transfer surface is necessary and higher contact factors are common.
1.5 m3 s“1 of moist air at a state of 28°C dry-bulb, 20.6°C wet-bulb (sling) and 101.325 kPa flows across a cooler coil and leaves the coil at 12.5°C dry-bulb and 8.336 g per kg of dry air.
Determine (a) the apparatus dew point, (b) the contact factor and (c) the cooling load.
Figure 3.5 shows the psychrometric changes involved and the values immediately known from the data in the question. From CIBSE tables (or from a psychrometric chart) it is
Established that h3 = 59.06 kJ kg and that hb = 33.61 kJ kg. The tables also give a value for the humid volume at the entry state to the coil, of 0.8693 m3 kg-1.
Ga =12.10 g/kg
Gb = 8.336 g/kg
Fig. 3.5 The psychrometry for Example 3.5.
(a) Mark state points A and B on a psychrometric chart. Join them by a straight line and extend the line to cut the saturation curve. Observation of the point of intersection shows that this is at a temperature of 10.25°C. It is not easy to decide this value with any exactness unless a large psychrometric chart is used. However, it is sufficiently accurate for present purposes (and for most practical purposes) to take the value read from an ordinary chart. Hence the apparatus dew point is 10.25°C.
(b) Either from the chart or from tables, it can be established that the enthalpy at the apparatus dew point is 29.94 kJ kg-1. Using the definition of contact factor,
59.6 — 29.94 One might also determine this from the temperatures:
28° — 12.5°
28° — 10.25°
Clearly, in view of the error in reading the apparatus dew point from the chart, the value of P obtained from the temperatures is quite accurate enough in this example and, in fact, in most other cases.
(c) Cooling load = mass flow rate x decrease of enthalpy
= n oiL x (59.06 — 33.61) = 43.9 kW U. oo93
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