Partial load operation
The vast majority of cooler coils used in air conditioning perform under conditions of partial load for the greater part of their life. Operation under design conditions of full load is confined to a few hours per year in many cases, at least so far as the UK is concerned. In hot climates, full load conditions occur more often.
There are two ways in which the load on a cooler coil may reduce:
(i) by reduction in the enthalpy of the moist air entering the coil, and
(ii) by a reduced demand on the part of the air conditioning system, necessitating an increase in the enthalpy of the air leaving the coil.
Lines of constant wet-bulb temperature are almost parallel to lines of constant enthalpy, on the CIBSE psychrometric chart. A reduction in the wet-bulb temperature of entering air is, therefore, one way in which a partial load condition can arise. Figure 10.7(a) illustrates this case. With a fall in load a reduction must occur in the temperature rise suffered by the chilled water passing through the coil. This, in turn, must mean a drop in the value of the mean coil surface temperature. Thus, the position of A falls to A’, along the saturation curve, as the entering wet-bulb reduces. For a given cooler coil the contact factor is constant if the ratio of air flow to water flow remains constant. Equation (10.8) implies this for the air side of the coil, and equation (10.6) suggests that, because the water-side resistance depends largely on the velocity of flow of the water through the tubes, the same is true for the water side. The constancy of the contact factor is an important point which allows geometrical methods to be used on the psychrometric chart in assessing the performance of coils under partial load conditions. Since changes in the state of the air entering the coil and variations in the temperature of the coolant have virtually no effect on the contact factor, the position of W, a point representing the state of the air leaving the coil, may be easily calculated from equation (3.1), defining the contact factor:
Fig. 10.7 The psychrometry for partial load on a cooler coil.
O _ H0 h
P ~ u
It is possible that although the wet-bulb does not alter, the dry-bulb of the entering air may reduce. While this has practically no impact on the load (because the enthalpy is virtually constant), it does have an effect on the state of the air leaving the cooler coil. Figure 10.7(6) shows such a case. The comparative absence of a load change means that the mean coil surface temperature is unaltered, and so the position of A is fixed. The
position of W is again easily found, using the definition of the contact factor given by equation (3.1). It can be seen that although the temperature of W’ is less than that of W, its moisture content is higher. This is an aspect of the performance of a cooler coil that ought to be considered during selection, as well as operation with the design entering dry-bulb. The load may be unaltered but the performance may be quite unsatisfactory under such reduced dry-bulb conditions if this has not been taken account of.
As was discussed in chapter 8, a reduction in the sensible or latent heat gains in the conditioned space may require the supply of air at a state different from its design value. For example, if the latent heat gains fall off (the sensible heat gains remaining unaltered), the moisture content of the supply air must be elevated if the relative humidity in the room is to be kept constant. (This would be coupled with the supply of air at the same dry-bulb temperature, achieved by means of a reheater, as described in section 8.4.) There are three ways in which the state of air leaving a cooler coil using chilled water may be altered:
(i) by varying the rate of water flow, (ii) by changing the temperature of the chilled water flowing on to the coil and by the use of face and by-pass dampers, dealt with in section 8.6.
If the state of the air entering a coil remains fixed but the rate of flow of the chilled water is reduced, the temperature rise suffered by the water increases. The mean coil surface temperature therefore goes up and the position of A rises up the saturation curve. At first, when this happens, the value of the contact factor stays virtually unaltered, little variation in the value of taking place. So it is possible to draw a set of condition lines O-W, all having the same contact factor, p. In due course a limiting point, AL, is reached. Any further reduction in the flow rate of the water causes such an increase in the value of Rw that the value of P can no longer be regarded as constant. The states of the air leaving the coil can be joined by a broken line from W to WL, as shown in Figure 10.7(c). Above the limiting state, WL, the value of P falls away and the locus of the air-leaving state is represented in the figure by the chain-dotted line which runs from WL to O. This last part of the locus will not be parallel to a line of constant moisture content (except perhaps at its very end) since, the entering water temperature being assumed constant, some small amount of dehumidification always takes place.
A variation in the temperature of the chilled water flowing on to a coil with the flow rate of water constant, produces a condition rather similar to the one just considered. A progressive increase in the value of the temperature of the chilled water causes the mean coil surface temperature to rise and A moves up the saturation curve to a limiting position at AL, as in Figure 10.7(d). Before this point the contact factor may be determined by geometrical methods on the psychrometric chart but afterwards it may not, although the value of P remains constant. As was mentioned in section 10.6, a chilled water temperature is eventually reached which results in the coil executing sensible cooling only. Under these conditions, air leaves the coil at state Ws and the locus from this point to O is along a line of constant moisture content.
Posted in Engineering Fifth Edition