Mixing and adiabatic saturation with reheat
Figure 3.13(a) shows a plant arrangement including an air washer which, although undesirable for comfort air conditioning, is retained here to illustrate the psychrometry involved. Air at state R is extracted from a conditioned room and partly recirculated, the remainder being discharged to atmosphere. The portion of the extracted air returned to the air conditioning plant mixes with air at state O, drawn from outside, and forms a mixture state M. The air then passes through an air washer, the spray water of which is only recirculated and adiabatic saturation occurs, the state of the air changing from M to W (see Figure 3.13(b)) along a line of constant wet-bulb temperature (see sections 3.5 and 3.6). An extension of the line MW cuts the saturation curve at a point A, the apparatus dew point. To deal with a particular latent heat gain in the conditioned room it is necessary to supply the air to the room at a moisture content gs, it being arranged that the difference of moisture content gt — gs, in conjunction with the mass of air delivered to the room, will offset the latent gain. In other words, the air supplied must be dry enough to absorb the moisture liberated in the room.
It is evident that the moisture content of the air leaving the washer must have a value gw, equal to the required value, gs. This is amenable to calculation by making use of the definition of the effectiveness of an air washer, in terms of ga, gw and gm (see section 3.5).
EXAMPLE 3.14
If the room mentioned in example 3.13 is conditioned by means of a plant using a mixture of recirculated and fresh air, of the type illustrated in Figure 3.13(a), calculate:
(a) the percentage of the air supplied to the room by mass which is recirculated, and
(b) the humidifying efficiency of the air washer.
Answer
(a) Since the wet-bulb scale is not linear, it is not accurate enough to calculate the mixing proportions on this basis. Instead, one must make use of changes of enthalpy or moisture content. Bearing in mind that lines of constant enthalpy are not parallel to lines of constant wet-bulb temperature, some slight inaccuracy is still present if the assumption is made that the change of state accompanying a process of adiabatic saturation is along a line of constant enthalpy. However, this is unavoidable, and so such an assumption is made.
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Referring to Figure 3.13(b) it can be seen that ha — hw — hm
From tables and Figure 3.12(b) it is established that hw (at 12°C dry-bulb and 6.831 g kg’1) is 29.30 kJ kg“1.
From the principles set out in section 3.2, governing the change of state associated with a mixing process, it is clear that the percentage of recirculated air, by mass,
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X 100 |
^ m hp
Ht — K
_ 29.30-0.298 inf)
39.14 — 0.298
= 75 per cent
Thus, 75 per cent of the air supplied to the room, if recirculated and mixed with 25 per cent of air from outside, will have an enthalpy of 29.30 kJ kg-1 and a wet-bulb of 10°C (sling). If adiabatic saturation is then to produce a state of 12°C dry-bulb and 6.831 g kg-1, the humidifying efficiency of the washer used can no longer be the value used for example 3.13, namely, 85 per cent. An entirely different washer must be used for the above calculations to be valid and this must have an effectiveness which may be calculated as follows:
(.b) Since efficiency is expressed in terms of moisture content, it is necessary to determine the value of gm, the values of g. d and gw being already known.
8m = 0-75 gr + 0.25 g0
= 0.75 x 7.497 + 0.25 x2.137 = 6.157 g kg“1
Humidifying efficiency = x 100
Јa ~ Sm
6.831 — 6.157 w lf)ft " 7.659 — 6.157 X iUU = 45 per cent
In practical terms, this is a low efficiency.
If the washer used in this example had an efficiency of 85 per cent, as in example 3.13, then the calculations would not have been so easy. The line AWM would have had to have been at a lower wet-bulb value in order to fulfil two requirements:
(i) gw = 6.831 g kg-1
(ii) x 100 = 85 per cent
Ga — gm
For this to be the case, the dry-bulb temperature of W must obviously be less than 10°C. The easiest way to achieve a practical solution is by drawing a succession of lines representing processes of adiabatic saturation on a psychrometric chart and calculating several values of efficiency until one of acceptable accuracy is obtained.
Posted in Air Conditioning Engineering