Humidification

This, as its name implies, means that the moisture content of the air is increased. This may be accomplished by either water or steam but the present section is devoted to the use of water only, section 3.7 being used for steam injection into moving airstreams.

There are three methods of using water as a humidifying agent: the passage of moist air through a spray chamber containing a very large number of small water droplets; its passage over a large wetted surface; or the direct injection of water drops of aerosol size into the room being conditioned. (A variant of this last technique is to inject aerosol-sized droplets into an airstream moving through a duct.) Whichever method is used the psychrometric considerations are similar.

It is customary to speak of the humidifying efficiency or the effectiveness of an air washer (although neither term is universally accepted) rather than a contact or by-pass factor. There are several definitions, some based on the extent to which the dry-bulb temperature of the entering moist airstream approaches its initial wet-bulb value, and others based on the change of state undergone by the air. In view of the fact that the psychrometric chart currently in use by the Chartered Institution of Building Services Engineers is constructed with mass (moisture content) and energy (enthalpy) as oblique, linear co-ordinates, the most suitable definition to use with the chart is that couched in terms of these fundamentals. There is the further advantage that such a definition of effectiveness, E, is identical with the definition of contact factor, p, used for cooler coils.

Although humidifying efficiency is often expressed in terms of a process of adiabatic saturation, this is really a special case, and is so regarded here.

Figure 3.6(a) shows an illustration of the change of state experienced by an airstream as it passes through a spray chamber.

The effectiveness of the spray chamber is then defined by

And humidifying efficiency is defined by

R| = 100E (3.6)

It is evident that, because moisture content is the other linear co-ordinate of the psychrometric chart and the points A, B and C lie on the same straight line, C being obtained by the extension of the line joining AB to cut the saturation curve, then it is possible to put forward an alternative and equally valid definition of effectiveness, expressed in terms of moisture content:

9c 9t

9a 9e 9d

(a)

9c 9b

(b)

Fig. 3.6 (a) The change of state when air flows through a spray chamber having spray water at a controlled temperature, (b) Adiabatic saturation when the spray water is entirely recirculated and Neither cooled nor heated.

R| = 100Ј, as before

Figure 3.6(a) shows that hh is greater than ha. This implies that there is a heat input to the spray water being circulated through the spray chamber. Although it is not recommended

It could be easily accomplished by means of, say, a calorifier in the return pipe from the washer tank. If the spray water had been chilled, instead of heated, then a change of state might have been as shown dotted on Figure 3.6(a), from the point A to the point B’. Under these circumstances, the effectiveness would have been expressed by

_ 8a ~ 8b’ 8a ~ 8c’

Or

Consider the special case of adiabatic saturation: for this to occur it is necessary that

(i) the spray water is totally recirculated, no heat exchanger being present in the pipelines or in the washer tank;

(ii) the spray chamber, tank and pipelines are perfectly lagged; and

(iii) the feed water supplied to the system to make good the losses due to evaporation is at the temperature of adiabatic saturation (see section 2.18).

Under these conditions it may be assumed that the change of state follows a line of constant wet-bulb temperature (since the Lewis number for air-water vapour mixtures is unity), the system having been given sufficient time to settle down to steady-state operation. There must be a change of enthalpy during the process because feed water is being supplied at the wet-bulb temperature (virtually) and this will not, as a general rule, equal the datum temperature for the enthalpy of water, 0°C. Strictly speaking then, it is incorrect to speak of the process as being an adiabatic one. The use of the term stems from the phrase ‘temperature of adiabatic saturation’ and so its use is condoned. One thing is fairly certain though: at the temperatures normally encountered the change of enthalpy during the process is neligible, and so effectiveness must be expressed in terms of change of moisture content. Figure 3.6(b) shows a case of adiabatic saturation, in which it can be seen that there is no significant alteration in enthalpy although there is a clear change in moisture content. It can also be seen that a fall in temperature accompanies the rise in moisture content. This temperature change provides an approximate definition of effectiveness or efficiency which is most useful, and, for the majority of practical applications, sufficiently accurate. It is

The way in which the psychrometric chart is constructed precludes the possibility of this being an accurate expression; lines of constant dry-bulb temperature are not parallel and equally spaced, such properties being exclusive to enthalpy and moisture content.

EXAMPLE 3.6

1.5 m3 s_1 of moist air at a state of 15°C dry-bulb, 10°C wet-bulb (sling) and 101.325 kPa barometric pressure, enters the spray chamber of an air washer. The humidifying efficiency of the washer is 90 per cent, all the spray water is recirculated, the spray chamber and the tank are perfectly lagged, and mains water at 10°C is supplied to make good the losses due to evaporation.

Calculate (a) the state of the air leaving the washer, (b) the rate of flow of make-up water from the mains.

Answer

(a) This is illustrated in Figure 3.7.

Using the definitions of humidifying efficiency quoted in equations (3.6) and (3.7), and referring to tables of psychrometric data for the properties of moist air at states A and C, one can write

90 gb ~ 5.558 100 7.659 — 5.558

Hence,

Gb = 7.449 g per kg of dry air

The state of the moist air leaving the air washer is 10°C wet-bulb (sling), 7.449 g per kg and 101.325 kPa barometric pressure. The use of equation (3.8) shows that the approximate dry-bulb temperature at exit from the washer is 10.5°C.

(b) The amount of water supplied to the washer must equal the amount evaporated. From tables (or less accurately from a psychrometric chart), the humid volume at state A is

0. 8232 m3 kg-1. Each kilogram of dry air passing through the spray chamber has its associated water vapour augmented by an amount equal to gb — ga, that is, by 1.891 g.

Fig. 3.7 Adiabatic saturation along a wet-bulb line.

Thus, the rate of make-up is

1.5 x 0.001891 x 3600 “ 0.8232

= 12.40 kg of water per hour

It is to be noted that this is not equal to the pump duty; the amount of water the pump must circulate depends on the humidifying efficiency and, to achieve a reasonable value for this, the spray nozzles used to atomise the recirculated water must break up the water into very small drops, so that there will exist a good opportunity for an intimate and effective contact between the airstream and the water. A big pressure drop across the nozzles results from the atomisation, if this is to be adequate. Spray water pumps used with this type of washer have to develop pressures of the order of 2 bar, as a result.

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