Water injection

The simplest case to consider, and the one that provides the most insight into the change of state of the airstream subjected to humidification by the injection of water, is where all the injected water is evaporated. Figure 3.8 shows what happens when total evaporation occurs.

Water injection

Fig. 3.8 A humidification process by the entire evaporation of injected spray water.

Air enters a spray chamber at state A and leaves it at state B, all the injected water being evaporated, none falling to the bottom of the chamber to run to waste or to be recirculated. The feed-water temperature is /w. It is important to realise that since total evaporation has occurred, state B must lie nearer to the saturation curve, but just how much nearer will depend on the amount of water injected.

Two equations, a heat balance and a mass balance, provide the answer required. Striking a heat balance, we can write

K "i" hw hb

And, in a similar way, the mass balance may be written as

Ma + mw — mb

Knowing the amount of feed water evaporated, the mass balance gives the necessary information about the moisture content of the airstream leaving the area of the water injection. Applying the mass balance to the water vapour only (since the associated kilogram of dry air may be ignored),

Јb = ga + mw

Where mw is the amount of feed water evaporated in kg per kg of dry air flowing through the spray chamber.

Applying the heat balance:

/la + Ay

= (1.007fb — 0.026) + gb(2501 + 1.84fb) (2.24)

One thing is immediately apparent: if feed water is injected into the airstream at a temperature of 0°C there will be no alteration in the enthalpy of the airstream, since 0°C is the temperature datum of zero enthalpy for the water associated with 1 kg of dry air. Under these circumstances the change of state between A and B will follow a line of constant enthalpy.

One further conclusion may be drawn: if the feed is at a temperature equal to the wet — bulb of the airstream, the change of state will be along a line of constant wet-bulb temperature. This is a consequence of the fact that the Lewis number of air-water vapour mixtures at normally encountered temperatures and pressures, is virtually unity—as was discussed in sections 2.17 and 2.18.

To see what happens at other water temperatures, consider water at 100°C injected into a moving moist airstream and totally evaporated.

EXAMPLE 3.7

Moist air at a state of 21°C dry-bulb, 15°C wet-bulb (sling) and 101.325 kPa barometric pressure enters a spray chamber. If, for each kilogram of dry air passing through the chamber, 0.002 kg of water at 100°C is injected and totally evaporated, calculate the moisture content, enthalpy and dry-bulb temperature of the moist air leaving the chamber.

Answer

From CIBSE tables of psychrometric data,

Ha = 41.88 kJ per kg dry air ga = 0.008 171 kg per kg dry air

Since the feed water has a temperature of 100°C, its enthalpy is 419.06 kJ per kg of water injected, from CIBSE tables of properties of water at saturation.

Use of the equation for mass balance yields the moisture content of the moist air leaving the spray chamber:

Gb = 0.008 171 + 0.002

= 0.010 171 kg per kg dry air

Use of the energy balance equation gives the enthalpy of the air leaving the chamber and hence, also, its dry-bulb temperature by equation (2.24)

Hb = 41.88 + 0.002×418.06 = 42.716 kJ per kg dry air = (1.007?b — 0.026) + 0.010 171(2501 + 1.84fb)

Thus,

42.716 = 1.007rb — 0.026 + 25.44 + 0.0187fb tb = 16.9°C

Reference back to Figure 3.8 shows a summary of what happens with different feed — water temperatures: change of state from A to Bj is along a line of constant enthalpy and is for a feed-water temperature of 0°C; change of state from A to B2 occurs when the water is injected at the wet-bulb temperature of the entry air and takes place along a line of constant wet-bulb temperature; and change of state from A to B3 (the subject of example 3.7) is for water injected at 100°C. For all cases except that of the change A to B1; an increase in enthalpy occurs which is a direct consequence of the enthalpy (and, hence, the temperature) of the injected water, provided this is all evaporated. In general, the condition line AB will lie somewhere in between the limiting lines, AB^ = 0°C) and AB3(rw = 100°C). The important thing to notice is that the angular displacement between these two condition lines is only about 7 degrees and that one of the intermediate lines is a line of constant wet-bulb temperature. It follows that for all practical purposes the change of state for a process of so-called adiabatic saturation may be assumed to follow a line of wet-bulb temperature. It is worth noting that although the process line on the psychrometric chart still lies within the 7° sector mentioned, the warmer the water the faster the evaporation rate, since temperature influences vapour pressure (see equations (4.2) and (4.3)).

Ultrasonic acoustic humidifiers are also used to atomise water. A piezo-electric crystal submerged in water converts a high frequency electronic signal into oscillations which drive particles of water from the surface into the airstream. The psychrometry is similar to that shown in Figure 3.8. As with all humidifiers that inject droplets into an airstream demineralised water must be used. Otherwise the salts of hardness and other pollutants remain in the humidified airstream as airborne solids after the droplets have evaporated and are delivered to the conditioned space.

The air washer is not used today for humidifying airstreams supplied to occupied spaces. It is not hygienic (see Pickering and Jones (1986)). It is also expensive in capital and maintenance costs, occupies a lot of space and is thermally inefficient when compared with a cooler coil. These objections are more than enough to offset the advantages it offers of giving thermal inertia to the system (with improved stability for dew-point control of the airstream leaving the washer) and simple ‘free’ cooling in winter.

Posted in Air Conditioning Engineering