Distinction between cooler coils and air washers
The evaporative cooling of air by adiabatic saturation (see sections 3.5 and 3.6) may be improved by lowering the temperature of the feed water, but little improvement results if all the feed water is evaporated. The picture changes, however, if the rate of injection of feed water is increased so that not all of it is evaporated, but some falls into a sump and is recirculated. When the rate of water flow is speeded up its temperature has an increasingly important influence on the heat exchange with the airstream. For example, if an infinitely large quantity of water were circulated at a temperature fw, then the air would leave the spray chamber also at a temperature fw, in a saturated condition.
In air washers used for cooling and dehumidification processes (see section 3.4), the quantities of water circulated are very large compared with the quantity that could be totally evaporated. The temperature of the water thus plays a major part in determining the state of the air leaving the washer. Figure 10.1 illustrates the psychrometric considerations.
Direct cooling of this sort is not necessarily the most effective method. True contra-flow heat exchange, or even a reasonable approximation to it, cannot be easily obtained in an air washer. On the other hand, when chilled water is circulated through a cooler coil of more than three rows a good measure of contra-flow is realised. This permits a closer approach between the leaving air temperature and the entering-water temperature with cooler coils using chilled water or brine.
In comparison with the cooler coil the washer suffers from a number of other disadvantages:
(i) It is more bulky.
(ii) Corrosion is a greater risk.
(iii) Maintenance is more expensive.
(iv) It uses an open chilled water circuit, thus causing the deposition of scale, rust, slime, etc. in the water chiller (evaporator), reducing its heat transfer efficiency and increasing the cost of the refrigeration plant.
(v) Micro-organisms tend to establish colonies in the recirculated water tank. These may constitute a health risk if carried by the airstream into the conditioned space. Pickering and Jones (1986) discuss how an allergic response (humidifier fever) may be induced in some susceptible people who develop symptoms similar to those of mild influenza at the beginning of the working week. Immunity is acquired by the middle of the week but is lost over the weekend. The cycle can repeat.
Humidification by the injection of dry steam (see section 3.7) in the right place in a system is much better than the use of an air washer. It is sterile if not wet and perfectly safe.
Fig. 10.1 Possibilities for cooling and humidification.
There is one point strongly in favour of air washers and sprayed cooler coils: the presence of the large mass of water in the sump tank and spray chamber gives a thermal inertia to the system, smoothing out fluctuations in the state of the air leaving the coil or washer, and adding stability to the operation of the automatic controls.
However, in spite of these favourable aspects, the air washer is very much out of fashion because the cooler coil is more efficient, and because of the five points mentioned.
Posted in Engineering Fifth Edition