Sprayed cooler coils
Although they have fallen out of use for hygienic reasons, as Pickering and Jones (1986) explain, and would not be used today in commercial applications, sprayed cooler coils are worth considering as an academic illustration of a psychrometric process.
A sprayed cooler coil is a coil, fed with chilled water or liquid refrigerant in the usual way, positioned over a recirculation tank, with a bank of stand pipes and spray nozzles located a short distance from its upstream face. Mains water is fed to the tank through a ball-valve to make good any evaporative losses that may occur. Water is drawn from the tank by a pump and is delivered through the nozzles on to the face of the coil. This water then falls down the fins into the tank and is largely recirculated. Figure 10.8 illustrates this.
Chilled water out
Chilled water in
Fig. 10.8 A simple diagram of the plant arrangement for a sprayed cooler coil.
The prime function of the sprayed cooler coil is to provide humidification for operation of the air conditioning plant in winter. A cooler coil offers a very large surface area to the airstream passing over it, and it is by wetting this very large area that the spray nozzles achieve humidification. The nozzles, therefore, should not have a very large pressure drop across them, since this is associated with an atomisation effect, but should be placed close enough to the face of the coil to ensure that all the water delivered by them goes on to the coil. Only one bank of nozzles is necessary and, since this faces downstream, the humidifying efficiency associated with an atomised spray is small: of the order of 50 per cent. On the other hand, if atomisation is not relied on, but the surface of the coil is properly wetted instead, a better humidifying efficiency can be obtained. Only tests carried out by the manufacturer in the works or by the user on site, can provide reasonably accurate data as to the efficiencies likely. But, the contact factor of the coil itself provides some clue. If the coil were completely wetted throughout its depth, then it would perhaps be reasonable to take the humidifying efficiency as approaching the value of the contact factor. Since the coil is not completely wetted, the actual value will be less than this; a practical value is 80 per cent.
An effective distribution of water is about 0.82 litres per second on each square metre of face area of the coil. The pressure drop across the nozzles should be about 42 kPa. If more pressure than this is lost little is gained in effectiveness, but a penalty is paid in extra running costs for the pump.
When chilled water flows through the inside of the coil and spray water is circulated over the outside of the coil, accurate control of dew point is possible for a wide range of entering air conditions. Figures 10.9 and 10.10 illustrate how this may be achieved. If the coil is not sprayed, and is chosen to cool and dehumidify air from state 1 to 2 under design conditions in summer, it will produce a state 2′ if the entering air state is Y, during autumnal or spring weather. This has a moisture content which is too low if control over dew point is necessary. If a sprayed coil is used, however, summer operation will, for all practical purposes, be the same as before but when the entering air state is Y it is now possible to produce a leaving state 2. For purposes of illustration the process can be regarded as one of adiabatic humidification from 1′ to 1", followed by cooling and dehumidification to 2. This is shown in Figure 10.10. So, as with an air washer, the sprayed cooler coil, when properly controlled, can execute a process of cooling and humidification, as typified by the change Y to 2.
Fig. 10.9 Using a sprayed coil it is possible to achieve close control over the state of the air leaving the coil. Thus state 2 can be obtained for a variety of different entering states, such as 1 and V. If the coil were unsprayed the leaving state would be 2′ when the entry state was V. Sprayed cooler coils are rarely used, for hygienic reasons.
Fig. 10.10 One way of regarding the process of change across a sprayed cooler coil is to consider adiabatic saturation (state Y to state 1") followed by cooling (state 1" to state 2).
There is one further advantage that the sprayed coil shares with the air washer. The presence of the large mass of water in the tank, at a temperature which is virtually the same as the leaving air temperature, provides a thermal reservoir which gives inertia to the control of the cooler coil. This is of value if a constant air leaving state is required, particularly in direct-expansion coils, which are sprayed. Inadequacies in the control over the coil output or fluctuations in the state of the air leaving the cooler coil are smoothed out. Stability is added.
During winter, of course, the wetted surface acts as an adiabatic humidifier, just as does an air washer. Control may be effected by means of a pre-heater or variable mixing dampers for fresh and recirculated air, as was discussed in sections 3.9 and 3.10. Under such circumstances the refrigeration plant would not be working, although, as is remarked in the next section, it may be desirable to run the pump used to circulate chilled water from the chiller to the coil.
Posted in Engineering Fifth Edition