Methods of rejecting heat
All heat gains dealt with by an airconditioning system must be rejected at the condenser. To accomplish this, the practical condenser for air conditioning must be water-cooled, evaporative-cooled or air-cooled.
Air-cooled, dry heat exchangers have also been used. These are capable of producing cooling water at a temperature within 11 K of the ambient dry-bulb. Thus cooling water might be produced at 40°C if the ambient air was at 29°C dry-bulb. Such temperatures are too high to be of practical value in air conditioning. The heat exchangers are also bulky and expensive in both capital and running costs.
Except in the comparatively rare cases where a supply of lake or river water may be drawn on, or the even rarer cases where mains water is available, the water used by a water-cooled condenser must be continuously recirculated through a cooling tower. Figure
11.1 Illustrates what occurs. Water is pumped through the condenser and suffers a temperature rise of (fwl — ?w2) as it removes the heat rejected by the refrigerant during the process of condensation. The cooling water then flows to the top of a cooling tower (an induced — draught type is illustrated) whence, in falling to a catchment tank at the bottom of the tower, it encounters airflow. Contra-flow heat exchange occurs, the water cooling by evaporation from fwl to fw2 and the air becoming humid in the process. The heat lost by the water constitutes a gain of enthalpy to the air. The water is then ready for recirculation to the condenser at the desired temperature. Some water is necessary from the mains, to make good the evaporative losses from the system but, since 1 kg of water liberates about 2400 kJ during a process of evaporative cooling (whereas 1 kg rising through 5 degrees absorbs only 21 kJ), the amount of make up required is quite small, being of the order of 1 per cent of the circulation rate in this respect. There is some loss also due to the carryover of droplets by the emergent airstream but, even allowing for this, the loss is still little more than 1 per cent. This loss is termed drift and it must be reduced to virtually zero, by the provision of drift eliminators, to minimise the risk of contaminating the local surroundings. Drift should not be confused with pluming: this consists of steam which has condensed to water droplets and appears as a cloud coming out of some cooling towers under certain conditions of operation (see section 3.12).
Evaporative condensers operate in a similar fashion but more directly. In Figure 11.2 it can be seen that the pipe coils of the condenser are directly in the path of the air and
Fig. 11.1 Induced draught cooling tower. Control is shown by Cl and R1 over the condensing pressure associated with a reciprocating machine. Immersion thermostat C2 cycles the fan. An alternative arrangement for centrifugal machines is shown by a chain dotted line. Immersion thermostat C3 controls the flow water temperature on to the condenser by means of a motorised butterfly valve R3.
Water streams and that evaporative cooling takes place directly on the outer surface of the tubes.
With air-cooled condensers, no water is used at all. The refrigerant condenser consists of a coil of finned tube, and over this air is drawn or blown by means of a propeller, axial flow or centrifugal fan. As with cooling towers and evaporative condensers, several fans may be used.
The evaporative condenser is probably the most efficient way of rejecting heat to the atmosphere, if it is properly designed. This is reflected in the typical air quantities required:
30-60 litres s“1 kW“1 for evaporative condensers,
40-80 litres s“1 kW-1 for cooling towers and 140-200 litres s-1 kW-1 for air-cooled condensers.
ASHRAE (1996) suggest that 80-160 litres s"1 kW“1 are likely with face velocities of 2-4 ms“1.
Fig. 11.2 Evaporative condenser. Cl is a pressure controller which keeps the refrigerant condensing at a constant pressure through the agency of the motorised modulating dampers Rl.
Posted in Engineering Fifth Edition