Systems are normally designed to work satisfactorily during maximum ambi­ent conditions, and the condenser will be sized for this. In colder weather, the condensing temperature and pressure will fall and the resulting lower pressure difference across a thermostatic expansion valve may lead to malfunction. A drop of pressure difference to half the normal figure may reduce mass flow below that required, and it will be necessary to prevent the condenser pressure from falling too low.

With air-cooled condensers and water cooling towers it is possible to reduce the air flow by automatic dampers, fan speed control, or switching off fans, where two or more are fitted. The control should work from pressure but can be made to work from temperature.

Water-cooled condensers can be fitted with a directly controlled water- regulating valve operated by condenser pressure, or may have a three-way blending valve in the water circuit.

A condenser pressure regulator can be in the form of a pressure-operated bleed valve in a bypass across the condenser, to divert hot gas to the receiver. The valve diaphragm is balanced by a pre-set spring and will open the bypass if the condensing pressure falls. A similar effect can be obtained by a pressure — operated valve between the condenser and the receiver, to restrict the flow and allow liquid to accumulate in the condenser, reducing its efficiency. For operat­ing economy, it is important that such valves are not set at too high a pressure.

Where evaporative condensers and water cooling towers have only one fan (or fan drive motor), coarse control can be effected by on-off switching. The time lag will then depend on the mass of water in the circuit, and the sensing element needs to have a wide differential to prevent frequent motor starts.

Towers should have thermostatic control of the fan to prevent water freezing on the packing in winter.

An integrated control circuit with an electronic expansion valve can be arranged to permit the condensing pressure to fall, providing the valve can pass the refriger­ant flow required to meet the load. This gives lower compressor energy costs.

In all forms of condenser pressure control, the minimum maintained pres­sure should be the lowest which will give satisfactory operation, in the interests of running economy. An indication of the relative electricity costs for a 350 kW air-conditioning plant is given in Table 9.1.

Table 9.1

Condensing temperature (°C)

Coefficient of performance

Weekly electricity costs (Ј, @ 10 p/unit)

35 (summer maximum)






25 (probable minimum)



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