Air-cooled condensers

Unlike evaporative condensers, air-cooled condensers have a capacity which is related to the dry-bulb temperature of the ambient air, rather than to its wet-bulb temperature. If working condenser pressures are not to become excessively high, making the plant expensive to run, large condenser surface areas must be used. This has set a limit on the practical upper size of air-cooled condensers. Their use in air conditioning has been commonly confined to plants having a capacity of less than 70 kW of refrigeration, although they have been used for duties as high as 2000 kW, in temperate climates.

The hot gas discharged from the compressor is desuperheated over approximately the first 5 per cent of the heat transfer surface, followed by condensation over the succeeding 85 per cent, with a small drop in the condensing temperature, related to the frictional pressure loss. A certain amount of sub-cooling of the liquid can then occur. Additional heat transfer surface may be provided to assist the sub-cooling, achieving an increase of about

0. 9 per cent in the cooling capacity for each degree of liquid temperature drop, according to ASHRAE (1996).

Propeller fans, direct-coupled to split-capacitor driving motors, are most commonly used to promote airflow, although axial flow or centrifugal fans are also sometimes adopted. Fan powers are about 20 to 40 W for each kW of refrigeration capacity. Noise is often a problem and this is made worse if there are obstructions in the inlet or outlet airflow paths. Although vertical fan arrangements are possible, with horizontal cooling airflow paths, these are susceptible to wind pressures and it is recommended that the condenser coils should be horizontal with vertical cooling airflow paths. Propeller fans will not deliver airflow against any singificant external resistance. It follows that ducting connections are then not possible if these fans are used.

A 20-degree difference between the entering dry-bulb temperature and the condensing temperature is often consistent with the avoidance of excessively large condenser surface areas. Air-cooled condensers are increasing in popularity because of the absence of water piping, the consequent simplicity of operation and the freedom from any health risk associated with the use of spray water.

One objection to their use is that the capacity of the refrigeration plant does not gradually reduce as the ambient dry-bulb rises but ceases suddenly when the high pressure cut-out operates. A partial solution is to arrange for some of the compressor to be unloaded when the condensing pressure rises, before it reaches the cut-out point. Continued operation at a reduced capacity is then possible beyond the design ambient dry-bulb. It is a good plan to select air-cooled condensers to operate in an ambient temperature two or three degrees higher than the design value chosen for the rest of the air conditioning system.

EXAMPLE 11.2

Assuming an airflow rate of 0.15 m3 s-1 kW"1, and a difference of 20 degrees between the ambient dry-bulb and the condensing temperature of R 134a, determine (a) the condensing

Pressure and (b) the leaving air temperature for an installation using an air-cooled condenser in London if the refrigeration duty is 1 kW.

Answer

(a) The design outside dry-bulb temperature for London is about 28°C (see section 5.11).

At 48°C, the absolute condensing pressure of R134a is 1252.7 kPa (12.36 bar) (see Table 9.1). This is a manageable pressure.

(b) To cool at a rate of 1 kW in the evaporator, about 1.25 kW must be rejected at the condenser, assuming a coefficient of performance of 4. All this heat is rejected into the airstream and so, using equation (6.6), the leaving air temperature will be

F_,oo. 1.25 w 273 + 28)

‘~28 0Л5 358

= 35°C

Posted in Engineering Fifth Edition