In an air-cooled condenser, air is used to absorb the latent heat of condensation released during desuperheating, condensation, and subcooling. An air-cooled condenser consists of a condenser coil, a subcooling coil, condenser fans, dampers, and controls.

There are refrigeration circuits in the condensing coil. Condensing coils are usually made of copper tubes and aluminum fins. The diameter of the tubes is 1/4 to 3/4 in., typically 3/8 in., and the fin density is 8 to 20 fins/in. On the inner surface of the copper tubes, microfins, typically 60 fins/in. with a height of 0.008 in., are used.

A condensing coil usually has only two to three rows due to the low pressure drop of the propeller-type condenser fans. A subcooling coil is located at a lower level and is connected to the condensing coil. Hot gas from the compressor enters the condensing coil from the top.

When the condensate increases, part of the condensing area can be used as a subcooling area. A receiver is necessary only when the liquid refrigerant cannot all be stored in the condensing and subcooling coils during the shut-down period in winter.

Cooling air is drawn through the coils by a condenser fan(s) for even distribution. Condenser fans are often propeller fans for their low pressure and large volume flow rate. A damper(s) may be installed to adjust the volume flow of cooling air.

In air-cooled condensers, the volume flow of cooling air per unit of total heat rejection is 600 to 1200 cfm/ton of refrigeration capacity at the evaporator, and the optimum value is about 900 cfm/ton. The corresponding cooling air temperature difference — cooling air leaving temperature minus outdoor temperature (Tca.l – To) — is around 13°F.

The condenser temperature difference (CTD) for an air-cooled condenser is defined as the difference between the saturated condensing temperature corresponding to the pressure at the inlet and the air intake temperature, or (Tcon.i – To). Air-cooled condensers are rated at a specific CTD, depending on the evaporating temperature of the refrigeration system Tev in which the air-cooled condenser is installed.

For a refrigeration system having a lower Tev, it is more economical to equip a larger condenser with a smaller CTD. For a comfort air-conditioning system having a Tev of 45°F, CTD = 20 to 30°F. A higher condensing temperature Tcon, a higher condensing pressure pcon, and a higher compressor power input may be due to an undersized air-cooled condenser, lack of cooling air or low value, a high entering cooling air temperature at the roof, a dirty condensing coil, warm air circulation because of insufficient clearance between the condenser and the wall, or a combination of these. The clearance should not be less than the width of the condensing coil.

If pcon drops below a certain value because of a lower outdoor temperature, the expansion valve in a reciprocating vapor compression system may not operate properly. At a low ambient temperature To, the following controls are often used:

• Duty cycling, turning the condenser fans on and off until all of them are shut down, to reduce cooling air volume flow
• Modulating the air dampers to reduce the volume flow
• Reducing the fan speed

Some manufacturers’ catalogs start low ambient control at To = 65°F and some specify a minimum operating temperature at To = 0°F.

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