Handbook of air conditioning and refrigeration / Shan K

10.26 CHAPTER TEN
Part-Load Operation
Condenser Capacity. Parameters that influence the condenser capacity Q rej of a shell-and-tube condenser
are mainly U o, A o, �T m, and Q rej/A o. Size and capacity of the condenser must be matched with
the evaporator and compressor. Condenser capacity is always rated at certain operating conditions.
When the heat flux Q rej/A o reduces at the condensing surface of the shell-and-tube condenser during
part-load operation, h con increases. Therefore, the reduction of Q rej at part-load operation in a shelland-tube
condenser is mainly caused by the drop in condensing temperature T con and, therefore, a
smaller �T m between refrigerant and condenser water. At part-load operation, the degree of subcooling
is also reduced because of the drop in condensing temperature T con.
10.7 AIR-COOLED CONDENSERS
Construction of Air-Cooled Condenser
Air-cooled condensers use air to extract the latent heat of condensation released by the refrigerant
during condensation. An air-cooled condenser generally consists of a condenser coil in which there
are a main condensing coil section and a subcooling coil section connected in series, with several refrigerant
circuits to condense the gaseous refrigerant to liquid, and a subcooled refrigerant at a lower
level, as shown in Fig. 10.11. The condenser coil is usually equipped with copper tubes and aluminum
fins when halocarbon is used as refrigerant. The diameters of the tubes are usually between
1 �4
3 and �4 in. (6.5 and 19 mm), and the fin spacing is generally 12 to 20 fins/in. (1.3 to 2 fins/mm).
On the inner surface of the copper tubes, microfins (typically 60 fins/in. or a fin spacing of 0.4 mm)
with a height of 0.008 in. (0.2 mm) are used, as in DX coils. The condenser coil usually has two to
three rows of tubes because of the lower air-side pressure drop provided by the propeller fan.
Hot gas from the compressor enters the refrigerant circuits at the top. This arrangement provides
flexibility between the condensing and subcooling areas. As the condensate increases, part of the
condensing area can be used as subcooling for the storage of liquid refrigerant. A receiver is necessary
only when not all the liquid refrigerant can be stored in the condenser coil during the shutdown
period of the refrigerant plant in winter. Cooling air is usually forced through the coil by a propeller
fan, as shown in Fig. 10.11. A propeller fan has a lower fan total pressure and large volume flow
rate, which make it more suitable for air-cooled condensers. Fans are usually located downstream
from the coils in order to provide an even airstream through the coils. A damper may be installed
after or before the fan to modulate the air volume flow rate. In a small air-cooled condenser, the
coils, propeller fan, and damper may be installed in line horizontally. In large air-cooled condensers,
condensing and subcooling coils are usually located on two sides, and the propeller fans
and dampers are at the top of the unit.
Heat-Transfer Processes and Temperature Curves
Heat-transfer processes in a typical air-cooled condenser using HCFC-22 as refrigerant are divided
into three stages, as shown in Fig. 10.11b: desuperheating of hot gas, condensation of liquid refrigerant,
and subcooling. Desuperheating uses about 5 percent of the condensing surface area, condensation
uses 85 to 90 percent, and subcooling uses 5 to 10 percent.
For a condenser coil with a depth of two to three rows, airflow and refrigerant flow are usually in
a combined counterflow and cross-flow arrangement. Air enters the condenser coil at a temperature
equal to the summer outdoor design dry-bulb temperature T o. In a year of average weather, T o is the
temperature whose annual cummulative frequency of occurrence is about 0.01 � 8760 � 88 h. If
the air-cooled condenser is installed on the roof, an additional 3 to 5°F (1.7 to 2.8°C) should be