Handbook of air conditioning and refrigeration / Shan K

It is important to recognize that the shell-side condensing coefficient h con is a function of
1/(T con � T s). Here T s is the surface temperature of tube wall. Therefore, the condensing coefficient
can be calculated as
1
hcon � ho � Ccon� Qrej / A � o 1/3
(10.18)
where C con � constant. The difference between the boiling and condensing heat-transfer coefficient
is that the greater the heat flux Q/A o, the higher the boiling heat-transfer coefficient h b and the
lower the condensing coefficient h con.
Parameters That Influence the Performance of Shell-and-Tube Condensers
REFRIGERATION SYSTEMS: COMPONENTS 10.25
Temperature Difference T cl � T ce. Temperature difference T cl � T ce determines the condenser
water flow rate. When the condenser water is from the cooling tower, the temperature difference
between the condenser water leaving and entering the shell-and-tube condenser T cl � T ce has a
direct impact on condensing temperature T con and the compressor’s power input, condenser water
pump power, and cooling tower fan power input. The performance of the cooling tower must also
be considered. A temperature difference T cl � T ce � 8°F (4.4°C) means a condenser water flow rate
of 3 gpm/ton (0.054 L/s�kW), and T cl � T ce � 12°F (6.7°C) means a flow rate of 2 gpm/ton
(0.036 L/s�kW).
Kirsner (1996) showed that for a three-stage centrifugal chiller using HCFC-123 as refrigerant,
a 3 gpm/ton (0.054 L/s�kW) condenser water flow rate had a compressor’s kW/ton of 0.596 (5.9
COP) at T ev � 37.6°F (3.1°C), whereas a 2 gpm/ton (0.036 L/s�kW) flow rate had a 0.644
kW/ton (5.46 COP), an increase of compressor power input of about 8 percent. At the same time,
condenser water flow rate of 2 gpm/ton (0.036 L/s�kW) showed a saving of condenser water
pump power of 3.4 percent and a cooling tower fan power saving of about 3 percent compared to
the flow rate of 3 gpm/ton (0.054 L/s�kW). In addition, the initial cost of a condenser water flow
rate of 2 gpm/ton was lower than the flow rate of 3 gpm/ton.
When lake, river, well, or seawater is used, the scarcity of water and water temperature are factors
to be considered. A cost analysis is often required to determine the temperature difference T cl � T ce.
Condensing Temperature T con. In a shell-and-tube condenser, T con is closely related to the
temperature of condenser water T ce available. If T cl � T ce has been selected, then T cl is a fixed value
and T con is determined by choosing an optimum value of T con � T cl. This value is directly related to
the size of the shell-and-tube condenser, as shown in Fig. 10.10b. As in the shell-and-tube liquid
cooler, a smaller T con � T cl means a lower T con and a large condensing surface area, whereas a
larger T con � T cl indicates a higher T con and a smaller condenser. Condenser manufacturers usually
adopt a T con � T cl value between 6 and 10°F (3.3 and 5.6°C), and it is 4 to 7°F (2.2 and 3.9°C) for
energy-efficient models.
Effect of Oil. If the lubrication oil is miscible with the refrigerants (like many halocarbon refrigerants),
experiments show that there is no significant effect on condensing coefficient h con when oil
concentration is lower than 7 percent. In actual operation, oil concentration is usually lower than
3 percent.
Subcooling. Subcooling increases the cooling capacity and coefficient of performance. However,
subcooling also uses part of the surface area in the condenser to cool the liquid refrigerant at the
bottom of the condenser. The surface heat-transfer coefficient on the subcooled liquid refrigerant
side is smaller than the condensing coefficient h con. Subcooling also depends on the temperature
of entering condenser water T ce. For a shell-and-tube condenser, it may vary from 2 to 8°F (1.1 to
4.4°C).