Handbook of air conditioning and refrigeration / Shan K

25.22 CHAPTER TWENTY-FIVE
Condenser Model
For a shell-and-tube water-cooled condenser, from Eqs. (10.16a) and (13.7), the rate of heat transfer
at the condenser Q rej, in Btu/h (kW), or the total heat rejection, can be calculated as
(25.15)
From Eq. (25.8) power input Pcom is known only when condensing temperature Tcon, work input
Win, and mass flow rate of the refrigerant all have been calculated. Therefore, from Eq. (10.17)
Qrej � FrejQev; assume a heat rejection factor Frej first. Then Tcon, Win, and Pcom can be calculated.
If the calculated Pcom and Qrej do not equal the assumed values, try another Frej until the assumed
and calculated values are equal to each other. For comfort air conditioning, Frej usually varies from
1.20 to 1.35.
As in the evaporator model, from Eq. (13.8), the overall heat-transfer coefficient based on the
outer surface area of the condenser Uo,con, in Btu/h � ft2� °F (W/m2� °C), is
(25.16)
From Eq. (13.11), the log-mean temperature difference between the condensing refrigerant and the
condenser water �Tcon, in °F (°C), is
(25.17)
From Eq. (13.10), the condensing coefficient hcon, in Btu/h � ft2 � °F (W/m 2 �Tcon �
� °C), is
T Qrej � Qev �
� AconUo,con �Tcon m˙ r
1
Uo,con �
1 / (�fhcon) � AconRf / Ai � Rg � Acon / (Aihi) cl � Tcon � (Tce � Tcon) ln[(Tcl � Tcon) / (Tce � Tcon)] 2545Pcom �mot 1
hcon � Ccon� Q � rejAcon In a filmwise condensation shell-and-tube water-cooled condenser having -in or -in.- (15.9- or
19.1-mm-) diameter copper tubes with integrated fins and using HCFC-123 as refrigerant, constant
Ccon can be taken as 10,500.
In a condenser using a well-maintained cooling tower water with proper water treatment, a fouling
factor Rf � 0.00025 h � ft2� °F/Btu (0.000044 m2 � °C/W) is recommended. In industrial areas,
if a brush cleaning system is installed, Rf � 0.0002 h � ft2 � °F/Btu (0.000035 m2 � °C/W).
The operating pressure of HCFC-123, like CFC-11, is lower than atmospheric pressure, so air
and other noncondensable gases leak into the evaporator. The compressor transports them to a
higher level and then they accumulate in the condenser. Noncondensable gases reduce the condensing
area and raise the condensing pressure. Their effect is similar to that of a gas-side resistance Rg, in h � ft2 � °F/Btu (m2 5 3
�8 �4
� °C/W), at the condenser as follows:
As in the evaporator model, C is calculated as follows:
Condensing temperature is therefore calculated as
R g � 0.00778 � 0.0173R load � 0.0114R load 2 (25.18)
T cl � T ce
Q rej / (A conU o,con)
� C
T con � eC T cl � T ce
e C � 1
1 / 3
(25.19)