Handbook of air conditioning and refrigeration / Shan K

From Eq. (13.8), the overall heat transfer coefficient based on the outer surface area of the copper
tubes U o,ev in Btu/h� ft 2 � °F (W/m 2 � °C), is given as
U o,ev �
(25.10)
The boiling coefficient for HCFC-123 is slightly lower than that for refrigerant CFC-11. From Eq.
(13.9), for a shell-and-tube flooded-type evaporator, the boiling coefficient of HCFC-123 can be
evaluated as
According to the experimental results in Webb and Pais (1991) and Jung and Radermacher (1991),
for HCFC-123 in copper tubes with integrated fins of 26 fins/in. (0.98 mm), constant Cb can be
taken as 2.5 and exponential index n is approximately 0.7. For enhanced surfaces, according to the
results of Webb and Pais (1991), ho is 35 percent higher.
The copper integrated-fin tubes currently used for evaporators usually have a fin spacing of 19 to
35 fins/in. (1.3 to 0.73 mm), typically 26 fins/in. (0.98 mm). The outside diameter of copper tubes
5 3 varies from �8 to �4 in. (15.9 to 19.1 mm). The ratio of outer surface area to inner surface area Ao/Ai is often between 3 and 4.
For a closed-circuit chilled-water system for evaporators with conventional water treatments, the
fouling factor R f can be taken as 0.00025 h � ft 2 � °F/Btu (0.000044 m 2 � °C/W).
From Eq. (10.9), water-side heat transfer coefficient h i, in Btu/h � ft 2 � °F (W/m 2 °C), can be calculated
as
NuD � (25.11)
From Eq. (13.11), the log-mean temperature difference between refrigerant and chilled water �Tev can be calculated as
hiDh k � 0.023 ReD 0.8Pr 0.4
(25.12)
The mass flow rate of chilled water m˙ w,ev,
in lb/min (kg/min), flowing through the copper tubes
in the evaporator usually remains approximately constant during operation. Temperature Tel is
often set and reset according to the requirement of the air system and the energy efficiency of
the system.
From Eq. (13.13)
T ee � T el
Q ev / (A evU o,ev)
The evaporating temperature can then be determined as
ENERGY MANAGEMENT AND GLOBAL WARMING 25.21
h o � C b� Q ev
A ev � n
�T ev � T ee � T ev � (T el � T ev)
ln [(T ee � T ev) / (T el � T ev)]
Tev � (25.13)
At design load, Qev is equal to the design refrigeration load Qev,d, in Btu/h (kW). In part-load
operation,
eB Tel � Tee eB � 1
Q ev � R loadQ ev,d
� B
and the evaporating temperature at part load T ev,p can be similarly calculated.
1
1 / (� f h o) � A oR f / A i � A o / (A ih i)
(25.14)