Page 421 - Design and Operation of Heat Exchangers and their Networks

404 Design and operation of heat exchangers and their networks

transfer coefficients using measured wall temperatures and found that they
agreed well for the plain tube, but for the finned tube, the deviations were
significant.
Heat transfer coefficients for enhanced tubes are typically measured indi-
rectly using the Wilson plot method to first characterize the thermal perfor-
mance of the one side (heating or cooling supply) and then to obtain the heat
transfer data for the enhanced side based on the Wilson plot results.
Jin (2003) used the Wilson plot method for determining the boiling and
condensation heat transfer coefficients of a refrigerant on horizontal tubes
and convective heat transfer coefficient of water inside the tubes in two
steps: at first determine the thermal performance of the tube inner side (heat-
ing or cooling supply) and then evaluate the heat transfer data for the
enhanced outside surface based on the Wilson plot results. The tubes were
manufactured with 3-D-structured enhanced surfaces on both sides. The
heat transfer coefficients for single-phase and two-phase heat transfer are
expressed in the following forms, respectively:

0:14
λ i 0:8 1=3 μ i
α i ¼ C i Re Pr (8.53)
i i
d i μ w,i
α o ¼ C o q n (8.54)
o
We take the test for high-efficiency evaporation heat transfer tubes as an
example to illustrate the test procedure. In the evaporator, the hot water is
heated in an electric heater and flows through the tested tube driven by a
pump. The water flow rate is controlled by a frequency converter. Being
heated by the hot water inside the tube, the refrigerant in the shell side boils
on the tube outside surface. The vapor flows upward along a pipe into the
condenser, where the vapor is cooled by one or more high-efficiency con-
densation heat transfer tubes and condenses on the tube outside surface. The
cooling water from the chiller flows through the tube and back to the chiller.
The flow rate of the cooling water is regulated with a frequency converter.
The condensate flows downward to the evaporator again through another
pipe (downcomer). The saturation temperature in the system can be regu-
lated by the manipulation of the hot water temperature or the chiller
temperature.
The evaporation experiments consist of two parts: (1) test runs for deter-
mining unknown C i and (2) test runs for determining unknown C o and n.
In the first test set, six or more test runs are carried out under the specified
hot water Reynolds numbers whose logarithmic values are equally divided.