Handbook of air conditioning and refrigeration / Shan K

12.24 CHAPTER TWELVE
FIGURE 12.10 A heat pipe heat exchanger: (a) heat exchanger and airstreams; (b) heat pipe.
These two airstreams flow in a counterflow arrangement for greater effectiveness. The airstreams
are separated by a sealed partition to prevent cross-contamination.
The performance of a heat pipe heat exchanger is indicated by its sensible heat effectiveness � T.
Both the effectiveness and the airstream pressure drop �p of a heat pipe heat exchanger depend
mainly on its face velocity v face, fin spacing, and the number of rows of heat pipes in the direction of
airflow. If two heat pipe heat exchangers are connected in series, the number of rows of such a
series is the total number of rows of the two heat exchangers.
The greater the total number of rows of heat pipe and fins per inch, the higher the � T and �p.
The lower the face velocity v face of the heat pipe exchanger, the higher the � T and the lower the �p.
However, the volume flow rate and the capacity of the heat pipe heat exchanger will also be smaller.
The design face velocity of a heat pipe heat exchanger is between 400 and 800 fpm (2 and 4
m/s). The total number of heat pipe rows varies from 6 to 10 rows. Sensible heat effectiveness
varies from 0.45 to 0.75. For a heat pipe with 14 fins/in. (1.8-mm fins) and a total of 8 rows with
a face velocity v face � 500 fpm (2.5m/s), its sensible heat effectiveness � T � 0.65 and its pressure
drop �p is about 0.6 in. WC (150 Pa). Refer to the manufacturer’s data for detailed
information.
The capacity and rate of heat transfer of a heat pipe can be controlled by varying the slope or tilt
of the heat pipe. This adjustment increases or decreases the liquid flow inside the heat pipe and,
therefore, its capacity.