Page 410 - Design and Operation of Heat Exchangers and their Networks

Experimental methods for thermal performance of heat exchangers 393

and “ΔP” a differential pressure sensor. For a gas, we need the pressure and
temperature to determine its properties such as density, thermal capacity,
thermal conductivity, and viscosity. For a liquid fluid, the pressure value
is not important. The saturation liquid properties can be used for common
data evaluation.

8.1.1.2 Two-phase heat exchangers
For a heat exchanger operated under a two-phase state (evaporation or con-
densation), we shall ensure that the thermodynamic states of both fluids at
the exchanger inlets and at least the thermodynamic state of one fluid at
the exchanger outlet are known. Furthermore, for the fluid with phase
change, the heat load is calculated with

Q h ¼ _m h h h,in h h,out Þ (8.7)
ð
ð
Q c ¼ _m c h c,out h c,in Þ (8.8)
Take an evaporator used in a vapor compression refrigeration system as
an example, as is shown in Fig. 8.2A. The heat load is calculated with the
measured flow rate and inlet and outlet temperatures of the chilled water

Fig. 8.2 Measurement schematic for the evaporator of a vapor compression
refrigeration system. (A) Completely evaporated in the evaporator. (B) Partially
evaporated in the evaporator.