Page 411 - Design and Operation of Heat Exchangers and their Networks

394 Design and operation of heat exchangers and their networks

(hot fluid) by Eq. (8.1). The measured temperature and pressure of the
refrigerant (cold fluid) before entering the expansion valve are t 1 and p 1 ,
and those after the expansion valve are t 2 and p 2 . Using these measured data,
we have the enthalpy of the subcooled refrigerant before entering the expan-
sion valve as h 1 ¼h(t 1 , p 1 ) and the enthalpy after the expansion valve h 2 ¼h 1 .
The vapor mass fraction at the evaporator inlet can be approximately eval-
uated by the thermodynamic vapor quality x 2 :

h 2 h s,l
_ x 2 x 2 ¼ (8.9)
h s,v h s,l
In the experiment, the outlet state of the refrigerant should be kept in the
superheated range, that is, t 3 >t s (p 3 ). In such a case, h 3 ¼h(t 3 , p 3 ), the heat
load can be obtained from the refrigerant side with Eq. (8.8), and the energy
balance error can be evaluated by Eq. (8.6). If the refrigerant at the outlet of
the evaporator lies in the two-phase region (t 3 t s,3 , h(t 3 , p 3 )<h s,v ), the out-
let state of the refrigerant has to be determined by the energy balance
between the refrigerant and the chilled water, which yields

_ m w c p,w,m t 4 t 5 Þ
ð
h 3 ¼ h 2 + (8.10)
_ m r
h 3 h s,l
_ x 3 x 3 ¼ (8.11)
h s,v h s,l
In such a case, the energy balance error of the evaporator cannot be eval-
uated. To evaporate the refrigerant completely, we can add a heater to the
refrigerant pipe after the evaporator, as is shown in Fig. 8.2B. Neglecting the
pressure drop and heat loss in the heater and using the measured heat duty of
0,
the heater Q 3 and the measured temperature after the heater t 3 we can
express the heat load of the refrigerant flow as

ð
½
0
Q r ¼ _m r h r t 3 , p 3 Þ h 2 Š Q 3 (8.12)
The enthalpy at the outlet of the evaporator is calculated with
(8.13)
h 3 ¼ h 2 + Q r = _m r
The vapor mass fraction and energy balance error can then be evaluated with
Eq. (8.11), (8.6), respectively.

8.1.2 Measurement of heat transfer coefficient
For the research and development purposes, people have more interest on
experimental determination of heat transfer coefficients of different heat