Page 416 - Design and Operation of Heat Exchangers and their Networks

Experimental methods for thermal performance of heat exchangers 399

After flowing through the tube, the fluid flows through a cooler where the
fluid is cooled by the cooling water. We can regulate the valve to control the
flow rate of the cooling water and keep the inlet fluid temperature t 1 at the
specified value.
The heat load from the steam side is determined with the measured mass
flow rate and the evaporation enthalpy Δh v at the measured saturation tem-
perature t s as

Q h ¼ _m h Δh v (8.25)
The heat load from the fluid side can be obtained from the measured inlet
and outlet fluid temperatures t 1 and t 2 :
Q c ¼ _m c c p,m,c t 2 t 1 Þ (8.26)
ð
where c p,m,c is determined with the mean fluid temperature (t 1 +t 2 )/2. The
evaluated heat load of the tube is expressed with Eq. (8.5) for the further data
evaluation. The energy balance error can then be evaluated by Eq. (8.6).
To evaluate the heat transfer coefficient inside the tube, we assume the
heat transfer coefficients at both sides of the tube are constant along the tube;
the heat load of the tested tube can be expressed as (see Eq. 2.77)

ð t s t 1 Þ t s t 2 Þ
ð
Q ¼ kAΔt LM ¼ kπd i L (8.27)
ln t s t 1 Þ= t s t 2 Þ½ ð ð Š
where k is the overall heat transfer coefficient based on the tube inner area
1
ð
d i d i ln d o =d i Þ 1
k ¼ + + (8.28)
α o d o 2λ t α i
and can be determined with the measured data.
The mean condensation heat transfer coefficient at the tube outside can
be estimated with the Nusselt film condensation equation (from Eq. 4.161):

3 1=4
ð
gρ ρ ρ Þλ Δh v
l
v
l
l
α o ¼ 0:728 (8.29)
μ t s t w Þd
ð
l
in which t w is the mean value of the outside wall temperatures at the tube
inlet and outlet, t w ¼(t w,1 +t w,2 )/2, and
kd i
t w, j ¼ t s t s t j ð j ¼ 1, 2Þ (8.30)
α o d o
For the determination of t w,1 and t w,2 , an iteration between Eqs. (8.29) and
(8.30) is needed.