Civil Engineering Reference
In-Depth Information
The procedure for evaluating various types of heat exchangers involves the
following steps:
1. The heat flow rate
Q
of the stream is calculated using the relationships given
above (
m
h
).
2. The log-mean difference of temperatures
Δ
T
LM
is calculated on the basis of the
inlet/outlet temperatures and then corrected by factor FT.
3. The ratio
Q
/
Δ
T
LM
should be calculated for each type of exchanger.
4. If
U
is estimated (see Tables
15.3
and
15.4
), the exchanger surface is obtained
and then the cost of the exchanger. Alternatively, curves or tables which give the
cost per unit of W/K (
¼
W/
C) or of W/
F corresponding to different values of
U
can conveniently be used to determine the cost of the exchanger.
Δ
15.5.2 Effectiveness-NTU Method
This approach is based on the introduction of a parameter defined as number of
transfer units NTU
U
/
C
min
where
A
is the effective heat transfer area,
U
is the
effective overall heat-transfer coefficient, and
C
min
is the minimum heat capacity
flow rate. The parameter NTU, which is an adimensional quantity, is indicative of
the size of the heat exchanger.
The effectiveness of typical heat exchangers versus the parameter NTU is
expressed by means of curves. Different curves can be described for different
values of
C
min
/
C
max
(Figs.
15.4
and
15.5
). These curves are plotted using the
log-mean difference expression.
Notice that a linear relationship exists between effectiveness and the parameter
NTU until the value of the latter reaches 1; after this point the curve begins to knee,
which means that effectiveness can be increased with a corresponding higher
increase of the NTU parameter. This involves paying more and more for
exchangers with high effectiveness.
This method is very useful when the inlet and outlet temperatures in the heat
exchanger are not known.
The following steps outline a preliminary design for a heat exchanger made by
the NTU method, if only input temperatures and heat capacity flow rates of the
fluids are known:
1. A value of the overall heat-transfer coefficient
U
is estimated.
2. The heat capacity flow rates of the two fluids are known (
m
¼
A
c
).
3. For a given effectiveness, which corresponds to a given output temperature of
one fluid, the value of the NTU parameter is derived from curves like those in
Figs.
15.4
and
15.5
.
4. The exchange surface value can be calculated from the NTU expression
(NTU
U
/
C
min
) and the unknown outlet temperature is derived from the
effectiveness expression.
Alternatively
3
0
. For a given heat exchanger surface the NTU parameter is calculated.
¼
A
