Power Plant Engineering

9.3.2. INTERCOOLERS AND HEAT EXCHANGERS
The intercooler is generally used in gas turbine plant when the pressure ratio used is sufficiently
large and the compression is completed with two or more stages. The cooling of compressed air is
generally done with the use of cooling water. A cross-flow type intercooler is generally preferred for
effective heat transfer.
The regenerators, which are commonly used in gas turbine plant, are of two types, recuperator
and regenerator.
In a recuperative type of heat ex-
changer, the air and hot gases are made to flow
in counter direction as the effect of counter-
flow gives high average temperature difference
causing the higher heat flow.
A number of baffles in the path of air-
flow are used to make the air to flow in con-
tact for longer time with heat transfer surface.
The regenerator type heat exchanger
consists of a heat-conducting member that is
exposed alternately to the hot exhaust gases
and the cooler compressed air. It absorbs the
heat from hot gases and gives it up when ex-
posed to the air. The heat, capacity member is
made of a metallic mesh or matrix, which is
rotated slowly (40-60 r.p.m.) and continuously
exposed to hot and cold air.
In
le
t Gu
id
e
V
a
n
e
s
Fig. 9.6. Axial Flow Air Compressor.
 Fig. 9.7. Ritz Regenerative Heat Exchanger.
Exhaust to
Atmosphere
Rotating Mesh Type
Heat Exchanger
Fixed Casing
Air to Combustion
Chamber
Air from
Compressor
Exhaust from
gas turbine

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274
POWER PLANT ENGINEERING
Prof. Ritz suggested the first application of regenerative heat exchanger to gas turbine plants of
Germany and the heat exchanger was titled against his name. The arrangement of Ritz heat exchanger is
shown in Fig. 9.7.
The heat-exchanging element A is slowly rotated by a drive from the gas turbine via shaft S. The
rotation places the heat-transferring element A in the exhaust gas passage for one half of the time re-
quired for one r.p.m. and in the air supply passage for the remaining half. The heat element absorbs heat
from the hot gases, when exposed to hot gases and gives out the same heat to the cold air when the
heated part moves in the air region. By suitable design of the speed of rotation of transfer element and its
mass in relation to the heat to be transferred, it is possible to secure a high effectiveness, values of 90%
are claimed. The principal advantages claimed of this heat exchanger over the recuperative type are
lightness, smaller mass, and small size for given effectiveness and low-pressure drop.
The major disadvantage of this heat exchanger is, there will be always a tendency for air leakage
to the exhaust gases as the compressed air is at a much higher pressure than exhaust gases. This ten-
dency of leakage reduces the efficiency gain due to heat exchanger. Therefore, the major problem in the
design of this type of heat exchanger is to prevent or minimize the air loss due to leakage.
Recently very special seals are provided to prevent the air leakage. This seal stands at very high
temperature and pressure and allows the freedom of movement.
The performance of the heat exchanger is determined by a factor known as effectiveness. The
effectiveness of the heat exchanger is defined as
ε
= 
actual heat transfer to the air
maximum heat transfer theoretically possible
The effectiveness is given by
ε
= 
5
2
4
2
C
(T
T )
C
(T
T )
−
−
pa
a
pg
g
m
m
where m
a
and m
g
are the masses of the air and exhaust gases and C
Pa
and C
Pg
are the corresponding
specific heats.
If the mass of the fuel compared with mass of the air, is neglected and C
Pa
= C
Pg
is assumed, then
the effectiveness is given by an expression
ε
= 
5
2
4
2
T
T
T
T
′ −
−