Power Plant Engineering

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 =^52

42

C(TT)

C(TT)

−

−

pa a

pg g

m

m

where ma and mg are the masses of the air and exhaust gases and CPa and CPg are the corresponding
specific heats.

If the mass of the fuel compared with mass of the air, is neglected and CPa = CPg is assumed, then
the effectiveness is given by an expression

ε =^52

42

TT

TT

′−

−

9.3.3 Combustion Chambers

The gas turbine is a continuous flow system; therefore, the combustion in the gas turbine differs
from the combustion in diesel engines. High rate of mass flow results in high velocities at various points
throughout the cycle (300 m/sec). One of the vital problems associated with the design of gas turbine
combustion system is to secure a steady and stable flame inside the combustion chamber. The gas tur-
bine combustion system has to function under certain different operating conditions which are not usu-
ally met with the combustion systems of diesel engines. A few of them are listed below:

1. Combustion in the gas turbine takes place in a continuous flow system and, therefore, the
   advantage of high pressure and restricted volume available in diesel engine is lost. The chemical
   reaction takes place relatively slowly thus requiring large residence time in the combustion
   chamber in order to achieve complete combustion.