Civil Engineering Reference
In-Depth Information
Fig. 9.4
Steam turbine types
for cogeneration systems: (a)
straight non-condensing (b)
single extraction
non-condensing (c) double
extraction condensing
where,
η
T
¼
turbine efficiency,
η
G
¼
electric generator efficiency from the shaft to
the electric output section.
The electric output is then the ratio between the Actual Steam Flow (ASF)
through the turbine as required by the process and the ASR:
P
e
(kW)
¼
ASF (kg/h)/ASR (kg/kWh)
Typical values are 10,000 kg/h of steam to produce 1,000 kW of electric
power with a pressure drop of 4 MPa (580 psi).
If ASR is established, depending on the turbine cycle and operating
characteristics, the relationship between the inlet and outlet steam enthalpy is as
follows:
h
out
¼
h
in
AE
η
T
¼
h
in
3,600/(
η
G
ASR) (kJ/kg)
Depending on the size of the turbine, on the quantity and quality of process
steam demand and other operating factors, several options are available (see
Fig.
9.4
):
straight non-condensing turbine,
single or multiple extraction
non-condensing turbine.
Figure
9.5
illustrates a typical industrial steam turbine cycle. It consists of a
high-pressure boiler, generally 4-10 MPa (580-1,450 psi) generating superheated
steam for admission to a backpressure or non-condensing steam turbine. The steam
turbine drives either an electric generator or other equipment such as compressors,
pumps, etc. The majority of the steam energy content remains in the outlet steam
which will be utilized in the process; the energy required for mechanical power and
related losses is delivered between the inlet and outlet of the turbine.
The main factors governing the optimal exploitation of a steam turbine cycle
based on a fixed exhaust pressure and a constant net heat to process can be
summarized as follows:
• The overall efficiency of a steam turbine plant is influenced by the inlet volume
flow, inlet-outlet pressure ratio, geometry associated with turbine staging, throt-
tling losses, mechanical coupling, and electric generator losses;
