Chapter 8
Vapor Power Systems
Regenerative Vapor Power Cycle Using
an Closed Feedwater Heater
►Regenerative feedwater heating also can be
accomplished with closed feedwater heaters.
►Closed feedwater heaters are shell and tube
type recuperators.
►The feedwater temperature increases as the
extracted steam condenses on the outside of the
tubes carrying the feedwater.
►Since the two streams do not mix, they can be at
different temperatures.
Regenerative Vapor Power Cycle Using
an Closed Feedwater Heater
 A regenerative vapor power cycle having one closed feedwater
heater with the condensate trapped into the condenser:
 A trap is a type of valve that permits only liquid to pass through to a region
of lower pressure.
 The expansion through the trap is a throttling process. The enthalpy of
steam remains constant during this throttling process.
h7=h8
Regenerative Vapor Power Cycle Using
an Closed Feedwater Heater
 The expansion through the trap is a throttling process.
h7=h8
 In a throttling device a significant reduction in pressure can be
achieved simply by introducing a restriction into a line through which
a gas or liquid flows.
 This is commonly done by means of a partially opened valve or a
porous plug.
Multiple Feedwater Heaters
• The open and closed feedwater heaters can be compared as follows:
• Open feedwater heaters are simple and inexpensive and have good heat
transfer characteristics. For each heater, however, a pump is required to
handle the feedwater.
• The closed feedwater heaters are more complex because of the internal
tubing network, and thus they are more expensive. Heat transfer in closed
feedwater heaters is also less effective since the two streams are not allowed
be in direct contact. However, closed feedwater heaters do not require a
separate pump for each heater since the extracted steam and the feedwater
can be at different pressures.
• Most steam power plants use a combination of reheat stage with a number of
open and closed feedwater heaters.
• The number of feedwater heaters used is based on economic considerations,
since incremental increases in thermal efficiency achieved with each additional
heater must justify the added capital costs (heater, piping, pumps, etc.).
Multiple Feedwater Heaters
 Actual power plants have many of the same basic features as the one
shown in the figure:
Example:
Consider a regenerative vapor power cycle with two feedwater heaters, a
closed one and an open one, as shown in Figure.
Determine for the cycle:
(a) the heat transfer to the
working fluid passing through
the steam generator, in kJ
per kg of steam entering the
first stage
turbine.
(b) the thermal efficiency.
(c) the heat transfer from the
working fluid passing through
the condenser to the cooling
water, in kJ per kg of steam
entering
the
first-stage
turbine.
Example:
Steam enters the first turbine stage at 12 MPa, 480°C, and expands to
2 MPa.
Some steam is extracted at 2 MPa and fed to the closed feedwater
heater.
The remainder expands through the second-stage turbine to 0.3 MPa,
where an additional amount is extracted and fed into the open
feedwater heater operating at 0.3 MPa.
The steam expanding through the third-stage turbine exits at the
condenser pressure of 6 kPa.
Feedwater leaves the closed heater at 210°C, 12 MPa, and condensate
exiting as saturated liquid at 2 MPa is trapped into the open feedwater
heater.
Saturated liquid at 0.3 MPa leaves the open feedwater heater.