U.S. patent application number 11/336865 was filed with the patent office on 2006-08-03 for rankine cycle system.
This patent application is currently assigned to HONDA MOTOR CO., LTD.. Invention is credited to Shigeru Ibaraki, Akihisa Sato, Kensaku Yamamoto.
Application Number | 20060168963 11/336865 |
Document ID | / |
Family ID | 36755042 |
Filed Date | 2006-08-03 |
United States Patent
Application |
20060168963 |
Kind Code |
A1 |
Sato; Akihisa ; et
al. |
August 3, 2006 |
Rankine cycle system
Abstract
A Rankine cycle system includes: an evaporator for heating a
liquid-phase working medium with thermal energy of exhaust gas of
an engine so as to generate a gas-phase working medium; a
displacement type expander for converting the thermal energy of the
gas-phase working medium generated by the evaporator into
mechanical energy; a target pressure setter for setting a target
pressure for the gas-phase working medium based on actual flow rate
and temperature of the gas-phase working medium supplied to the
expander; an estimated flow rate calculator for calculating an
estimated flow rate of the gas-phase working medium supplied to the
expander based on an output and a rotational speed of the engine;
and a target rotational speed calculator for calculating a target
rotational speed for the expander based on the estimated flow rate
calculated by the estimated flow rate calculator and the target
pressure set by the target pressure setter.
Inventors: |
Sato; Akihisa; (Wako-shi,
JP) ; Ibaraki; Shigeru; (Wako-shi, JP) ;
Yamamoto; Kensaku; (Wako-shi, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
HONDA MOTOR CO., LTD.
Tokyo
JP
|
Family ID: |
36755042 |
Appl. No.: |
11/336865 |
Filed: |
January 23, 2006 |
Current U.S.
Class: |
60/645 |
Current CPC
Class: |
F01K 23/065
20130101 |
Class at
Publication: |
060/645 |
International
Class: |
F01K 13/00 20060101
F01K013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2005 |
JP |
2005-14995 |
Claims
1. A Rankine cycle system comprising: an evaporator for heating a
liquid-phase working medium with thermal energy of exhaust gas of
an engine so as to generate a gas-phase working medium; a
displacement type expander for converting the thermal energy of the
gas-phase working medium generated by the evaporator into
mechanical energy; target pressure setting means for setting a
target pressure for the gas-phase working medium based on actual
flow rate and temperature of the gas-phase working medium supplied
to the expander; estimated flow rate calculation means for
calculating an estimated flow rate of the gas-phase working medium
supplied to the expander, based on an output and a rotational speed
of the engine; and target rotational speed calculation means for
calculating a target rotational speed for the expander based on the
estimated flow rate calculated by the estimated flow rate
calculation means and the target pressure set by the target
pressure setting means.
2. A Rankine cycle system comprising: an evaporator for heating a
liquid-phase working medium with thermal energy of exhaust gas of
an engine so as to generate a gas-phase working medium; a
displacement type expander for converting the thermal energy of the
gas-phase working medium generated by the evaporator into
mechanical energy; target pressure setting means for setting a
target pressure for the gas-phase working medium based on actual
flow rate and temperature of the gas-phase working medium supplied
to the expander; estimated flow rate calculation means for
calculating an estimated flow rate of the gas-phase working medium
supplied to the expander, based on a throttle opening degree or an
accelerator opening degree of the engine, and a rotational speed of
the engine; and target rotational speed calculation means for
calculating a target rotational speed for the expander based on the
estimated flow rate calculated by the estimated flow rate
calculation means and the target pressure set by the target
pressure setting means.
3. The Rankine cycle system according to claim 1, wherein the
target rotational speed calculation means calculates a target
rotational speed for the expander based on the estimated flow rate
calculated by the estimated flow rate calculation means, the target
pressure set by the target pressure setting means, and the
temperature of the gas-phase working medium supplied to the
expander.
4. The Rankine cycle system according to claim 3, wherein the
estimated flow rate calculation means calculates an exhaust gas
flow rate based on the throttle opening degree and the rotational
speed of the engine, calculates an exhaust gas energy from the
exhaust gas flow rate and an exhaust gas temperature, and
calculates the estimated flow rate from the exhaust gas energy and
a target temperature of the gas-phase working medium supplied to
the expander.
5. The Rankine cycle system according to claim 3, wherein the
estimated flow rate calculation means calculates an exhaust gas
energy based on the throttle opening degree and the rotational
speed of the engine, and calculates the estimated flow rate from
the exhaust gas energy and a target temperature of the gas-phase
working medium supplied to the expander.
6. The Rankine cycle system according to claim 2, wherein the
target rotational speed calculation means calculates a target
rotational speed for the expander based on the estimated flow rate
calculated by the estimated flow rate calculation means, the target
pressure set by the target pressure setting means, and the
temperature of the gas-phase working medium supplied to the
expander.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority under 35 USC 119 to
Japanese Patent Application No. 2005-14995 filed on Jan. 24, 2005
the entire contents of which are hereby incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a Rankine cycle system that
includes an evaporator for heating a liquid-phase working medium
with thermal energy of exhaust gas of an engine so as to generate a
gas-phase working medium, and a displacement type expander for
converting the thermal energy of the gas-phase working medium
generated by the evaporator into mechanical energy.
[0004] 2. Description of the Related Art
[0005] Japanese Patent Application Laid-open No. 2004-60462
discloses a Rankine cycle system in which, in order to control the
steam pressure at the entrance of an expander at a target pressure
with good precision without changing the amount of steam supplied
to an evaporator, a feedforward value is calculated based on the
target pressure and a steam flow rate at the exit of the
evaporator, a feedback value is calculated by multiplying a
deviation of the steam pressure at the entrance of the expander
from the target pressure by a feedback gain calculated based on the
steam flow rate, and the rotational speed of the expander is
controlled based on the sum/difference of the feedforward value and
feedback value.
[0006] In order to maintain the Rankine cycle system in a highly
efficient state, it is necessary to control the temperature and
pressure of steam supplied to the expander at optimum values with
respect to variations in the load of the engine; in conventional
general control the steam temperature is controlled by changing the
amount of water supplied to the evaporator, and the steam pressure
is controlled by changing the rotational speed of the expander.
[0007] In accordance with this control, as shown in FIG. 12, when
the accelerator opening degree is increased stepwise by depressing
an accelerator pedal, the throttle opening degree increases
stepwise and the engine output also increases stepwise. When the
exhaust gas energy increases accompanying the increase in engine
output, the temperature of steam generated in the evaporator
increases beyond a target steam temperature (ref. region a), but
due to the heat capacity of the evaporator the increase in steam
temperature has a time lag relative to the increase in the engine
output. When the steam temperature increases in this way, feedback
control is carried out so as to increase the amount of water
supplied to the evaporator in order to suppress the increase in
steam temperature (ref. region b). Since, due to the increase in
the amount of water supplied, the amount of steam supplied from the
evaporator to the expander increases and the steam pressure
increases, feedback control is carried out so as to increase the
rotational speed of the expander in order to decrease the steam
pressure. However, if the increase in steam pressure is rapid, the
rotational speed of the expander reaches a maximum rotational speed
and the steam pressure cannot be decreased sufficiently (ref.
region c), and there is a possibility that the steam pressure might
overshoot an upper limit pressure (ref. region d) and the operating
efficiency of the expander might be degraded or the durability
might be adversely affected.
[0008] In this way, in the case in which the steam pressure is
controlled by changing the rotational speed of the expander, even
if an attempt is made to decrease the steam pressure by increasing
the rotational speed of the expander after detecting a steam flow
rate or a steam pressure at the entrance of the expander, phase
changes (liquid phase.fwdarw.saturation, saturation.fwdarw.gas
phase) are accelerated accompanying a decrease in the pressure in
the interior of the evaporator, and as a result the steam flow rate
increases, thus causing the problem that a time lag occurs before
the steam pressure decreases.
SUMMARY OF THE INVENTION
[0009] The present invention has been accomplished under the
above-mentioned circumstances, and it is an object thereof to
control, in a Rankine cycle system, the pressure of a gas-phase
working medium supplied from an evaporator to an expander at a
target pressure with good responsiveness.
[0010] In order to achieve the above-mentioned object, according to
a first feature of the invention, there is provided a Rankine cycle
system comprising: an evaporator for heating a liquid-phase working
medium with thermal energy of exhaust gas of an engine so as to
generate a gas-phase working medium; a displacement type expander
for converting the thermal energy of the gas-phase working medium
generated by the evaporator into mechanical energy; target pressure
setting means for setting a target pressure for the gas-phase
working medium based on actual flow rate and temperature of the
gas-phase working medium supplied to the expander; estimated flow
rate calculation means for calculating an estimated flow rate of
the gas-phase working medium supplied to the expander, based on an
output and a rotational speed of the engine; and target rotational
speed calculation means for calculating a target rotational speed
for the expander based on the estimated flow rate calculated by the
estimated flow rate calculation means and the target pressure set
by the target pressure setting means.
[0011] With the first feature, since the target pressure setting
means sets the target pressure for the gas-phase working medium
supplied to the expander based on the actual flow rate and
temperature of the gas-phase working medium, the estimated flow
rate calculation means calculates the estimated flow rate of the
gas-phase working medium supplied to the expander based on the
output and the rotational speed of the engine, and the target
rotational speed calculation means calculates the target rotational
speed for the expander based on the estimated flow rate and the
target pressure, the pressure of the gas-phase working medium can
be controlled at the target pressure with good responsiveness using
the estimated flow rate of the gas-phase working medium, which
responds instantaneously to a change in the output of the engine,
without being affected by a response lag in the actual flow rate of
the gas-phase working medium supplied to the expander.
[0012] According to a second feature of the present invention,
there is provided a Rankine cycle system comprising: an evaporator
for heating a liquid-phase working medium with thermal energy of
exhaust gas of an engine so as to generate a gas-phase working
medium; a displacement type expander for converting the thermal
energy of the gas-phase working medium generated by the evaporator
into mechanical energy; target pressure setting means for setting a
target pressure for the gas-phase working medium based on actual
flow rate and temperature of the gas-phase working medium supplied
to the expander; estimated flow rate calculation means for
calculating an estimated flow rate of the gas-phase working medium
supplied to the expander, based on a throttle opening degree or an
accelerator opening degree of the engine, and a rotational speed of
the engine; and target rotational speed calculation means for
calculating a target rotational speed for the expander based on the
estimated flow rate calculated by the estimated flow rate
calculation means and the target pressure set by the target
pressure setting means.
[0013] With the second feature, since the target pressure setting
means sets the target pressure for the gas-phase working medium
supplied to the expander based on the actual flow rate and
temperature of the gas-phase working medium, the estimated flow
rate calculation means calculates the estimated flow rate of the
gas-phase working medium supplied to the expander based on the
throttle opening degree or the accelerator opening degree and the
rotational speed of the engine, and the target rotational speed
calculation means calculates the target rotational speed for the
expander based on the estimated flow rate and the target pressure,
the pressure of the gas-phase working medium can be controlled at
the target pressure with good responsiveness using the estimated
flow rate of the gas-phase working medium, which responds
instantaneously to a change in the throttle opening degree or the
accelerator opening degree of the engine, without being affected by
a response lag in the actual flow rate of the gas-phase working
medium supplied to the expander.
[0014] According to a third feature of the present invention, in
addition to the first or second feature, the target rotational
speed calculation means calculates a target rotational speed for
the expander based on the estimated flow rate calculated by the
estimated flow rate calculation means, the target pressure set by
the target pressure setting means, and the temperature of the
gas-phase working medium supplied to the expander.
[0015] With the third feature, since the target rotational speed
calculation means calculates the target rotational speed for the
expander from, in addition to the estimated flow rate and the
target pressure, the temperature of the gas-phase working medium
supplied to the expander, the target rotational speed for the
expander can be calculated with better precision.
[0016] According to a fourth feature of the present invention, in
addition to the third feature, the estimated flow rate calculation
means calculates an exhaust gas flow rate based on the throttle
opening degree and the rotational speed of the engine, calculates
an exhaust gas energy from the exhaust gas flow rate and an exhaust
gas temperature, and calculates the estimated flow rate from the
exhaust gas energy and a target temperature of the gas-phase
working medium supplied to the expander.
[0017] With the fourth feature, since the estimated flow rate
calculation means calculates the exhaust gas energy from, in
addition to the exhaust gas flow rate, the exhaust gas temperature,
and calculates the estimated flow rate of the gas-phase working
medium from, in addition to the exhaust gas energy, the target
temperature for the gas-phase working medium supplied to the
expander, the estimated flow rate can be calculated with yet better
precision.
[0018] According to a fifth feature of the present invention, in
addition to the third feature, the estimated flow rate calculation
means calculates an exhaust gas energy based on the throttle
opening degree and the rotational speed of the engine, and
calculates the estimated flow rate from the exhaust gas energy and
a target temperature of the gas-phase working medium supplied to
the expander.
[0019] With the fifth feature, since the estimated flow rate
calculation means calculates the estimated flow rate of the
gas-phase working medium from, in addition to the exhaust gas
energy, the target temperature for the gas-phase working medium
supplied to the expander, the estimated flow rate can be calculated
with yet better precision.
[0020] The above-mentioned object, other objects, characteristics,
and advantages of the present invention will become apparent from
preferred embodiments that will be described in detail below by
reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 to FIG. 5 show a first embodiment of the present
invention;
[0022] FIG. 1 is a diagram showing the overall arrangement of a
Rankine cycle system,
[0023] FIG. 2 is a block diagram of a control system for the
rotational speed of an expander,
[0024] FIG. 3 is a diagram showing a map used for control of the
rotational speed of the expander,
[0025] FIG. 4 is a diagram showing a map in which a target steam
pressure is looked up from steam energy and steam temperature,
and
[0026] FIG. 5 is a time chart of the control system for the
rotational speed of the expander.
[0027] FIG. 6 is a block diagram of a control system for the
rotational speed of an expander related to a second embodiment.
[0028] FIG. 7 is a diagram showing a map in which exhaust gas
energy is looked up from an accelerator opening degree and an
engine rotational speed.
[0029] FIG. 8 is a block diagram of a control system for the
rotational speed of an expander related to a third embodiment.
[0030] FIG. 9 is a diagram showing a map in which an estimated
steam flow rate is looked up from an accelerator opening degree and
an engine rotational speed.
[0031] FIG. 10 is a flowchart of a changeover routine of control
for the rotational speed of an expander related to a fourth
embodiment.
[0032] FIG. 11 is a graph explaining the flowchart of FIG. 10.
[0033] FIG. 12 is a time chart of a conventional control system for
the rotational speed of an expander.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0034] FIG. 1 shows the overall arrangement of a Rankine cycle
system R to which the present invention is applied. The Rankine
cycle system R, which recovers thermal energy of exhaust gas of an
engine E and converts it into mechanical energy, includes an
evaporator 11, an expander 12, a condenser 13, and a water supply
pump 14, the evaporator 11 heating water with the exhaust gas
discharged by the engine E so as to generate high temperature, high
pressure steam, the expander 12 being operated by the high
temperature, high pressure steam generated by the evaporator 11 so
as to generate mechanical energy, the condenser 13 cooling
decreased temperature, decreased pressure steam that has completed
work in the expander 12 so as to turn it back into water, and the
water supply pump 14 pressurizing water discharged from the
condenser 13 and re-supplying it to the evaporator 11.
[0035] As shown in FIG. 2, estimated flow rate calculation means M1
estimates, without a response lag, a steam flow rate, which is
conventionally obtained by directly detecting a flow rate of steam
supplied from the evaporator 11 to the expander 12 or by estimation
from an amount of water supplied to the evaporator 11, and includes
exhaust gas flow rate calculation means M2, exhaust gas energy
calculation means M3, and steam flow rate calculation means M4.
[0036] The exhaust gas flow rate calculation means M2 calculates an
exhaust gas flow rate Qg by applying an accelerator opening degree
AP (that is, a throttle opening degree TH) and an engine rotational
speed Ne to the map shown in FIG. 3A. The reason for the rate of
increase in the exhaust gas flow rate Qg in response to an increase
in the accelerator opening degree AP decreasing in region i of FIG.
3A is because when the engine rotational speed Ne is high, a fuel
injection quantity increases and an air/fuel ratio A/F
decreases.
[0037] The exhaust gas energy calculation means M3 calculates an
enthalpy Hg of the exhaust gas by applying an exhaust gas
temperature Tg and the air/fuel ratio A/F to the map shown in FIG.
3B, and calculates an exhaust gas energy Eg by multiplying the
enthalpy Hg by the exhaust gas flow rate Qg calculated by the
exhaust gas flow rate calculation means M2. The steam flow rate
calculation means M4 calculates an estimated steam flow rate Qs by
applying a target steam temperature Tobj and the exhaust gas energy
Eg calculated in FIG. 3B to the map shown in FIG. 3C.
[0038] Target pressure setting means M5 sets a target steam
pressure by applying an actual flow rate and a temperature of steam
supplied from the evaporator 11 to the expander 12 to the map of
FIG. 4. This target pressure corresponds to a steam pressure at
which the expander 12 is operated with maximum efficiency.
[0039] Target rotational speed calculation means M6 includes
feedforward rotational speed calculation means M7 and feedback
rotational speed calculation means M8. The feedforward rotational
speed calculation means M7 calculates a feedforward rotational
speed of the expander 12 by applying the estimated steam flow rate
Qs calculated by the estimated flow rate calculation means M1 and
the target steam pressure set by the target pressure setting means
M5 to the map shown in FIG. 3D. The feedback rotational speed
calculation means M8, into which a deviation of the actual steam
pressure from the target steam pressure is inputted, calculates a
feedback rotational speed by multiplying the deviation by a
predetermined gain. The target rotational speed calculation means
M6 outputs as a target rotational speed for the expander 12 a value
obtained by subtracting the feedback rotational speed from the
feedforward rotational speed.
[0040] A motor/generator 15 is connected to the expander 12; since
when the generator load of the motor/generator 15 decreases, the
rotational speed of the expander 12 increases, and when the
generator load increases, the rotational speed of the expander 12
decreases, the rotational speed of the expander 12 can be
controlled freely. Increasing the rotational speed of the expander
12 allows the steam pressure at the entrance of the expander 12 to
be decreased, whereas decreasing the rotational speed of the
expander 12 allows the steam pressure at the entrance of the
expander 12 to be increased.
[0041] A deviation of the actual rotational speed (feedback
rotational speed) of the expander 12 from the target rotational
speed of the expander 12 outputted by the target rotational speed
calculation means M6 is inputted into PI feedback term calculation
means M9, and by making the motor/generator 15 generate a target
torque calculated in the PI feedback term calculation means M9 the
rotational speed of the expander 12 can be feedback controlled at
the target rotational speed.
[0042] The above-mentioned control results are summarized by
reference to the time chart of FIG. 5. When the accelerator opening
degree is increased stepwise by depressing an accelerator pedal,
the throttle opening degree increases stepwise and the engine
output also increases stepwise. The future estimated flow rate Qs
of steam supplied from the evaporator 11 to the expander 12 is
calculated based on this increase in engine output (that is, the
increase in the accelerator opening degree or the throttle opening
degree), the target rotational speed for the expander 12 is
calculated based on this estimated flow rate Qs, and it is
therefore possible to control the rotational speed of the expander
12 at the target rotational speed simultaneously with an increase
in the engine output without being affected by a time lag between
when the engine output increases and when the steam flow rate
actually increases (ref. region g).
[0043] As a result, even if the temperature of steam generated by
the evaporator 12 increases beyond the target temperature
accompanying an increase in the engine output (ref. region e), the
amount of increase can be made smaller than that with conventional
control (ref. region a in FIG. 12). Furthermore, feedback control
is carried out so that the amount of water supplied to the
evaporator 11 increases in order to suppress an increase in steam
temperature (ref. region f), but the increase in the amount of
water supplied can be made smaller than that with conventional
control (ref. region b in FIG. 12).
[0044] Because of this, the steam pressure hardly deviates from the
target pressure (ref. region h), and a situation in which the steam
pressure overshoots an upper limit pressure is reliably avoided,
thus preventing the operating efficiency of the expander 12 from
deteriorating or the durability from being adversely affected.
[0045] FIG. 6 and FIG. 7 show a second embodiment of the present
invention; FIG. 6 is a block diagram of a control system for the
rotational speed of an expander, and FIG. 7 is a diagram showing a
map in which an exhaust gas energy is looked up from an accelerator
opening degree and an engine rotational speed.
[0046] In the second embodiment, the arrangement of the estimated
flow rate calculation means M1 of the first embodiment shown in
FIG. 2 is simplified. The estimated flow rate calculation means M1
of the first embodiment includes the exhaust gas flow rate
calculation means M2, the exhaust gas energy calculation means M3,
and the steam flow rate calculation means M4, but estimated flow
rate calculation means M1 of the second embodiment does not include
exhaust gas flow rate calculation means M2, and only includes
exhaust gas energy calculation means M3 and steam flow rate
calculation means M4.
[0047] The exhaust gas energy calculation means M3 of the second
embodiment calculates an exhaust gas energy Eg by applying an
accelerator opening degree AP (a throttle opening degree TH) and an
engine rotational speed Ne to the map shown in FIG. 7. In the map
shown in FIG. 7, when the accelerator opening degree AP is
increased at the same engine rotational speed Ne, due to an
increase in the exhaust gas temperature and an increase in the
exhaust gas flow rate, the exhaust gas energy Eg increases
quadratically (ref. region j). When the engine rotational speed Ne
is high, the fuel injection quantity increases, the air/fuel ratio
decreases, and the exhaust gas flow rate decreases (ref. region
k)
[0048] In accordance with the second embodiment, since the exhaust
gas energy Eg is calculated only from the accelerator opening
degree AP (the throttle opening degree TH) and the engine
rotational speed Ne without using the exhaust gas temperature Tg, a
future estimated steam flow rate Qs can be calculated yet more
quickly without waiting for a change in the exhaust gas temperature
Tg. Since the air/fuel ratio A/F changes instantaneously based on a
fuel injection command, even if it is excluded from calculation of
the exhaust gas energy Eg, the responsiveness is not affected.
[0049] FIG. 8 and FIG. 9 show a third embodiment of the present
invention; FIG. 8 is a block diagram of a control system for the
rotational speed of an expander, and FIG. 9 is a diagram showing a
map in which an estimated steam flow rate is looked up from an
accelerator opening degree and an engine rotational speed.
[0050] In the third embodiment, the arrangement of the estimated
flow rate calculation means M1 of the second embodiment shown in
FIG. 6 is simplified. The estimated flow rate calculation means M1
of the second embodiment includes the exhaust gas energy
calculation means M3 and the steam flow rate calculation means M4,
but estimated flow rate calculation means M1 of the third
embodiment does not include exhaust gas energy calculation means
M3, and only includes steam flow rate calculation means M4.
[0051] The steam flow rate calculation means M4 of the third
embodiment calculates an estimated steam flow rate Qs by applying
an accelerator opening degree AP (a throttle opening degree TH) and
an engine rotational speed Ne to the map shown in FIG. 9. This
estimated flow rate Qs is a steam flow rate when the steam
temperature can be controlled at an optimum temperature, and since
a target rotational speed for the expander 12 that corresponds to
the estimated flow rate Qs is set directly, a final estimated flow
rate Qs can be calculated without depending on the amount of water
supplied to the evaporator 11.
[0052] In the first to third embodiments, when the range in engine
output is small, if the rotational speed of the expander 12 is
increased by estimating an increase in the steam pressure, there is
a possibility that the steam pressure might instead decrease
excessively. In a fourth embodiment, whether or not control by
estimating a steam flow rate is determined based on the flowchart
of FIG. 10 is carried out.
[0053] That is, if in step S1 an engine output Pse exceeds a
threshold value PSESW and there is a possibility that the steam
pressure might exceed an allowed maximum pressure (ref. the solid
line in FIG. 11), then in step S2 control by estimating a steam
flow rate of the first to third embodiments is carried out, whereas
if in step S1 the engine output Pse does not exceed the threshold
value PSESW and there is no possibility that the steam pressure
might exceed the allowed maximum pressure (ref. the dotted-dashed
line in FIG. 11), then in step S3 control by estimating a steam
flow rate of the first to third embodiments is not carried out, but
conventional normal control is carried out. This enables a decrease
in the output of the expander 12 due to an excessive decrease in
the steam pressure to be avoided effectively.
[0054] Although embodiments of the present invention have been
explained above, the present invention can be modified in a variety
of ways as long as the modifications do not depart from the subject
matter of the present invention.
* * * * *