Note: Descriptions are shown in the official language in which they were submitted.
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This is a division of commonly assigned copending Canadian
application Serial No. 356,838 filed July 23, 1980.
BACKGROUND OF THE INVENTION
This invention relates to heat engine control systems,
and in particular to a control system for a Cheng dual~fluid
engine.
Commonly assigned U.S. Patent 4,128,994 entitled
"Regenerative Parallel Compound Dual-Fluid Heat Engine'',
issued December 12, 1978 (referred to hereinafter as the
prior Cheng cycle patent) describes the dual-fluid (Cheng)
cycle heat engine. This engine, which employs parallel
Rankine and Brayton cycles, requires a critical balance of
operating parameters to produce high thermal efficiencies.
For any given set of cycle parameters, the prior Cheng cycle
patent referred to above, teaches that an efficiency peak
exists only at a unique ratio of Rankine to Brayton fluids.
Either too much or -too little Rankine fluid leads to reduced
cycle efficiency.
The prior Cheng cycle patent defines the peak :
operating condition cycle parameters to design an engine for
100 percent load. Because of the parallel combined nature
of the Brayton and Rankine cycles in this engine~ the
quantity and quality of steam that can be generated by a
given engine
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configuration can be varied freely over a range. The control
path for the steam cycle is esser,tially independent of that
for the gas turbine cycle. The control path for throttling
the engine is essentially free or undefined. Thus to reduce
engine pDwer from the peak operating points to reach partial
load output conditions poses a difficult control problem that
involves precision control of the air flow, fuel flow, and
steam flow.
In addition, because of the nature of the parallel
compound fluid engine, several independent parameters are
defined somewhat arbitrarily by the designer or fixed by some
operational constraint such as synchronous speed of a
generator for example. These include the compression ratio
(CPR), turbine inlet temperature (TI~), compressor RPM and
work turbine RP~, as well as those determined by the air,
fuel and steam flows, which are air-fuel ratio (A/F),
specific heat input rate (SRIR~, steam-to-air ratio (Xmix),
and total mass flow. Among the constraints on operating this
engine at variable load conditions are the boiler surface
area, boiler pressures, and the degree of superheat of the
steam. Taken together this array of parameters makes design
of a control system both difficult and unique.
The waste heat boiler for the dual-fluid engine
system is normally designed for the peak efficiency condition
at design loa~. Of course, once the heat exchanger is built,
the surface area for the heat exchanger is fixed. If one
desires to operate the engine at over~load conditions, the
required surface area to generate more steam is not available
unless the system has been designed with a boiler that is
oversized for the dcsign load condition. On the other hand,
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when the engine is op~Lated under partial load conditions,
the area of th~ heat exchanger is in excess of needs, thus
permitting operation at decreased differences in exhaust gas
and boiler temperature.
~ or a given turbine inlet temperature and
compression ratio, peak work output efficiency of the dual-
fluid cycle engine occurs only at a certain steam-to-air
ratio~ That ratio of steam-to-air is precisely define~ as
corresponding to maximum recovery of exhaust heat by the
steam within designated turbine temperature limits of the
engine~ Steam is generated by recovering the exhaust waste
heat at pressures that are relatively low when compared to
the pressures usually used in a steam Rankine cycle following
a gas turbine, the so-called combined gas/steam (COGAS)
system.
In the Cheng dual-fluid cycle system the steam is
injected into the engine before the work turbine and both
combustion gases and steam deliver work to the turbine.
Since the energy of the steam is derived from the exhaust of
the same work turbine, or turbines, the system contains a
feedback loop which must be solved in designing a control
system.
The Cheng cycle is complicated in other ways.
Unlike a gas turbine engine the exhaust temperature of the
Cheng cycle tucbine at a given inlet temperature and fixed
pressure ratio is no longer uniquely defined by the turbine
characteristics. It also depends on the steam-air mixture,
Xmix. Steam and combustion air have different thermodynamic
properties, namely, specific heats, and their ratio. Air has
a higher gamma function, i.e., specific heat ratio, than
steam. In expanding a mixture of combustion air and steam
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through a turbine more work is produced for a given pressure ratio expansion
than can be produced by expanding the air and the steam separately through
the same pressure ratio.
The details of this synergistic effect were disclosed in the
prior Cheng patent. As discussed there the peak efficiency can be identified
with a minimum "effective" temperature. But because the "effective"
temperature is a measure of the thermodynamic potential that cannot be
directly measured by a thermometer or thermocouple device, the feedback
control design is even more difficult. In this invention a control system
is disclosed to resolve these difficulties.
As disclosed in the prior Cheng patent, the maximum heat recovery
rate does not occur at the lowest waste heat boiler gas exit temperaturesO
The latent heat of evaporation of the steam in the mixture gas is generally
not recovered. Physically, if too much steam is used, the exit (engine
injection) temperature of the steam from the waste heat boiler is low due
to the large amount of water used to recover the waste heat. The heat loss
due to the latent heat content of thé exhaust gas exiting the boiler is very
large. On the other hand, if the steam quantities are insufficient the heat
exchanger exit temperatures of the exhaust products become excessive, and
the engine will not have reached its improved efficiency potential. For
a given set of parametric constraints the peak efficiency occurs at the
steam-air ratio corresponding to the maximum rate of waste heat recovery.
This is not known unless the constraints on the boiler design are given.
Traditionally, prior art control systems for gas turbines
adjust for the load on the gas turbine by merely varying the injection
rate of fuel, thereby increasing or
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decreasing the turbine inlet ~emperature. According to
thermodynamic laws a higher working temperature provides not
only higher work content but, generally, a higher thermal
efficiency. One would presume that in the dual-fluid cycle
however, the maximum continuous-operation turbine inlet
temperature corresponds to the maximum efficiency design point
of the engine. One would also presume that the partial load
condition could be obtained by merely reducing the amount of
fuel and steam injected into the engine system while maintaining
the maximum turbine inlet temperature~ However, neither of
these presumptions are correctO
SUMMARY OF THE INVENTION
In accordance with the invention of the parent
application higher efficiencies are maintained in the Cheng dual- -
- fluid cycle engine under partial load conditions by reducing
rather than maintaining the turbine inlet temperature. This
is accomplished by properly adjusting the steam-to-air ratio
and air-fuel ratio through a positive control system having
independent fuel and steam control loops. One of the
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impoLtant features of the subject invelltion is the linking of
all operating parameters at conditions that yield maximum
work output efficic-ncy for every partial and ov~r-load
condition across the entire load range This becomes a
primary objective of thé control system.
In accordance with the present invention, on the other hand,
i~ the engine is to be used for both work output and steam
- generation (co-generation) a different objèctive is demanded
of the control system~ The co-generation control path as a
boundary for controlling the engine operation is also set
forth~
The sets of combinations of engine operating
parameters for highest efficiency at each partial load is
-- computed by methods similar to those described in the prior
~ Cheng patent using engine-component specifications and
ambient conditions, except that the engine configuration is
fixed rather than "rubberized". In other words, in
initially designing the dual-fluid cycle engine, the
designer is free to use whatever component sizes he wishes,
but once the engine is designed and built, such freedom does
: not exist when the engine is operated at other than 100~
load conditions~ Consequently some of the fixed quantities
in the engine design using the referenced patents such as
temperature differences in the "neck" and "top" of the heat
exchanger, become variables in deriving a control scheme
while the heat exchanger surface area, which was free to
float to any necessary value to meet the temperature
difference specification, is now fixed~
In summary of the above, therefore, the invention of
the parent appllcation broadlv provides a control system for a
dual-fluid srayton/Rankine cycle engine comprising control means
for following a control path defined by the locus of peak
efficiency points at reduced loads, the control means including
(a) a first control system for controlling the srayton cycle
part of the dual fluid cycle engine; and (b) a second control
system for controlling the Rankine cycle part of the dual-fluid
cycle engine; whereby following the control path results in a
declining inlet temperature of a turbine portion of the engine
as the load decreases.
The invention of the parent application may also be
seen as contemplating the method of operating a dual-fluid
heat engine at partial load conditions, which engine comprises
a chamber; compressor means for introducing a first gaseous
working fluid into the chamber, the compressor means having a
predetermined pressure ratio (CPR); means for introducing a
seco.d liquid-vapor working fluid in the form of a vapor within
the chamber at a defined second/first working fluid ratio
(XMIX); means for heating the first gaseous working fluid and
the second working fluid in the vapor form in the chamber at a
defined specific heat input rate (SHIR); turbine means
responsive to the mixture of the first and second working
fluids for converting the energy associated with the mixture
to mechanical energy, the temperature of the mixture entering
the turbine means defining the turbine inlet temperature
(TIT); counter~low heat exchanger means for transferring
residual thermal energy from the exhausted mixture of first
and second working 1uids
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to the incoming second working fluid, the method comprising
the steps of: ~reheating the second working fluid in the
heat e~changer to a superheated vapor state prior to its
introduction within the chamber; and selecting XMIX and SHIR
so that (a) for a given value of TIT, XMIX is substantially
equal to or is greater than XMIXpeak, where XMIXpeak occurs
by hoth (i) maximizing the temperature of the superheated
second working fluid vapor; and (ii3 minimizing the effective
temperature of the exhausted mixture of the first and second
working fluids; and (b) TIT decreases as engine load decreases.
The present invention broadly provides a method of
operating a dual-fluid Brayton/Rankine cycle engine under
partial load conditions for maximi.zing co-generation of steam
comprising following an engine control path which maintains
tur~ine inlet temperature constant at varying loads, by
a) controlling the Brayton cycle part of the dual-fluid cycle
engine by means of a first control system; b) controlling
the Rankine cycle part of the dual-fluid. cycle engine by
means of a second control system; and c) setting the desired
operating points of the first and second control systems by
providing predetermined settings for all load conditions, the
settings comprising at least fuel flow rates and water flow
rates.
Furthermore, the present invention contemplates a heat
engine with steam co-generation comprising: a chamber;
compressor means for introducing a first gaseous working fluid
into the chamber, the compressor means having a predetermined
pressure ratio (CPR); means for introducing a second liquid-
vapor working fluid in the form of a vapor within the chamber at
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at a defined second~first working fluid ratio (XMIX); means for heating the
first gaseous working fluid and the second working fluid in
the vapor form in the chamber at a defined specific heat input
rate (SHIR); turbine means responsive to the mixture of the
first and second working fluids for converting the energy
associated with the mixture to mechanical energy, the
temperature of the mixture entering the turbine means defining
the turbine inlet temperature (TIT); counterflow heat
exchanger means for transferring residual thermal energy from
the mixture of first and second working fluids exhausted from
the turbine means to the incoming second working fluid;
means for co-generating process steam by diverting some or all
of the water.vapor exiting the heat e~changer from the
- chamber; means for preheating the second working fluid in the
heat exchanger to a superheated vapor state prior to its intro-
duction within the chamber; and means for selecting XMIX and
SHIR so that for a given value of TIT, XMIX is substantially
equal to or is greater than XMIX peak, where XMIX peak. occurs
by both; i) maximizing the temperature of the superheated
second working fluid vapor; and ii) minimizing the effective
temper~ture Of the exhausted mixture of the first and second
working fluids.
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3RIEF` DESCIRIPTIC)N OF TI~E DRAWINGS
Pig. lA is a schematic repr~sentation of a
simple gas turbine; Fig, lB of a simple steam turbine ~ngine;
and Fig. lC of a simple Cheng dual~fluid cycle ~ngine.
Fig. 2 is a hlock schematic representation of
a standard feedback control systém for a simple gas turbine
engine.
Fig. 3 is a block schematic representation of
a standard feedback control system for a simple steam turbine
engine.
Fig. 4 is a graphical plot of efficiency of,a
dual-fluid cycle engine as a function of air-fuel ratio at
various constant TIT.
Fig. 5 is a graphical plot of shaft horsepower
output for a dual-fluid cycle engine at various constant TIT
as a function of air-fuel ratio,
Fig. 6 is a graphical plot for a dual-fluid
cycle engine of Xmix vs. the air-fuel ratio at various
constant TIT.
Fig. 7 is a block schematic representation of
a ,control system for a dual-fluid cycle engine, in accordance
with the present invention~
Fig. 8 is a graphical plot of the waste heat
boiler (heat exchanger) of a dual-fluid cycle engine as a
function of air-fuel ratio both for a design point and "real"
heat exchanger.
Fig. 9 is a graphical plot of engine
efficiency as a function of horsepower per pound of air-flow
per second for various constant TIT, for a typical dual-fluid
cycle engine. ~
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Fig. 10 is a graphical plot of fuel flow as
funetion of horsepower per pound of air flow per seeond, at
various eonstant TIT, for a typical dual-fluid cycle engine.
Fig. 11 is a typical graphical plot of steam
flow as a function of horsepower per pound of air flow per
seeond, at various eonstant TIT, and for a dual-fluid eyele
engine at various load conditions.
Fig. 12 is a typical graphical plot derived
from Pigs. 9-11 of steam-to-fuel as a function of horsepower
per pound of air flow per seeond or at various load
eond tinns of ~ typieal du~l-fiuiù eyele enqine.
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DESCRIPTION O~ THE PREFE~ED EMBODIMENTS
To facilitate understanding the con~rol system for
such a unique engine system~ the Cheng dual-fluid cycle
engine can be superficially dissected into two parallel
operating cycles as shown in Figs. lA and lBD Fig. lA
illustrates a simple gas turbine engine 10 which includes a
compressor 12, a combustion chamber 14, a turbine 16, and a
load 18. Ordinarily, the controlling parameterS are the
turbine inlet temperature, TIT, the compression ratio, CPR,
and the engine RPM
Sometimes, due to the reluctance to put measuring
probes before the highly-stressed rotating turbine, the
turbine e~haust temperature is used as a measure of TIT~ The
turbine outlet temperature TeXh, is linked in a one-to-one
corresponding fashion with the turbine inlet temperature
through knowledge of the turbine characteristics. Thus the
controlling parameter is the fuel flow to the system which
effects the turbine inlet temperature of the engine directlyr
The engine has operating limits, of course, due in
part to the inertial nature of the heavy engine rotor 20.
Therefore the fuel control system must have a differential
loop that anticipates the engine response and programs the
fuel requirement to produce the response to load variation
without exceeding turbine inlet temperatures. The gas
turbine has an inherent partial feedback loop because of the
fact that the t~rbine-produc~d power drives the compressor.
Therefore, only the power in excess of that reyuired by the
compressor is available to drive the load~ -
FigD lB depicts a steam (Rankine) cycle enqine 22
operating within a Cheng-cycle duai-fluid engine includin~
fired boiler 24, heat input 26, turbine 28, load 18, waste
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heat boiler or heat exchanger 32, and pump 34. This SyStem
is unioue in that the steam expanded through the turbine 2~
gives away much of its discharge heat to the oncoming water
and steam in line 36 in waste heat boiler 32, After
discharge from the boiler the exhaust steam, line 3~, is
condensed in condensor 40 for return and reuse in the cycle
or wasted in an open cycle. In either case feed water is
pumped through the waste heat boileL 32 to become superheated
steam, The steam is ~ed into the fired boiler 24 and heated
further to the designed gas turbine inlet temperature, deter-
mined by the turbine inlet temperature of the gas turbine of
Fig lA before it is expanded through turbine 28. The ~ired
boiler 24 is eliminated when the two cycles are operating in
parallel because the additional heating of the steam is done
by mixing with the air combustion products. If the steam
cycle were not operating in parallel with the gas turbine,
the steam would have to be heated by an external heat source
in the fired boiler to reach such high temperatures.
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Fig. lC is a block schematic diagram of the Cheng
dual-fluid cycle engine 42 illustrating the two cycles in
parallel. Where applicable the reference numera~s of
components of Figs. l~ and lB are used in Fig. lC. A
detailed description of this engine cycle is set forth in the
prior Cheng patent referenced above. Added to the heat
engine 4~ of Fig. lC is a waste water clean-up 43 before pump
34.
An important feature of operating the two cycles in
parallel is that the waste heat of the combustion gas and
steam passing through line 44 is recovered by the recycled
steam passing out of the waste heat boiler 32 through line
46,
The combining or superimposition of the two cycles
in a parallel arrangement simplifies the component mechanical
arrangement of the powerplant. The engine has only one
output shaft 4~; additional heating of the steam by the
combustion products is slmple; and the steam turbine is
eliminated. This represents a significant saving in costs of
producing such an engine as compared to the typical "combined
cycle" engine. However, the control functions become more
complex.
Although the Cheng cycle can be dissected
super~icially into two parallel cycles, the mutual
interaction between the gaseous working fluid of the Brayton
cycle and the liquid vapor working fluids of the Rankine
cycle produces more than parallel work output. Air has a
gamma function close to 1.4 and steam has a gamma function
close to 1.28. The gamma function ~specific heat ratio) is
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implicitly ~ep~ndent on both tempe;ature and pressure. The
mixing of the working fl~ids makes the expansion process
dependent on the mixture ratio of the working fluids as well,
because the proportional relationship between turbine inlet
temperature and exhaust temperature of a simple gas turbine or
a simple steam turbine at a given pressure ratio and component
efficiency does not hold for a Cheng-cycle system.
Por partial load operation conditions, both the
turbine inlet temperature corresponding to best efficiency and
the resPective working fluid ratios have to be computed for an
engine of fixed hardware components. Einding the partial load
efficiency peak o' a given engine experimentally, while
possible, would be time consuming and difficult due to the
double feedback nature of the Cheng cycle. Instead, the
correct operating parameters for each point representing a
range of partial load operations can be calcuiated using the
method indicated in patent 4,128,994 except that the
calculations must be made at partial load and with real
(fixed) engine component characteristicsO The highest
efficiencies at reduced load occurs, surprisingly, at reduced
turbine inlet temperatures, which the control system produces
as a result rather than as a controlled reference variable.
To illustrate the differences in the control system
of the present invention a control system concept applicable
to the series co~bined cycle (COGAS) engine is first
described. In Fig. 2, a standard feedback control system 50
for a simple gas turbine ;s illustrated schematically. R
indicates a desired setting based upon the engine loadO R
is compared with a feedback signal indicative of the actual
output setting at comparator 52. An error signal El results
which is sent to control unit 54, which provides a signal
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to increase or decr~ase the fuel flow dep~nding on ~rror El-
As the fuel flow changes, a new f~edback signal Cl based upon
the new actual fuel flow, is sent back to the comparator 52.
Control unit 54 has certain boundary limits in order to avoid
compressor stall. The control system 50 must have a built-in
auxiliary control loop for engine start-up, and shut-down but
it is not discussed here.
The controlling parameter in this case is the fuel
flow rate, which establishes a certain air-fuel ratio to
obtain the turbine inlet temperature necessary to produce the
desired temperature.
In Fig. 3, the same simple control loop 50' is
diagrammed for the Rankine or steam cycle system. In a COGAS
system, the stea~ cycle follows the gas turbine cycle in a
serial fashion. A preset load condition, as given by R2, is
compared with the feedback signal C2 at comparator 52, which
in this cas~ can be the steam temperature, the turbine
exhaust temperature, boiler pressure, etc~, to produce an
error signal E2. This signal is sent to the controller 54
The controller 54 either increases or decreases the steam
flow rate as the control parameter instead of the fuel flow
rate in the qas turbine system 50.
To compensate for the time lag of components
response, such as the inertia of the turbine wheels for
acceleration and deceleration, boiler pressure build up and
blowdown, it is necessary to incorporate integrated response,
proportional control and differential controls described
above into both the gas turbine 50 control loop and the steam
cycIe control loop 50'. The integrating control smooths out
small, fast variations-within a characteristic time, so that
the engine control does not have to chase the high frequency,
small perturbations. If a sudden increase of engine load is
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requir~d, the differential response advances the fuel and
steam rate beyond the steady state flow rates temporarily,
then transfers the control function to proportional control,
Gl which drives the control parameters to steady-state values
for the load. The reset times, having characteris~ic time
tailored to the component characteristics, must be matched
with component systems dynamics~ ~f the engine is unable to
reach the steady state condition within the reset times T
and T2, then a reset mode is started.
With the dual-fluid cycle engine the air, fuel,
and steam flow rates can be adjusted to maintain the
continuous rating maximum turbine inlet temperature~ As
pointed out previously, however, the design continuous-
rating maximum turbine inlet temperature does not produce
the highest efficiency at partial loads. ~1hen the DFC
engine is operated over a range from idle condition to
design load condition, the fuel/air ratio and steam air
ratio operating parameters are essentially free unless some
discipline is imposed to control them~ This freedom is
attributable to the somewhat independent nature of two
feedback loops and the parallelism of the two cycles.
There are significant differences in finding the
peak efficiency for off-load conditions for an engine which
has already been built, compared with calculation of such
peaks during t~he design of a dual-fluid cycle engine, which
is described in the prior Cheng patent. These differences
relate directly to the fact that a "real" engine system
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has specific fixed components~ T}.ese components have
characteristic performance values which must be taken into
account in determining best engine efficiencies at loads
different from the design.load~
Thus, in designing the parameters for maximum
efficiency for off-design load conditions for a "real"
engine, one must take into account the following: 1) In
the design case the surface area of the boiler is a
computed result at given temperature difference
constraints; but for a "real" engine the surface area of
the boiler is fixed and the temperature differences are
variable. 2) The turbine and compressor efficiencies of a
real engine vary somewhat with air, fuel and steam flow
rates, and engine RPM, and thus component efficiency maps
must be programmed into the control to account for these
performance variations~ 3) Pressur.e losses through the
system, particularly through the combustorj the steam
injector, and the boiler also vary with flow rates, and
these variations must be programmed into the control
functions. 4) The component response time in the engine
systèm, particularly in the thermal lag of generating steam
in the boiler and in the inertia of the rotor mass, must be
accounted for in designing the control response. This is a
factor not apparent in designing a "steady-state" operating
condition for the en~ine.
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Figs; ~, 5 and 6 are a series of plots for a
sp~cific enqine to illustrate the control functions for
that engine, in accordance with the present inv~ntion.
Fig~ 4 illustrates the thermal efficiencies for a given
known, single-shaft turbine engine in terms of air/fuel
ratio of a dual-fluid cycle engine at different load
conaitions. A number of examples of turbine inlet
temperature conditions, namely 1400~F, 1450F and 1500DF
are shown, as an independent parameter within the plot.
For this example, the engine has a compression ratio of 7.3
to 1, inlet pressure of 14.3 PSIA exhaust pressure of 15.1
PSIA, inlet temperature of 80F and 60% relative humidity~
Performance is plotted against air/fuel ratio in
Fig. 4 because this represents the energy input to the
engine 42. For a given turbine inlet temperature and
compression ratio, there is only one air-fuel ratio
corresponding to each load requirement at which the thermal
efficiency reaches the peak. An engine design point
control path 60 links all the peak efficiency design points
at different TITs with a fixed ~T "top" and "neckn~ This
graph shows clearly the unexpected result that maximum
efficiencies are achieved by a lowering of the turbine
inlet temperature as the partial load gets smaller.
Fig.~S represents the power output per pound of
air flow through the turbine of Figure 4 for the same
turbine inlet t~mperaturés, as a function of the air-fuel
ratio, One can see that the peak thermal efficiency for
such a single-shaft, constant RPM system, for which the
compressor air flow rate is nearly constant, occurs at
'discrete air-fuel ratios. The loci of such points is
indicated by the control path 63. If, in throttling the
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engine for lesse~ load (increas~d air-fuel ratio) one
changes the air-fuel ratio and the steam-to-air ratio to
maintain a fixed turbine inlet temperature at the lower
power output of the engine, the engine efficiency falls off
faster than if the turbine inlet temperature is also allowed
to fall, and the steam-air and air fuel ratios re-optimized.
In other words a constant TIT at varying load conditions is
not desirable to maximize enaine efficiency.
Fig. 6 presents the data already shown in Figs. ~
and 5 as a cross plot of steam-air flow ratio, Xmix, versus
air-fuel flow ratio, at varying partial loads. This plot
shows that the Steam-tO-air flow rate along a constant TIT
operating path is highly nonlinear and as a matter of fact
is discontinuous around peak efficiency points. The optimum
efficiency operating line for a Cheng dual-1uid engine thus
involves changing the air-fuel ratio, steam-air ratio and
turbine inlet temperature simultaneously when going from the
design load condition ~o a lesser load condition~
Normally the engine design is optimized at the
maximum efficiency point corresponding to a continuous-
running maximum turbine inlet temperature. Assuming this to
be the case, to accommodate power loads above the design
load of the engine, requires departure from the (apparent)
optimum efficiency operating line in order not to exceed the
assumed limiting turbine inlet temperature. Thus the "best"
operating line has a discontinuity ~change of slope) at or
near the maximum efficiency point 72 for 100 percent load.
From the foregoing, the preferred engine operating
line, corresponding to a variable turbine inlet temperature,
are specified such that for a given load condition the
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correct air-fuel ratio and steam-air ratio to maintain high
thermal efficiency at partial loads can be obtainedO ~7hen '
fixed component conditions are realized, the engine can not -
follow the design point operating line at over load due to
the lack of heat exchanger surface area. At less than 100~
load the surface area allows the temperature differences ~T
"top" and ~T "neck" to be reduced below that allowed in the
design. The "real" control path is shown as 60' in Figs.4, 5
and 6. This is explained in detail later. The control
system described next is designed to achieve this result.
Fig. 7 is a block schematic of a control system 80
for a Cheng dual-fluid cycle engine. Control system 80 uses
a pre-computed multi-dimensional contoured map stored in
master memory 82. This stored map contains operating
conditions which link the operating parameters for h;gh
efficiency operation at each load condition for the dual-
fluid cycle engine. The control load se~ting R triggers pre-
determined signals Rlo and R20 from me~ory 82 to separately
establish desired operating points for a gas turbine servo
system 84 and a steam servo system 86. The respective servo
lo~ps 84 and 86 then operate independently of each other to
control, respectively, the fuel flow rate and the steam flow
rate so that the engine operates at the locus of the highest
peak efficiencies for that load, i.e. along the optimum
control path shown in Figs. 4-6. No over-all control
feedback is provided; only feedback is provided within the
separate gas turbine and steam control loops.
The components of the gas turbine control loop 84
and steam control loop 86 are basically the same as those
~hown in Figs. 2 and 3 respectively for a gas turbine and
steam turbine engine. Thus where appropriate, identical
numerals used in Figs. 2 and 3 are used in the following
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description. Referring to the gas turbine portion 84 of the
control SyStem 80, the input load setting Rlo, set by memory
82 goes to the error comparison box 52 where the actual fuel
flow is compared with the desired setting Rlo to produce
error signal ~1 This signal is acted upon by control unit
54 which provides a control feedback signal Cl for
controlling the fuel flow. Controlling fuel flow basically
controls the engine heat input rate. Controls to compensate
for barometric pressure, ambient temperature, etc. are not
described here but are within the scope of skill in the art
of gas turbine controls.
Since steam passes through a dual-fluid cycle
engine in parallel with the combustion products of air, the
turbine inlet temperature is no longer uniquely determined by
the fuel flow at a given load. The steam flow rate is
equally importantt The steam flow rate is controlled by the
steam servo system 86. The input load setting R20, set by
memory 82 in response to a desired engine load condition,
goes to comparator 52, producing an error signal E2. This
signal is acted upon by steam controller 54, which produces a
feedback control signal C2 which establishes the correct
steam flow rate. The steam controller senses the boiler
pr~essure and temperature. The combination of the control
signals Cl and C2 provides the right control operating
signals C3 to accommodate the actual load output such that
the engine can be operated on the peak efficiency contour for
the given load requirement.
The differential, proportional, and the integratins
signal processo~s in the respective steam and gas control
loops 86 and 84 provide conventional servo-system control.
The flow rates of air and steam may be momentarily
increased or decreased beyond the ultimate steady state flow
rates corresponding to the new load command. The flow rate
overshoots are designed a priori from knowledge of the engine
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compon~nt dyr)~mic characteristics so that physical or
characteristic performance limits of the equipment ar~ not
exceeded. In particular the fuel-air-steam ratios must be
kept within the limits that would cause ov~rheating of the
turbine, and varied in a way to avoid compressor stall or
surge. Fuel flow may not be dropped so suddenly with
decreased load as to cause combustor flame out.
These dynamic limits are keyed into the
differential processor, which compares the anticipated
variation with the rate of change of fuel and steam flow
rate That is, it differentiates the flow rates with respect
to time. Upon increase or decrease of load the differential
processor controls the fuel and steam flow rate variation to
ini~ially overshoot the steady state flow requi.ement, then
rapidly adjusts the overshoot to steady-state flow rates for
the load~ `
The differential response is in action only if the
rate of change of flow rate is larger than a preset rate of
change of flow rate. The threshold function can be proviaed
by a mechanical-hydraulic system, such as a spring loaded
check valve, or by an electrical system having a threshold
trigger voltage with a ramp function. If the rate of change
of the flow rate is smaller than the preset rate of change of
flow rate, the control signal simply goes to proportional
control. The proportional control must be provided with a
certain gain in terms of mechanical advantages or electrical
amplification. The fuel flow rate is set according to the
master control 82 commandj
The error signals El and E2 are processed through
integrating signal processors in both the gas and steam
controllers. Any control system can be plagued with some
high frequency short-duration noise inputs which are not true
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I
commands from the operator; mechanical vibration to the
control handle transmitted through the floor, for example.
Because the turbine system of a rotary machin~ has rotational
energy stored in the rotor the engine does not have to chase
short-time duration changes. Therefore, an integrated
processor is used to filter out noises as much as possible.
The characteristic time of the integrated processor can be
determined a priori from the known engine system component
dynamics.
The boiler 32 is an energy storage system that
warrants the reguirement of an integrating processor. For
the boiler 32, in contrast to the turbine 16, inertia of the
turbine wheels is replaced by thermal lags.
In accordance with the invention, the operating
contour is determined a priori. The memory system 82 stores
precomputed values for all the operating parameterS, i e.,
different RPM's, pressure ratios and temperatures. The
memory 82 produces the two signals Rlo and R20 dependent upon
the particular set of operating parameteYs called for, and
the load condition. This memory control system 80 need only
obtain the command of an engine operator for a different load
condition to determine what operating parameters are
necessary to regulate the engine for operation at the peak
efficiency.
The controlled parameters are: (1) fuel flow rate
and (2) the steam flow rate. The memory 82 essentially
eliminates the freedom of the control paths of the parallel
combination of Brayton and Rankine cycle. As a result, for a
given load condition away from peak load condition, the
turbine inlet temperature is no longer maintained at the
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highest turbine inlet temperature for a given throughput.
Rather, the turbine inlet temperature is computed for the
load setting from a known engine operating map and stored in
the memory system 82.
Therefore the control of the engine is accomplished
by reference to the memory 82 in such a way that the load
setting provides commands to the fuel setting and the steam
flow rate setting without an over-all feedback control. The
memory control system 80 constitutes an open-loop, positive
control system rather than a feedback control system on top
of two individual feedback systems~
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~.
` I
DESIG~ OF Tll~ CONTROL SYSTEM
To design a control system 80, for an actual Cheng
cycle engine it is r~quired to know the control path a priori
and to store it in the memory 82. Figs. 4, 5 and 6 show
typical steady state operating characteristics of a single-
shaft dual-fluid cycle engine. These allow one to determine
the limitations and characteristics of all the possible
control paths, as will be explained.
The prior Cheng patent allows one to determine peak
operating regions for a particular dual-fluid cycle engine
design. A temperature difference ~Ttop and ~Tneck' for
the heat exchanger 32 is assumed on the grounds of heat
exchanger size and economic trade off. The required surface
area for the heat exchanger varies at different turbine inlet
temperatures, TIT, and compression ratios, CPR.
Once the engine is designed~ the heat transfer
surface areas for superheater, evaporator and economi7er for
heat exchanger 32 are fixed. The temperature differentials
p and ATneCk are no longer design constraint
considerations. ~nder partial load conditions excess surface
area is available in the waste heat boiler. Th~s, the heat
transfer area is greater than that required to maintain ~Ttop
and ~TneCk at less than 100 percent load. Consequently
these temperature differences decrease. The fixed surface
area of heat exchanger 32 is inadequate, however, to maintain
the design ~Ttop and ~TneCk along the overload control
path above 100~ load.
The overload and partial load surface area
requirements are shown in Figure 8 which plots the heat
exchanger 32 area per unit of air flow as a function of air-
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~77~63
fuel ratio both for the design case 90 and an actual heat
exchanger case 92. The n real" heat exchanger case 52 is a
hori~ontal line passing through the design operating curve 90
at 100% load. The additional heat exchanger surface area
available at partial loads means smaller ~ Ttopr hence
higher engine overall efficiency a~ partial load. This is
indicated by the cross-hatched area 94.
An example is now given to show how one chooses the
control path of the Chena cycle engine with given component
sizes and to show how to create a control map to be stored in
the master memory 82. As explained, one follows the
teachings in U.S. Patent No. 4,128,994 to first create engine
design maps as shown in Figs. 4, 5 and 6. Since peak
efficiency conditions will shift due to changed ambient
conditions, temperature, relative humidity and atmospheric
pressure become direct inputs to the master mernory 82 to call
out the corrections. For purposes of this description,
however, it is assumed that only constant ambient conditions
exist.
Once the 100~ load point is picked, the surface
area of the waste heat boiler 32 is fixed and the engine no
longer operates exactly according to the design point peaks,
i.e. along control path 60 shown in Figs. 4, 5 and 6. This
is indicated by the actual optimum control path 60' in Figs.
4, 5 and 6. This optimum control path is derived as follows.
Referring to Fig. 8 if one picks TIT=1500DF, and
the peak efficiency point is designed for 100~ of load, then
the operating map is generated as a perturbation over that of
the engine design map. This is accomplished by examining the
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I 1l~7263
horizontal control path 92 of ~realn heat exchanger 32. As
explained, improved efficiency results in the par~ial load
region due to the availabie excess heat exchanger area. As
also explained, during partial load conditions the prescribed
maximum temperature difference between the exhaust gas
mixture and the incoming steam are removed. The new peak
operating point is computed based upon a steam/air ratio
which satisfies the two conditions, set forth in the prior
Cheng patent, simultaneously~ maximum heat recovery from
the exhaust gases, i.e., lowest effective temperature of the
exhaust gases and t2) maximum superheat steam temperature
prior to entering the combustion chamber 14.
These seemingly conflicting conditions can be
obtained due to the fact that superheater portion of heat
exchanger 32 heats steam from the evaporator portion to as
high a temperature as possible; and the evaporator of heat
exchanger 32 heats as much water into steam as possible. So
the two conditions are realized at different parts of the
system. Thus, the two limiting factors to the operation of
the heat exchanger 32 are: (1) its fixed surface area (2)
its self-limiting temperature profile. So to compute the
peak operating oonditions for partial load conditions for an
engine with a fixed heat exchanger one must use the given
superheater evaporator areas as boundary conditions and
remove the temperature difference constraints used in the
prior Cheng patent.
Referring again to Fig. 8, if one wants to operate
in the overload region, there is not enough surface area in
heat exchanger 32 to generate the superheat steam, so the
control parameters must change in a way which is below the
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~L17726~
designed maximum efficiency points. This is shown in Figs.
4, 5 and 6, above 100% load.
The control memory 82 reauires information on 103d
versus steam flow and fuel flow, at given ambient
temperature, relative humidity and atmospheric pressure. The
peak efficiency points defining one optim~m control path are
therefore recalculated from the information in Figs. 4, 5 and
6. ~his is compiled in ~igs. 9-11.
Fig. 9 is a plot of efficiency, Fig. 10 is a plot
of fuel flow, and Fig. 11 is a plot of steam flow all as a
function of horsepower per pound of air flow per second.
The control path hn shows the control path of maximum
i efficiency. The load output in terms of horsepower for full
and partial load is normalized to one pound per second air
flow through the compressor, such that for a single-shaft
constant-RPM engine for generating electricity the air flow
rate is essentially a constant. Other engines having power
turbines with variable RPN load, follow essentially the same
procedure only the normali~ation factor is connected by air
flow, due to RPM change according to a compressor map.
~ The steam flow and fuel flow, as a function of
load control path is then taken directly from Figs. 10 and
11 and is stored in master memory 82.
As an alternative method to store control path
information, one can l;nk fuel control to steam flow rate.
From Figs. 9, 10 and 11 one can cross plot the control path
60 as a ratio of steam-fuel flow ratio as a function of
horsepower per lb. of air flow per second as shown in Fig.
12. These relationships can be stored in memory 82 to link
the fueI control with steam control.
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. 1177;~63
Of cours~ the pa~ametric relationships stored in
memory 82, bas~d upon predetermin~d calculations May have to
be altered sligh~ly for actua~ better engine operation, Such
"fine tuning" is accomplished experimentally.
CO-GENERATION
Efficiency of an engine system can be ~easured in
either of two ways; one as a fraction of the available energy
that is converted to work (shaft horsepower) output; the
other the fraction of the total available energy that is
utilized as either work or heat, Co-generation is a total
energy system which utilizes the waste heat to generate
process steam. The energy of the process steam is counted as
an output of the system so that the overall efficiency is
increased,
From Fig. ~, it is apparent that an alternative
control path 96 for the engine is to reduce the steam-flow
rate while reducing the load output so that the engine runs
at maximum designed turbine inlet temperature. The excessiv~
boiler 32 surface area is then used for co-generation, such
that only the proper amount of steam is used in the Cheng
cycle engine 42 at a given load.
. The co-generation path 96 represents another limit
of the choice of control path. Here the control path is
bounded by constant maximum TIT, The first limit is the
control path l~nking all the highest possible efficiency
points at constant heat exchanger surface area. This follows
the horizontal path 92. Other paths can be chosen in the
region between path 92 and 96 to satisfy specific
applications but generally they are bounded by the co-
generation path 96 and the optimum engine efficiency control
path 92~
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SINGLE SHAFT VARIABLE-SPEED ~NGINE
Pigs. 4, 5 and 6 show the complex nature of
identifying the peaks for a single-shaft single-speed dual-
fluid cycle engine. If the variable load can be accommodated
by changing RP~ then air 10w and pressure ratio become
dominant operating variables~ Changing RPM occurs in the case
for pumping a fluid, in contrast to driving a utility
generator, where synchronous turbine-generator speed must be
maintained. To identify the operating conditions for peak
efficiencies for an engine with these additional variable
parameters, two more operating maps are required. One can
regard Figs. 4, 5 and 6 as representative cross sections of
three-dimensional maps where the additional dimension
represents either compression ratio or air flow. Such plots
can be readily made using the methods similar to those defined
in the prior Cheng patent but with some d;fferences in
parametric conditions for each compression ratio or air flow,
and then plotted or prepared as a contoured multiple-
dimensional surface map.
D~AL SHAFT ENGINES
The use of a dual-shaft arrangement for the Cheng-
cyrle engine system perrnits the use of a variable-speed turbine
to drive the compressor to supply whatever air flow is required
to accommodate the load at high efficiency. To permit such
variable-speed compressor cperation the first turbine, or core
turbine, that drives the compressor can be equipped with
variable stator vanes. The variable stator vanes in such a
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li77263
case do not add another degree of freedom to the engine
¦operation because the position of the guide vanes is
¦determined by the required air and steam flow~
¦ The added freedom of variable air flow and
¦compression ratio required to drive the work turbine, which in
¦turn may be either constant-speed or variable-speed, under its
¦load condition, constitutes in essence a problem similar to
¦that discussed above for the single-shaft variable-speed
¦engine. The computations required to identify the maximum
efficiency conditions corresponding to each load become
¦increasingly complex. The best-efficiency optimum operating
¦condition can be represented by a multiple-dimensional
¦contoured surface in which each point represents the
¦simultaneous variation of several of the engine operating
¦parameters~ It is evident that this control must be able to
¦process the additional (compressor RPM or pressuse ratio)
input in determining how to establish the new operating
parameters to accommodate the load, and it must be equipped
¦with differential, proportional and integral control responses
¦ that operate on the additional input. From the multi-
¦ dimension control surface, a unique control path is singled
out for a specific Cheng cycle engine.
¦ It is apparent that such plots justify preparation
¦ only with specific engine component performance figures, figs.
¦ 4, 5 and 6, being for a specific engine configuration. The
above discussion is considered sufficient to illustrate the
¦ method of the invention. The task of the control device that
constitutes this invention is to operate along a control path
on the multi-dimensional contour surface that represents the
¦ highest efficiency operating condition within the mechanical
¦ constraints of the equipment for each load.
It is clear that the design of a control system for
a particular dual-fluid engine is highly specific to the
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characteristics of the components of that particular engine
configuration~ Furthe-rmore,-it is also clear that there are or
can be innumerable mechanical, hydraulic or pneumatic servo
devices involved in such control systems, including analog or
digital computer devices to relate the inputs which are not
disclosed herein but which are within the capabilities of one
skilled in the art. For example protective devices would be
employed. RPM governors, high temperature turbine inlet
temperature limiters, boiler pressure limiters, and so on,
which are not shown in the controller block background, w~uld
be integrated into the control system as in other prior art
systems
The link of the two parallel cycles is through the
memory system 82. The memory 82 is preprogrammed
electronically in a memory storage bank or by a mechanical cam
and gear system or by a combination of both. For highest
efficiency operation at any load the system must be programmed
to guide the dual-fluid cycle to operate along the path which
links the peak efficiency points. If the engine is ~o be us~d
to co-generate power and heat, then a different control path
which maintains highest continuous rating turbine inlet
temperature and maximizes steam production at that operating
condition is used. All dual-fluid cycle engine control paths
will ordinarily be bounded by conditions corresponding to their
ob~ectives,
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