Boiler economizer and control system

An increased efficiency boiler is provided, which, instead of trying to eliminate steaming in the economizer, designs the economizer to permit steaming, and a control is provided for the boiler which takes into account the heat input to the boiler as well as the water level in the steam drum and the reliability of the economizer.

BACKGROUND OF THE INVENTION 
The present invention relates to boilers, and, in particular, to a boiler 
which includes an evaporator and an economizer. 
In boilers of the type referred to above, water enters the economizer at a 
relatively low temperature and, in the economizer section of the boiler, 
is usually heated to just below the boiling point. Then, the hot water 
passes into the evaporator portion of the boiler, where it boils. The 
water and steam are separated in a drum, and the steam may then go on to a 
superheater, where it is heated to a temperature higher than its boiling 
temperature. The steam which leaves the boiler may then go to a turbine, 
where it performs work. 
In the prior art, there have been many problems with these boilers. There 
is sometimes a problem with vaporization taking place in the economizer. 
In many cases, in order for the boiler to work most efficiently, the water 
which leaves the economizer must be close to the boiling point. However, 
if the water begins to boil in the economizer section, it can cause 
problems. The vapor can become trapped, causing vapor lock and water 
hammering, as well as fatigue, which can damage the boiler. 
This problem occurs often under transient conditions. For example, if there 
is a need for a greater steam flow, the valve in the steam output line 
from the boiler is opened, reducing the pressure in the boiler. With the 
reduced pressure, more fluid boils in the evaporator. The rising volume of 
steam bubbles in the boiling water causes the water level in the drum to 
rise. If the water level goes too high, the steam quality is reduced, with 
some water entrained in the steam, and some water can enter the 
superheater and eventually damage it. Even if the steam does not go on to 
a superheater, the steam quality is important, and the water level in the 
drum must be maintained in order to maintain the steam quality. To prevent 
the water level from becoming too high, the water input to the boiler is 
reduced. With less water flow into the economizer, the water in the 
economizer is more likely to boil, creating the vapor lock, water hammer, 
and fatigue problems. 
A common solution to this problem is to put a control valve or a small 
orifice in the line between the economizer and the evaporator, controlling 
the feed water supply, in order to raise the pressure in the economizer, 
making it more difficult for the water to boil. However, that means that 
the boiling takes place in the control valve or orifice instead, causing 
the valve or orifice to fail. It also means that more power is consumed, 
because the feed water pump must pump water across that large pressure 
drop, thus decreasing the efficiency of the power plant. 
Another common solution is, once boiling begins in the economizer, to cause 
the feed water to bypass the economizer and go directly to the evaporator. 
This means that the economizer is not functioning for a good part of the 
time the boiler is operating, thereby greatly reducing the efficiency of 
the boiler. It also means that the economizer cycles between hot and cold 
as it goes from dry to wet, which causes wear and tear on the economizer. 
U.S. Pat. No. 4,582,027 "Cuscino" shows a boiler in which the problem of 
boiling in the economizer is partially addressed. In this patent, a 
well-known bypass is provided, so that, under low load and start-up 
conditions, some of the fluid that has gone through the economizer does 
not go to the evaporator but is, instead, returned to the economizer. This 
keeps flow rates high enough to prevent boiling in the economizer. The 
teaching of this patent is intended to solve the problem of steaming in 
the economizer only during start-up and low load conditions, and for short 
periods of time--not during high flow rate conditions, where the boiler 
should be operating to be most efficient. 
SUMMARY OF THE INVENTION 
The present invention provides a boiler which is very efficient, because 
its economizer can operate continuously, whenever the boiler is in 
operation. 
One embodiment of the present invention provides an economizer which 
includes at least one upwardly-flowing, single pass module at the end of 
the economizer so that, if the fluid boils at the end of the economizer, 
there is no problem. Instead of making various efforts trying to prevent 
steaming in the economizer section, as taught in the prior art, the 
present invention designs the economizer section so that steaming in at 
least part of the economizer does not create a problem. This means that 
the economizer can operate in the most efficient temperature range, 
bringing water right up to the boiling point, without causing problems. 
This also eliminates the need for valving or orifices to cause the 
pressure to be much higher in the economizer than in the evaporator. 
One embodiment of the present invention provides a control system which 
effectively controls the feed water supply to the boiler so that the 
boiler continues to operate reliably, even under transient conditions. 
This control system can function with an economizer comprised of vertical 
tubes as shown in the drawings as well as with other types of economizers, 
including, for example, those with horizontal or inclined tubes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a combined cycle power plant 10, in which the exhaust from a 
gas turbine 12 is used to provide heat for a steam boiler 16. The steam 
from the boiler 16 drives a turbine 14, which drives a load 18, such as an 
electrical generator. 
The steam boiler 16 includes a horizontal gas duct 20, having a gas inlet 
22 at the upstream end and a gas outlet 24 at the downstream end. 
The steam boiler 16 receives water from a feedwater supply pump 45. The 
water passes through a water inlet control valve 62, through a water inlet 
conduit 47, and into the boiler 16. The water first passes through an 
economizer section 30, where the water is heated to a temperature that is 
close to boiling, then through a conduit 56 to a drum 27. The water then 
passes down through a conduit 29 in the evaporator section 26. In the 
evaporator section 26, the water is heated to the boiling point. 
It should be noted that, when referring to the flow of heating gas in this 
description, upstream is in the direction from which the heating gas 
enters the boiler (generally left in FIG. 1), and, when referring to the 
flow of water in this description, upstream is in the direction from which 
the water enters the boiler (generally right in FIG. 1). Since the water 
and heating gas flow in generally opposite directions, the upstream 
direction will also be generally opposite, depending upon whether the 
description is of the heating gas or the water. 
The evaporator section 26 includes vertical modules 31, which extend across 
the duct 20 so that the hot gas passes the vertical modules 31 and heats 
the water in the modules 31. The vertical modules 31 receive water from 
the conduit 29 through a header 33 and feed a steam/water mixture back to 
the drum 27 through the modules 31 and the risers 28. The risers 28 to the 
right of the conduit 29 feed into a common collection header 52, which has 
an outlet 58 into the drum 27. Each of the modules 31 in the evaporator is 
an upward-flowing, single pass module. 
The boiling water passes upwardly through the risers 28 to the drum 27, 
which separates the water and steam. The steam then goes on to the 
superheater 17. The steam leaves the superheater 17 through a steam 
conduit 19, through a steam control valve 25, and to the steam turbine 14. 
When the hot gas enters through the gas inlet 22, it first encounters the 
superheater 17, then the evaporator section 26, and then the economizer 
section 30. As shown in these drawings, the hot gas is the exhaust from a 
gas turbine, but it could be from another heat source, such as a burner, 
or it could be a combination of gas turbine exhaust and a supplemental 
heater. 
The economizer section 30 includes two types of vertical modules. The 
upstream modules in the first embodiment are multiple pass modules 38, as 
shown in more detail in FIG. 3. The multiple pass module, shown in FIG. 3, 
includes a bottom header 45, a top header 47, and a plurality of tubes 
extending between and in fluid communication with the bottom header 45 and 
the top header 47. Both the bottom header 45 and the top header 47 include 
baffles 49 so that fluid must make multiple passes up and down within the 
multiple pass module before it can exit the module. In the multiple pass 
modules, the water enters at the bottom inlet 44, makes several passes up 
and down as it works its way across the module 38, and exits at the bottom 
outlet 40. The outlet 40 of one module 38 is connected to the inlet 44 of 
the next module 38 downstream, so that the water flows serially from one 
module 38 to the next, becoming warmer as it moves downstream. Multiple 
pass modules 38 are the preferred type of module in the economizer 
section, because they provide the necessary high water velocities, which 
provide the best heat transfer from the hot gas to the water. 
At the downstream end of the economizer section 30 are one or more 
upwardly-flowing single pass modules 36, which form the steaming section 
32 of the economizer 30. In this embodiment, two such modules 36 are 
shown. Another view of the single pass module 36 is shown in FIG. 4. Water 
leaves the outlet 40 of the downstream-most multiple pass module 38, and 
enters a bottom inlet manifold 50, which feeds the water to the bottom 
headers 46 of the two upwardly-flowing single pass modules 36. While two 
upwardly-flowing single pass modules are shown here, the number of single 
pass modules at the end of the multiple-pass module portion may vary. The 
water goes up through the single pass modules 36 to the top headers 48 of 
the modules 36, then to a top outlet manifold 54, which leads to the 
conduit 56. This permits the output from the economizer 30 to use the same 
inlet 58 to the drum 27 as is used by some of the evaporator modules 31. 
While the single pass modules do not provide the same velocities as the 
multiple pass modules and therefore are not as efficient and do not 
transfer as much heat per unit area of module, they play an important role 
in the present invention. The single pass modules 36 at the downstream end 
of the economizer section 30 provide for heat transfer and permit boiling 
at the downstream end of the economizer section without any problems being 
caused due to the boiling. Since the water becomes warmer and warmer as it 
progresses downstream along the economizer section, the boiling is most 
likely to take place near the downstream end of the economizer section. 
Putting the upwardly-flowing, single-pass modules 36 at the downstream end 
of the economizer section 30 means that, in the area where boiling is most 
likely to occur, the economizer 30 is designed so that boiling causes no 
problems. 
The concept of designing the economizer section to permit boiling is 
contrary to the teaching in the art which says that various techniques 
must be used to prevent boiling in the economizer section. 
In the preferred embodiment, a collection header 52 is located to collect 
the flow from the economizer 30 and the flow from some of the modules 31 
in the evaporator section 26. The steam/water mixture flows through the 
collection header 52 into the steam drum 27. 
Steam from the steam drum 27 passes into the superheater module 17, where 
it is heated above the boiling temperature and then leaves the boiler 16. 
The boiler 16 shown in FIG. 1 also includes a by-pass system 60, which 
provides a second path for water that is leaving the economizer 30. The 
by-pass line 64 runs from the economizer output conduit 56 to a bypass 
valve 66, and then either out of the boiler (as shown) or back to the 
water inlet 45. The by-pass line 64 can be used to keep sufficient water 
flowing through the economizer 30 while cutting back on the amount of 
water flow to the evaporator 26 during transient conditions, as will be 
described later. 
FIG. 5 shows the feed water control system 68, which operates the water 
inlet valve 62 and the bypass valve 66. The control system 68 includes a 
water level sensor 70, which is located in the drum 27 to sense the level 
of water in the drum. The control system also includes a controller 72 
which controls the water inlet valve 62 and the bypass valve 66. The 
controller 72 also receives signals from the water level sensor 70 in the 
drum 27. The control system also includes an inlet valve position sensor 
74, which senses the position of the inlet valve 62 (by measuring the 
stroke of the valve or the flow rate in the inlet line 47), and a bypass 
valve sensor 76, which similarly senses the position of the bypass valve. 
Both the inlet valve sensor 74 and the bypass valve sensor 76 communicate 
with the controller 72. The control system also includes a load 
transmitter 82, which tells the controller 72 how much heat is coming into 
the gas inlet 22. The load transmitter 82 preferably determines the amount 
of heat input by measuring the position of the fuel valve for the fuel 
that is used to make the heat. This would be true whether there is a gas 
turbine upstream of the boiler, whether the fuel is being burned just to 
make heat for the boiler, or whether the heat input is a combination of 
heat from the gas turbine upstream and from a burner associated just with 
the boiler. (This would occur when a heat recovery steam generator is 
operating in fired mode.) 
The controller 72 is preferably an electronic controller, which includes 
logic, control, and data processing capability, but it may be a 
combination of devices--electrical and/or mechanical--which perform the 
functions that are described below. 
FIG. 6 shows two curves, which can be calculated or determined by testing 
for any given boiler system. The curves, A and B, show the position the 
water input valve 62 should take for any given heat input to the boiler. 
The "A" curve shows the position the water input valve 62 should take 
under steady state conditions, and the "B" curve shows the minimum 
position the water input valve 62 should take under transient conditions 
to make the economizer reliable. 
If the economizer does not include a portion that is designed to permit 
steaming, then the "B" curve would be the minimum valve position which 
would prevent steaming in the economizer. If the economizer does include a 
portion that is designed to permit steaming, then the "B" curve would be 
the minimum valve position which would prevent steaming in the portion of 
the economizer that is not designed for steaming (i.e., for the boiler 
shown in FIG. 1, the minimum valve position to prevent steaming in the 
multiple pass modules 38). 
The curve "B" is programmed into the controller 72, so that, for any given 
heat input signal from the heat input transmitter 82, the controller 72 
determines a minimum water input valve set point from the "B" curve. 
When the power plant 10 is operating at steady state, the controller causes 
the water input valve 62 to open to the position on the "A" curve which 
permits enough water to enter the boiler to make up for the amount of 
steam leaving the boiler, and causes the bypass valve 66 to be closed. 
If the load 18 rapidly increases, the steam turbine 14 will require more 
steam, so the steam output valve 25 is opened relatively rapidly. Now, the 
condition of the boiler changes from steady state to a dynamic or 
transient state of operation. Opening the steam output valve 25 to permit 
more steam flow to the turbine 14 causes the pressure in the boiler to 
drop. With the drop in pressure, more of the water in the evaporator will 
boil. The sudden increase in steam volume in the tubes 31 and in the 
risers 28 will push the water level in the drum 27 up. Under these 
conditions, the most urgent problem is to maintain the proper water level 
in the drum 27 in order to maintain the necessary steam quality. Also, it 
is desirable to provide enough water flow to the multiple pass modules 38 
in the economizer section 30 to prevent steaming in the multiple pass 
modules 38. (Remember, steaming in the multiple pass modules 38 would 
create a steam hammer effect or fatigue problems, which are destructive to 
the modules.) With the design of this embodiment, we do not care if 
steaming occurs in the single-pass, upwardly-flowing modules at the end of 
the economizer section. 
In the prior art, during normal operation of the boiler, the controller 
would simply look at the water level in the drum and reduce the flow 
through the water input valve to prevent the water level in the drum from 
becoming too high. However, in the present invention, the control operates 
differently. 
To simultaneously maintain the water level in the drum 27 and provide 
reliable operation of the economizer 30, the present invention maintains a 
sufficiently large feed water flow to the economizer 30 to prevent boiling 
in the multiple pass modules 38 while providing a small supply of water to 
the drum 27. 
As was mentioned earlier, for any heat input transmitted from the heat load 
transmitter 82 to the controller 72, the controller 72 determines a 
minimum set point for the water input valve 62. The controller knows that, 
no matter what, it is not to permit the water input valve 62 to close down 
more than that minimum set point. 
If the controller 72 receives a signal from the sensor 70, telling it that 
the water level in the drum 27 is getting too high, it will cause the 
water input valve 62 to move from its first position, on the "A" curve, to 
a second position, which is either between the "A" and "B" curves or on 
the "B" curve, but which is not below the minimum set point position 
defined by the "B" curve. If the water input valve 62 has been closed to 
the position on the "B" curve, and the water level in the drum 27 is still 
too high, then the controller will begin to open the bypass valve 66 to 
allow water to flow through the bypass conduit 64, bypassing the drum 27, 
to maintain the proper level in the drum 27 while maintaining enough water 
flow through the economizer to prevent problems with boiling in the 
economizer. 
The controller continues to monitor the water level in the drum 27, and, as 
the water level goes down, it gradually shuts off the bypass valve 66, and 
then opens the water input valve 62, until the water input valve 62 is 
again at the point on the "A" curve corresponding to the steady state 
position for the heat input to the boiler. 
If there is a decrease in steam demand from the steady state operating 
position (with the bypass valve 66 closed and the water input valve 62 at 
the position on the "A" curve), the steam output valve 25 will be closed 
down somewhat, causing an increase in pressure in the evaporator 26. This 
will cause some of the steam in the evaporator to condense, and the 
decreased volume of steam in the modules 31 and risers 28 will cause the 
water level in the drum 27 to go down. 
The water level sensor 70 will tell the controller 72 that the water level 
has dropped below the desired level. The controller 72 will then gradually 
open the water inlet valve 62 until the water level in the drum 27 again 
reaches the correct level. The controller 72 can open the water inlet 
valve 62 until it is completely open, but it will not close the water 
inlet valve 62 down below the minimum set point defined by the "B" curve. 
Thus, in summary, the controller receives input telling it the water level 
in the drum, the heat input to the boiler, and the flow rates or valve 
positions for the water input line and the bypass line, and, based on that 
information and based on the curves "A" and "B", it controls the water 
input valve position and the bypass valve position to maintain the proper 
water level in the drum 27 while preventing steaming in the multiple pass 
portion of the economizer. 
The system shown in FIG. 7 is a second embodiment of the invention. This 
embodiment is the same as the first embodiment, except that, in this 
embodiment, the economizer section 130 has several single-path modules 37 
connected together in series, so that water flows up the first module 37, 
down the second module 37, up the third module, and so forth. At the 
downstream end of this series of single-path modules are two 
upwardly-flowing single pass modules 36 connected in parallel. As with the 
first embodiment, there would be a problem if steaming occurred in or 
upstream of any downward-flowing portion of the economizer. As with the 
first embodiment, there is no problem if steaming occurs in the 
single-path, upwardly-flowing modules at the downstream end of the 
economizer section 130. 
This second embodiment is controlled in the same manner as the first 
embodiment. An "A" curve and "B" curve are developed for the boiler, 
either empirically or by calculation. The "A" curve represents the 
positions of the water input valve 62 at steady state for any given heat 
input to the boiler, and the "B" curve represents the minimum set point 
positions of the water input valve 62 for any given heat input to the 
boiler. 
As with the first embodiment, if the controller 72 notes that the water 
level in the drum 27 is becoming too high, it first reduces the water 
input flow by closing the water input valve 62 (never closing it below the 
minimum set point). If reducing the water input flow to the "B" set point 
is not sufficient to maintain the proper water level in the drum 27, then 
the controller 72 will begin opening the bypass valve 66 until the proper 
water level is reached in the drum 27. The controller 72 will then 
gradually close the bypass valve 66 until it is completely closed and will 
then gradually open the water inlet valve 62 until the water level in the 
drum 27 is where it should be. 
FIG. 8 shows a third embodiment of the invention. This would be a very 
unusual arrangement, which could be used, for example, when the boiler is 
designed for operation at low pressure. In FIG. 8, everything is the same 
as in the first embodiment, with two exceptions. First, in this 
embodiment, the entire economizer section 230 is made up of single pass, 
upwardly-flowing modules 36 connected in parallel. Since the economizer 
section 230 of this embodiment does not include any downwardly-flowing 
paths, there can be steaming in any portion of this economizer section 230 
without encountering any problems. Second, since steaming in the 
economizer section will not cause any problems, there is no need for a 
bypass system as in the two previous embodiments. 
In the embodiment of FIG. 8, there is a bottom header 46 at the bottom of 
each module 36 and a top header 48 at the top of each module 36. A bottom 
inlet manifold 50 provides water to all the bottom headers 46 and receives 
water from the inlet pump 45. The collection header 56 collects the flow 
from all the top headers 48. The water or steam/water mixture from the 
collection header 56 enters the collection header 52, then flows through 
the drum inlet 58 into the drum 27. 
Control of this system differs from the control of the previous embodiment, 
in that there is no bypass valve to control, and there is no concern about 
steaming in the economizer section, so this system can be controlled in 
the straightforward method of the prior art, which is simply to monitor 
the water level in the steam drum 27 and open or close the water input 
valve 62 to maintain the proper water level in the drum 27. 
It will be obvious to those skilled in the art that modifications may be 
made to the embodiments described above without departing from the scope 
of the present invention.