Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates in general to hot water
generators and more particularly to a hot water generator
operating at high temperatures so the heat exchanger of the
generator will produce a maximum rate of heat absorption and
therefore temperature rise without nucleation.
Heretofore, hot water generators have operated well
below the maximum heat release obtainable because of heat
exchange and pressure control systems which would result in
generating transient pockets of vapor, some of which might
become superheated. Pressure transients produced by these
vapor pockets generate "hammer" or load pressure transients
in the heat exchanger portion of the generator which is objec-
tionable in damage caused to the heat exchanger and audible
noise emitted. Superheated vapor pockets localize overheating
of heat exchanging surfaces that result in decreased life.
Vaporization of water causes film temperatures at sufficient
high levels which may produce scaling and/or corrosion.
Accordingly, it is important that the water heated hy the
generator never enter the vapor phase.
A prior known water heating apparatus is shown in
; U. S. Patent 3,282,257, wherein a heat exchange structure
similar to that of the present invention is used in
connection with a steam drum to produce controlled quality
steam. This heater operates at approximately seventy-five
percent of the maximum heat release due to variations in
load pressure.
A boiler for producing rapid water temperature
increases is shown in U. S. Patent 3,651,790, which concerns
the use of complex heat transfer structures for increasing
the heat exchange surface. Structures of this type are
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difficult and expensive to manufacture. Moreover, such struc-
tures have non-uniform internal pressure distribution during
thermal and flow transients which result in the generation of
undesirable vapor pockets.
Another complex type of heat transfer water generator
is shown in U. S. Patent 3,773,019. This structure would like-
wise be difficult and expensive to manufacture.
The present invention provides a method of thermal
shoc~. testing a high performance fabricated piping component by
subjecting the component to a rapid water temperature rise with-
out nucleation, wherein the method includes use of a hot water
generator having a high turndown ratio for rapidly increasing the
temperature of hot water to a point of substantially theoretical
maximum heat absorption and temperature rise without nucleation,
wherein the generator includes a housing defining connected coil
and combustor sections, a coil in the coil section with an open
central area for receiving combustion gases and defining a water
flow path, an inlet and an outlet for said coil, piping connected
between the inlet and outlet defining a closed loop water path, a
load in the piping, a pump in the piping forcing constant water
circulation through the closed loop, a fuel and air induction
system including fuel and air controls coacting to deliver the
proper mixture of fuel and air to the combustor section, said fuel
and air induction system including a fuel line delivering fuel to
the combustor section, a combustion air line delivering combustion
air to the combustor section, and means continually bleeding air
directly into the fuel line to maintain stable combustion in the
combustor section at minimum fire conditions, a fast response
actuator for said fuel and air controls, and a temperature con-
troller providing operating signals to the actuator in response toa set point temperature and the water temperature of the coil
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outlet to operate the fuel and air controls and cause a rapid
rate of temperature rise in the outlet water temperature without
nucleation, said method comprising the steps of mounting said
component in said piping connected between the inlet and outlet
of the coil, operating said generator between low and high firing
rates in a short period of time to produce the maximum heat flux
absorbable by the load without incurring nucleate boiling, and
observing the function of said component.
The present invention utilizes a heat exchanger struc-
ture and a combustor similar to that shown in the aforementionedU. S. Patent 3,282,257,~ together with a unique control system for
obtaining the maximum rate of water temperature increase without
generation of vapor pockets. For example, the system of the in-
vention can operate to heat water from a temperature of about 562;
degrees F. and a pressure of about 2282 psig to a temperature of
about 627 degrees F. anld a pressure of about 2266 psig in about
fifty seconds. This allows the maximum heat flux to be absorbed
by the heat exchange structure without moving into the area of
nucleate or more advanced stages of boiling corresponding to the
pressures involved. Accordingly, the hot water generator of the
~ invention is capable of shock testing high performance fabricated
; piping components such as valves, regulators, heat exchangers and
the like, by subjecting a component to a rapid water temperature
rise without nucleation or the generation of vapor pockets.
The controls for operating the generator to obtain the
rapid temperature rise ln the water heated includes a temperature
controller monitoring the outlet temperature of the water heated
by the generator and producing a signal for operating a high speed
air fuel actuator to provide the maximum permissible combustion
rate capable of being handled by the heat exchanger structure of
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the generator without generating vapor pockets in the water being
heated. A blower generating combustion air for the combustor has
its output controlled by a damper connected to the actuator.
fuel metering valve delivering fuel to the combustor is also con-
trolled by the actuator. A 24-to-1 turndown ratio of the gener-
ator permits generation of a "wall of water" which produces rapid
increases in the water outlet temperature within accurately con-
trolled flow and pressure limits.
The high speed hot water generator used in the method
of this invention is capable of producing a high rate of temper-
ature increase approaching the theoretical maximum change in wat-
er temperature without generating vapor pockets.
The hot water generator and method for shock testing
high performance fabricated piping components subject a component
to a rapid temperature rise of water without nucleation.
The invention is also concerned with the prov~sion of
a high speed hot water generator including controls that are
capable of operating the generator to provide the maximum rapid
rate of temperature change without generating vapor pockets in
the liquid.
The hot water generator has a heat exchanger which may
be efficiently and inexpensively manufactured and which has long
service life where controlled heat input and extraction produces
a maximum rate of heat absorption and therefore temperature rise.
Other features and advantages of the invention will be
apparent from the following detailed disclosure, taken in con-
junction with the accompanying sheets of drawings, wherein like
reference numerals refer to like parts, in which:
Fig. 1 is a somewhat diagrammatic and~block view of
the hot water generator according to the invention together
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with the controls and water flow loop;
Fig. 2 is a somewhat diagrammatic view of a system
including a plurality of hot water generators connected in
parallel in connection with the shock testing of a piping com-
ponent;
Fig. 3 is a graphical illustration of the maximum
allowable water temperature at the hot water generator outlet as
a function of outlet pressure for a hot water generator according
to the present invention;
Fig. 4 is a graphical illustration of the maximum out-
let water pressure and temperature of a hot water generator
according to the invention as a function of water flow rate for
prevention of boiling in the heat exchanger of the generator;
Fig. 5 is a typical boiling curve illustrating boiling
regimes for hot water generators;
Fig. 6 is a graphical illustration of the time response
plotted against heater water outlet temperature illustrating the
low fire to high fire conditions of the generator according to
the present invention and the maximum temperature rise without
nucleation;
Fig. 7 is a graphical illustration showing the steady
state of maximum fire rates allowable for operation between low
fire outlet heater temperature and maximum allowable outlet tem-
perature without boundary layer boiling;
Fig. 8 is a block diagram of the air fuel control sys-
tem of the generator of the present invention;
Fig. 9, 10 and 11 are graphical illustrations of
operations pursuant to a change in the set point temperature of
the controller in relation to the control signal, the damper and
the temperature of the hot water at the outlet of the heat
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exchanger; and
Figs. 12, 13 and 14 show the relationship during the
change in load between the temperature of the hot water coming
into the heat exchanger of the generator, the hot water leaving
the heat exchanger and the flue gases transferred from the com-
bustor to the heat exchanger.
Referring now to the drawings, and particularly to Fig.
l, the hot water generator of the invention is diagrammatically
illustrated as including a housing 20 enclosing a combustor sec-
tion 21 and a coil section 22. A choke 23 is provided within the
housing between the combustor section and the coil section to
direat the flow of gases from the combustor section to the coil
section. Within the coil section 22, a coil or heat exchanger in
the form of a coil and designated generally by the numeral 24
includes a plurality of watertubes. The coil is annularly shaped
and helical in form and where;`n the watertubes are arranged in
radially arranged layers and longitudinally spaced or arranged
rows with an inlet 25 at the outermost layer of watertubes and an
outlet 26 at 'the innermost layer of watertubes. Combustion or
flue gases generated in the combustor section 2~1 flow through thechoke 23 into the central area of the coil 24 outwardly and
through a flue gas outlet 27. While the generator is shown
diagrammatically, it will be appreciated that this part of the
generator is essentialIy of a type shown in the aforementioned
U. S. Patent 3,282,257. The helical watertubes define a path for
the water which picks up heat from the coil that is absorbed from
the flue gases which involves the transfer of heat to the minimum
amount of water moving through the coils. It is important that
minimum amount of water is involved in the heat transfer process
in order to obtain the desired rapid rise in temperature of the
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water.
Combustion gases are generated from an appropriate
fossil fuel and air mixture. Fuel may be a natural gas or other
suitable fossil fuel which is delivered from a suitable source of
supply through a fuel line 30 to the combustor section or the
combustion chamber in the combustor section. A metering valve
31 controls the fuel flow in the combustor section.
Primary air or combustion air is generated by a blower 32
and delivered through a duct 33 to the combustor section 21. A
damper 34 is provided in the duct to control the air flow through
the duct. In-order to maintain stability of combustion at low
fire condition, a small amount of air is bled from the duct 33
ahead of the damper into the fuel line 30 by a bleed line 35 to
provide some mixture of air with the fuel that is delivered by
the fuel line into the combustor section. This is important in
order to provide a high turndown ratio from high fire to low fire.
More specifically, it assists in providing about a 24-to-1 turn-
down ratio in order to provide the necessary increase in tempera-
ture and obtain the necessary shock testing in connection with
the shock testing of fabricated components according to the
invention.
A high speed air fuel actuator 38 is mechanically
coupled to the metering valve 31 and the damper 34 for control-
ling the operation thereof and therefore the air fuel flow into
the combustion section 21. This actuator receives a signal from
a temperature controller 39 of a suitable type which responds to
a set point indicator 40 and the temperature of the hot water at
the outlet 26 of the heat exchanger 24 by virtue of a suitable
temperature measuring instrument 41.
A pipe or conduit 45 is connected between the inlet end
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and outlet end of the coil or heat exchanger 24. A pump 46 is
provided in the piping 45 to cause constant flow of water through
the entire water path or loop at all times at a desired flow rate.
Therefore, forced circulation of water is established through the
coil 24. A loa~ 47 is also provide~ in the pipe 45 for loading
the system. As shown, the load is in the form of a heat exchanger
but may be of any desirable form although it should be such as to
provide a continous path in the piping 45. In the event that a
fabricated component to be shock tested is a heat exchanger, such
can be inserted at the load 47 position. Should the component to
be tested be in the form of a valve, a regulator or the like,
suc'n can be mounted in the piping 45 at the position
48 ahead of the load 47 or at the position 49 between the load
and the pump 46. The load 47 would be matched with the coil
capacity of the generator.
As seen in Fig. 2, in the event that the load is of
such a magnitude that a single hot water generator would be in-
sufficient, a plurality of hot water generators may be connected
in parallel, as shown by the hot water generators 20A, 20B and
20C. Suitable manifolds at the inlet and outlet will be provided
to facilitate the parallel connecting of such generators. While
three generators are shown in Fig. 2, it can be appreciated that
any number may he connected in parallel depending on the load.
It can be appreciated that suitable balancing valves may be pro-
vided at the outlets of parallel connected generat.ors so that the
same temperature and pressure conditions of each generator is de-
livered into the outlet manifold.
The generators may use any suitable type of fossil fuel
as already mentioned. The water used in the system will be de-
aerated, de-mineralized, and perhaps otherwise treated.
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The air fuel actuator 38 may be of any suitable type
which provides a fast enough response which is necessary in order
to enable the rapid rise in temperature of the liquid in the heat
exchanger of the generator and also to pre~ent nuc~eation. ~or
example, if the actuator will provide a full stroking time in two
seconds or less, it will be able to provide the necessary quick
response needed for the generator of the present invention. The
response must ~e faster than the heat absorption capacity of the
hèat exchanger of the generator.
The typical boiling curve illustrating boiling regimes
for hot water generators is shown in Fig. 5. Most hot water
systems involve operating in a state of unsteady equilibrium
where several of the boiling regimes illustrated in Fig. 5 are
traversed. It is preferable that a system operate in the incipi-
ent nucleate ~oiling range indicated at 51 which is just below
the nucleate boiling point A, where the most efficient heat trans-
fer is available. The system of the present invention operates at
point B. Overshoot of the maximum heat rate can move the opera-
tion of a system into the unstable boiling regions resulting in
damage or destruction of the heat exchange surfaces. The curve
of Fig. 5 shows the change in temperature plotted along the hori-
zontal and the heat flow rate along the vertical.
The rate of rise in temperature is dictated by the maxi-
mum heat flux which can be absorbed by the heat exchanger of the
generator without moving into the area of nucleate or more ad-
vanced stages of boiling corresponding to the pressures involved.
An illustration of the temperature-time response of a generator
according to the present invention is shown in Fig. 6 wherein
time in seconds is plotted along the horizontal and heater outlet
water temperature is plotted along the vertical. As illustrated,
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the time taken to move from low fire to high fire or one hundred
percent firing rate of the generator is about forty seconds,
where the low-fire temperature is 573 degrees F. and the high-
fire temperature is about ~i27 degrees F. Likewise, the rate of
drop in temperature is dic~ated by the maximum heat flux absorb-
able by the heat exchanger. The "trip" point A and normal opera-
ting point B are also shown here.
The curve in Fig. 3 relates to the maximum allowable
water temperature at the outlet of the coil as a function of out-
let pressure for the generator of the invention to illustrate whattemperature and pressure relationship must be maintained to pre-
vent nucleation. The region on the left side of the curve repre-
sents no boundary layer boiling for the coil, while the shaded
region on the right side of the curve represents the area to be
avoided where coil boundary layer boiling does exist. According-
i ly, during temperature rise of the water, the relationship be-
tween the temperature and the pressure must be in the region to
the left of the curve to prevent boiling. The loci of point A
from Fig. 5 is shown for water flow rates from fifty to one
hundred percent if the fuel inputs of ~ig. 7 are adhered to.
The curves shown in Fig. 4 represent the limiting hot
water generator outlet water pressure and temperature as a func-
tion of the water flow rate for the prevention of boiling in the
coil boundary layer therein. Heater outlet temperature in degrees
Farenheit is plotted along the horizontal, while heater outlet
pressure in psia is plotted along the vertical. Numbers 55, 56
and 57 respectively represent fifty percent, ninety percent and
one hundred percent water flow rates. Regions of no boiling are
to the left of the curves, while regions of boiling are to the
right of the curves. The steady state maximum temperatures allow-
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able are reached at approximately 2400 psia at temperatures of630, 638 and 642 degrees F. for the fifty, ninety and one hundred
percent water flow rates. Accordingly, the 2400 psia limitation
is indicated as a "trip" point which is a pressure that must be
maintained or exceeded in order to prevent entering the boiling
region which is shown in the shaded portion in Fig. 3.
The operation of the generator of the present invention
according to changing from low fire to high fire conditions in-
volves raising a nominal or residual temperature at low fire and
establishement of the water flow rate at a value corresponding to
the amount of heat required. This is illustrated in Fig. 7. The
curves in Fig. 7 show heat transfer to the water in btu's per hour
times 106 along the horizontal and heater outlet temperature in
degrees Farenheit along the vertical. These curves illustrate
maximum fuel fire rates allowable for operation between the low
fire heater outlet temperature and maximum allowable outlet temp-
erature without boundary layer boiling. The percent water flow
curves are shown to be fifty, sixty, seventy, eighty, eighty-
five, ninety, ninety-five, one hundred. The one hundred percent
water flow rate represents the maximum heat output of the system.
These curves concern a generator system with a capability of an
output of about twelve million btu's per hour. The maximum fuei
firing rate will vary depending upon the water flow rate required.
Rapid application of the correct fuel and air mixture to approxi-
mately fifty percent of the rated flow capacity of the generator
of the present invention results in a temperature increase along
the path of curve 60 where the desired heat flux capability is
reached at 627 degrees F. and the associated controls for hand-
ling the fuel and air mixture trip and thereby remove the heat
input. A temperature higher than 635 degrees F. will cause
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boiling at this water flow rate. Further, it is not possible to
use a higher maximum fire rate. The time required to meet the
627 degree point is approximately fifty seconds.
It can be appreciated that a definite relationship
exists between pressure, tempera-ture and flow rate for water
heated at one hundred percent maximum firing rate without creat-
ing vapor pockets. Exemplary of flow rate capacity for a genera-
tor having the characteristics illustrated in Fig. 7 are as
follows:
Water Flow atMinimum Flow VelocityFlow Capacity
Maximum FuelInside Tube Inside Tube
Firing Rates (~) (ft/sec) (lb/hr)
100 5.8 124,833
5.5 118,591
5.2 112,350
4.9 106,108
4.6 99,866
4.1 87,3~3
~,5 74,900
2.9 62,417
The block diagram of the system shown in Fig. 8
illustrates the temperature control 39 as receiving the set
point~ temperature and delivering a signal to a summing junction
65 which receives a feedback from the temperature of the hot
water out (Tho) which is taken at the temperature measuring
device 41 in the outlet of the heat exchanger 24. The summation
of the feedback signal and the signal from the temperature con-
troller is delivered to the servo motor or actuator 38 which con-
trols the operation of the gas valve 31 and the damper 34. The
operation of the gas valve 31 and the damper 34 results in the
weight and temperature of the flue gas in the combustor and the
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heat exchanger 21, 24. Load 47, which removes heat and therefore
determines the temperature of the hot water into the heat ex-
changer, effects the overall operation of the system.
The response to the change in set point of the tempera-
ture controller during an increase in temperature is illustrated
in Figs. 9, 10 and 11 as plotted against time. In Fig. 9 the
control signal variation of the set point temperature (Tsp) is
illustrated as being increased along the full range of temperature
increase desired of about 573 to 627 degrees F. The temperature
and weight of the flue gases (Tfq and Wfg) are shown in Fig. 10
in relation to the damper and valve operation. The rise in the
temperature of the hot water out (Tho) of the heat exchanger is
illustrated in Fig. 11. Note the dead zone 68 which is also
illustrated at 69 in Fig. 6.
The change in load of the heat exchanger indicated by a
decrease in temperature is illustrated with the relationship
between the hot water temperature into the heat exchanger, the
hot water temperature out of the heat exchanger, and the tempera-
ture and weight of the flue gases in the combustor and heat
exchanger sections is shown in Figs. 12, 13 and 14.
Accordingly, it will be seen from the foregoing that
the hot water generator of the invention may be utilized in the
method of shock testing of high performance piping components
wherein it is desired to produce maximum heat flux rate without
nucleation so as to provide a hot water generator with a heat
exchanger of long service life.
It will be understood that modifications and variations
may be effected without departing from the scope of the novel con-
; cepts of the present invention, but it is understood that this
application is to be limited only by the scope of the appended
claims.
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