U.S. patent application number 11/882747 was filed with the patent office on 2008-03-06 for boiler and combustion control method.
This patent application is currently assigned to Miura Co., Ltd.. Invention is credited to Tatsuya Fujiwara.
Application Number | 20080057451 11/882747 |
Document ID | / |
Family ID | 39133614 |
Filed Date | 2008-03-06 |
United States Patent
Application |
20080057451 |
Kind Code |
A1 |
Fujiwara; Tatsuya |
March 6, 2008 |
Boiler and combustion control method
Abstract
Provided is a boiler (100) including: a burner (5); a fuel
supply unit (10) for supplying fuel to the burner (5); a blowing
unit (20) for supplying air to the burner (5); and a control unit
(30) for adjusting an amount of fuel to be supplied to the burner
(5) and a quantity of air to be supplied to the burner (5), in
which the control unit (30) has a reference amount computing
portion for calculating a reference fuel amount and a reference air
quantity to be supplied to the burner (5) with respect to a
required load, an air quantity computing portion that corrects the
reference air quantity based on a temperature of the air to be
supplied to the burner (5) and a temperature of the fuel to be
supplied to the burner (5) and calculates the corrected air
quantity as a supply air quantity, and a control portion that
controls combustion at the burner based on the reference fuel
amount and the supply air quantity.
Inventors: |
Fujiwara; Tatsuya;
(Brantford, CA) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
Miura Co., Ltd.
Matsuyama-shi
JP
|
Family ID: |
39133614 |
Appl. No.: |
11/882747 |
Filed: |
August 3, 2007 |
Current U.S.
Class: |
431/12 |
Current CPC
Class: |
F23N 3/002 20130101;
F23N 2233/08 20200101; F23N 2225/20 20200101; F23K 2900/05001
20130101; F23N 2241/04 20200101; F23K 2400/201 20200501 |
Class at
Publication: |
431/12 |
International
Class: |
F23N 1/02 20060101
F23N001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 30, 2006 |
JP |
2006-234371 |
Claims
1. A boiler comprising: a control means for adjusting a quantity of
air to be used for combustion based on a temperature change in an
air and fuel to be used for combustion.
2. A boiler according to claim 1 further comprising: a burner; a
fuel supply means for supplying fuel to the burner; a blowing means
for supplying air to the burner; and an amount of fuel to be
supplied to the burner and a quantity of air to be supplied to the
burner, which are adjusted by the control means, wherein the
control means has a reference amount computing portion for
calculating a reference fuel amount and a reference air quantity to
be supplied to the burner with respect to a required load, an air
quantity computing portion that corrects the reference air quantity
based on a temperature of the air to be supplied to the burner and
a temperature of the fuel to be supplied to the burner and
calculates the corrected air quantity as a supply air quantity, and
a control portion that controls combustion at the burner based on
the reference fuel amount and the supply air quantity.
3. A boiler according to claim 2, further comprising thermistors
used as a means for measuring the temperature of the air to be
supplied to the burner and as a means for measuring the temperature
of the fuel to be supplied to the burner.
4. A boiler according to claim 3, wherein the air quantity
computing portion calculates the supply air quantity such that a
correction amount for correcting the reference air quantity is in
proportion to 1/(1+R.sub.TH1/R.sub.s+R.sub.TH1/R.sub.TH2), where
R.sub.TH1 is a resistance value of an air temperature measuring
thermistor, R.sub.TH2 is a resistance value of a fuel temperature
measuring thermistor, and R.sub.s is a resistance value of a fixed
resistor.
5. A boiler according to claim 2, wherein the air quantity
computing portion calculates the supply air quantity such that a
correction amount for correcting the reference air quantity is in
proportion to T.sub.a/(T.sub.g).sup.1/2, where T.sub.g is the
temperature of the fuel measured by a fuel temperature measuring
means, and T.sub.a is the temperature of the air measured by an air
temperature measuring means.
6. A combustion control method by which combustion is performed at
a predetermined air ratio, and in which NOx in an exhaust gas is
suppressed to a level within a predetermined range, the combustion
control method comprising: calculating a reference fuel amount and
a reference air quantity corresponding to a required load of the
boiler; correcting the calculated reference air quantity based on
temperatures of a fuel and an air to be used for combustion; and
performing combustion with the corrected air quantity and the
reference fuel amount.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a boiler and a combustion
control method making it possible to perform combustion at a
predetermined air ratio and to suppress generation of NOx in the
exhaust gas.
[0003] 2. Description of the Related Art
[0004] It is important for a boiler to be capable of performing
stable combustion with high thermal efficiency. In view of this,
there have been proposed air ratio control boilers or combustion
apparatuses in which control is performed such that combustion is
performed at a predetermined air ratio.
[0005] For example, JP 2001-272030 A proposes an air-fuel ratio
control (air ratio control) monitoring method for a burner in a
boiler and an air-fuel ratio control monitoring apparatus for
executing the method. In the method and the apparatus, in response
to a load command from a control panel, the pressure of the
combustion air to be supplied to the burner and the pressure of the
fuel to be supplied to the burner or the pressure of the fuel
returned from the burner are monitored to make a judgment as to
whether the air amount and the fuel amount are being properly
controlled.
[0006] JP 10-47654 A discloses an air ratio automatic correction
system for a combustion apparatus in which combustion air is
preheated and supplied. In the system for a combustion apparatus,
each of a combustion air supply path and a fuel supply path is
provided with a pressure gauge and a thermometer, the fuel supply
path is equipped with an equalizing valve for equalizing the fuel
supply pressure and the combustion air supply pressure, an impulse
line of the equalizing valve is provided with an orifice and a
bleeding valve, a fuel supply pressure for supplying the requisite
fuel for maintaining a predetermined air ratio is obtained from the
actual air temperature and air supply pressure and fuel temperature
measured by each thermometer and pressure gauge, and the fuel
supply pressure thus obtained and the actually measured fuel supply
pressure are compared with each other to adjust the bleeding valve
such that those become equal to each other.
[0007] Further, from the viewpoint of environmental hygiene, there
is a demand for a boiler capable of suppressing discharge of
harmful exhaust gas. In order to meet this demand, JP 2000-46302 A,
for example, discloses a boiler in which there is provided a
rectangular combustion space with vertical water tubes standing
close together between water tube walls arranged in parallel so as
to be spaced apart from the burner front surface. In the boiler,
there is provided a relatively long gas passage extending from the
burner to a gas outlet via the inter-space between the vertical
water tubes, thereby suppressing the flame combustion temperature
to a level of A approximately 1200 to 1300.degree. C. to thereby
reduce NOx to 70 to 80 ppm and to reduce CO to 50 ppm or less.
[0008] However, none of the above-mentioned conventional techniques
proposes a boiler or a combustion control method capable of
controlling both the air ratio and the NOx generation in the
exhaust gas and suppressing generation of NOx in a stable manner
even if there is a change in outside temperature due to seasonal
variations or the like. As an air ratio control boiler, the
air-fuel ratio control monitoring method or the air-fuel ratio
control monitoring device as disclosed in JP 2001-272030 A is
advantageous in that it is a simple method or device. However, it
is intended for the control of the air ratio when the air ratio is
outside a predetermined range, but it is not capable of accurately
controlling the air ratio. The air ratio automatic correction
system for a combustion apparatus proposed in JP 10-47654 A has a
problem in that it requires a complicated construction and control.
As a boiler for suppressing NOx in the exhaust gas, the boiler as
proposed in JP 2000-46032 A can effectively achieve a reduction in
NOx. However, there is a demand for a boiler capable of achieving a
further reduction in NOx generation in a stable manner.
SUMMARY OF THE INVENTION
[0009] In view of above-mentioned requirement and the problems in
the prior art, it is an object of the present invention to provide
a boiler and a combustion control method which are of a relatively
simple construction, in which combustion is performed at a
predetermined air ratio, and which can suppress generation of NOx
in the exhaust gas.
[0010] A boiler according to the present invention includes: a
control means for adjusting a quantity of air to be used for
combustion based on a temperature change in an air and fuel to be
used for combustion. Further, a boiler according to the present
invention includes: a burner; a fuel supply means for supplying
fuel to the burner; a blowing means for supplying air to the
burner; and an amount of fuel to be supplied to the burner and a
quantity of air to be supplied to the burner, which are adjusted by
the control means, in which the control means has a reference
amount computing portion for calculating a reference fuel amount
and a reference air quantity to be supplied to the burner with
respect to a required load, an air quantity computing portion that
corrects the reference air quantity based on a temperature of the
air to be supplied to the burner and a temperature of the fuel to
be supplied to the burner and calculates the corrected air quantity
as a supply air quantity, and a control portion that controls
combustion at the burner based on the reference fuel amount and the
supply air quantity.
[0011] In the above-mentioned aspect of the present invention,
thermistors are preferably used as a means for measuring the
temperature of the air to be supplied to the burner and as a means
for measuring the temperature of the fuel to be supplied to the
burner. The air quantity computing portion calculates the supply
air quantity such that a correction amount for correcting the
reference air quantity is in proportion to
1/(1+R.sub.TH1/R.sub.s+R.sub.TH1/R.sub.TH2), where R.sub.TH1 is a
resistance value of an air temperature measuring thermistor,
R.sub.TH2 is a resistance value of a fuel temperature measuring
thermistor, and R.sub.s is a resistance value of a fixed
resistor.
[0012] Further, the air quantity computing portion calculates the
supply air quantity such that a correction amount for correcting
the reference air quantity is in proportion to
T.sub.a/(T.sub.g).sup.1/2, where T.sub.g is the temperature of the
fuel measured by a fuel temperature measuring means, and T.sub.a is
the temperature of the air measured by an air temperature measuring
means.
[0013] A combustion control method according to the present
invention is a method by which combustion is performed at a
predetermined air ratio and in which NOx in an exhaust gas is
suppressed to a level within a predetermined range, the combustion
control method including: calculating a reference fuel amount and a
reference air quantity corresponding to a required load of the
boiler; correcting the calculated reference air quantity based on
temperatures of a fuel and an air to be used for combustion; and
performing combustion with the corrected air quantity and the
reference fuel amount.
[0014] The boiler according to the present invention has a
relatively simple construction, and is capable of performing
combustion at a predetermined air ratio, suppressing generation of
NOx in the exhaust gas in a stable manner, and performing stable
combustion with high thermal efficiency. Further, according to the
combustion control method of the present invention, it is possible
to suppress generation of NOx to a level of 12 ppm or less in
normal combustion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic view of the construction of a boiler
according to the present invention;
[0016] FIG. 2 is a circuit diagram showing a construction example
of a blower control portion of a control device of the boiler of
FIG. 1;
[0017] FIG. 3 is a graph showing an example of how the O.sub.2
amount in the exhaust gas is controlled by a control device having
the blower control portion of FIG. 2;
[0018] FIG. 4 is a graph showing the relationship between ambient
temperature and fuel temperature;
[0019] FIG. 5 is a graph showing an example of how the NOx amount
in the exhaust gas is controlled by the control device having the
blower control portion of FIG. 2;
[0020] FIG. 6 is a graph showing the relationship between the NOx
amount and the O.sub.2 amount in the exhaust gas;
[0021] FIG. 7 is a graph showing the relationship between blower
frequency and supply air temperature when the control device having
the blower control portion of FIG. 2 is used;
[0022] FIG. 8 is a graph showing the relationship between the
O.sub.2 amount in the exhaust gas and fuel temperature when the
control device having the blower control portion of FIG. 2 is used
and when the supply air temperature is fixed at 20.degree. C. or
40.degree. C.; and
[0023] FIG. 9 is a graph showing the relationship between the
O.sub.2 amount in the exhaust gas and fuel temperature when, in the
case of FIG. 8, fixed resistance R.sub.s, reference frequency
f.sub.0, and maximum frequency f.sub.m are varied.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0024] In the following, a boiler according to an embodiment of the
present invention will be described with reference to the drawings.
The boiler according to the present invention is provided with a
control device for adjusting air quantity based on temperature
changes of air and fuel. FIG. 1 is a schematic view of an
embodiment thereof. As shown in FIG. 1, a boiler 100 has a burner
5, a fuel supply device 10 for supplying fuel to the burner 5, a
blowing device 20 for supplying air to the burner 5, and a control
device 30 for controlling the amount of fuel supplied to the burner
5 and the quantity of air supplied to the burner 5; further, the
boiler 100 has a fuel temperature measuring device 35 for measuring
the temperature of the fuel supplied to the burner 5 and
transmitting a corresponding signal to the control device 30, and
an air temperature measuring device 36 for measuring the
temperature of the air supplied to the burner 5 and transmitting a
corresponding signal to the control device 30.
[0025] The boiler 100 according to the present invention operates
as follows: Fuel (e.g., natural gas) is sent from the fuel supply
device 10 and spouted at the forward end of the fuel supply device
10 (in the vicinity of the right-hand end of the fuel supply device
10 as seen in FIG. 1); this fuel is supplied to the burner 5 while
being mixed with combustion air supplied from the blowing device
20, and is burned by the burner 5. The burned gas passes gaps
between a plurality of water tubes (water tube group) 40, and is
gradually cooled while performing heat exchange with water in the
plurality of water tubes 40, and is then sent to a flue 50 before
being discharged into the atmosphere.
[0026] It is possible to use a well-known burner as the burner 5.
There are no particular limitations regarding the type of burner.
In the case of the boiler of the embodiment shown in FIG. 1, there
is used a perfectly premixed burner having a planar burning
surface.
[0027] As the fuel supply device 10, it is possible to use a
well-known fuel supply device. For example, it is possible to use a
fuel supply device having a pump, a control valve, and a control
device for controlling them, and capable of supplying a
predetermined amount of fuel corresponding to a load.
[0028] It is possible to use a well-known blowing device as the
blowing device 20. For example, it is possible to use an inverter
type blower having a blower, a drive source, and an inverter for
controlling the RPM of the blower, and capable of supplying a
predetermined quantity of air corresponding to the fuel. It is also
possible to use a so-called damper type blower capable of supplying
a predetermined quantity of air corresponding to the fuel.
[0029] The control device 30 has a reference amount computing
portion, an air quantity computing portion, and a control portion.
The reference amount computing portion serves to calculate a
reference fuel amount and a reference air quantity corresponding to
the load required of the boiler 100 by the heat engine. The air
quantity computing portion serves to correct a reference air
quantity calculated by the reference amount computing portion based
on the output from an air temperature measuring device 36 for
measuring the temperature of the air to be supplied to the burner 5
and the output from a fuel temperature measuring device 35 for
measuring the temperature of the fuel to be supplied to the burner
5, calculating the corrected air quantity as the supply air
quantity. The control portion serves to supply the burner 5 with
the quantity of supply air as obtained by the air quantity
computing portion with respect to the reference fuel amount already
calculated to perform required combustion.
[0030] It is only necessary for the fuel temperature measuring
device 35 and the air temperature measuring device 36 to be capable
of measuring the temperatures of the fuel and the air and supplying
the control device 30 with signals corresponding to the
temperatures. For example, it is possible to use a thermistor as
the fuel temperature measuring device 35 or the air temperature
measuring device 36, whereby it is possible to form the fuel
temperature measuring device 35 or the air temperature measuring
device 36 in a simple and compact structure.
[0031] The boiler 100, constructed as described above, is used as
follows. First, when a certain amount of load is required of the
boiler 100 by the heat engine or the like, the reference amount
computing portion of the control device 30 calculates a reference
fuel amount and a reference air quantity with respect to the
required load. The reference fuel amount and the reference air
quantity are theoretically computed under a predetermined air
ratio. Next, the calculated reference air quantity is corrected by
the air quantity computing portion based on the temperatures of the
fuel and the air to be supplied to the burner 5. That is, with
respect to the load momentarily required by the heat engine or the
like, the control device 30 according to the present invention
serves to supply the burner 5 with a corrected air quantity
described below based on the theoretical reference fuel amount and
the temperatures of the actually supplied fuel and air, making it
possible to perform the required combustion.
[0032] In the present invention, this reference air quantity is
corrected according to a principle described below. A case will be
described in which an inverter type blower is used as the blowing
device 20. Suppose that the temperature of the air supplied to the
burner 5 (supply air temperature) is T.sub.a, that the air density
is .rho..sub.a, that the volume flow rate is Q.sub.a, and that the
RPM of the blower is N. Since the volume flow rate Q.sub.a is in
proportion to the RPM N, and the air density .rho..sub.a is in
inverse proportion to the supply air temperature T.sub.a, the
following formula (1) holds true:
.rho..sub.aQ.sub.a .alpha. N'/Ta (1)
[0033] The fuel is supplied to the burner 5 such that the flow
velocity of the fuel supplied is a predetermined flow velocity,
that is, the difference in pressure between the pressurization
source side and the boiler 100 side is fixed, so the following
equation holds true:
.DELTA.P.sub.g=a.times..rho..sub.gQ.sub.g.sup.2=fixed value (a is a
constant), Q.sub.g .alpha. (T.sub.g).sup.1/2 (2)
where .DELTA.P.sub.g is the difference in pressure, T.sub.g is the
fuel temperature, and .rho..sub.g is the fuel density.
[0034] Since the fuel density .rho..sub.g is in inverse proportion
to the fuel temperature T.sub.g, from the above equation (2), the
following formula (3) holds true:
.rho..sub.gQ.sub.g .alpha. 1/(T.sub.g).sup.1/2 (3)
[0035] To maintain a fixed air ratio, it is necessary to keep
.rho..sub.aQ.sub.a/.rho..sub.gQ.sub.g at a fixed value. That is, it
can be seen from formulas (1) and (3) that to maintain a fixed air
ratio, it is necessary to keep N.times.(T.sub.g).sup.1/2/Ta at a
fixed value. It can be seen that the RPM N of the blower is to be
adjusted as shown in the following equation (4), in which k is a
constant:
N=k.times.T.sub.a/(T.sub.g).sup.1/2 (4)
[0036] Equation (4) shows that it is possible to perform combustion
at a fixed air ratio by making an adjustment such that the RPM of
the blower is in proportion to the supply air temperature T.sub.a
and in inverse proportion to (T.sub.g).sup.1/2, where T.sub.g is
the fuel temperature. That is, by supplying the burner 5 with a
quantity of air (supply air quantity) corrected by taking into
consideration the reference air quantity and the supply air
temperature, it is possible to cause the boiler 100 to perform
combustion at a predetermined air ratio. The pressure of the air
supplied to the burner 5 is not necessarily an essential monitoring
factor for performing combustion at a fixed air ratio. During use
of the boiler, there are various fluctuations in conditions, and it
can happen that the predetermined air ratio is deviated from. In
such cases, it is effective to monitor the air pressure.
[0037] In the present invention, the reference air quantity is
corrected based on the supply air temperature and the fuel
temperature, and the corrected air quantity (supply air quantity)
and the already calculated reference fuel amount are supplied to
the burner 5 by the control portion of the control device 30 to
thereby perform combustion. The correction amount for the
correction of the reference air quantity is an amount in proportion
to T.sub.a/(T.sub.g).sup.1/2.
[0038] This correction of the reference air quantity based on the
supply air temperature and the fuel temperature can be executed by
providing in the air quantity computing portion a program allowing
computation of a predetermined correction amount based on the
signals from the fuel temperature measuring device 35 and the air
temperature measuring device 36, and a computer for executing the
program. However, as described below, it is possible to form a
control device 30 of a simple structure by a control device having
the fuel temperature measuring device 35 and the air temperature
measuring device 36 consisting of thermistors, and a blower control
portion capable of directly controlling the blower based on signals
from the thermistors.
[0039] FIG. 2 shows an example of the blower control portion of the
control device 30. As shown in FIG. 2, the blower control portion
of the control device 30 has a fixed resistor R.sub.s, a fuel
temperature measuring thermistor R.sub.TH2 connected in parallel
thereto, and an air temperature measuring thermistor R.sub.TH1
connected in series to them. The symbols represented by R and their
subscripts indicate resistance values (.OMEGA.). Assuming that the
frequency (reference frequency) at the time of minimum voltage
applied is f.sub.o, and that the frequency (maximum frequency) at
the time of maximum voltage is f.sub.m, the inverter frequency f
with respect to the voltage V input to the inverter can be
expressed as follows: f=f.sub.o+(f.sub.m-f.sub.o).times.V; when
this output is performed, assuming that the maximum voltage applied
to the inverter is V.sub.o, the voltage V input to the inverter can
be expressed as follows:
V=V.sub.o.times.1/(1+R.sub.TH1/R.sub.s+R.sub.TH1/R.sub.TH2), so the
frequency f can be expressed as follows:
f=f.sub.o+(f.sub.m-f.sub.o).times.V.sub.o.times.1/(1+R.sub.TH1/R.sub.s
+R.sub.TH1/R.sub.TH2)
[0040] FIG. 3 shows the O.sub.2 amount in the exhaust gas when the
boiler 100 is controlled by the control device 30 having the blower
control portion shown in FIG. 2. In FIG. 3, the horizontal axis
indicates the supply air temperature, and the vertical axis
indicates the O.sub.2 amount in the exhaust gas. As described
above, curves A.sub.1 and A.sub.2 represent examples of the present
invention in which combustion is performed in the boiler 100 with
the supply air quantity and the reference fuel amount. The curve
A.sub.1 indicates the case in which the supply air quantity is
obtained on the assumption that the fuel temperature and the supply
air temperature are equal to each other. The curve A.sub.2
indicates the case in which the supply air quantity is obtained on
the assumption that the fuel temperature changes in a proportion of
1/2 with respect to the change amount of the supply air
temperature. Curve B indicates a case (conventional example) in
which it is assumed that the supply air temperature and the fuel
temperature are equal to each other and in which combustion is
performed in the boiler 100 with an air quantity obtained by
correcting the reference air quantity taking solely the supply air
temperature into consideration and the reference fuel amount.
[0041] The curves of FIG. 3 are obtained through calculation of the
O.sub.2 amount (%) in the exhaust gas when theoretical combustion
is performed under the following conditions: Assuming that the
temperature of the fuel or the supply air is T(.degree. k), the
resistance value of the fuel temperature measuring thermistor and
the air temperature measuring thermistor is
R.sub.TH1=R.sub.TH2=15000.times.exp (3450(1/T-1/273)) (.OMEGA.).
The resistance value R.sub.s of the fixed resistor is 4500 .OMEGA..
The reference frequency f.sub.o is 50 Hz, and the maximum frequency
f.sub.m is 73 Hz.
[0042] According to FIG. 3, the curves A.sub.1 and A.sub.2 are of a
fixed linear configuration, with the O.sub.2 amount being 6%. Thus,
it can be seen that, in the present invention; combustion is
performed at a fixed air ratio in the supply air temperature range
of 10 to 50.degree. C. In contrast, the curve B is an inclined
straight line indicating a change in O.sub.2 amount of 5.7 to 6.7%
within the supply temperature range of 10 to 50.degree. C. Thus, it
can be seen that combustion at a fixed air ratio is not performed
within the temperature range of 10 to 50.degree. C. in the case of
the conventional example.
[0043] In FIG. 3, the curve A.sub.1 is a graph indicating the case
in which the supply air quantity is obtained on the assumption that
the fuel temperature and the supply air temperature are equal to
each other, and the curve A.sub.2 is a graph indicating the case in
which the supply air quantity is obtained on the assumption that
the proportion of the change in fuel temperature is 1/2 of the
proportion of the change in supply air temperature. As described
below, actually, however, it is to be assumed that the O.sub.2
amount can be controlled to a value within the range surrounded by
the curves A.sub.1 and A.sub.2.
[0044] FIG. 4 is a graph showing the relationship between the
temperature around piping and the fuel temperature at the inlet of
the boiler 100 in a case in which fuel (natural gas or LPG) flows
through the piping, which has a total length of approximately 50 m
and is exposed on the ground, at a maximum flow velocity of
approximately 500 Nm.sup.3/h. In FIG. 4, the horizontal axis
indicates the ambient temperature, and the vertical axis indicates
the fuel temperature. FIG. 4 indicates that the fuel temperature is
in a high correlation with the ambient temperature; assuming that
the fuel temperature is T.sub.g and that the ambient temperature is
T.sub.a, T.sub.g can be expressed as follows: T.sub.g=0.75 T.sub.a
(T.sub.g/T.sub.a=0.75). That is, it can be seen that the fuel
temperature changes at a proportion of 3/4 with respect to the
change amount of the ambient temperature. Further, according to
certain data, when high combustion is performed by using LPG fuel
in the boiler 100 with gas piping which is exposed on the ground
and of a length of approximately 10 m, the fluctuation width of the
fuel temperature at the inlet of the boiler 100 is approximately
1/2 of the fluctuation width of the ambient temperature. Taking
into account the above data and the possibility of the gas piping
being buried in the ground, it is to be assumed that the
fluctuation width of the fuel temperature at the inlet of the
boiler 100 is generally approximately 1/2 or less of the
fluctuation width of the ambient temperature. It is to be assumed
that the supply air temperature is substantially equal to the
ambient temperature. That is, in the present invention, it is to be
assumed that the O.sub.2 amount can be controlled within the range
surrounded by the curves A.sub.1 and A.sub.2 shown in FIG. 3.
[0045] In this way, in the boiler 100 according to the present
invention, combustion can be performed at a predetermined air
ratio. Further, as described below, the boiler 100 according to the
present invention can perform combustion in which the NOx
generation amount is small. FIG. 5 is a graph showing the
relationship between the supply air temperature and the NOx
generation amount. In FIG. 5, the horizontal axis indicates the
supply air temperature, and the vertical axis indicates the NOx
amount (ppm). The curves shown in FIG. 5 are obtained by converting
the O.sub.2 amounts of the curves A.sub.1, A.sub.2 and B shown in
FIG. 3 into NOx amounts based on the characteristic curve of FIG. 6
showing the relationship between the NOx amount and the O.sub.2
amount. FIG. 6 is a graph obtained through a combustion test on a
surface combustion burner type boiler 100 using premixed air fuel
mixture with a performance of an evaporation amount of 3130 kg/h.
As in FIG. 3, in FIG. 5, of the symbols A.sub.1, A.sub.2 and B, the
symbols A.sub.1 and A.sub.2 indicate examples of the present
invention, and the symbol B indicates a conventional example. The
symbol A.sub.1 indicates a case in which the supply air quantity is
obtained on the assumption that the fuel temperature is equal to
the supply air temperature, and the symbol A.sub.2 indicates a case
in which the supply air quantity is obtained on the assumption that
the proportion of the change in the fuel temperature is 1/2 of the
proportion of the change in the supply air temperature. This also
applies to the following description.
[0046] According to FIG. 5, in the case of the present invention,
the NOx amount is 10.6 to 11.2 ppm within the supply air
temperature range of 10 to 50.degree. C. Thus, it can be seen that
the NOx amount is kept at a level below 12 ppm.
[0047] FIG. 7 is a graph showing the relationship between the
control frequency and the supply air temperature in an inverter
type blower under the same conditions as in the case of the graph
shown in FIG. 2 or 3. In FIG. 7, the horizontal axis indicates the
supply air temperature, and the vertical axis indicates the
frequency of the blower. According to FIG. 7, the curves A.sub.1,
A.sub.2 and B are all substantially linear, with the curve A.sub.1
exhibiting the minimum gradient and the curve B the maximum
gradient. From the results as shown in FIG. 7 and those shown in
FIG. 3, it can be seen that in the case of the conventional
example, an excessive quantity of air is supplied to the burner 5
upon an increase in the supply air temperature. That is, it can be
seen that in the present invention, not only is it possible to
perform combustion in the boiler 100 at a fixed air ratio and with
low NOx, but it is also possible to perform an economical operation
with the boiler 100.
[0048] As described above, in the present invention, it is possible
to form a simple and compact control device 30 by a control device
using a thermistor. The boiler 100 having the control device 30 can
perform combustion within a supply temperature range of 10 to
50.degree. C., at a fixed air ratio, and with low NOx. When the
boiler 100 is installed in a room, the room temperature is
generally higher than the outside temperature, with its upper limit
being approximately 50.degree. C. Thus, it suffices to consider the
characteristics of the boiler 100 within the supply temperature
range of 10 to 50.degree. C. However, the supply air temperature is
outside the above-mentioned range in some cases. Such cases can be
coped with by selecting a fixed resistor and a thermistor that are
in conformity with the conditions of use thereof.
[0049] FIGS. 8 and 9 show the results of an examination of the
effects of the fixed resistance R.sub.s, the reference frequency
f.sub.o of the inverter, and the maximum frequency f.sub.m on the
O.sub.2 amount in the exhaust gas when the supply air temperature
is fixed at 20.degree. C. or 40.degree. C. In FIGS. 8 and 9, the
horizontal axis indicates the fuel temperature, and the vertical
axis indicates the O.sub.2 amount (ppm) in the exhaust gas. In the
cases shown in the drawings, the supply air temperature is
20.degree. C. or 40.degree. C. In the case of FIG. 8, the fixed
resistance R.sub.s, the reference frequency f.sub.o, and the
maximum frequency f.sub.m are the same as those of FIG. 3. In
contrast, in the case shown in FIG. 9, the fixed resistance R.sub.s
is 3000 .OMEGA., the reference frequency f.sub.o is 51 Hz, and the
maximum frequency f.sub.m is 74 Hz. In FIGS. 8 and 9, the curve B
indicates a conventional case.
[0050] Comparison of FIGS. 8 and 9 shows that in the case of FIG.
8, the O.sub.2 amount in the exhaust gas is controlled to a fixed
level, and that in the case of FIG. 9, no conspicuous effect of the
control is to be observed. That is, it can be seen that it is
necessary to select the fixed resistance R.sub.s, the thermistor
resistors R.sub.TH1 and R.sub.TH2, the reference frequency f.sub.o
of the inverter, and the maximum frequency f.sub.m according the
conditions of use of the boiler 100.
* * * * *