GB1582307A

GB1582307A - Smoke-tube boilers

Info

Publication number: GB1582307A
Application number: GB3776577A
Authority: GB
Original assignee: Nederlandse Gasunie NV
Current assignee: Nederlandse Gasunie NV
Priority date: 1976-09-10
Filing date: 1977-09-09
Publication date: 1981-01-07
Also published as: DE2740023A1; FR2364403A1; NL7610044A; IT1089839B; BE858457A

Description

(54) SMOKE-TUBE BOILERS (71) We, N.V. NEDERLANDSE GASUNIE, a Netherlands Limited Liability Company of Gronigen, the Netherlands, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to boilers wherein the passage of flue gas through smoke tubes may be controlled in response to the load on the furnace, and to boilers incorporating control means for effecting such control automatically with varying load of the boiler.

Fire-tube boilers are used for example for generating superheated steam. Such boilers usually comprise two parallel sets of smoke tubes through a fire tube section wherein steam is generated by heat-exchange. The flue gases originating from the combustion chamber can be distributed over the sections by means of valves in the smoke box, which is the box connecting the smoke tubes with the chimney. The object is to keep the temperature of the generated superheated steam as constant as possible at varying load by means of the control means, but hitherto it has been difficult to control the efficiency of the boiler with varying load.

The heated surface area (HSA) of a boiler is calculated on the basis of the nominal load (full load) or capacity. This means that with a partial load (approximately 1/4 to 1/3 of the full load) the HSA is too large, as a result of which proportionately more heat in the flue gases is transferred to the boiler water than at full load, and a lower chimney temperature is obtained. This brings about a great difference between the chimney temperatures at nominal load and partial load and temperature differences of between 50 and 1500C occur.

At the adjustment of the boiler the flue gas temperature at full load is so chosen that at partial load the flue gas temperature remains above the dew point. For full load the chimney temperature then lies in most instances between 200 and 250 , while 100" would still be sufficiently high. This lower temperature can be reached by enlargement of the HSA or by installation of retarders. Too high a temperature means that the efficiency is riot optimal and the loss, depending on the 2 - or CO2- content of the flue gases, amounts to from 2 to 5%in an absolute sense, or to about 2 to 7% referred to the efficiency achieved.

If both at full load and at partial load a like part of the supplied heat is transferred, it is possible at full load as well as at partial load, to work at a virtually equal and low chimney temperature, by which a constent high efficiency is obtained.

The object of the invention is to provide an installation with the aid of which the efficiency of the boiler is automatically optimally adapted to the load by maintaining the chimney temperature, with varying boiler load, at a low value as constant as possible, and at the same time to provide an installation with the aid of which the chimney temperature of a fire tube shell boiler for hot water is automatically kept at such a temperature that no condensation takes place in the flue gas discharge channels if the water temperature should be lower than the dew point.

To this end, according to the invention, the smoke tubes are provided at least in part with thermally sensitive valves, which open or close with the rising or falling flue gas temperature respectively resulting.

The invention thus provides a boiler comprising a combustion chamber which opens into a combustion chamber connected with one or more bundles of smoke tubes arranged in series or in parallel, characterized in that at least a proportion of the said smoke tubes are provided with thermally operating valves which open or close with rising or falling temperature of the flue gas resulting from variations in the load of the furnace, merely to regulate the total gas flow within the tubes.

Preferably the thermally-sensitive valves are arranged at an end of the smoke tubes, and preferably at the end of a bundle of smoke tubes entering the smoke box as the temperature in the smoke box determines the result to be achieved.

The valves are known as such and are used for example for shutting off a chimney at zero load of a boiler. However in the present invention the valves control the flue gases passing through the smoke tubes. Said valves preferably utilize bimetal action. A useful valve is described in Dutch Patent specification 7,110,457. However other known valves may be used.

In the smoke tubes, the heat is transferred principally by convection, usually achieved by heat transfer, according to the equation Q = a F(Trg- Tw) kcal/h where Q = the heat transferred, in kcal/h; a = the heat transfer coefficient, in kcal/h.m2. C; F = the surface area of the effectively active part of the HSA (at full load F = HSA), in m2; Trg = the flue gas temperature, in "C; Tw = the wall temperature, in "C.

The heat transfer at partial load can be decreased relative to that at full load by reducing the effectively active heated surface area F. This will then result in a practically equal chimney temperature at full load as well as at partial load. a depends to some extent on the flow velocity, but the influence is so small that it may be disregarded.

At an increase in load the temperature of the flue gases originating from the combustion zone and a first bundle of smoke tubes rises because of the increased fuel supply. The warmer flue gas is led through the smoke tubes. Through the enlargement of F more heat will be transferred by convection and hence it will be avoided that the outlet temperature of the bundle of smoke tubes and the chimney temperature rise too much. If then the load is decreased again the reverse effect will take place. If the load of the boiler drops, a number of tubes are shut off by valves because the temperature has dropped below a certain value in the smoke box. Virtually all the flue gas will then flow through the non-closed tubes. Since the valves do not close the tubes completely, a minor leakage flow will occur through the tubes.

The temperature of the flue gas in these tubes will be lower at the side of the valves than in the tubes not shut off and will be approximate to the water or steam temperature. As a result of this the valves remain closed.

If subsequently the load of the boiler is increased, the flue gas volume increases, as well as the pressure difference between the inlet and the outlet of the tubes. This will cause the flue gas temperature at the outlet of the non-closed tubes, i.e. in the smoke box, to rise and also the quantity of leakage flow over the closed tubes to increase. This results in a higher temperature at the site of the valve so that this will open and thus a larger effectively active surface area will become available for heat transfer, so that according to the aforesaid formula, more heat can be transferred. As the opening of the valve is a function of the temperature an equilibrium condition will occur, which is determined by the flue gas temperature and the constructive properties of the valve.These properties are such that the average flue gas temperature lies only little above the temperature at the higher load.

At a decrease of the load the reverse will take place. The average temperature in the smoke box decreases, as well as the pressure drop over the tubes. As a result of this the temperature at the valves will fall, so that they will close and a smaller effectively active surface area becomes available for heat transfer. In that case the flue gas temperature will be higher than it would be if all tubes were available, but it will lie slightly below the temperature at full load.

The invention is hereinafter particularly described and illustrated in the accompanying drawing, which is a schematic longitudinal elevation of one embodiment of a fire tube shell boiler which may be adapted according to the invention.

Referring to the drawing, a fuel injection device is connected to a combustion chamber 2. A first bundle of smoke tubes 3 connect the said combustion chamber with a deflector box 4. A second bundle of smoke tubes 5 disposed to lie parallel to the aforesaid first bundle of smoke tubes, connect the said deflector box with a chimney 6.

The thermally-operable valves may be positioned, according to the invention, at the discharge ends 7 of the said second bundle of smoke tubes.

The installation according to the invention is particularly suitable for natural gas-fuelled boilers. With use of natural gas pollution effects do not occur in practice so that the automatically operating valves will continue to function without the frequent cleaning necessary with the use of other fuels.

Because it is possible to apply small valves the investment is small. For an installation having a capacity of 200 m3/h of natural gas the pay-out amounts to less than one year through efficiency improvement.

The advantages of the invention are further illustrated by tests using two commerciallyavailable fire tube-shell boilers, one with and one without the valves according to the invention. The boilers are hereinafter referred to as Boiler A and Boiler B. The boilers were fuelled with natural gas consisting of 81.3 vol.% of CH4 and 14.35 vol.% of N2, the temperature of the combustion air being 15"C. The valves functioned on a temperature difference of 20"C The results are set forth in the accompanying Table.

Boiler Boiler Flue CO2 Chimney Radiation Boiler load gas cont., losses, losses, efficiency, tlhour temp. in % in % in % in SO OC Boiler "A" 20 100 7.50 4.51 7.50 87.99 without 40 110 8.00 4.77 3.80 91.43 valves 60 120 9.50 4.87 2.50 92.93 80 140 10.00 5.21 1.90 92.89 100 160 10.50 5.81 1.50 92.69 Boiler "A" 20 100 7.50 4.51 7.50 87.99 with 40 105 8.00 4.52 3.80 91.68 valves 60 110 9.50 4.13 2.50 93.37 80 115 10.00 4.17 1.90 93.93 100 120 10.50 4.21 1.50 94.29 Boiler "B" 20 122 8.25 5.23 7.50 87.27 without 40 147 9.00 6.00 3.80 90.20 valves 60 178 9.80 6.91 2.50 90.59 80 200 10.20 7.59 1.90 90.51 100 227 10.60 8.43 1.50 90.07 Boiler "B" 20 122 8.25 5.23 7.50 87.27 with 40 127 9.00 5.09 3.80 91.11 valves 60 132 9.80 4.96 2.50 92.54 80 137 10.20 5.01 1.90 93.09 100 142 10.60 5.05 1.50 93.45 WHAT WE CLAIM IS: 1. A boiler comprising a combustion chamber which opens into a combustion chamber connected with one or more bundles of smoke tubes arranged in series or in parallel, characterized in that at least a proportion of the said smoke tubes are provided with thermally operating valves which open or close with rising or falling temperature of the flue gas resulting from variations in the load of the furnace, merely to regulate the total gas flow within the tubes.

2. A boiler according to Claim 1, wherein the said valves are arranged at an end of the said smoke tubes.

3. A boiler according to Claim 2, wherein the said valves are arranged at the end of a bundle of smoke tubes entering a smoke box of the combustion chamber.

4. A boiler according to any of Claims 1 to 3, wherein the said valves are bimetallic valves.

5. A boiler according to any of Claims 1 to 4, wherein the said boiler is a natural gas-fuelled boiler.

6. A boiler according to Claim 1, substantially as hereinbefore described and illustrated in the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

The advantages of the invention are further illustrated by tests using two commerciallyavailable fire tube-shell boilers, one with and one without the valves according to the invention. The boilers are hereinafter referred to as Boiler A and Boiler B. The boilers were fuelled with natural gas consisting of 81.3 vol.% of CH4 and 14.35 vol.% of N2, the temperature of the combustion air being 15"C. The valves functioned on a temperature difference of 20"C The results are set forth in the accompanying Table.

Boiler Boiler Flue CO2 Chimney Radiation Boiler load gas cont., losses, losses, efficiency, tlhour temp. in % in % in % in SO OC Boiler "A" 20 100 7.50 4.51 7.50 87.99 without 40 110 8.00 4.77 3.80 91.43 valves 60 120 9.50 4.87 2.50 92.93

80 140 10.00 5.21 1.90 92.89

100 160 10.50 5.81 1.50 92.69 Boiler "A" 20 100 7.50 4.51 7.50 87.99 with 40 105 8.00 4.52 3.80 91.68 valves 60 110 9.50 4.13 2.50 93.37

80 115 10.00 4.17 1.90 93.93

100 120 10.50 4.21 1.50 94.29 Boiler "B" 20 122 8.25 5.23 7.50 87.27 without 40 147 9.00 6.00 3.80 90.20 valves 60 178 9.80 6.91 2.50 90.59

80 200 10.20 7.59 1.90 90.51

100 227 10.60 8.43 1.50 90.07 Boiler "B" 20 122 8.25 5.23 7.50 87.27 with 40 127 9.00 5.09 3.80 91.11 valves 60 132 9.80 4.96 2.50 92.54

80 137 10.20 5.01 1.90 93.09 100

142 10.60 5.05 1.50 93.45 WHAT WE CLAIM IS: 1. A boiler comprising a combustion chamber which opens into a combustion chamber connected with one or more bundles of smoke tubes arranged in series or in parallel, characterized in that at least a proportion of the said smoke tubes are provided with thermally operating valves which open or close with rising or falling temperature of the flue gas resulting from variations in the load of the furnace, merely to regulate the total gas flow within the tubes.
2. A boiler according to Claim 1, wherein the said valves are arranged at an end of the said smoke tubes.
3. A boiler according to Claim 2, wherein the said valves are arranged at the end of a bundle of smoke tubes entering a smoke box of the combustion chamber.
4. A boiler according to any of Claims 1 to 3, wherein the said valves are bimetallic valves.
5. A boiler according to any of Claims 1 to 4, wherein the said boiler is a natural gas-fuelled boiler.
6. A boiler according to Claim 1, substantially as hereinbefore described and illustrated in the accompanying drawing.