Algunas objeciones a la fulanidad

This boiler design concept is built around the ‘‘waterwall.’’ This refers to the tubes connecting the steam drum to the mud drum. The tubes are attached to each other with a continuous strip of metal (called a membrane), usually this strip is 3/16–1/4 in. thick by 1.0 in. wide and whatever length that is required.

This forms a continuous wall of tubes filled with water at the bottom and steam at the top, thus waterwall. This waterwall design is used in the outerwalls and also in some cases, the dividing wall between the furnace section and the convec-tion secconvec-tion.

This boiler wall design is very efficient in heat transfer, especially in the furnace area. The majority of the radiant heat that strikes the membrane bar is transferred to the tube on either side. This waterwall design also acts as a gas-tight wall, which also adds to the efficiency of the boiler.

As you know, boiler walls were originally mostly refractory. The problems were many. The normal furnace temperature is 2200–2400°F when firing natural gas. It is higher when firing fuel oil. This can reach a maximum temperature of 3200°F. This high temperature is very detrimental to refractory. The continuous operation of the furnace at these high temperatures will, over time, cause the disintegration of the refractory. Refractory maintenance was a very high dollar item in the power plant budget.

Waterwalls were added to existing watertube boilers before they were in-corporated into the newly designed package units. The waterwalls allowed the boilers to be operated continuously at the maximum firing rate and still realize good operating economics. The waterwalls cut down on outages, greatly reduced or eliminated refractory maintenance, and also permitted the efficient firing of lower grade fuels.

A secondary effect of waterwalls is the lowering of the furnace tempera-ture. Most of this is due to the increased absorption of radiant heat by the wa-terwall. This lowering of furnace temperature directly affects the formation of NOx. As the furnace temperature decreases, the formation of combustion NOx

decreases.

FIGURE3.2 Two types of packaged boilers built with membrane waterwalls.

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TABLE3.2 Fuel Oil Atomization

Steam atomization Mechanical atomization

1. Best suited to variable load. 1. Best suited to steady load and high capacity.

2. Wide capacity range without 2. Limited capacity range for any changing tip or gun assembly. given tip size; wide-range sys-tem overcomes this somewhat.

3. Frequent cleaning of tip unneces- 3. Frequent cleaning of tip neces-sary, as openings are relatively sary to maintain efficient spray.

large and can be quickly blown Owing to relatively small

open-out with steam. ing, entire gun assembly must

be removed so sprayer plate and tip orifices may be carefully cleaned.

4. Capacity up to 120 million Btu 4. Capacity up to 100 million Btu per nozzle per hour. per nozzle per hour.

5. Considerable flexibility for shap- 5. No flexibility in flame shape.

ing flame to conform with fur-nace conditions.

6. Relatively low oil temperature re- 6. Relatively high oil temperature re-quired (approximately 185°F), as quired (approximately 220°F), as viscosity need only be low viscosity must be low enough enough (40 SSU) to readily per- (180–220 SSU) to produce

satis-mit pumping. factory atomization.

7. Oil pressure 2–125 psi. 7. Oil pressure 50–250 psi.

8. Steam for atomization may vary 8. Steam for pumping and heating from 0.7 to 5.0%. The approxi- varies from 0.5 to 1.0%, and is mate average for careful opera- governed by the equipment in-tion is 1.25%. stalled, rather than by operation.

9. Steam for atomization is lost up 9. Exhaust steam from pump and the stack, and must be consid- heater set may be returned to ered when there is a question of hotwell, thereby minimizing

makeup water. makeup.

10. Lower air pressure required, be- 10. Higher air pressure required, be-cause aspirating effect of the cause of the absence of aspirat-steam jets makes up for some of ing effect with the mechanically the pressure drop in the register. produced spray.

11. Lower fixed charges, because of 11. Fixed charges high, owing to lower temperature and furnace cost of equipment to provide for

requirements. high-pressure and

high-tempera-ture requirements.

Source: Ref. 15.

TABLE3.3

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TABLE3.4

TABLE3.5

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TABLE3.6

TABLE3.7

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TABLE3.8

TABLE3.9

As the prices of natural gas and fuel oil slowly edge upwards, the efficient operation of a boiler becomes more and more important. This leads to the follow-ing considerations in packaged boiler design.

1. All boiler outer walls are to be of the membraned waterwall design.

2. All outer tube walls are to be connected by membrane with no corner joints.

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3. The complete packaged membrane waterwall boiler to be 100% gas tight.

4. The dividing wall between the furnace and convection sections is to be a gas-tight membrane waterwall.

5. A refractory is to be used only to protect the steam drum and mud drum from flame impingement and thus disruptive heat transfer affect-ing water circulation.

When a boiler manufacturer follows these design parameters, a highly effi-cient, compact boiler can be shop-assembled and shipped. Size is limited only by rail or road clearance. If shipment by water is available to the plant, then the size limit to a packaged waterwall boiler seems to be about 350,000 lb of steam per hour. Packaged waterwall boilers of over 3000 psig operating pressure have been built and are in operation. Steam temperature up to 1000°F is standard design.

A large packaged boiler, say 150,000 lb of steam per hour with steam at 650 psig and 710°F, built to the foregoing parameters, operating at capacity around the clock, using the best controls, using the best low NOxburner, burning natural gas, will operate with an efficiency of 84.0–85.0%.

Figure 3.2 shows for two typical packaged waterwall boiler designs. Tables 3.2–3.9 give fuel oil atomization, oil and gas analysis, combustion constants, minimum autoignition temperatures, and natural gas and fuel oil examples and formulas.

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