锅炉水循环和水处理外文翻译

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1、 中文 3822 字 附 录 专业外文资料翻译 一 .英文资料 Boiler Circulation Water Treatment An adequate flow of water and water-steam mixture is necessary for steam generation and control of tube metal temperatures in all the circuits of a steam-generating unit. At supercritical pressures, this flow is produced mechanically

2、 by means of pumps. At subcritical pressures, circulation is produced either naturally by the action of the force of gravity, by pumps, or by a combination of the two. Fig-1 Simple natural-circulation circuit (diagrammatic) including primary steam separator in drum The force of gravity available to

3、produce flow in natural circulation comes from the difference between the densities (1b/cuft) of the fluids in the downcomer (downflow) and riser (upflow) portions of the circuit (Fig-1). Maximum pumping effect occurs if the fluid in the downcomers is water at or slightly below saturation temperatur

4、e and free of steam bubbles. Heat-absorbing rises at saturation temperature convey to the boiler drum a water-steam mixture of less density than that of the water in the downcomers. This difference in density establishes the force available for circulation. The flow in the various circuits of boiler

5、 units designed for forced circulation at subcritical pressures, is produced by mechanical pumps. There are two general types of forced-circulation systems, a “once-through ” system and a “recirculating” system. The “once-through” force-circulation type receives water from the feed supply, pumping i

6、t to the inlet of the heat-absorbing circuits. Fluid heating and steam generation take place along the length of the circuit until evaporation is complete. Further progress through the heated circuits results in superheating the vapor. Conventionally this type of force circulation requires no steam-

7、and-water drum. A modification of the “once-through” type evaporates to partial dryness(90%quality) removeing the excess water in a separator. The “recirculation” forced-circulation-type unit has water supplied to the heat-absorbing circuits through a separate circulating pump. The water pumped is c

8、onsiderably in excess of the steam produced and, like a natural-circulation boiler, a steam-and-water drum is required for steam separation. The separated water together with feedwater from the feed pump is returned through downcomer circuits to the circulating pump for another “round trip.” In the

9、recirculating type of forced circulation there is a net thermal loss for the boil unit because of the separate circulating pump. While practically all the energy required to drive the pumps reappears in the water as added enthalpy, this energy originally came from the fuel at a conversion-to-useful-

10、energy factor of less than 1.0. If an electric motor drive is used, the net energy lost, referred to the fuel input in a plant with 33%thermal efficiency, is about twice the energy supplied to the pump motor. Natural circulation In a natural-circulation system, circulation increases with increased h

11、eat input (and increased steam output) until a point of maximum fluid flow is reached. Beyond this point, any further increase in heat absorption results in a flow decrease. The from of the curve, shown in Fig-2, is produced by two opposing forces, An increase in downcomers and risers as the heat ab

12、sorption increases. At the same time, the friction and other flow losses in both downcomers and risers also increase. When the rate of increase in these losses (caused primarily by the increase in specific volume in the riser circuits) becomes greater than the gain from increasing density difference

13、, is to design all the circuits to operate in the region of the rising part of the curve, to the left of the peak in Fig-2. When design conditions are limited to the rising portion of the circulation curve, a natural-circulation boiler tends to be self-compensating for the numerous variations in hea

14、t-absorption conditions encountered in an operating unit. These include sudden overloads, change in heat-absorbing-surface cleanliness, nonuniform fuel bed or burner conditions,and even the inability to forecast precisely actual conditions over the operating lifetime. Fig-2 typical relationship betw

15、een circulation in a boiler circuit (at a given pressure) and amount of steam steam produced(scale arbitraiy) No similar compensating effect is inherent in a forced-circulation unit operating at subcritical pressures, since a large part of the total resistance of the riser circuits, much greater tha

16、n the natural circulation effect, is caused by flow-distribution devices required at the circuit inlets. Under these conditions, because of the disproportionately large resistance of the distributors, an increase in heat absorption to an individual circuit or group of circuits causes only a slight c

17、hange in the flow rate. The method of producing flow in boiler circuits, whether natural or mechanical, has virtually no bearing on the effectiveness of heat-absorbing surfaces as long as the inside surface is werred at all times by the water in a water-steam mixture of suitable quality to maintain

18、nucleate boiling. Provided this fundamental requirement is met, the water-film resistance to heat flow is negligibly small, and the overall heat conductance depends on gas-side and tube-wall resistances. Within the nucleate boiling regime, boiler heat-absorbing surface in the furnace or convection p

19、ortion of the unit absorbs absorbs substantially the same amount of heat per sq ft regardless of whether the circulating flow is produced by natural methods or by pumps. With either type of circulation, any departure from the nucleate boiling regime requires special consideration of the forced-convection stearn-film heat transfer