锅炉水循环和水处理外文翻译.docx

1、锅炉水循环和水处理外文翻译 专业外文资料翻译一.英文资料Boiler CirculationWater TreatmentAn 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 by me

2、ans 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 drumThe force of gravity available to produce

3、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 temperature and fr

4、ee 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 units de

5、signed 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 it to the i

6、nlet 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-and-water

7、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 considerably

8、 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 recirculatin

9、g 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-energy facto

10、r 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 circulationIn a natural-circulation system, circulation increases with increased heat input (and

11、 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 absorption incre

12、ases. 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, is to design

13、 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 heat-absorption co

14、nditions 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 between circulation

15、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 than the natural cir

16、culation 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 change in the flow

17、 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 nucleate boiling.

18、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 portion of the unit

19、 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 h

20、eat transfer coefficient and its relation to permissible metal temperatures.Forced or natural circulationUnder certain conditions forced circulation can be usefully applied to steam generation. Mechanical means to move the fluid within the circuits are employed for boilers designed to operate above

21、or near the critical pressure (3208.2psia.) There are instances, also, in the process and waste-heat fields where temperature control or consolidating heat pickup from widely separated points can be economically effected by the use of pumps. The condition where forced circulation can be applied to a

22、dvantage are quite specific.Natural circulation is most effectively emploved when large changes in density usually restricted to subcritical pressure designs where thereis a considerable difference in density between steam and water. At pressures above 2900 psi a natural-circulation system becomes i

23、ncreasingly large and costly,and a pump may be more economical to assure positive flow.The forced-circulation principle, however, is equally operable in both the supercritical and subcritical pressure ranges. The selection of the identifying name “Universal-Pressure” boiler reflects the broad applic

24、ability of the once-through forced-circulation principle. Its choice,as opposed to the retention of natural circulation in the subcritical range, is essentially determined by the economics of the installation.Fig-3 Densities of steam and water at saturated steam temperature for pressures from atmosp

25、heric to criticalThe differential in densities of steam and water for the range of 14.7 to 3208.2 psia is noted in Fig-3. A substantial differential persists well up toward the critical pressure. As long as the maximum effectiveness off this differential is maintained by the efficient separation of

26、the steam from the water in the circnlating system, as with the use of cyclone steam separator, mechanical aid to circulation is not essential.Internal Treatment of WaterThere are numerous chemicals ,some called boiler compounds, on the market and recommended for “internal” water softening and other

27、 treatment. Their duty is to stabilize hardness agents, prevent scaling or residual make deposits easily removable. Such chemicals are also used for neutralization of residual hardness in systems after external treatment. Chemicals of this nature are introduced at a more or less constent rate in dis

28、solved state into feed tanks or systems. The most frequently used chemicals with calcium and magnesium salts results in consolable calcium and magnesium phosphates. Other internal treatment agents are calcium hydroxide, sodium carbonate, sodium hyates, tannins and amines.The most effective method of

29、 introducing internal treatment compounds to ensure a reasonable degree of quantity control is by means of dosage apparatus consisting of one or more containers having manual or motor driven agitators and a dosage pump. As a general rule internal treatment alone, for waters of much more than 5（UK）（p

30、.p.m）hardness is not recommended because system and make-up quantities and water composition may become critical and precipitated sludge and salts are liable to increase density of water and solidification of sludge with consequent propensity to foaming and priming in steam plant and circulation com

31、plications. Internal treatment is generally used within limits for smallish systems, say up to 23MW or as residual treatment and to raise pH value to a requisite amount.External Treatment of WaterExternal softening of system and make-up water is the more effective and comprehensive method of water t

32、reatment for any size of plant and network and it can suit most water characteristics. Nowadays external treatment involves the principle of ion-exchange, which ensures water of virtually zero hardness.The ion-exchange process can be described as follows. Molecules of dissolved salts are dissociated

33、 in from of free electrically charged ions in liquid solution. In the forch field they tend to be attracted to opposite charges: positive cations to the negative cathodes and negative anions to the positive anode. Ion-exchange materials are insoluble artificial resins to which activated chemical groups with tied dissociabl