Boilers steam temperature controls

Boiler control systems include combustion controls, superheat steam temperature controls and FW controls. Control systems are used to maintain steam pressure, boiler load, drum water levels, and fuel-air ratios. 

How is steam temperature controlled in a drum boiler What feature of a once-through boiler enables manipulation of feedwater flow to control temperature ... 

Humidification. For wiater operation, or for special process requirements, humidification maybe required (see Simultaneous HEAT and mass transfer). Humidification can be effected by an air washer which employs direct water sprays (see Evaporation). Regulation is maintained by cycling the water sprays or by temperature control of the air or water. Where a large humidification capacity is required, an ejector which direcdy mixes air and water in a no22le may be employed. Steam may be used to power the no22le. Live low pressure steam can also be released directly into the air stream. Capillary-type humidifiers employ wetted porous media to provide extended air and water contact. Pan-type humidifiers are employed where the required capacity is small. A water filled pan is located on one side of the air duct. The water is heated electrically or by steam. The use of steam, however, necessitates additional boiler feed water treatment and may add odors to the air stream. Direct use of steam for humidification also requires careful attention to indoor air quahty. 

The hot gases from the combustor, temperature controlled to 980°C by excess air, are expanded through the gas turbine, driving the air compressor and generating electricity. Sensible heat in the gas turbine exhaust is recovered in a waste heat boiler by generating steam for additional electrical power production. 

Steam can be contaminated with soHds even when carryover is not occurring. Contaminated spray attemperating water, used to control superheated steam temperature at the turbine inlet, can introduce soHds into steam. A heat exchanger coil may be placed in the boiler mud dmm to provide attemperation of the superheated steam. Because the mud dmm is at a higher pressure than superheated steam, contamination will occur if leaks develop in the cod. 

Also shown in Figure 23.17 are let-down stations between the steam mains to control the mains pressures via a pressure control system. The let-down stations in Figure 23.17 also have de-superheaters. When steam is let down from a high to a low pressure under adiabatic conditions, the amount of superheat increases. Desuperheating is achieved by the injection of boiler feed-water under temperature control, which evaporates and reduces the superheat. There are two important factors determining the desirable amount of superheat in the steam mains. 

Radiant heat transfer in furnaces is roughly proportioned to the difference in the fourth power ol the absolute temperatures of the radiating and receiving surfaces. The water wall surface is approximately at boiler saturation lemperuture. while the superheater surfaee varies from this to somewhat above the temperature of the steam al the superheater outlet. However ihe mean radiating temperature of Ihe furnace gases is usually over 1204 C. The fourth power of the receiving surface temperature is thus seen to be small compared to the fourth power of the transmitting surface temperature consequently the latter controls the transmittance, and boiler lube temperature does not need to be considered a variable to be accounted for. 

The synthesis loop consists of a recycle compressor, feed/effluent exchanger, methanol reactor, final cooler and crude methanol separator. Uhde s methanol reactor is an isothermal tubular reactor with a copper catalyst contained in vertical tubes and boiling water on the shell side. The heat of methanol reaction is removed by partial evaporation of the boiler feedwater, thus generating 1-1.4 metric tons of MP steam per metric ton of methanol. Advantages of this reactor type are low byproduct formation due to almost isothermal reaction conditions, high level heat of reaction recovery, and easy temperature control by... 

Figure S-27a and b shows variations in the response of a distributed lag to a step change in load for different combinations of proportional and integral settings of a PI controller. The maximum deviation is the most important criterion for variables that could exceed safe operating levels, such as steam pressure, drum level, and steam temperature in a boiler. The same rule can apply to product quality if violating specifications causes it to be rejected. However, if the product can oe accumulated in a downstream storage tank, its average quality is more important, and this is a function of the deviation integrated over the residence time of the tank. Deviation in the other direction, where the product is better than specification, is safe but increases production costs in proportion to the integrated deviation because quality is given away.

In the boiler above, it is the pressure of the saturated steam in the superheater, it is steam temperature. Each unit must be controlled to deliver whatever rate of product is required at a consistent quality if the next unit is to fulfill its function. 

Steam temperature in a drum boiler is controlled principally with spray. The once-through boiler has one less controlled variable (liqnid level), which frees a manipulated variable (feedwater flow) for temperature control. 

A supercritical fossil-fired power plant with a once-through steam cycle is usually operated with a sliding pressure the turbine governor valve is kept open in the upper load range and the boiler outlet temperature is kept constant such that the boiler outlet pressure increases proportionally with the steam mass flow and thus with load. Consequently, the boiler is operated at subcritical pressure below approximately 80—90% load. However, such control is not permitted for the SCWR because dryout... 

The chemical injection pumps meter the solution into a mixing header with flow rates automatically controlled by continuous monitoring of the boiler load, temperature, and NO, concentration (Nalco, 1990 Epperly and Hofmann, 1989). The concentrated NO,OUT A solution is diluted with water and flows to the injector modules through the distribution piping. Typically, a total of S-7 scfm of plant air per injector has been used for atomization and for the wall-injector outer cooling jackets (Grisko, 1992). When steam is used for atomization, about 50 Ib/hr of 50 psig steam per injector is required. 

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