Forced circulation boilers

There are two simple rules to minimize boiler leaks 1) keep total dissolved solids (TDS) in the boiler blowdown below 2,500 ppm 2) keep all boiler tubes submerged in water. Watch the steam drum liquid level closely. For forced circulation boilers, circulate 10-15 lb of water for each pound of steam generated. Damage to the ferrules in the front-end tube sheet is another common cause of boiler leaks. 

In the case of forced circulation boilers, a pump provides water circulation (Fig. 11.4). This system is important when boilers operate over a wide range of capacity. 

Certain boilers employ forced circulation, whereby a pump helps impart the circulation through the downcomer lines to the waterwaH header, particularly to improve or control circulation at low loads. Forced-circulation pumps are also required in high pressure and supercritical pressure boilers, because once the pressure within a boiler approaches the critical pressure, 22.1 MPa (3208 psia), the densities of the water and steam become similar, limiting or eliminating the potential for natural circulation. 

Water is evaporated from purified brine using multiple-effect or vapor recompression evaporators (Figs. 3 and 4). Multiple-effect systems typically contain three or four forced-circulation evaporating vessels (Fig. 4) connected together in series. Steam from boilers suppHes the heat and is fed from one evaporator to the next to increase energy efficiency in the multiple-effect system. 

Coil boilers are available as fully automatic package units, typically ranging in size from 15 to 300 boiler hp (500 lb/hr-10,000 lb/hr). Most designs employ forced circulation and a single, continuous spiral, helical-wound coiled tube that hangs inside the furnace. The coil is subjected to intense heat release from a gas or oil burner. 

Fouling produces dirty and inefficient steam-water systems, impeding the natural or forced circulation within a boiler, limiting the flow in... 

Values in the table for electrode, high-voltage, forced circulation jet-type boilers assume the existence of a mechanical deaerator. ASME recommends that chemical deaeration be provided in all cases, especially if mechanical deaeration is nonexistent or inefficient. 

In the NSM process (see Figure 10.5) the reactor pressure is about 4.5 bar and the temperature is between 170 and 180°C. Forced circulation and a thermal siphon effect circulate the solution through the reactor. Some of the heat of reaction is used to generate pure steam in an external boiler and some vaporizes water in the reactor to make process steam. This process steam is used to concentrate the NH4NO3 solution from less than 80% up to 95%295. 

The process gases enter a forced-circulation-type waste heat boiler at 800-1000 °C. Several changes have been made to the mechanical rapping equipment and cooling pipe bundles of the boiler to improve the performance and maintenance access. The gases exit the waste heat boilers at a temperature of 300-350 °C and pass through the hot cyclone separators and electrostatic precipitators into the mercury removal towers. Two electrostatic precipitators are installed simultaneously in both lines to ensure continuous processing. 

Boiler Type Forced circulation waste heat type... 

A horizontal steam boiler for the forced circulation of gases to produce saturated steam, and having the characteristics of 50 tonne/h at 400 atm and 400 C including five evaporator bundles and two superheater bundles, will be built. A steam condenser of 40 tonne/h and the associated service installations will also be installed. A DM water plant of 90 m /h capacity to feed the boiler will also be constructed. 

A typical steam-generation thermosiphon circuit is shown in Exhibit 7-48. Briefly, this circuit is the difference of the water-steam mixture and the static head in the downcomer that maintains fluid circulation. Locating the steam drum at the top of the furnace provides the static head and collects the steam being generated Steam returns to the top portion of the drum, while the water lines come off die bottom. Loops and pockets must be avoided when layipg out die downcomer, and riser piping and any horizontal lines must slope toward the vwiste heat boiler or convection coil. Wh pumps are used in this circuit, it is called forced circulation. 

Each steam generator was of single-wall tube-in-shell design. The evaporators were of the forced-circulation type, with each of the three circuits having a steam drum and a boiler circulating pump. Superheated steam from the three circuits flowed to a common header to drive a 300MW turbo-alternator. The main feed was via a 100% duty steam-driven pump with... 

The circulation in steam water circuits is of two types (a) natural or (b) forced circulation (Fig. 11.4). Natural circulation in boilers is induced by the difference in the density between the cooled water and heated water. The tubes which transport the hotter water are called riser tubes, and those which transport the cooler water are called downcomer tubes. The difference of mean density between the cooler water in the downcomer tubes and heated water in the riser tubes (furnace evaporate tubes) generates a driving force sufficient to produce natural circulation in the steam water circuit. Too high heat... 

Fire-tube boiler A type of boiler in which flue gases are inside the tubes in contact with water outside the tubes. Forced circulation The use of an external device, such as a pump, to force the circulation of water. 

Steam-Generator Circulation System Circulation systems for utility application are generally classified as natural circulation and forced or pump-assisted circulation in drum-type boilers, and as once-through now in subcritical- and supercriticaf-pressure boilers. The... 

Thermal power plant auxiliaries such as flow control of primary air fan, ID fan and forced-draught fans, boiler feed pumps circulating water pumps and condensate pumps, coal handling plant (e.g. ball mill, wagon tippler, and stacker reclaimer)... 

These applications have considerably more stringent performance requirements than any other application. Circulating water pumps, boiler feed pumps, forced-draught (FD) and induced-draught (ID) fans, pulverizers (ball mills) and condensate pumps are components in a thermal power station that may require extra safety in a standard motor to make it able to fulfil these requirements and withstand abnormal service conditions and system disturbances. Abnormal operating conditions may be one or more of the following ... 

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