Forced circulation Pumps

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. 

In the saturator process (see Figure 12.7), neutralization and crystallization are carried out in the same vessel. The sulfuric acid is delivered to the suction side and the ammonia to the pressure side of the forced circulation pump. Crystallization of the metastable solution gives particle sizes generally between 0.5 and 3 mm. The salt is continuously discharged at the lower end of the saturator. The salt is separated in centrifuges, dried, and cooled. The mother liquor is returned to the saturator. Impurities in the sulfuric acid can adversely affect crystallization. Small quantities of phosphoric acid, urea, or inorganic salts are added to promote crystal growth295. 

A Kryslai type of vacuum evaporator-crystallizer B Forced-circulation pump C Slurry-redrculation pump D Mother-liquor recycle pump E Mother-liquor tank F Continuous or batch centrifuge G Slurry concentrator H Drier conveyor I Rotary drier J Dried-product conveyor K Vacuum condenser-ejector unit... 

Forced-circulation rehoiler A pump is used to force liquid through the rehoiler. 

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... 

The treated water contains sufficient concentration of surface film-forming agents if cold water spends about 12 min and warm water at least 20 min in the tank [19]. Sudden temperature variations over 10°C must be prevented because the active form of Al(OH)3 is sensitive to them [20]. If mixing with cold water or subsequent warming cannot be avoided, a short-term electrolytic aftertreatment must be provided in a small reaction tank. The development of undisturbed protective films in the tubing assumes continuous water flow with forced circulation by pumps [20]. 

Forced Circulation System—Using foreed eireulation in a waste heat reeovery system allows the use of smaller tube sizes with inherent inereased heat transfer eoeffieients. Flow stability eonsiderations must be addressed. The reeireulating pump is a eritieal eomponent from a reliability standpoint and standby (redundant) pumps must be eonsidered. In any event, great eare must go into preparing speeifieations for this pump. 

A low-pressure-drop liquid cyclone is sometimes used to clarify liquor discharged from the evaporator. The driving force is the pressure drop across the circulating pump. Thickened slurry is returned through a wide-open cyclone underflow connection to the circulating piping before the pump suction. 

Pumpen-benzin, n. pump gasoline, pump petrol, -kolben, m. pump piston, -kiihlung,/. forced-circulation cooling, punkt, adv. punctually exactly, just. 

Forced circulation Viscous and solid-containing liquids can be circulated. Enables an erosion-fouling balance. Circulation rate can be controlled. Relatively expensive due to extra shell volume. Cost of pump and pumping. Leakage of material at stuffing box. 

In forced circulation evaporators the liquid is pumped through the tubes. They are suitable for use with materials which tend to foul the heat transfer surfaces, and where crystallisation can occur in the evaporator. 

Forced circulation, Figure 12.50 in which the fluid is pumped through the exchanger, and the vapour formed is separated in the base of the column. When used as a vaporiser a disengagement vessel will have to be provided. 

The major difference between a stirred-tank batch reactor and a stirred-tank flow reactor is that, in the latter, provision must be made for continuous flow of material into and out of the reactor by gravity flow or forced-circulation flow with a pump, together with appropriate block and relief valves. 

A single-effect evaporator is used to concentrate 0.075 kg/s of a 10 per cent caustic soda liquor to 30 per cent. The unit employs forced circulation in which the liquor is pumped through the vertical tubes of the calandria which are 32 mmo.d. by 28 mmi.d. and 1.2 m long. Steam is supplied at 394 K, dry and saturated, and the boiling-point rise of the 30 per cent solution is 15 degK. If the overall heat transfer coefficient is 1.75 kW/m2 K, how many tubes should be used, and what material of construction would be specified for the evaporator The latent heat of vaporisation under these conditions is 2270 kJ/kg. 

Increasing the velocity of flow of the liquor through tubes results in a significant increase in the liquid-film transfer coefficient. This is achieved in the forced circulation units where a propeller or other impeller is mounted in the central downcomer, or a circulating pump is mounted outside the evaporator body. In the concentration of strong brines, for example, an internal impeller, often a turbine impeller, is fitted in the downtake, and this form... 

Figure 14.21. Forced circulation evaporator with an external pump...

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