FC forced circulation

I - Circuit I depressurization II - Circuit II depressurization III - Circuit III depressurization (a) before TEGs (b) area of TEGs TEG - Thermoelectric generator NC - natural convection FC - forced circulation... 

EHRS - Emergency heat removal system ECCS - Emergency core cooling system FC - Forced circulation NC - Natural circulation FWP - Feedwater pump CP - Condensate pump... 

EHRS - emergency heat removal system FC - forced circulation NC - natural circulation... 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

FIGURE 64.7 A forced-circulation (FC) crystallizer. (Reprinted from Mersmann, A., Ed., Crystallization Technology Handbook, Marcel Dekker, New York, 1995. With permission.)... 

Pri. Primary Circuit FC Forced (Pumped) Circulation C/R Conduction/Radiatlon... 

If crystallization is only used for separating substances, it is sufficient to create crystallized masses that can be readily separated. In such cases, a normal elbow-type circulation pump can be used, as is the case in the forced circulation crystallizers (Figure 11.2a). If coarser crystals are to be produced, a circulation pump that rotates more slowly will have to be used and the pressure drop of the system reduced. This is the case in the so-called DTB crystallizers (Figure 11.2b) that are equipped with an internal draft tube propeller pump. It is not unusual for these pumps to have a diameter of >lm up to about 2.5 m, with significantly lower tip speeds in comparison to the external pumps in the FC crystallizers (approximately 8-12 m/s instead of 16-20 m/s). In the case of evaporative crystallization, this crystallizer design offers the above-mentioned advantage that the system resistance H for the internal circulation pump is reduced and therefore the required specific energy input is considerably lower. Fewer nuclei are produced and the crystals become coarser. 

On the left hand side, it is shown a sketch of a (forced circulated) FC-type crystalliser, a crystalliser with external circulation loop. On the right hand side, one can find a simplified solubility system in which the metastable limit is marked with a dashed line. What happens within one circulation loop is indicated by numbers in the solubility diagram as well as in the FC-crystalliser sketch. 

The rated thermal output of MONJU [5.63, 5.64] is transported through the primary heat transport system (PHTS) and intermediate heat transport system (IHTS) loops to the steam generators. Shutdown heat removal is normally by forced circulation (FC) provided by pony motors associated with each of the loop pumps. Heat is rejected to air at the air blast heat exchanger of the intermediate reactor auxiliary cooling system (ACS) which branches off from each IHTS loop. Thus the auxiliary cooling system (ACS) of the Monju reactor is coupled with the secondary system which also has the role as decay heat removal system. 

In Fig. 4.6 a scheme of cooling circuit related to FCS for automotive application is shown. The main components are the pump for liquid circulation, the water reservoir and the heat exchanger with fan. For small size stacks (from 100 to 500 W) it is possible to use only air forced by fan to cool directly the stack, while for higher powers, more suitable for automotive requirements (1-100 kW), it is reasonable to use an internal coolant circuit fed by a liquid, such as de-ionised water or ethylene glycol-water mixtures, to improve the heat removing capacity of an order of magnitude with respect to the gas. 

The reactants can be mixed in the circulation piping of an FC-type crystallizer or in the draft tube of a DTB-type crystallizer where a large volume of slurry is mixed continuously with the reactants so as to minimize the driving force (supersaturation created by the reaction). Removal of heat is conveniently done by vaporizing water or other solvents as in a conventional evaporative-type crystallizer. 

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