Evaporators Once-through

This hydrochloric arid gas co-produccd is rccydcd after absorption, extraction, and possible reconcmtiation in evaporators. Once-through conversion is about 10 to 15 per cent, and the hydrolysis yield is 85 to 95 molar per cent... 

FIG. 11-125 Flow sheets for seawater evaporators, a) Multiple effect (falling film), (h) Multistage flash (once-through). (c) Multistage flash (recirculating). 

Once-through boilers may be either sub- or super-critical. Sub-critical boilers are clearly at some potential risk of on-load corrosion owing to the presence of the evaporator zone. Measures aimed at avoiding on-load corrosion include—keeping the overall solute concentration low, keeping the ionic balance matched, and maintaining the waterside oxide suitably thin. With super-critical plant, there is no chance of on-load corrosion whilst it is operating in the super-critical mode, as there is no phase boundary. The risk is present, however, when the plant is run in the sub-critical mode, as all super-critical plant must be at times. 

Lee, D. H., 1970, Studies of Heat Transfer and Pressure Drop Relevant to Subcritical Once-through Evaporator, Paper IAEA-SM-130/56, Symp. on Progress in Sodium-Cooled Fast Reactor Engineering, Monte Carlo, Monaco. (4)... 

Nguyen et al. [205] used a technique in which a constant mass flow rate of water-saturated air was forced through a water-saturated sample. It was explained that the shear force of the gas flow dragged water out of the sample. In addition, the saturated air was needed in order to prevent water loss from the sample by evaporation. Once a steady state was achieved, the pressure difference between the inlet and outlet of the apparatus was recorded. After the tests were completed, the sample was weighed to obtain its water content. Thus, the relative permeability was calculated from the pressure drop, the water content in the sample, and the mass flow rate [205]. 

Noncontact cooling water is normally supplied to several processes from the utilities area. The system is either a loop that utilizes one or more evaporative cooling towers, or a once-through system with direct discharge. 

In batch processes, the time of exposure to heat is well defined. This is also true of continuous processes in which the liquid to be evaporated is passed only once through the heater. In continuous processes, is where the liquid is recirculated through the heater, the average residence time, a, is given by the ratio. 

Evaporators may be operated either as once-through units, or the liquid may be recirculated through the heating elements. In once-through operation, all the evaporation is accomplished in a single pass. The ratio of... 

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