Evaporators operating problem

Flooded refrigeration systems are a version of the closed-cycle design that may reduce operating problems in some appHcations. In flooded systems, the refrigerant is circulated to heat exchangers or evaporators by a pump. Figure 11 shows the flooded cycle, which can employ any of the simple or compound closed-refrigeration cycles. 

Control of an evaporator requires more than proper instrumentation. Operator logs snould reflect changes in basic characteristics, as by use of pseuao heat-transfer coefficients, which can detect obstructions to heat flow, hence to capacity. These are merely the ratio of any convenient measure of heat flow to the temperature drop across each effect. Dilution by wash and seal water should be monitored since it absorbs evaporative capacity. Detailed tests, routine measurements, and operating problems are covered more fuUy in Testing Procedure for Evaporators (loc. cit.) and by Standiford [Chem. Eng. Prog., 58(11), 80 (1962)]. 

CYCLE TIME FOR MINIMUM COST PER UNIT OF HEAT TRANSFER There are many different circumstances which may affect the minimum cost per unit of heat transferred in an evaporation operation. One simple and commonly occurring case will be considered. It may be assumed that an evaporation unit of fixed capacity is available, and a definite amount of feed and evaporation must be handled each day. The total cost for one cleaning and inventory charge is assumed to be constant no matter how much boiling time is used. The problem is to determine the cycle time which will permit operation at the least total cost. 

The characteristic features of liquefaction described above necessarily result in unstable operation and high cost of liquefaction via pyrolysis. Recently, the operational problems have been solved by adding calcium hydroxide to control the evaporation and decomposition of terephthalate components such as calcium terephthalate [5]. 

In Chapter 3, Ruslan Khamizov and coworkers describe the economic recovery of minerals from seawater and brines formed in desalination plants and solar evaporation operations. The employment of ion-exchange technology for this purpose is critically reviewed. They carefully examine the economic aspects of optimal ion-exchange recovery of minerals from these sources by combining various techniques of the operation discussed in Chapter 2. The great rapid depletion of land-based mineral resources of the world make this a problem of great interest. 

Seawater containing 3.5 wt% dissolved salts is to be desalinated in an adiabatic six-effect evaporator. (See Problem 8.58.) Backward feed is to be used the seawater is fed to the last evaporator, and successively concentrated brine solutions flow countercurrent to the direction of flow of steam from one effect to the next. Saturated steam at P = 2 bar is fed to the tube bundle in the first effect. The operating pressures in bars of the six effects are, respectively, 0.9,0.7,0.5,0.3,0.2, and 0.1. The brine leaving the first effect contains 30 wi% salt. The flowchart shows Effects 1,5. and 6. 

The experiments have proved that membrane distillation can be applied for radioactive wastewater treatment. In one-stage installation the membrane retained all radionuclides and decontamination factors were higher than those obtained by other membrane methods. The distillate obtained in the process was pure water, which could be recycled or safely discharged into the environment. It seems the process can overcome various problems of evaporation such as corrosion, scaling, or foaming. There is no entrainment of droplets, which cause the contamination of condensate from thin-film evaporator. Operation at low evaporation temperature can decrease the volatility of some volatile nuclides present in the waste, such as tritium or some forms of iodine and ruthenium. The process is especially economic for the plants, which can utilize waste heat, e.g., plants operating in power and nuclear industry. 

Many of the problems in operation and control of a given evaporator system will be specific to the application. However, all systems need to answer such questions as how to evaluate performance, how best to schedule periodic boil outs (cleaning), how to measure and control variables typified by temperature, pressure, fluid level, fluid flow rate, composition, etc., and how to detect faults in evaporator operation quickly and efficiently. ... 

Chen, I.Y. KocamustafaoguUari, G. An experimental study and practical correlations for overall heat transfer performance of horizontal tube evaporator design. In Heat Transfer Equipment Fundamentals, Design, Applications, and Operating Problems Shah, R.K., Ed. The American Society of Mechanical Engineers New York, 1989 108, 23-32. 

Nearly every supplier of evaporation equipment and systems maintains a pilot plant facility where, for a fee, different evaporation schemes can be set up. Data obtained from several days of testing on small laboratory or pilot plant units can be good predictors of evaporator performance, and these data are very helpful in the scaling-up calculations for production-sized installations. Samplers obtained from the test work can be used to check the mass balances, concentrations, and product quality. Serious operational problems like foaming, plugging, and fouling can occur in even short pilot plant tests and can point to the need for alternative evaporator types or modified designs. 

Since relatively small, briquetted, almond or pillow-shaped ice can be easily poured and metered, it became a superior cold-packing material. Roller presses were preferred for this operation because the thermal contraction of machine parts does not cause operational problems. Dry ice, compressed and shaped solid COj, is an even better material for cold packing because it evaporates rather than melts and its cooling capacity is almost three-times higher than that of water ice. Dry ice compacts are typically made with hydraulic presses and are still used today. 

The small operation range and a minimum circulation rate are drawbacks in the case of evaporators operated due to natural convection. In addition, problems may arise for evaporators under vacuum or for liquid mixtures with a wide range of boiling temperatures. 

When froth exists, the froth density is a function of the heat transfer. As a result, apparent liquid levels often vary as steam flow varies. This can result in operational problems and flooding of systems, especially if trays are provided in the evaporator body. Excessive entrainment can also result. 

Water-soluble scales and calcium carbonate account for many evaporator scaling problems. Calcium scales have a much greater impact on evaporator capacity and pose the more serious problem. The rate of calcium scaling is very dependent on temperature and is also directly proportional to solids content. Calcium scales can be controlled by minimizing calcium inputs and, to a great extent, through operation techniques. Special treatments are also sometimes effective. 

Large centrifugal compressors have proven to be highly reliable in evaporator systems. Most operational problems have been caused by improper matching of compressor and evaporator and excessive entrainment. Several actions are recommended to maximize compressor reliability ... 

In spite of precautions taken during the design of evaporator systems, problems do arise during startup and operation. Parameters which cannot always be precisely determined make it nearly impossible to define all the problems during the design stage. These parameters include ... 

Evaporators -handles TDS > 100,000 ppm -virtually no pre-treatment needed -distillate not significantly affected by influent quality -energy and capital intensive -scaling may be an operating problem S, 15, IS... 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

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