Ejectors condensers

Figures 6-1 lA, B, and C indicate the capacity of various ejector-condenser combinations for variable sucdon pressures when using the same quandty of 100 psig modve steam. Each point on these curves represents a point of maximum efficiency, and thus any one curve may represent the performance of many different size ejectors each operating at maximum efficiency [1]. Good efficiency may be expected from 50%-115% of a design capacity. Note that the performance range for the same type of ejector may vary widely depending upon design condidons.

Colder cooling water lor ejector condensers Seal-weld HVGO and TOO drawoff trays Increase the number of bottom-stripping trays More superheat of exhaust stripping steam Add velocity steam... 

For clarity, let s assume for Fig. 25.5 that valve A is closed and valves B and C are open. Note that the inlet pressure to the second-stage jet (380 mm Hg) is double its design pressure. Hence the poor vacuum (100 mm Hg) in the upstream surface condenser. If a leak develops in the partition plate in the ejector condensers, then noncondensables (i.e., air) will leak back from the 780 mm Hg discharge... 

Figure 25.5 Steam turbine surface condenser vacuum is bad due to a leak in the ejector condenser shell-side partition plate.

The solution to this problem was not to repair the defective ejector condenser. This had been done before, but without preventing a repeat failure. As I ve explained in Section 25.2.2 of this text, the final or second-stage condenser serves no function. It s a design error. So in this case, as shown in Fig. 25.5, valves B and C were shut and the local atmospheric vent valve A was opened. This allowed the second-stage jet to discharge directly to the atmosphere. As a result, the pressure in the surface condenser dropped by about one-half. 

Cavitation can lead to the shutdown of a desalination plant. If there is cavitation in an ejector condensate pump, it will fafl to reach the discharge pressure and the required amount of condensate would not be extracted. One of the requirements of pumping liquid is that the pressures in any point in the suction arm should never be reduced to the vapor pressure of the liquid as this causes boiling (at reduced pressure). Too low a pressure at the pump suction must always be avoided so that cavitation is not caused. Cavitation in pumps is noticed by a sudden increase in the noise level and its inability to reach discharge pressure [114]. 

SOPLENKOV, K.I., et al., Design and testing of passive heat removal system with ejector-condenser . Progress in Design, Research and Development and Testing of Safety Systems for Advanced Water Cooled Reactors, IAEA-TECDOC-872, Vienna, 1996. 

The carbamate solution from the scmbber flows to a high pressure ejector. The NH feed pressure induces enough head to convey the carbamate solution from the scmbber to the carbamate condenser. 

Del y for Dec y. Nuclear power plants generate radioactive xenon and krypton as products of the fission reactions. Although these products ate trapped inside the fuel elements, portions can leak out into the coolant (through fuel cladding defects) and can be released to the atmosphere with other gases through an air ejector at the main condenser. 

The prevacuum technique, as its name implies, eliminates air by creating a vacuum. This procedure faciUtates steam penetration and permits more rapid steam penetration. Consequendy this results in shorter cycle times. Prevacuum cycles employ either a vacuum pump/steam (or air) ejector combination to reduce air residuals in the chamber or rely on the pulse-vacuum technique of alternating steam injection and evacuation until the air residuals have been removed. Pulse-vacuum techniques are generally more economical vacuum pumps or vacuum-pump—condenser combinations may be employed. The vacuum pumps used in these systems are water-seal or water-ring types, because of the problems created by mixing oil and steam. Prevacuum cycles are used for fabric loads and wrapped or unwrapped instmments (see Vacuum technology). 

Evaporative crystalli rs generate supersaturation by removing solvent, thereby increasing solute concentration. These crystallizers may be operated under vacuum, and, ia such circumstances, it is necessary to have a vacuum pump or ejector as a part of the unit. If the boiling poiat elevation of the system is low (that is, the difference between the boiling poiat of a solution ia the crystallizer and the condensation temperature of pure solvent at the system pressure), mechanical recompression of the vapor obtained from solvent evaporation can be used to produce a heat source to drive the operation. 

Ejector (steam-jet) refrigeration systems are used for similar apph-cations, when chilled water-outlet temperature is relatively high, when relatively cool condensing water and cheap steam at 7 bar are available, and for similar high duties (0.3-5 MW). Even though these systems usually have low first and maintenance costs, there are not many steam-jet systems running. 

The condenser design, surface area, and condenser cooling water quantity should be based on the highest cooling water temperature likely to be encountered, if the inlet cooling water temperature becomes hotter then the design, the primaiy booster (ejector) may cease functioning because of the increase in condenser pressure. 

When the steam supply to one ejector of a group is closed, some means must be provided to for preventing the pressure in the condenser and flash tank from equ zing through that ejector, A com-partmental flash tank is frequently used for such purposes. With this... 

Vacuum is applied to the chamber and vapor is removed through a large pipe which is connected to the chamber in a manner such that, if the vacuum is broken suddenly, the inrushing air will not greatly disturb the bed of material being dried. This line leads to a condenser where moisture or solvent that has been vaporized is condensed. The noncondensable exhaust gas goes to the vacuum source, which may be a wet or diy vacuum pump or a steam-jet ejector. 

Figure 1. A wide range of pressures can be achieved by using various combinations of ejectors and condensers. The same steam consumption is used for each design here. Note Curves are based on 85°F condensing water. If warmer water is used, curves shift to the left—cooler water, shift right.

Air is usually the basic load component to an ejector, and the quantities of water vapor and/or condensable vapor are usually directly proportional to the air load. Unfortunately, no reliable method exists for determining precisely the optimum basic air capacity of ejectors. It is desirable to select a capacity which minimizes the total costs of removing the noncondensable gases which accumulate in a process vacuum system. An oversized ejector costs more and uses unnecessarily large quantities of steam and cooling water. If an ejector is undersized, constant monitoring of air leaks is required to avoid costly upsets. 

Vacuum Distillation - Heavier fractions from the atmospheric distillation unit that cannot be distilled without cracking under its pressure and temperature conditions are vacuum distilled. Vacuum distillation is simply the distillation of petroleum fractions at a very low pressure (0.2 to 0.7 psia) to increase volatilization and separation. In most systems, the vacuum inside the fractionator is maintained with steam ejectors and vacuum pumps, barometric condensers, or surface condensers. 

Cracking imposes an additional penalty in a vacuum unit in that it forms gas which cannot be condensed at the low pressures employed. This gas must be vented by compressing it to atmospheric pressure. This is accomplished by means of steam jet ejectors. Ideally, it would be possible to operate a vacuum pipe still without ejectors, with the overhead vapors composed only of steam. In practice, however, leakage of air into the system and the minor cracking which occurs make it necessary to provide a means of removing non-condensibles from the system. In addition to the distillation of atmospheric residuum, the lube vacuum pipe still is also used for rerunning of off specification lube distillates. 

The VPS overhead consists of steam, inerts, condensable and non-condensable hydrocarbons. The condensables result from low boiling material present in the reduced crude feed and from entrainment of liquid from the VPS top tray. The noncondensables result from cracking at the high temperatures employed in the VPS. Inerts result from leakage of air into the evacuated system. Steam and condensable hydrocarbons are condensed using an overhead water-cooled condenser. The distillate drum serves to separate inerts and non-condensables from condensate, as well as liquid hydrocarbons from water. Vacuum is maintained in the VPS using steam jet ejectors. 

Cracking imposes an additional penalty in a vacuum unit in that it forms gas which carmot be condensed at the low pressures employed. This gas must be vented by compressing it to atmospheric pressure. This is accomplished by means of steam jet ejectors. 

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