Ejector control

Ejectors do not respond to wide fluctuations in operating variables. Therefore, control of these systems must necessarily be through narrow ranges as contrasted to the usual control of most other equipment. 

Absolute Pressure Ins. Hg Temperature °F. Volume cu. ft./lb. Absolute Pressure Ins. Hg Temperature °F. Volume cu. ft./lb. Absolute Pressure Ins. Hg Temperature °F, Volume cu. ft./lb. 

Values obtained directly or by interpolation from Keenan Keyes— Thermodynamic Properties of Steam, John Wiley Sons, 1930 by permission and Courtesy C. H. Wheeler Co., Philadelphia, Pa. 

The controls for larger systems involve about the same principles unless special performance is under consideration. 

Applied Process Design for Chemical and Petrochemical Plants 

---

Figure 6-31. Single-stage ejector control with varying load.

Dusts, particle sizes, 225 Dusts, hazard class, 521-523 Explosion characteristics, 524 Efficiency, centrifugal pumps, 200 Ejector control, 380 Ejector systems, 343, 344, 351 Air inleakage, table, 366, 367 Applications, 345 Calculations, 359-366 Chilled water refrigeration, 350 Comparison guide, 357, 375 Evacuation lime, 380, 381 Charts, 382 Example, 381 Features, 345... 

RTM moulds are typically made of metal and often incorporate temperature control and part ejectors. Control of the temperatiue during the injection phase allows a better control of the resin viscosity and flow advancement ramping up the temperatiue after the injection phase can greatly accelerate the curing of the resin and allow cycle time. The use of an injection pump capable of mixing the resin to the desired ratio, and self-flushing injection heads, can eliminate any contact of the operator with the hquid resin. 

Humidification. For wiater operation, or for special process requirements, humidification maybe required (see Simultaneous HEAT and mass transfer). Humidification can be effected by an air washer which employs direct water sprays (see Evaporation). Regulation is maintained by cycling the water sprays or by temperature control of the air or water. Where a large humidification capacity is required, an ejector which direcdy mixes air and water in a no22le may be employed. Steam may be used to power the no22le. Live low pressure steam can also be released directly into the air stream. Capillary-type humidifiers employ wetted porous media to provide extended air and water contact. Pan-type humidifiers are employed where the required capacity is small. A water filled pan is located on one side of the air duct. The water is heated electrically or by steam. The use of steam, however, necessitates additional boiler feed water treatment and may add odors to the air stream. Direct use of steam for humidification also requires careful attention to indoor air quahty. 

Capacity Control The simplest way to regulate the capacity of most steam vacuum refrigeration systems is to furnish several primary boosters in parallel and operate only those required to handle the heat load. It is not uncommon to have as many as four main boosters on larger units for capacity variation. A simple automatic on-off type of control may be used for this purpose. By sensing the chilled-water temperature leaving the flash tank, a controller can turn steam on and off to each ejector as required. 

In dry compressors, shaft end seals are generally one of five type.s. These are labyrinth, restrictive ring, mechanical contact, liquid film, and dry gas seal. The labyrinth type is the most simple but has the highest leakage. The labyrinth seal is generally ported at an axial point between the seals in order to use an eductor or ejector to control leakage and direct it to the suction or a suitable disposal area. Alternatively, a buffer gas is used to prevent the loss of process gas. Appendix D presents a calculation method for use with labyrinth seals. 

This type of technology is a part of the group of air pollution controls collectively referred to as "wet scrubbers." Venturi scrubbers are also known as venturi jet scrubbers, gas-atomizing spray scrubbers, and ejector-venturi scrubbers. The technology is based on the removal of air pollutants by inertial and diffusional interception. 

The air bleed is u.sed to maintain a constant condition. However, a control valve may be used instead. Control or hand valves in the lower pressure vapor lines to an ejector are not recommended, as they must be paid for in system pressure drop and ejector udlity requirements. 

Reasonable factors of safety should be applied to the various loads in order to insure adequate capacity. Excess ejector capacity can be handled by pressure control and some adjustment in steam flow and pressure, but insufficient capacity may require ejector replacement. Factors of 2.0 to 3.0 are not uncommon, depending upon the particular qqte of system and knowledge of similar system operauons. 

Figure 6-31 illustrates control schemes for the single stage unit which allow greater stability in performance. As the load changes for a fixed suction pressure, the process fluid is replaced by an artificial load (usually air Figure 6-31, item 1) to maintain constant ejector operation. An artificial pressure drop can be imposed by valve (2), although this is not a preferred scheme. Wlten the addi-... 

Figure 6-32 illustrates ejector systems with large condensable loads which can be at least partially handled in the precondenser. Controls are used to maintain constant suction pressure at varying loads (air bleed), or to reduce the required cooling water at low process loads or low water temperatures [2]. The cooler W ater must not be throttled below the minimum (usually 30%-50% of maximum) for proper contact in the condenser. It may be controlled by tailwater temperature, or by the absolute pressure. 

Blatchley, C. G., Controlling Ejector Performance, 1956, Schutte and Koerting Co. 

In general, the number of boosters determines the operational flexibility of the unit with respect to the refrigeration load. A single booster unit operates continuously, regardless of load. A two booster unit can operate at 50% load by shutting off one unit at lower load levels it uses a pressure controller on the steam actuated by the condenser pressure. Because jets are not usually very flexible with respect to steam consumption and vacuum, load control may be in increments as compared to continuous variation. If a 100-ton unit is expected to operate an appreciable portion of the time at 25% of load, it may prove economical to install a four-booster unit and to operate only one for this period. Auxiliary ejectors remove uncondensed water vapor and air from the main condenser. 

The application of instrumentation to the automatic control of a sequence of operations, e.g., injection moulding processes. Once the mould has been loaded with inserts (assuming a rubber to metal bonded part) into the press, the operation of a push-button starts the controller which closes the press, injects the rubber, controls the cure cycle, recharges the injection unit, opens the press, operates the ejectors and presents the mould for cleaning and loading of inserts. 

No evidence of injuries or disease. Yellow fluorescent particles found in mouth appeared like those placed in NaCN ejector mechanisms used in predator control. However, blood cyanide concentration was similarto that found in nonexposed vultures, including two captive California condors... 

In-plant management practices may often control the volume and quality of the treatment system influent. Volume reduction can be attained by process wastewater segregation from noncontact water, by recycling or reuse of noncontact water, and by the modification of plant processes. Control of spills, leakage, washdown, and storm runoff can also reduce the treatment system load. Modifications may include the use of vacuum pumps instead of steam ejectors, recycling caustic soda solution rather than discharging it to the treatment system, and incorporation of a more efficient solvent recovery system. 

---