Equipment Ejectors

When the mold is opened, the part should be easily removable. Cavities are made with a slight taper to reduce frictional drag of the part on the mold. The half of the mold attached to the movable platen is equipped with ejector pins, which push the part out of the cavity while the mold is being opened. When the mold is closed, the pins are flush with the cavity surface. Release agents or lubricants facilitate ejection and shorten the mol ding cycle. Some complex parts require that the mold open in several directions in addition to the direction of the platen movement. For a threaded part, eg, a bottle cap, part of the mold must be rotated to remove the article from the mold. 

Energy costs ate not direcdy related to the energy efficiency of the process (6,42). Even if the thermal efficiency of a steam ejector, for example, is less than that of mechanical equipment mn by an electdc motor, the overall cost of the energy to mn the steam ejector may still be less. 

A useful summary of the typical equipment used for developing and maintaining process system vacuum is presented in Table 6-1. Also see Birgenheier [33]. The positive displacement type vacuum pumps can handle an overload in capacity and still maintain essentially the same pressure (vacuum), while the ejectors are much more limited in this performance and cannot maintain the vacuum. The liquid ring unit is more like the positive displacement pump, but it does develop increased suction pressure (higher vacuum) when the inlet load is increased at tlie lower end of the pressure performance curve. The shapes of these performance curves is important in evaluating the system flexibility. See later discussion. 

Wlien ejectors pull non-condensables and other vapors from a direct contact water condenser (barometric, low level jet, deaerator) there is also a release of dissolved gases, usually air, from water. This air must be added to the other known load of the ejector. Figure 6-22 presents the data of the Heat Exchange Insdtute [10] for the amount of air that can be expected to be released when cooling water is sprayed or otherwise injected into open qqae barometric or similar equipment. 

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. 

Combinations of steam jet ejectors operating in conjunction th mechanical pumps can significandy improve the overall s) stem efficiency, especially in the lower suction pressure torr range of 1 torr to 100 torn They can exist beyond the range cited, but tend to fall off above 200 torr. Each system should be examined indhadually to determine the net result, because the specific manufacturer and the equipment size enter into the overall assessment. Some effective combinations are ... 

Although the thermal efficiencies of various mechanical vacuum pumps and even steam jet ejectors vary with each manufacturer s design and even size, the curves of Figure 6-34 present a reasonable relative relationship between the types of equipment. Steam jets shown are used for surf ace intercondensers with 70°F cooling water. For non-condensing ejectors, the efficiency would be lower. 

Ejectors, steam/water requirements, 371 Electrical charge on tanks, 537 Electrical precipaiaiors, 280 Applications, 280, 282 Concept of operation, 281 Emergency relief, 450 Engineering, plant development, 46 Equipment symbols, 19—2 L Abbreviations, 25 Instruments, 21, 26. 29 Piping, 22 Valve codes, 26 Equivalent feel (flow), 86 Estimated design calculation time,... 

Vacuum capacities and operating ranges, table, 344, 355 Ejectors, 344, 357 Integrated systems, 344 Liquid ring pumps, 344 Rotary lobe blowers, 344 Rotary piston pumps, 344 Rotary vane pumps, 344 Vacuum equipment, 343 Applications diagram, 352 ASME Code, 344 Pumps, 382 Steam jets, 357 Vacuum flow,... 

The efficiency of the condenser is reduced by poor air removal (and the presence of other noncondensable gases), so surface condensers usually are equipped with vacuum pumps but also may incorporate older style, single or multistage multielement, steam-jet air ejectors. Under most normal operations, the residual oxygen level is below 20 to 40 ppb 02. 

A backyard barbeque grill contains a 20-lb tank of propane. The propane leaves the tank through a valve and regulator and is fed through a 1/2-in rubber hose to a dual valve assembly. After the valves the propane flows through a dual set of ejectors where it is mixed with air. The propane-air mixture then arrives at the burner assembly, where it is burned. Describe the possible propane release incidents for this equipment. 

Similarly, vacuums can be created when a blower, fan, compressor, or jet ejector removes gases from equipment. The magnitude of the vacuum attainable will be governed by the performance characteristics of the device. Other mechanisms for generating a vacuum, which have been demonstrated by industry experience, include the following. 

Both scrubbers will be supplied with three sources of electrical power - the main grid supply, a back-up grid supply from a different substation and an emergency diesel generator. In the emergency chlorine scrubber, critical equipment items will be backed-up by automatic start-up of stand-by equipment. A gravity head tank of caustic soda and a nitrogen ejector will also be provided to allow safe neutralisation of chlorine vents in the event of total power failure. 

On the one hand, the differential reactor with recycle permits kinetic measurements of high accuracy. On the other hand, a transfer equipment is required to recycle a fraction of the reaction mixture. This can be difficult when the pressure is high. For this purpose, a jet loop reactor was developed which is equipped with an ejector to recycle the fluid. The design of the jet loop reactor is described in Chapter 4.3.4. 

Uncondensed vapors are removed at the top of the column with a one-stage steam jet ejector equipped with a barometric condenser. 

Figure 3.5. Vacuum control with steam jet ejectors and with mechanical vacuum pumps, (a) Air bleed on PC. The steam and water rates are hand set. The air bleed can be made as small as desired. This can be used only if air is not harmful to the process. Air bleed also can be used with mechanical vacuum pumps, (b) Both the steam and water supplies are on automatic control. This achieves the minimum cost of utilities, but the valves and controls are relatively expensive, (c) Throttling of process gas flow. The valve is larger and more expensive even than the vapor valve of case (a). Butterfly valves are suitable. This method also is suitable with mechanical vacuum pumps, (d) No direct pressure control. Settings of manual control valves for the utilities with guidance from pressure indicator PI. Commonly used where the greatest vacuum attainable with the existing equipment is desired.

Ithough liquids particularly can be transported by operators carrying buckets, the usual mode of transport of fluids is through pipelines with pumps, blowers, compressors, or ejectors. Those categories of equipment will be considered in this chapter. A few statements will be made at the start about piping, fittings, and valves, although for the most part this is information best... 

Steam jet ejectors are used primarily to evacuate equipment but also as pumps or compressors. They are discussed in Section 7.7. 

Application ranges of the various kinds of devices for maintenance of subatmospheric pressures in process equipment are shown in Table 7.3. The use of mechanical pumps—compressors in reverse— for such purposes is mentioned earlier in this chapter. Pressures also can be reduced by the action of flowing fluids. For instance, water jets at 40psig will sustain pressures of 0.5-2.0psia. For intermediate pressure ranges, down to O.lTorr or so, steam jet ejectors are widely favored. They have no moving parts, are quiet, easily installed, simple, and moderately economical to operate, and readily adaptable to handling corrosive vapor mixtures. A specification form is in Appendix B. 

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