Ejectors Steam requirements

Steam pressure. The main boosters can operate on steam pressures from as low as 0,15 bar up to 7 bar gauge. The quantity of steam required increases rapidly as the steam pressure drops (Fig, 11-106), The best steam rates are obtained with about 7 bar. Above this pressure the change in quantity of steam required is prac tically negligible. Ejectors must be designed for the highest available steam pressure, to take advantage of the lower steam consumption for various steam-inlet pressures. 

The secondaiy ejector systems used for removing air require steam pressures of 2,5 bar or greater. When the available steam pressure is lower than this, an electrically driven vacuum pump is used for either the final secondaiy ejector or for the entire secondaiy group. The secondary ejectors normally require 0,2-0,3 kg/h of steam per kW of refrigeration capacity,... 

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. 

Figure 6-25 presents estimated steam requirements for several ejector systems. Exact requirements can be obtained only from the manufacturers, and these will be based on a specific performance. 

Air/water vapor mixture, chart, 364,365 Air/water vapor, 359 Capacity at ejector suction, 369 Capacity for process vapor, 362 Evacuation time, 371, 380 Load for steam surface condenser, 367 Non-condensables, 362, 363 Size selection, 371 Steam pressure factor, 373 Steam requirements, 372 Steain/air mixture temperature, 361 Total weight saturated mixture, 362 Capacity, 358 Discharge, pressure, 358 Effect of excess steam pressure, 358 Effects of back pressure, 359 Effects of wet steam, 356 Inter-and-after condenser, 351 Load variation, 370 Materials of construction, 347 Molecular weight entrainment, chart, 360 Performance, 358, 370, 375 Relative comparison, 357... 

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,... 

Figure 7.31. Steam requirements of ejectors at various pressure levels with appropriate numbers of stages and contact intercondensers. Steam pressure 100 psig, water temperature 85°F. Factor for 65 psig steam is 1.2 and for 200 psig steam it is 0.80 (Worthington Corp).

Jet pumps are used to remove air, gases, or vapors from condensers and vacuum equipment, and the steam jets can be connected in series or parallel to handle larger amounts of gas or to develop a greater vacuum. The capacity of steam-jet ejectors is usually reported as pounds per hour instead of on a volume basis. Far design purposes, it is often necessary to make a rough estimate of die steam requirements for various ejector capacities and conditions. The data given in Table 3 can be used for this purpose. 

Figure 7 31. Steam requirements of ejectors at various pressure... 

THERMAL RECOMPRESSION. In a thermal recompression system the vapor is compressed by acting on it with high-pressure steam in a jet ejector. This results in more steam than is needed for boiling the solution, so that excess steam must be vented or condensed. The ratio of motive steam to the vapor from the solution depends on the evaporation pressure for many low-temperature operations, with steam at 8 to 10 atm pressure, the ratio of steam required to the mass of water evaporated is about 0.5. 

Reduce steam requirement by better heat integration in the plant, avoid slow rolling of steam turbines, replace steam turbine gland packing and ejector system with mechanical seals, replace steam jet system with liquid ring or dry vacuum pumps... 

The very high velocities that the ejector is exposed to subject the nozzle and diffuser throat to excessive wear. Low-quality steam will accelerate this erosion. A gradual loss in vacuum may be due to enlargement of the ejector clearances. It is a good practice to caliper these clearances when the system is out of service. A fuller description of ejector operation, steam requirements, and pressure capabilities can be found in Robert Frumerman s "Steam Jet Ejectors," Chemical Engineering, June 1956. 

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