Ejector capacity

Figure 6-12. Effects of excess steam pressure on ejector capacity. By permission, C. H. Wheeler Mfg. Co.

The sucdon pressure of an ejector is expressed in absolute units. If it is given as inches of vacuum it must be converted to absolute units by using the local or reference barometer. The suction pressure follows the ejector capacity curve, varying with the non-condensable and vapor load to the unit. 

Few vacuum systems are completely airtight, although some may have extremely low leakage rates. For the ideal system the only load for the ejector is the non-condens-ables of the process (absorbed gases, air, etc.) plus the saturated vapor pressure equivalent of the process fluid. Practice has proven that allowance must be made for air leakage. Considering the air and non-condensables. For base ejector capacity determine inert gases only by ... 

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. 

P s = design suction pressure of ejector, torr V = free volume of the process system, cu ft Wj = ejector capacity, 70°F dry air basis, Ib/hr... 

Wa = Air inleakage resulting from metal porosities and cracks along weld lines, lbs/hr Wm = Ejector capacity at final evacuation suction pressure, lbs/hr... 

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. 

The capacity of an ejector handling other than saturated vapor is a function of the molecular weight and temperature of the fluid. If motivating quantities are equal, the ejector capacity increases as the molecular weight of the load gas... 

Regardless of ejector capacity, if the vacuum tower uses stripping steam, the pressure at the top of the tower cannot be lower than the vapor pressure of water at the precondenser outlet temperature. 

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. 

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. 

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. 

For moderately tight, small chemical processing systems (say 500cu.ft.), an ejector air capacity of lOlbs/hr is adequate. For large systems, use 201bs/hour. For very tight, small systems, an air capacity of 2-51bs/hr is reasonable. 

The designer should consider the ejector price and its operating costs when he specifies the air capacity. Larger capacities may be economical with single-stage ejectors, for example. 

A reverse philosophy in sizing ejectors is occasionally applicable to a system in which even a small quantity of air leakage will upset the operation or contaminate the product. In such a system it may be desirable to install an ejector having a deliberately limited air handling capacity, so that the system cannot be operated until an injurious rate of air leakage is corrected. 

To estimate the time required for an ejector to evacuate a system from atmospheric pressure down to the design pressure, assume that the average air handling capacity during the evacuation period is twice the design... 

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. 

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.

An increase in steam pressure over design will not increase vapor handling capacity for the usual fixed capacity ejector. The increased pressure usually decreases capacity due to the extra steam in the diffuser. The best ejector steam economy is attained when the steam nozzle and diffuser are proportioned for a specified performance [8]. This is the reason it is difficult to keep so-called standard ejectors in stock and expect to have the equivalent of a custom designed unit. The throttling type ejector has a family of performance curves depending upon the motive steam pressure. This type has a lower compression ratio across the ejector than the fixed-type. The fixed-type unit is of the most concern in this presentation. 

Figure 6-11 A. Comparison guide for steam ejector performance. As absolute pressure is reduced, the number of stages increases for a given capacity. The same steam consumption is used for each design. By permission, Berkiey, F. D. [1].

The three modve steam pressure curves, 100%-90%-80%, are obtained from the ejector manufacturer as is the performance curve of sucdon pressure versus percent of ejector design capacity. This latter curve for an actual installation would show actual absolute suction pressures versus pounds per hour or cubic feet per minute of air or percent design capacity. 

Figure 6-11B. A typical relative comparison of various designs of steam jet ejectors. Based on same steam consumption, 100 psig steam pressure and 85°F water. Curves represent the capacity of ejectors designed for maximum air handling capacity at any one particular suction pressure. By permission, Graham Manufacturing Co.

The capacity of an ejector is expressed as pounds per hour total of non-condensable plus condensables to the inlet flange of the unit. For multistage units, the total capacity must be separated into pounds per hour of condensables and non-condensables. The final stages are only required to handle the non-condensable portion of the load plus the saturation moisture leaving the intercondensers. 

A surface condenser condensing the steam from a process turbine drive operates at 1.0 in. Hg abs. The condensing load is 85,000 Ibs/hr steam. WTat is the capacity of the ejector ... 

An evacuation booster or hogging ejector is sometimes used to remove air from a system on start-ups. Its capacity is set to bring the system pressure down to near operating conditions before the continuous operadng ejector system takes over. Figure 6-23 illustrates the instal-ladon of such a unit. 

---