Ejectors Discharge, pressure

Tower pressure cycling may be due to steam ejector underload and high ejector discharge pressure. 

Ejector discharge pressure, as controlled by the downstream condenser performance... 

Ejector Performance The performance of any ejec tor is a function of the area of the motive-gas nozzle and venturi throat, pressure of the motive gas, suction and discharge pressures, and ratios of specific heats, molecular weights, and temperatures. Figure 10-102, based on the assumption of constant-area mixing, is useful in evaluating single-stage-ejector performance for compression ratios up to 10 and area ratios up to 100 (see Fig. 10-103 for notation). 

For ejectors discharging to the atmosphere, steam pressures below 60 psig at the ejector are generally uneconomical [16]. If the discharge pressure is lower as in mul-... 

Discharge pressure of pump, psia = Intake pressure of pump Mth closed intake, psia = Final pressure in system, in. Hg abs Gas constant, = 1544/mol weight Pump speed, revolutions (or strokes) per second Pump speed, liters/sec Pump speed at P ", liters/sec Pump speed at P/, liters/sec Temperature, °R = 460 + °F Evacuation pump dowmtime, min Evacuation pump downtime, sec Ambient air temperature, °F Temperature of mixture at ejector suction, °F Temperature of steam on downstream side of nozzle, °F... 

Curves 1, 2 and 3 represent the maximum safe discharge pressure, as the system will operate along the capacity curve as long as the system discharge pressure from the ejector is less than the maximum value of the curve, all for a given suction pressure [4]. The slopes of the curves are a function of the type of ejector, its physical design and relative pressure conditions. Whenever the discharge backpressure exceeds the maximum safe dis-... 

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... 

Assuming that the discharge pressure from the ejector, that is in the steam-chest, is 170 kN/m2 at which the latent heat, A.0 = 2216 kN/m2, then a heat balance across the unit gives ... 

There is little danger, in injecting a controlled amount of water into a furnace inlet, when using a properly designed metering pump. Such pumps typically have a capacity of 1 to 10 GPM and provide a set flow, regardless of the discharge pressure. The injected water flashes immediately to steam inside the furnace tubes. Vfe have retrofitted several vacuum and delayed coker heaters with condensate injection systems, with no adverse downstream effects. Water from the hot well of a vacuum ejector system is our normal source of condensate for this environmentally friendly modification. 

The basic performance curve for an ejector operating in the critical range is determined by fixing the motive steam pressure and the discharge pressure and varying the load to the ejector suction. Corresponding values of suction pressure are observed and the performance curve shown in Figure 23-7 can be developed. 

The maximum discharge pressure curve droops at small loads. If the ejector of Figure 23-7 discharges against atmospheric pressure, it will be on its basic performance curve at loads down to 10 lb r. At loads below this, the actual discharge pressure exceeds the maximum discharge pressure of the ejector, and the ejector will operate in a "broken" state with its suction pressure higher than the value on the basic performance curve. 

If the motive steam pressure were reduced slightly, there would be an insignificant lowering of the suction-pressure curve, but the maximum discharge-pressure curve would be shifted downward significantly making the ejector "broken" at loads below 20 Ib/hr. 

In summary, for a typical ejector stage operating in the critical range, the basic performance curve is essentially fixed. The maximum discharge-pressure curve is somewhat variable, and the ejector will operate on its basic performance curve if the maximum discharge pressure is not exceeded. 

An ejector designed for noncritical operation has somewhat different performance characteristics and these are described in Chapter 18. The performance curve for noncritical ejectors is a function of the motive steam pressure and the actual discharge pressure. Thus, it has no broken mode of operation. 

Motive-steam pressure that is too high will also hurt the vacuum. This is because the steam nozzle will pass more steam than the diffuser throat was designed to handle. The diffuser chokes on the extra steam and the vacuum is adversely affected. Or, the downstream condenser may be overloaded by the increased steam flow and cause back pressure against the vacuum ejector discharge. 

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