Motive Steam

The efficiency of the Rankine cycle itself can be increased by higher motive steam pressures and superheat temperatures, and lower surface condenser pressures in addition to rotating equipment selection. These parameters are generally optimized on the basis of materials of constmction as well as equipment sizes. Typical high pressure steam system conditions are in excess of 10,350 kPa (1500 psi) and 510 °C. 

Incremental steam capacity is also convenient for exporting motive steam to other iategrated processes, such as urea. 

Condensa.te Recovery Pumps. The condensate recovery pump is a unique design that utilizes a motive steam or an air supply to move condensate. It can be appHed to capacities up to 23,000 kg/h (50,000 lb /h), providing valuable recovery of condensate or other Hquids, which otherwise would be wasted. 

The motive steam design pressure must be selected as the lowest expected pressure at the ejector steam nozzle. The unit will not operate stably on steam pressures below the design pressure [16]. 

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. 

In all cases it is important to describe the system, its requirements, control and method of operation in the specifications. The manufacturer needs complete data concerning the motive steam (air or water) and the condensable and non-condensable vapors. 

For the single stage ejector, the motive steam flow cannot be decreased below critical flow in the diffuser [2], (Figure 6-30). Units are usually designed for stable opera-... 

Pounds of motive steam per pound of mixture handled... 

Figures 6-11 A, B, and C indicate the capacity of various ejector-condenser combinations for variable suction pressures when using the same quantity of 100 psig motive 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 conditions.

The three motive steam pressure curves, 100%-90%-80%, are obtained from the ejector manufacturer as is the performance curve of suction 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. 

For one thing, steam produced from hot-lime-softened water will have some amount of silicates. These silicates tend to deposit on the rotor blades of turbines, which use the motive steam as a source of energy. The silicate fouling of the turbine blades reduces the turbine s efficiency. But, more importantly, from an operator s point of view, the silicate deposits eventually break off of the blades. This unbalances the rotor. An unbalanced rotor is the fundamental cause of vibration, Vibrations lead to damage of the shaft bearings and seals. Eventually, vibrations will destroy the turbine s internal components. 

The motive steam accelerates to such a great velocity that it can exceed the speed of sound (that is, exceed sonic velocity). This tremendous increase in velocity of the steam represents a tremendous increase in the kinetic energy of the steam. The source of this kinetic energy is the pressure of the steam. 

There is another type of jet surging, which is caused by the motive steam turning to ice. How is this possible Certainly, steam cannot turn to ice inside the jet But it can and does. 

If the motive steam were dry, these factors would reduce the 150-psig motive-steam saturated temperature from 350 to about 100°F. But the motive steam in Mobile was not dry. It had partly condensed in the steam supply line to the jet. If steam is wet and contains liquid water, the water will flash to steam if the steam pressure is suddenly reduced to a vacuum. But the heat of vaporization comes from the sensible-heat content of the steam. If the steam contains 10 percent moisture, it will... 

Calculating work available from motive steam... 

Step 1. The motive steam conditions are 375°F and 160 psia, which determine point A. Note that the motive-steam pressure is the pressure in the steam chest. 

You may have noticed that point B in Fig. 17.2 is below the saturated-steam envelope line. Does this mean that the exhaust steam would have appreciable amounts of entrained water Does this mean that the motive steam may sometimes partially condense inside a turbine Answer—yes Does this also mean that turbine exhaust steam may have too high a moisture content to be used in certain downstream services Well, it is fine for reboilers, but it would be unsuitable for steam jets (see Chap. 16). 

The higher the motive-steam superheat temperature, the drier the exhaust steam. 

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