Steam consumption

A four-stage ejector is to evacuate a system to 0.3Torr. The compression ratio in each stage will be 

The individual stage pressures and corresponding water bubblepoint temperatures from the steam tables are 

When barometric condensers are used, the effluent water temperature should be at least 5°F below the bubblepoint at the prevailing pressure. A few bubblepoint temperatures at low pressures are  

For each agitator with a standard stuffing box, 5 Ib/hr of air leakage is added. Use of special vacuum mechanical seals can reduce this allowance to 1-2 Ib/hr. 

For a conservative design, the rate from Eq. (7.59) may be supplemented with values based on Table 7.11. Common practice is to provide oversize ejectors, capable of handling perhaps twice the standard rates of the Heat Exchange Institute. 

Other Gases. The gas leakage rate correlations cited are based on air at 70°F. For other conditions, corrections are applied to evaluate an effective air rate. The factor for molecular weight M is 

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Minimising energy consumption per ton of alurnina while maintaining a steam-power balance is an industry-wide, ongoing effort. Reduction of steam consumption has been limited by the cost of purchased power to compensate for loss of power generation. 

While process design and equipment specification are usually performed prior to the implementation of the process, optimization of operating conditions is carried out monthly, weekly, daily, hourly, or even eveiy minute. Optimization of plant operations determines the set points for each unit at the temperatures, pressures, and flow rates that are the best in some sense. For example, the selection of the percentage of excess air in a process heater is quite critical and involves a balance on the fuel-air ratio to assure complete combustion and at the same time make the maximum use of the Heating potential of the fuel. Typical day-to-day optimization in a plant minimizes steam consumption or cooling water consumption, optimizes the reflux ratio in a distillation column, or allocates raw materials on an economic basis [Latour, Hydro Proc., 58(6), 73, 1979, and Hydro. Proc., 58(7), 219, 1979]. 

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. 

Determine total evaporation required, and estimate steam consumption for the number of effects chosen. 

FIG. 29-9 Steam consumption of a 400-hp iiniflow engine. To convert pounds per horsepower-hour to kilograms per Idlowatthoiir, multiply by 0.6084 to convert poiinds-force per square inch to Idlopascals, multiply hy 6.89. 

Example. To produce 20 tons of refrigeration while delivering 50°F chilled water, the steam consumption depends upon the quantity and temperature of the cooling... 

The vendor should also supply steam consumption data. However, for initial planning the process engineer needs to have an estimate. Use the following equations to calculate the horsepower required to compress noncondensing components from the jet inlet pressure and temperature to the outlet pressure. 

Figure 1. A wide range of pressures can be achieved by using various combinations of ejectors and condensers. The same steam consumption is used for each design here. Note Curves are based on 85°F condensing water. If warmer water is used, curves shift to the left—cooler water, shift right.

Smokeless Center Steam Cheapest steam-injection flare tip. Steam jet emerges at high velocity and penetrates to the exit plane of the flare without mixing completely with flare gas. Results are intense steam noise (much greater than with steam ring for the same steam rate) and higher steam consumption than the steam ring. 

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].

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.

Steam consumption read values on curves or interpolate. 

Total mixture to be handled = 60 Ibs/hr Suction pressure 4 in. Hg abs Steam pressure 125 psig Size selection Figure 6-26A 2 inch L Steam consumption 440 Ibs/hr at 90 psig... 

Figure 6-26A. Size ejector for 20 psig steam consumption, single stages, 3"-10" Hg abs. By permission, Worthington Corp.

Figure 6-26B. Steam consumption factor. By permission, Worthington Corp.

Step 3. Read Steam Consumption of unit selected off Capacity Factor Chart. See Table 6-16. 

In general, the number of boosters determines the operational flexibility of the unit with respect to the refrigeration load. A single booster unit operates continuously, regardless of load. A two booster unit can operate at 50% load by shutting off one unit at lower load levels it uses a pressure controller on the steam actuated by the condenser pressure. Because jets are not usually very flexible with respect to steam consumption and vacuum, load control may be in increments as compared to continuous variation. If a 100-ton unit is expected to operate an appreciable portion of the time at 25% of load, it may prove economical to install a four-booster unit and to operate only one for this period. Auxiliary ejectors remove uncondensed water vapor and air from the main condenser. 

Maximum temperature condenser water in, °F = 85 Steam consumption, booster, Ib/hr = 4,150... 

Superheated steam may be needed where steam distribution pipework in a plant is over extended distances, resulting in a loss of heat and increase in wetness of the steam. Another case may be where a process requires a temperature above the working pressure of the plant. The third case is where steam is used for turbines. Here it improves the performance of the turbine, where for every 6°C increase in steam temperature it can produce a saving of about 1 per cent reduction in steam consumption. Superheaters may also be supplied as independently fired units. These may be used when either the amount of superheated steam required is much less than the boiler evaporation or is only needed on an intermittent basis. 

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