Number of stages

Number of stages in a compressor is limited by the alLow le shaft length and critical speed. Usually, B to 12 stages are possible to be installed in one unit. Side load or interstage cooler will reduce the allowable stages to 

If number of stages required is exceeding the allowabte number of stages in one unit, two units in series can be used to achieve the required compression. 

---

The heat flow through the system can be manipulated by changing the number of stages. Figure 3.13c shows the effect of an increase from three to six stages. 

Figure 3.14 Variation of total cost with number of stages indicates that three stages is the optimal number for a stand-alone system in this case.

If the feed, solvent, and extract compositions are specified, and the ratio of solvent to feed is gradually reduced, the number of ideal stages required increases. In economic terms, the effect of reducing the solvent-to-feed ratio is to reduce the operating cost, but the capital cost is increased because of the increased number of stages required. At the minimum solvent-to-feed ratio, the number of ideal stages approaches infinity and the specified separation is impossible at any lower solvent-to-feed ratio. In practice the economically optimum solvent-to-feed ratio is usually 1.5 to 2 times the minimum value. 

The appHcation of this type of conversion coating can be by spray or immersion and is easily tailored to the needs of the user (see Coating processes). The number of stages may vary from two, ie, clean and phosphate then rinse, to as many as five, ie, clean, rinse, phosphate, rinse, and post-treat. The performance requirements dictate the number of operation stages as well as the need for a post-treatment. 

The benefits of high selectivity He in the abiUty to produce high purity cobalt in a limited number of stages. This minimises capital and operating costs. It is particularly important when the solution in question contains low concentrations of cobalt. Eor example, solutions derived from laterite deposits may only contain 0.5—2 g/L Co but 90—100 g/L Ni. 

Assuming for simplification that the temperature drop of the flashing brine, AT, is the same ia each stage and that the specific and latent heat of the brine remains the same throughout the plant, the relationship between the number of stages, n, and the performance ratio is expressed by... 

Insofar as the consumption of chemicals is concerned, it is obvious that the total consumption of reflux-producing chemicals is proportional to the interstage flows, or width of the cascade, but independent of the number of stages in series, or length of the system. 

The number of stages requited to span a given concentration difference in an ideal plant in which all stages have the same separation factor is therefore... 

Figure 5 shows an ideally tapered enricher that has been replaced by three square cascade sections, a process called squaring-off the cascade (3—6). During the squaring-off process, two essential requirements must be kept in mind The interstage flow in all-square sections must always exceed the local value of E at all points in the cascade, and the squared-off cascade must contain a total number of stages which exceeds N. In order for the... 

The need for a large number of stages and for the special equipment makes gaseous diffusion an expensive process. The three United States gaseous diffusion plants represent a capital expenditure of close to 2.5 x 10 dollars (17). However, the gaseous diffusion process is one of the more economical processes yet devised for the separation of uranium isotopes on a large scale. 

This quantity is minimised when the stage is operated at a pressure ratio across the barrier corresponding to r = 0.285. Furthermore, if power were the only economic consideration, the stage would be operated at this pressure ratio. However, as the value of ris decreased from this optimum, although the cost of power is increased, the number of stages required and hence the capital cost of the plant is decreased. Thus, ia practice a compromise between these factors is made. 

---