Equipment design thermal efficiency

Design Methods for Calciners In indirect-heated calciners, heat transfer is primarily by radiation from the cyhnder wall to the solids bed. The thermal efficiency ranges from 30 to 65 percent. By utilization of the furnace exhaust gases for preheated combustion air, steam produc tion, or heat for other process steps, the thermal efficiency can be increased considerably. The limiting factors in heat transmission he in the conductivity and radiation constants of the shell metal and solids bed. If the characteristics of these are known, equipment may be accurately sized by employing the Stefan-Boltzmann radiation equation. Apparent heat-transfer coefficients will range from 17 J/(m s K) in low-temperature operations to 8.5 J/(m s K) in high-temperature processes. 

Although the thermal efficiencies of various mechanical vacuum pumps and even steam jet ejectors vary with each manufacturer s design and even size, the curves of Figure 6-34 present a reasonable relative relationship between the types of equipment. Steam jets shown are used for surf ace intercondensers with 70°F cooling water. For non-condensing ejectors, the efficiency would be lower. 

In a later section, the characteristics and performances of the most widely used equipment will be described in some detail. Many types are shown in Figure 9.4. Here some comparisons are made. Evaporation rates and thermal efficiencies are compared in Table 9.2, while similar and other data appear in Table 9.3. The wide spreads of these numbers reflect the diversity of individual designs of the same general kind of equipment, differences in moisture contents, and differences in drying properties of various materials. 

Exit air usually is maintained far from saturated with moisture and at a high temperature in order to prevent recondensation of moisture in parallel current operation, with a consequent lowering of thermal efficiency. With steam heating of air the overall efficiency is about 40%. Direct fired dryers may have efficiencies of 80-85% with inlet temperatures of 500-550°C and outlet of 65-70°C. Steam consumption of spray dryers may be 1.2-1.81bsteam/lb evaporated, but the small unit of Table 9.19(b) is naturally less efficient. A 10% heat loss through the walls of the dryer often is taken for design purposes. Pressure drop in a dryer is 15-50 in. of water, depending on duct sizes and the kind of separation equipment used. 

The technology of gasification is under active development in both equipment and process design to reduce capital costs, and in the chemistry of the process to improve yields and obtain more favorable gas ratios. The latter includes the use of catalysts to enhance the process and to promote the formation of specific products, such as methane or ethylene for increased thermal efficiency or for feedstocks for chemical synthesis. 

The thermal efficiency of this cycle is that of a Carnot engine, given by (5.8). As a reversible cycle, it could serve as a standard of comparison for actui steam power plants. However, severe practical difficulties attend the operatk of equipment intended to carry out steps 2 3 and 4 1. Turbines that take i saturated steam produce an exhaust with high liquid content, which causes sevel erosion problems, t Even more difficult is the design of a pump that takes in mixture of liquid and vapor (point 4) and discharges a saturated liquid (poll 1). For these reasons, an alternative model cycle is taken as the standard, at lei for fossil-fuel-buming power plants. It is called the Rankine cycle, and diSei from the cycle of Fig. 8.2 in two major respects. First, the heating step 1 2 ... 

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