Dryers, cost

Based on dryer cost alone, indirect-heat dryers are more expensive to build and install than direct-heat dryers designed for the same duty. As environmental concerns and resulting restrictions on process emissions increase, however, indirect-heat dryers are more attractive because they employ purge gas only to remove vapor and not to transport heat as well. Dust and vapor recovery systems for indirect-heat dryers are smaller and less cosdy to supply heat for drying, gas throughput in direct-heat dryers is 3—10 kg/kg of water evaporated indirect-heat dryers require only 1—1.5 kg/kg of vapor removed. System costs vary directly with size, so whereas more money may be spent for the dryer, much more is saved in recovery costs. Wet scmbbers ate employed for dust recovery on indirect-heat dryers because dryer exit gas usually is close to saturation. Where dry systems are employed, all external surfaces must be insulated and traced to prevent vapor condensation inside. 

Distillation towers feed-tray location for, 10 optimum reflux ratio for, 371-376 specifications for, 16 (See also Bubble-cap contactors, Packed towers. Sieve trays, and Valve trays) Distribution costs, 194, 196, 207, 211 Distribution in statistical analyses, 745-746 Dividends, tax exemptions for, 259 Documentation, 137-149,452-476 Double-entry bookkeeping, 143-144 Downcomers in tray columns, 684-686 Drives, cost of 532-533 Dryers, cost of 713-716... 

Dryer cost data were obtained from manufocturers as a function of dryer volume, the latter being calculated by the model assuming the volumetric heat transfer coefficient to be a function of mean biomass moisture content. The form of the function was derived from manufacturers data. 

Installation. The installation involves costs for labor, foundations, platforms, construction expenses, etc. The installation cost may be taken as a percentage of the dryer cost, approximately 20 to 50 percent, depending upon its sophistication. 

Dryer Costs. Representative dryer costs on an installed 1958 price basis are given in Fig. 6-11. 

Note that unnecessarily stringent product specifications can cause significant increase in dryer costs, both capital and operating. 

An economic analysis of a superheated-steam drying of aluminum oxide from 40% to 2% moisture content and an output of2000 kg/h showed that the optimum inlet vapor temperature into such a dryer is 374°C [59]. The dryer cost was lower than that of a comparable air dryer, and the operating costs were lower by about 15%. Figure 54.11 shows a schematic diagram of a superheated drying system in a sugar factory [61]. 

Fixed cost elements. These are items unrelated to the amount of moisture removed from the product over the years. The dryer cost is the primary element because once the dryer is purchased, interest has to be repaid whether or not the dryer is actually in use. This item also includes the maximum demand charges for the electricity supply. 

Aside from the general formula, numerous data on dryer cost dependence on evaporation capacity exist in literature for the given dryers and definite operating parameters [20,28,29]. For example, cost of a spray dryer with a pressure nozzle atomizer, residence time of 16 s, inlet- and outlet-air temperature of 538°C and 121°C, respectively, can be calculated as [28] ... 

FIGURE 65.2 The dependence of dryer cost on evaporation capacity in kg moisture/h and inlet-air temperature in °C, according to Equation 65.3 (M S Index = 1100). 

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