Efficiency dryer

Another important class of dryer is the fluidized-bed dryers. Some designs combine spray and fluidized-bed dryers. Choice between dryers is usually based on practicalities such as the materials handling characteristics, product decomposition, product physical form (e.g., if a porous granular material is required), etc. Also, dryer efficiency can be used to compare the performance of different dryer designs. This is usually defined as follows -. 

If the total heat consumed is from an external utility (e.g., mains steam), then a high efficiency is desirable, even perhaps at the expense of a high capital cost. However, if the heat consumed is by recovery from elsewhere in the process, as is discussed in Chap. 15, then comparison on the basis of dryer efficiency becomes less meaningful. 

Fig. 20. Dryer efficiency improvement over time (19). To convert W/(m -K) to Btu/(h-ft -°F), multiply by 0.176.

Through-ckculation compartments employ perforated or screen bottom trays and suitable flow baffles so gas is forced through the material. If material is not inherently pervious to gas flow, it may be mechanically shaped iato noodles, pellets, or briquettes. These dryers are used ia small-scale operations to dry explosives, foods, and pigments. Dryer efficiency is 50—70%. Based on tray area, water vaporization rates are 1—10 kg/(h-m ). 

Gas flow in these rotary dryers may be cocurrent or countercurrent. Cocurrent operation is preferred for heat-sensitive materials because gas and product leave at the same temperature. Countercurrent operation allows a product temperature higher than the exit gas temperature and dryer efficiency may be as high as 70%. Some dryers have enlarged cylinder sections at the material exit end to increase material holdup, reduce gas velocity, and minimize dusting. Indirectly heated tubes are installed in some dryers for additional heating capacity. To prevent dust and vapor escape at the cylinder seals, most rotary dryers operate at a negative internal pressure of 50—100 Pa (0.5—1.0 cm of water). 

Combined indirect and indirect dryers pass the hot gases first through a jacket or tubes, and then wholly or in part through the open dryer. Efficiencies of such units are higher than of direct units, being in the range 60-80%. Table 9.10(d) shows performance data. Since the surfaces are hot, this equipment is not suitable for... 

Another useful classification is whether or not a dryer is a direct or indirect dryer. A direct contact dryer is one in which the material is dried by exposure to a hot gas, whereas in an indirect contact dryer, the heat required for evaporation is transferred from a heating medium through a metal wall to the material. Generally, direct heat dryers are more efficient. Dryer efficiency is defined by the fraction of energy supplied to the drying equipment which actually causes the evaporation of the liquid. As we shall see later in the chapter, heating is not always necessary to achieve drying. 

Pressure Pressure and equipment pressure drops are important to proper dryer operation. Most dryers are operated under vacuum. This prevents dusting to the environment, but excess leakage in decreases dryer efficiency. Pressure drops are especially important for stable fluid-bed operation. 

Overall system efficiency is the product of the boiler and dryer efficiencies ... 

Knit cotton golf shirt (study conducted by Cotton Incorporated, 2011) Average conditions — 54% cold wash/46% heated wash load size, medium washer efficiency, 70% conventional/30% Energy Star water heater type, 50% elec./50% nat. gas. Drying, 84% dryer/16% air dry dryer efficiency, 70% conventional/30% Energy Star 84% (approx. 1000 MJ/kg)... 

Dryer efficiency N/A N/A 70% Conv, 30% Energy Star B Conventional G... 

There are a wide variety of drying materials and types of equipment, so it is not possible to propose a generalized method for a standardized determination of dryer efficiency. Therefore, standard conditions at which a particular dryer should be tested have to be specified. This ultimately would lead to establishing standards for rating dryer capacity and energy performance. Such standards would allow dryer users to find out whether their dryers operate at or off the optimum conditions. 

Determine the effect of varying drying air temperature, ambient conditions, initial material moisture couteut, and temperature ou the dryer capacity and dryer efficiency. 

Alternatively, the reciprocal of the SMER is reported as the heat-pump dryer efficiency, which is the energy required to remove one kg of water. 

Thus, energy required to remove the unit mass of water is given by the heat-pump dryer efficiency (HPDE) defined as... 

Electronic monitoring from the compounding area to final inspection is reducing manpower requirements and increasing dryer efficiency dramatically. 

---