operation is tending toward superheated vapor drying, and in practice 40-50% of the gases in circulation are water vapor. This suggests that the specific heat of the gas will be approximately 0.34 Btu/lb °F compared with about 0.24 Btu/lb °F in a conventional total rejection dryer. Since the mass of gas for a given thermal capacity is appreciably less than in a conventional dryer and, as previously mentioned, oxygen levels are low, much higher operating temperatures may be used. This permits a reduction in the size of the closed-circuit dryer. For the case given in Table 4 and comparing the two dryers on the basis of their cross-sectional areas, the total rejection dryer would have rather more than double the area of the closed-circuit system. There are obvious limitations to the use of such a technique, but the advantages illustrated are very apparent, especially the savings in thermal energy alone, viz, 695,000 Btu/h representing about 29.2% of the total requirements of the conventional drying system.

Effect of Changing Feed Rates

Since it has a significant bearing on efficiency, another factor of major importance to consider when designing dryers of this type is the possible effect of reducing feed rate. What variation in quantity of feed is likely to occur as a result of operational changes in the plant upstream of the dryer and as a result, what turndown ratio is required of the dryer? With spray dryers and rotary dryers, the mass airflow can be varied facilitating modulation of the dryer when operating at reduced throughputs.

This, however, is not the case with pneumatic and true fluidized-bed dryers since the gases perform a dual function of providing the thermal input for drying and acting as a vehicle for transporting the material. Since the mass flow has to remain constant, the only means open for modulating these dryers is to reduce the inlet temperature. This clearly has an adverse effect on thermal efficiency. It therefore is of paramount importance to establish realistic production requirements. This will avoid the inclusion of excessive scale-up factors or oversizing of drying equipment and thereby maximize operating efficiency.

Figure 17 illustrates the effects on thermal efficiency of either increasing the evaporative capacity by increased inlet temperatures or alternatively, reducing the inlet temperature with the exhaust temperature remaining constant at the level necessary to produce an acceptable dry product. While the figure refers to the total rejection case of the previous illustration where design throughput corresponds to an efficiency of 62.4%, the curve shows that if the unit is used at only 60% of design, dryer efficiency falls to 50%. The converse, of course, also is true.

In this brief presentation, an attempt has been made to highlight some of the factors affecting efficiency in drying operations and promote an awareness of where savings can be made by applying new techniques. While conditions differ from one drying process to another, it is clear that economies can and should be made.


In the field of spray drying, the past 10 years have witnessed many new developments initiated mainly to meet three demands from food and dairy processors—better energy efficiency, improved functional properties of the finished powder, and production of new, specialized dried products. This has been accompanied by progress made in the automation of drying systems and in stricter requirements for the reduction of environmental impacts resulting from the operation of spray dryers.

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