1

Bag filter

Figure 28. Spray-drying plant with (top) liquid coupled recuperator, (middle) single-stage recuperator, and (bottom) two-stage recuperator.

drying process. This is due to the dust-laden air that is involved in the process and that tends to contaminate heat exchange surfaces. To be effective, therefore, recuperators for spray-dryer use should have the following properties:

• Modular system

• High thermal efficiency

• Low pressure drop

• Automatic cleaning

• Large temperature range

• Stainless steel

• Large capacity

Two such recuperators are the air-to-air tubular and the air-to-liquid plate designs.

Air-to-Air Heat Recuperators. For optimum flexibility, this heat exchanger is of modular design, each module consisting of 804 tubes welded to an end plate. Available in various standard lengths, the modules can be used for counterflow, cross-flow, or two-stage counterflow as sketched in Figure 29. While the first two designs are based on well-known principles, the two-stage counterflow (patent pending) system is an APV development. This recuperator consists of two sections, a long dry element and a shorter wet section. During operation, hot waste air is passed downward through the dry module tubes and cooled to just above the dew point. It then goes to the wet section where latent heat is recovered. The inner surfaces of these shorter tubes are kept wet be recirculating water. The system, therefore, not only recovers free and latent heat from the waste air but also scrubs the air and reduces the powder emission by more than 80%. Effectiveness and

Waste air inlet

Waste air outlet

Waste air outlet

Fresh air outlet

Fresh air intlet

Fresh air outlet

Fresh air intlet

Waste air inlet

Waste air inlet

Figure 29. Air-to-air tubular heat recuperators.

economy are such that the system replaces a conventional recuperator-scrubber combination.

Air-to-Liquid Recuperator. In cases where an air-to-air recuperator is impractical or uneconomical because of space limitations or the length of the distance between the outlet and inlet air to be preheated, a liquid heat-recovery system is available. This plate recuperator as shown in Figure 30 uses various standard plates as the base with waste air being cooled in counterflow by circulating a water/glycol mixture. At high temperature, circulating thermal oils are used. Heat recovered by this means gen

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