Heat-pump dryer

Alves-EUho, O. and Lystad, T., A new carhon dioxide heat pump dryer—An approach for better product quahty, energy use and environmentally friendly technology, in Proceedings of the 12th International Drying Symposium, Kerkhof, P.J.A.M., Coumans, W.J., and Mooiweer, G.D., Eds., Elsevier Science, Amsterdam, the Netherlands, 2000, Vol. B, Paper No. 51. 

An infrared-augmented heat pump dryer (HPD) can be used for faster removal of surface moisture during the initial stages of drying, followed by intermittent drying over the rest of the drying process. Therefore, an IR-assisted HPD offers the advantage of compactness, simplicity, ease of control, and low equipment cost when both surface and internal moisture are present (Mujumdar 2000). 

The three major advantages of heat pump dryers are [60] ... 

Typical drying temperatures in a heat pump dryer are in the range 30°C-60°C. It is expected that drying by this technique would improve the retention of volatile flavor, reduce the color degradation as well as the loss of heat-labile vitamins [61]. 

R.L. Mason, P.M. Britnell, G.S. Young, S. Birchall, S. Fitz-Paine, and B.J. Hesse, Development and application of heat pump dryers to the Australian food industry. Food Aus., 46(7) 319 (1994). 

A schematic diagram of a heat pump dryer-dehumidifier is shown in Figure 54.6a. The following processes take place in the system Humid gas A leaving the dryer is cooled by an evaporator (5) to a tanperature close to T below the dew point (B) and (2) consequently, part of the moisture from the air is... 

FIGURE 54.6 A heat pump dryer-dehumidifier (a) schematic diagram (1) compressor (2) condenser (3) liquid tank (4) expansion valve (5) evaporator (6) auxiliary fan (b) Ramzin-Mollier chart of air cycle refrigerant evaporation temperature refrigerant condensation... 

Compressors of heat pump dryers should be able to operate continuously for several days. 

T.M. Eikevik, O. Alves-Filho, and I. Stipmmen, Potential applications for a new COj heat pump dryer. The First Nordic Drying Conference, Trondheim, Norway (June 27-29, 2001). 

Heat Pump Dryer Incorporating Other Transfer Mechanisms.1115... 

The basic components of the heat pump system comprise an expansion valve, two heat exchangers (evaporator and condenser), and a compressor. A schematic diagram depicting the operation of a heat pump dryer is shown in Figure 55.2. 

Heat pump dryers can significantly improve product quality by drying at low temperatures. At low temperatures, the drying potential of the air can be maintained by further reduction of the air humidity. 

FIGURE 55.6 Aparallel flow batch mode heat pump dryer. 

Figure 55.13 shows another example of areal-time process control for the heat pump dryer to reduce nutrient degradation. Experiments have been carried out with hypodermic thermocouple needles to measure the transient temperature profiles of food products (Chou et al., 1997). These measured values make it possible to tune the drying air temperature to prevent the internal product tanperature from reaching a threshold value, hence reducing thermally induced nutrient degradation. 

FIGURE 55.12 An online feedback system for a heat pump dryer to control the color of the product. 

The evolution of the mean moisture content with time for drying of banana samples in a heat pump dryer is shown in Figure 55.18. It can be observed that step-down temperature profile is able to reduce the drying time to reach desired moisture content. Taking the moisture content of each product at 240 min as the basis for comparison, it can be observed that the step-down temperature profile was able to reduce the drying time for banana samples by 180 min. A noteworthy... 

Heat pump dryers are gaining recognition as energy efficient drying devices for production of quality products. The following sections are devoted to selected examples of industrial applications of the heat pump dryer. 

Energy Performance of a Heat Pump Dryer for Timber Drying... 

FIGURE 55.20 Schematic diagram of a heat pump dryer for timber drying. 

HEAT PUMP DRYER INCORPORATING OTHER TRANSFER MECHANISMS... 

Thus far, the research work has been primarily aimed at the performance of heat pump dryers for different products. Little work has been reported on the performance of heat pump dryers using external energy sources to compliment it. HPD is primarily a convective drying process. Use of infrared (IR) or radio frequency (RF) sources can be incorporated along with the usual HPD to enhance the drying rates while reducing the thermal load on the heat pump itself. 

A schematic layout of a fluidized bed heat pump dryer developed at the Norwegian Institute of Technology (NTNU) is shown in Figure 55.21 (Alves-Filho and Strpmmen, 1996 Strpmmen and Jonasen, 1996). The drying chamber receives... 

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