Elsevier

Journal of Food Engineering

Volume 50, Issue 4, December 2001, Pages 223-227
Journal of Food Engineering

Development of an air recirculating tray dryer for high moisture biological materials

https://doi.org/10.1016/S0260-8774(01)00024-3Get rights and content

Abstract

Fruits and vegetables are highly perishable and deterioration in quality and quantity occurs due to poor storage and processing facilities at the production site in developing countries like India. Dehydration of vegetables using a recirculatory dryer is possible but limited, in practice. The recirculatory dryers so far developed have a drawback of non-uniform drying along the length of the trays. Hence an effort was made to solve this problem. A recirculatory cabinet dryer of capacity 5 kg/batch using a central air distribution system was designed and developed. The dryer was tested with blanched potato chips. At a constant air flow rate of 1.5m3/min and 65°C temperature, it took about 3 h time to reduce the moisture content from 856.94% (dry basis) to 9.98% (dry basis). The heat utilization factor (HUF) and thermal heat efficiency (THE) of the developed dryer were found to be 18.94% and 22.16%, respectively.

Introduction

Fruits and vegetables play an important role in human diet and nutrition. India produced 46.8 million tonnes of vegetables in the year 1996–1997, but it is estimated that about 25–30% of the production is lost (Anon., 1999). In some developed countries the figure is far less. The main reasons for post harvest losses are poor handling and storage facilities for high moisture products at the production catchment area. One of the simplest methods to improve the shelf life of fruits and vegetables is to reduce their moisture content to such extent that the microorganism cannot grow.

Fruits and vegetables are not only high in moisture but also rich in nutrients. These nutrients restrict the temperature to which the product can be raised during drying. The rate of drying also affects the end quality of the dried product. Thus to limit the above boundary conditions the cabinet dryers available in the market are most suitable for drying fruits and vegetables.

The application of direct solar radiation for fruit and vegetable dehydration is most common in hot climates. But due to contamination of product from dust, insects and spoilage due to rains, use of sun drying is limited. Holdsworth (1971) reported that drying of grapes in cabinet dryers gave better quality than sun-dried grapes. Different solar dryers for perishables have been developed by various scientists (Kalra & Bharadwaj, 1981; Singh & Alam, 1982; Lutz, Wuhlbauer, Muller, & Reisinger, 1987). However, their application is limited due to its slow drying rates and weather dependency. High capacity electrically heated dryers for fruits and vegetables have been reported by Brown, Vanarsdel, and Lowe (1973) and Luh, Somogyi, and Mechan (1975), however, these are beyond the reach of small scale vegetable producers/processors. Small capacity electrical dryers for other biological materials have also been developed (Singh, Arrora, & Sehgal, 1978; Patil & Singh, 1984). But these cannot be used directly as the design of electrical dryer is influenced by various machine and drying air parameters which vary according to the type of product to be dried.

Vagenas and Marinos-Kouris (1991) presented a mathematical model for the design and optimization of an industrial dryer for Sultana grape and applied it to the determination of size and optimal operating conditions of the dryer. The optimization variables temperature and humidity of the drying air and product loading thickness on the trays were considered for a higher degree of recirculation of exhaust air.

Shukla and Patil (1992) reviewed different dryers and drying technology for food crops developed in India and expressed due to complex process of dehydration, most of the developed dryers have certain limitations. Hence there is need to develop simple technology for selected feasible dryer. The dryers he reported are having limitations of single crop basis or low capacity.

Singh (1994) developed a small capacity dryer for vegetable and tested with cauliflower, cabbage and onion. The drying time was in the range 11–14 h and overall energy efficiency was 28.21–30.83% with 55–65°C temperature.

Kiranoudis, Maroulis, and Morinos-Kouris (1995) stated that most design efforts in the field of fruits and vegetable dehydration face problems of extreme difficulty related to the complex drying conditions that involve many interconnected and opposing phenomenon, associated with material's complex nature of drying. Kiranoudis (1998) modeled design of batch-grape dryers considering all probable factors, which effect the dryer performance.

Literature on design aspect of recirculatory cabinet dryer is scanty. A recent work on design and development of a recirculatory tray dryer was carried out at IIT Kharagpur, India. Srivastav, Jain, and Das (1998) reported the dehydration characteristics of green mango slices in recirculatory tray dryer at a tray loading rate of 4–5kg/m2, 60°C, 65°C, 70°C temperature and 30–96% air recirculation. The analysis showed that the recirculation ratio did not significantly affect the drying time. However, 7 h of drying time was required to reduce the moisture content from 98.32% (d.b.) to 15.25% (d.b.) for mango slices without stone. The energy consumption to evaporate 1 kg of water was found to be 1.60 kW h at 60°C and 76.7% degree of recirculation, while the same was 1.86 kWh at 65°C and 50.4% degree of recirculation. The quality of the dried product was found better than similar products available in the market. Pelegrina et al., 1998, Pelegrina et al., 1999 designed a rotary semi-continuous drier for vegetables and developed a model for water removal rate and applied to see the effect of air recycling. It was observed that exhaust air recycling and mixing with fresh feed reduces the energy consumption. But for quality retention lower recycle fractions must be used.

In the commercially available cabinet dryer hot air (heated through electrical heaters or heat exchanger) is forced over the trays, where fruits or vegetables are spread in a thin layer. The hot air moves over the wet material and is discharged from the other end. Thus the total time available to the hot air to pick the moisture is very short and is normally less than 1 s (e.g., 2 m/s air velocity moves above the tray with total length of 1.2 m). Thus the utilization of heat supplied to the air is low. It results in poor efficiency of the dryer (sometimes less than 10%). Moreover, it leads to very slow drying in spite of the high temperature gradient that may allow microorganism to grow. Therefore, it is planned in the present design to use the outlet air and feed it back to the inlet fan. It will reduce the heat requirement for heating the air. Thus an air recirculatory tray drier is designed.

The existing recirculatory dryers have an air inlet into the drying chamber from one side and exhaust from the other side. This design causes non-uniform drying of the material, because the air carries with it moisture from the inlet side and hence the moisture removal at the exhaust end is less as compared to the inlet side. To avoid variation of the moisture removal from one end to the other end of the drying chamber, it is proposed to feed the hot air from the center. This will not only reduce variation but also decrease the heat loss to the surroundings. The recirculatory air duct will be at the top of the drying cabinet and will also help in reducing heat loss.

Section snippets

Experimental apparatus and procedure

The laboratory model of a recirculatory tray dryer consists of the following components:

  • 1.

    drying chamber,

  • 2.

    air blower,

  • 3.

    heating chamber,

  • 4.

    hot air distribution,

  • 5.

    air discharge/recirculation,

  • 6.

    control panel.

Drying chamber

The developed unit has two drying chambers (300×300×400mm3) separated by the inlet air duct (Fig. 1). The drying chamber is made of 1.5 mm thick aluminum sheet with an outer layer of glass wool. A layer of galvanized iron sheet further protects the insulation. Three rectangular slots (300×50mm2) on each side are cut for air inlet and outlet from the drying chambers. The slots are placed equidistant apart. The inlet ports have baffles to control the air flow rate in the chamber over the chamber

Air blower

To force the hot air through the chamber and for re-circulation over the trays at an air velocity of 2 m/s, a minimum blower capacity of 10.54m3/min is required. The blower was selected based on the total volumetric flow rate of air per second through the inlet ports. The blower is driven by a 0.75 kW universal motor which can be controlled by a variac for variable air flow.blowercapacity=airvelocity×totalportareablowerefficiency.

Heating chamber

The heating chamber is also made of mild steel sheet and is fitted with 1 kW heater, to heat the incoming fresh air from ambient temperature to 70°C. The heater capacity was calculated considering no heat loss to the surrounding during initial heating up of air in a time period of 15 min, with 0% air recirculation. A digital temperature controller fitted over the control panel controls the heater.heatercapacity=massflowrateofair×specificheatofair×riseintemperatureheaterefficiency.

Hot air distribution

The hot air for distribution over the trays into the drying chamber is passed through a plenum chamber (Fig. 2). The plenum chamber is a diverging pipe with a rectangular cross-section. The larger end is welded to the chamber. The dimension of the air distribution chamber is 400×300×100mm3. The smaller end of the air inlet duct is fitted to the heater outlet.

Air discharge/recirculation

The air after passing through the chamber is to be discharged through a discharge duct (Fig. 2). The air discharge duct of rectangular cross-section is proposed. There are two ducts on the opposite ends, which are connected, with a duct running over the chambers. The common duct is connected to twin outlet ducts separated by baffles, one being an exhaust to the outside and the other a duct for re-circulation. A throttle valve controls the exhaust to the outside or re-circulation. The discharge

Control panel

The control panel consists of switches and sockets, fuses, digital temperature controller, digital temperature indicator, voltmeter, ammeter and watt-meter.

Testing of the dryer

The developed drier was tested under:

  • 1.

    no-loading condition,

  • 2.

    loaded condition.

Results and discussion

The overall size of the drier is 1000×1400mm2 (including stands), and it has total weight of approximately 65 kg. It was mounted on a stand of 900 mm height to accommodate the blower and heating assembly. The baffles were adjusted to have about 95% re-circulation. The temperature of the ambient air, heated air and exhaust air were marked on the psychometric chart to calculate the approximate re-circulation percentage.

The air velocity in the chambers was measured. It was found that both the

References (19)

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