Thermal modeling of a natural convection greenhouse drying system for jaggery: An experimental validation
Introduction
Jaggery or “Gur” is one of the traditional, unrefined, whole sugars made by the concentration of sugarcane juice without the use of any chemicals/synthetic additives or preservatives and most important agro-processing industries under decentralised sector in India. Compared to 99.9% sucrose content of sugar, jaggery contains 70–85% sucrose, 10–15% reducing sugar, 1–2% minerals like calcium, iron and phosphorous, vitamins A and B, protein and fats (Rao and Lakshminarayana, 1999). The industry is meeting about 40% of the total requirement of sweeteners and giving employment to 25 lakhs people in rural areas (Alam, 1999). Manufacturing of jaggery starts by September/October and continues till March/April in most of the areas.
It has been reported that jaggery losses to the extent of 25% occurs in normal storage process. This loss is mainly due to the moisture present in the jaggery which triggers on biochemical activity—thus converting sucrose into reducing sugars (glucose and fructose) which further make jaggery more hygroscopic attracting more moisture and the process continues like chain reaction untill jaggery liquidifies. It is thus necessary to dry jaggery up to certain moisture content before storage (Uppal and Sharma, 1999a).
Recently, the concept of jaggery packaging for ease in handling, transport and distribution, besides hygiene has also received considerable attention in industry. Drying of jaggery up to an optimum moisture content thus becomes a pre-requisite for the effective packaging. Freshly prepared jaggery has moisture content in the range of 10–12% which is not desired and hence should be removed by the process of drying (Uppal and Sharma, 1999b).
The jaggery drying can be done by several methods namely open sun drying, cabinet drying and greenhouse drying. In open sun drying, humidity cannot be controlled and hence it takes considerably a longer time to dry due to the hygroscopic nature of jaggery. The drying can also be done with the help of a solar cabinet dryer but the high temperature inside the cabinet dryer may melt the jaggery which is not desirable. Another constraint is cost, as it is economic to dry the same volume of jaggery inside the greenhouse as compared to the cabinet dryer. However, the greenhouse dryer provides controlled environment in terms of moderate temperature and humidity which is benificial for the jaggery drying more effectively thus reducing the drying time (Prakash, 2004). The greenhouse drier is a system that uses the regular structure of a greenhouse, when the greenhouse is not used. This double function, greenhouse and drier, improves the return rate of the initial investment (Farkas, 1998, Condorí et al., 2001).
Rachmat and Horibe (1999) presented a mathematical model to predict the air temperature inside the greenhouse on the basis of ambient conditions. An optimal model for post-harvesting is developed for alfalfa drying considering the differences in drying behavior between stems and leaves of alfalfa, the heat and mass balances of the drying air and a model for a solar energy system. The complete model is used to calculate the dynamic optimal operation for alfalfa drying in a thin layer (Boxtel et al., 1996, Farkas, 2003). The performance of the solar tunnel drier for drying of pineapple slices under Bangladesh conditions was studied and proximate analysis indicated that the pineapple dried in a solar tunnel drier was a good quality dried product for human consumption (Bala et al., 2003).
Recently, mathematical models were presented to study the thermal behavior of crops (cabbage and peas) for open sun drying (natural convection) and inside the greenhouse under both natural and forced convection. The predicted crop temperature and crop mass during drying showed fair agreement with experimental values within the root mean square of percent error of 2.98% and 16.55%, respectively (Jain and Tiwari, 2004).
The present work focuses on the development of a thermal model for prediction of hourly jaggery temperature, greenhouse air temperature and moisture evaporation under natural convection mode of jaggery drying (Fig. 1).
Section snippets
Experimental observation
Jaggery pieces of either 0.75 or 2.0 kg having dimensions of 0.03 × 0.03 × 0.01 m3 were kept as thin layers in wire mesh tray of dimensions 0.4 × 0.24 m2 under the east–west oriented greenhouse for experimentation. A roof-type even span greenhouse with 1.20 × 0.78 m2 effective floor covering area having the central height and height of the walls as 0.60 and 0.40 m, respectively, was made of PVC pipe and UV film covering. An air vent with an effective opening of 0.0722 m2 was provided at the roof for natural
Working principle
The working principle of the greenhouse jaggery drying under natural convection condition is illustrated in Fig. 2. The plastic covered greenhouse traps the solar energy in the form of thermal heat within the cover and reduces the convective heat loss. The fraction of trapped energy will be received partly by the jaggery and partly by the floor and exposed tray area and the remaining solar radiation will heat the
Input values and computational procedure
Computer programs based on Matlab software were used to solve the mathematical models. The average hourly solar radiation on the different walls and roofs of the greenhouse was evaluated from the average hourly solar radiation on a horizontal surface with the help of Liu and Jordan formula (1962). Then, the average hourly total radiation received by the greenhouse is the sum of the average hourly radiations of the walls and roofs of the greenhouse. Thus, the average hourly total radiation
Results and discussion
The solar drying inside greenhouse was conducted in the month of February 2004. Each experiment was started at 10 am and continued till 5 pm. The thermal model developed for greenhouse jaggery drying under natural convection was solved for experimental data given in Table 1. The greenhouse air temperature (Tr) was calculated for initial values of jaggery and ambient air temperature by using Eq. (7). Then this value of Tr was used to calculate the jaggery temperature Tj by using Eq. (9).
Conclusions
The thermal model developed in this paper was validated with the experimental observations for the complete drying of jaggery under natural convection conditions. The predicted values and experimental observations were in good agreement with coefficient of correlation ranging between 0.90 and 0.98 for jaggery and greenhouse air temperature and 0.96–1.00 for the jaggery mass during drying. The present model will be beneficial to design greenhouse dryer for a given mass of jaggery with thin layer.
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