Evaluation of convective mass transfer coefficient during drying of jaggery G.N. Tiwari * , Sanjeev Kumar, Om Prakash Center for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Received 25 February 2003; accepted 25 July 2003 Abstract In this paper an attempt is made to evaluate the convective mass transfer coefficient during drying of jaggery in a controlled environment. In this method, different masses of jaggery were used. The jaggery was dried in the roof type even span greenhouse with a floor area of 1.2 · 0.8 m 2 in natural and forced convection mode at atmospheric pressure till it attained almost no variation in mass. The experimental data of mass evaporated, temperatures of jaggery, greenhouse room air and relative humidity were mea- sured and the data used to evaluate the convective mass transfer coefficient by regression analysis. It was found that the convective mass transfer coefficient is a strong function of mass of jaggery, temperatures and relative humidity for a given size of greenhouse. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Convective mass transfer; Drying of jaggery; Greenhouse 1. Introduction Jaggery is a traditional concentrated sugarcane juice and is a good source of minerals like calcium, phospho- rous, iron and has useful medicinal properties. In India, the jaggery is mainly produced in the months of No- vember to mid March and stored for the rest of year. The quality of stored jaggery mostly depends upon the moisture content which is favorable for inversion and developmentofdifferenttypesoffungiandbacteriainthe jaggery resulting in changes in tests and colors because of the formation of organic acid and complex decomposi- tion of products (Uppal & Sharma, 1999a, 1999b). Drying is basically a heat and mass transfer pheno- menon. The heat energy supplied to the jaggery surface is utilized in two ways i.e. to increase the jaggery surface temperature in the form of sensible heat and to vaporize the moisture present in jaggery through provision of latent heat of vaporization. The removal of moisture from the interior of the jaggery takes place due to in- duced vapor pressure difference between the jaggery and the surrounding medium. The desired difference in va- por pressure may be obtained either by increasing the vapor pressure of the jaggery surface or by decreasing the vapor pressure of the surroundings or by both. While drying under controlled conditions, all the above means may be employed with better control over the drying rate. This is not so in open sun drying, where it depends upon the weather conditions. The convective heat transfer coefficient depends on the temperature difference between the jaggery sur- face and air and the thermal physical properties of the humid air surrounding the jaggery. To find the convec- tive heat transfer coefficient for jaggery, dried under different operating conditions, an experimental setup was formulated and data for various parameters were recorded at regular interval for the following cases. (i) Jaggery drying under natural convection mode (Fig. 1(a)) (ii) Jaggery drying under forced convection mode (Fig. 1(b)) The objective of present study is to determine the convective mass transfer coefficient for the design of a dryer for jaggery to retain its quality for storage. 2. Literature review Jaggery is prepared for human consumption as a sweetening base for food in rural as well as urban areas Journal of Food Engineering 63 (2004) 219–227 www.elsevier.com/locate/jfoodeng * Corresponding author. Tel.: +91-11-2659-1258; fax: +91-11-2658- 1121/2686-2037/2659-2208. E-mail address: gntiwari@ces.iitd.ernet.in (G.N. Tiwari). 0260-8774/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2003.07.003