DESIGN, CONSTRUCTION AND TESTING OF AN EVAPORATIVE COOLING SYSTEM FOR STORING VEGETABLES

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DESIGN, CONSTRUCTION AND TESTING OF AN EVAPORATIVE COOLING SYSTEM FOR STORING VEGETABLES

PROPOSAL

An evaporative cooling system for the preservation of fresh vegetables was developed for extending the shelf life of tomatoes and carrots and its performance was evaluated. It consists of a pyramidal shaped with total storage space of 0.075 m3, made of galvanized mild steel, stainless steel and internally insulated with 0.025 m polystyrene foam, a suction fan of 4.3 m/s velocity air flow and 0.5 W (1250 rpm), cooling pad (Jute) of 0.06 m thickness and water pump with discharge capacity of 3.5 l/min as well as a power rating of 0.5 W. A water reservoir of capacity 62.5 m3 is linked to the cooling system at the bottom through a P.V.C. pipe supplying water to keep the cooling pad/mesh continuously wet. Study was conducted to check the freshness of tomatoes and carrots, and data were observed daily. Results of the transient performance tests revealed that the evaporative cooling system chamber temperature and relative humidity depression from ambient air temperature varied over 16-26°C and 33-88% respectively. Ambient air temperatures and relative humidity during the test periods ranged over 26-32°C and 18-31% respectively. The shelf life of the vegetable produce inside the evaporative cooling system was extended by fourteen days relative to ambient storage. Thus, the evaporative cooling system has the prospect of being used for short term preservation of vegetables soon after harvest and it will be very useful in a developing economy like Nigeria.

Background of the study

Most of the post-harvest losses incurred on fruits and vegetables in developing countries are due to lack of adequate storage facilities. While refrigerated cool stores are expensive to install and run, they are still the best method of preserving fruits and vegetables.Cooling through evaporation is an ancient but effective method of lowering temperature. The quality of fresh fruits and vegetables depends on post-harvest handling, transportation and storage (Haidar and Demisse, 1999). Compared with several temperate fruits and vegetables, tropical and subtropical vegetables such as tomatoes and carrots, present greater storage and transportation problems because of their perishable nature (Mitra and when available in order to ensure their constant supply throughout the year with their nutritional value still retained (CFNEU, 2003). In addition, preservation of fruits and vegetables is of great importance because it makes provision for delayed use and eliminates wastage (Aremu, 1975). Low temperature handling and storage have been described as the most important physical method for post-harvest loss control (Seyoum and Woldetsdik, 2004). Temperature of the surrounding air and produce can be reduced by forced air cooling, hydro cooling, vacuum cooling, and adiabatic cooling (Thompson et al., 1998). In developed countries, methods employed for extending shelf life and minimizing post-harvest losses of perishable produce include mechanical refrigeration, controlled atmospheres, hypobaric storage, and other sophisticated techniques. These techniques are highly capital intensive and for most developing countries, the required manpower is either lacking or inadequate. These cooling methods, except adiabatic cooling, are expensive for small scale peasant farmers, retailers and wholesalers, as they require electric power. Moreover, in the existing mechanical refrigerating systems, proper storage conditions are not often put into consideration as stored items (vegetables) were normally subjected to excessive chilling or freezing. The injurious effects this has on stored vegetable products are often very severe, hence, one of the major reasons for the low efficiency of this system in extending the shelf life of fresh vegetables. Low temperature and high relative humidity can be achieved by using less expensive methods of evaporative cooling (Seyoum and Woldetsadik, 2000; Seyoum and Woldetsadik, 2004). Evaporative cooling has been reported for achieving a favorable environment in greenhouses (Jain and Tiwari, 2002), animations and the storage structure for fruit and vegetables

MATERIALS AND METHODS

Design and construction

In this study an evaporative cooling system of 25 kg storage capacity, suitable for the preservation of fresh vegetables was constructed. The evaporative cooling system consists basically of the cabinet, cooling fan and the transmitting medium (cooling pad). It consists of a pyramidal shaped with total storage space of 0.075 m3, made of galvanized mild steel, stainless steel and internally insulated with 0.025 m polystyrene foam, a suction fan of 4.3 m/s velocity air flow and 0.5 W (1250 rpm), cooling pad (Jute) of 0.06 m thickness and water pump with discharge capacity of 3.5 l/min and power rating of 0.5 W. A water reservoir of capacity 62.5 m3 is linked to the cooling system at the bottom through a P.V.C. pipe supplying water to keep the cooling pad/mesh continuously wet. The basic principle relies on cooling by evaporation, when the system is set in operation, the dry air from the suction fan passes over the wet surface (cooling pad) and evaporated away the soaked water away from the cooling pad. When water evaporates, it draws energy from its surroundings (storage chamber) which produce considerable cooling effect in the storage chamber.

Assumptions

It was assumed that the weight loss of tomatoes and carrots was minimum when the commodities were stored in the evaporative cooling system chamber while it was maximum in ambient storage as presented in Figures 9 and 10. The physiological weight loss obtained for the tested samples that is, tomatoes and carrots are plotted in Figures 9 and 10 respectively. The weight of tomatoes and carrots stored in open air was maintained for only 7 days after which there was a sharp decline in weight from approximately 25 to 6 and 10 kg for the tested samples respectively after 2 weeks of storage, resulting into a loss in weight of about 18 and 14 kg for the samples respectively. Contrary to this observation, tomatoes and carrots kept in the evaporative cooling cabinet had their weight relatively maintained at 25 kg within 2 weeks of storage with only an approximate 5 and 3 kg loss in weight for the tested samples respectively after 2 weeks of storage.

Conclusions

The newly developed system performed up to expectation as tested samples maintained their fresh condition for the 14 days within which they were tested. The required storage temperature for the preservation of the selected vegetable samples was achieved at 16°C for the cabinet temperature at an ambient temperature of about 32°C.

With respect to the quality of the stored items (vegetable samples) results obtained show that there is a tremendous improvement over the mechanical multipurpose refrigerating system. The system developed maintained a higher quality of preservation when compared to the mechanical multipurpose refrigerating system. Hence, the excessive chilling or freezing effects normally experienced with vegetables stored using the latter method were naturally eliminated by the operating conditions of the newly developed evaporative cooling system, as the stored products were only exposed to their required storage temperatures. Hence this system can be used for preservation of fresh vegetables with their quality still maintained for at least fourteen (14) days.

The developed evaporative cooling system is easy to operate, efficient and affordable most especially for peasant farmers in developing countries who may find other methods of preservation quite expensive and unaffordable. This work has elucidated a cost effective means of preserving fresh vegetables, which if adopted will reduce postharvest losses, hence increase in income generated from agricultural produce.

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