An indirect evaporative cooling system for preserving the shell life of perishable farm product was designed using locally available materials such as copper tubes, fiber glass, galvanized mild steel, stainless steel and foam (pad). The system is divided into four parts viz; Heat exchanger, cooling chamber, body frame, and electrical part. The cooling chamber was constructed with galvanized metal steel, stainless steel, and polystyrene foam which were used for lagging. Upon completion of the system, it was tested for 10 days using fresh tomatoes to evaluate its performance and efficiency. From the test, an efficiency of about 70.37% was obtained for the designed system. It was shown that the average cabinet temperature (21.6oC) drops when compared to the average ambient temperature (31oC) while the relative humidity of the cabinet increases from 40% (ambient condition) to 75%. Hence, the conditions in the chamber therefore can be considered very conducive for storing fruits and vegetables because the storage temperature and the relative humidity were kept within the optimum level thus keeping the fruits and vegetables fresh. Experiments need to be performed to identify more locally available materials that can successfully be used as absorbents.
TABLE OF CONTENT
1.1 BACKGROUND OF THE STUDY
A Refrigeration system is the process of treating air so as to control simultaneously its temperature, humidity, air circulation and cleanliness to meet the requirement of the conditioned space (Anyanwu et., al 2010). When the purpose of refrigeration system is primarily for human comfort and health. If on the other hand, the so purpose is to provide a required environment conditions for product during manufacturing operations or storage, it is known as process or industrial refrigeration system. Since the air is conditioned to meet a certain need for both Humans, Plants, animal and processes, there is need to control the following parameters.
1. Temperature (through heating and cooling)
2. Humidity (through humidifying and de-humidifying)
3. Cleanliness (through filtering, fume and odors removal)
There are several methods of achieving this refrigeration system via; Vapour compression system, Vapour absorption system, Evaporative cooling system, Thermo-electric cooling, Stream ejector systematic. One of the methods is the evaporative refrigeration system which cools air and at the same time humidifies the air. This method involves the passage of air through a fine mist of water or water saturated medium which cools the air as the water extracts heat from the air and as well humidifies the air, this process is known as direct evaporative cooling. This method existed many years back and is regarded as the oldest and commonest form. On the other hand, when the air to be cooled is kept separate from the evaporation process and therefore not humidified as its cools, it is known as evaporative cooling. This method is most effective in hot and dry climates where high dry bulb temperature are associated over time with relatively low wet bulb temperatures such that the air temperature is uncomfortably high that any amount and kind of cooling will be considered a welcome relief.
Evaporative cooling occurs in nature near waterfalls and streams, over lakes and oceans even on wet surfaces of human skin. This cooling method has been known long time ago by the Egyptian and was used in cooling drinking waters and homes. In the same way, primitives evaporative cooling occurs today in canvas covered canteens for soldiers, drinking water jars of American Indians and Mexicans.
In 1800 B.C the New England textiles factory began to use the evaporative cooling systems to cool their mills. In the 1930s, the Beardmore tornado airship engine made use of this technology to reduce and completely remove the use of a radiator which reduces the effect of lag. Rusten, (1985) described some types of evaporative cooling that was been used in New Delhi, India in which a wetted mat with fan was used to cool a local restaurant. The concept of water-cooling a roof has a long history but it is estimated that less than 60 million square feet of roof have ever been water cooled (Tiwari et al., 2002).
During the nineteenth century, air washers and textile mill evaporative cooling were invented between 1900 and 1930 largely for industries. By 1932, according to Walt, (1997) there were thousands of those home-made coolers in Arizona. Zellweger (1989) at St Louis devised air washing fan whose blades and casting were kept wet to remove air-borne dust. The earliest were burlap-covered wooden frames, wet by dripping water as they are mounted outside window shaving electric motors equipped with fan a blade which frequently blows the cooled air indoor. This method of cooling is the direct evaporative cooling system, adding moisture to the air being cooled. However, this may be quite uncomfortable to some areas and individuals.
Refrigeration system is were developed to ameliorate the problem in direct evaporated coolers; these coolers performed cooling without adding moisture to the cooled air. This type developed chiefly in relatively and climates of Arizona and southern California where cooling towers were familiar sites. Two different style of refrigeration system were developed in Los Angeles where plate type heat exchangers were used in place of coils. In the more successful type, water sprays and air blasts were directed at the outside of hollow water plates, this cooled the air passing through them, without being humidified. This makes indirect coolers an excellent complement to the direct evaporative cooling system. While the concept and use of evaporative cooling has long been in existence, it was of recent that the technology has advance to a level that allows for its widespread use. A new development in materials and design now permits this method of cooling to be used in areas that were considered unsuitable.
Today, one of the main global challenges is how to ensure food security for a world growing population whilst ensuring long-term sustainable development. According to the FAO, food production will need to grow by 70% to feed world population which will reach 9 billion by 2050. Further trends like increasing urban population, shift of lifestyle and diet patterns of the rising middle class in emerging economies along with climate change put considerable pressure strain on the planets resources: Consequently, there is a need for an integrated and innovative approach to the global effort of ensuring sustainable food production and consumption (Nellemann et al., 2009). In the meantime, while the number of food insecure population remains unacceptably high each year and worldwide, massive quantities of food are lost due to spoilage and infestations on the journey to consumers (FAO, 2011). In some African, Caribbean and Pacific (ACP) countries, where tropical weather and poorly developed infrastructure contribute to the problem, wastage can regularly be as high as 40-50% (spore, 2011). Obviously, one of the major ways of strengthening food security is by reducing these losses there by increasing the interest in effective intervention for Post-Harvest Losses. The investment required to reduce post-harvest losses is relatively modest and the return on that investment rises rapidly as the price of the commodity increases. During a research prioritization exercise undertaken by ACP Food Security and Livelihoods sector (FSL) in 2011, postharvest handling was recognized as one of the important areas requiring attention.
The term “postharvest loss” refers to measurable quantitative and qualitative food loss in the postharvest system (de Lucia et al., 2010). This system comprises interconnected activities from the time of harvest through crop processing, marketing and food preparation, to the final decision by the consumer to eat or discard the food. Nowadays, interventions in Post-harvest losses reduction are seen as an important component of the efforts of many agencies to reduce food insecurity. A post-harvest loss is increasingly recognized as part of an integrated approach to realizing agricultures full potential to meet the worlds increasing food and energy needs.
Agricultural engineers are faced with the task of not only meeting the food requirements of the ever increasing global population, but to also maintain relatively low costs for food products. From basic economics principles, stabilizing the cost of a product undergoing an increase in its demand requires either an increase in its supply or a decrease in its cost of production. The only possible way left to stabilize the cost of food is to establish and implement new and efficient methods for crop production, post-harvest storage, as well as distribution and transportation. Although all of these aspects are important when trying to tackle the challenge of meeting stabilizing the cost and preservation of food, this design project only explores the post harvest storage aspect of food production.
Evaporative cooling is of specific interest to engineers concerned with the efficiency and energy demands of post-harvest storage, while providing a humid environment required for storage. The theory behind cooling effect from evaporation is simple: as water evaporates it absorbs latent heat from the surroundings (notably air) and as a result the ambient temperature is reduced. This phenomenon is precisely why a human being feels cooler when sweating. This project design system is aimed at solving problems encountered with the preservation of perishable agricultural produce by providing a less expensive means of persevering these produce thus extending their shelf life and retaining their nutrient during post harvest period. The project design involve the use of indirect evaporative system , utilizing air and water as the refrigerant to cool and preserve these produce at a certain temperature, preventing spoilage and decay of these produce.
1.2 PROBLEM OF THE STATEMENT
The aim of every project work is to solve a particular problem to help better our standard of living. Agricultural produce like fruits, tomatoes and vegetables are vital agricultural products for human consumption worldwide. They are rich in vitamins and minerals such as calcium, carotene (pro vitamin A), ascorbic acid, iron, iodine, etc. Deficiency of these nutrient and minerals can lead to wide spread of diseases and on a long run, lead to death. However these produce are not only seasonal but also are highly perishable having external factors outside of the food supply chain causing significant spoilage of these produce thus postharvest loss.
Postharvest loss can be defined as the degradation in both quantity and quality of a food production from harvest to consumption. Quality losses include those that affect the nutrient/caloric composition, the acceptability, and the edibility of a given product (Kader, 2002). Quantity losses refer to those that result in the loss of the amount of a product (Kitinoja et al., 2010). Cutting post-harvest losses could presumably add a sizable quantity to the global food supply, thus reducing the need to intensify production in the future.
To realize the relevance of this project, it is important to recognize the reasons for postharvest losses in relation to temperature and humidity. Losses related to temperature and humidity can include spoilage due to disease, over-ripening, negative physiological and compositional effects, loss of mass (produce water mass), and aesthetic appeal. The goals of post-harvest cooling are to counteract these effects by the following mechanisms:
Slow and inhibit water loss
Suppress enzymatic degradation and respiration
Slow and inhibit the growth rate and activity of pathogens.
Reduce the production of ethylene or minimize a commodity’s reaction to ethylene (Narayanasamy, 2006).
Some background on each aspect is necessary in understanding why it is important to analyze and correct this problem. The first problem of water loss is related to food structure, texture and appearance changes. Water loss after harvest is mainly dependent on two things:
1. The water vapour pressure deficit that exists between produce and its immediate environment.
2. The surface transfer resistance to water vapour movement (determined by shape and internal resistances as well as surface resistances) which occurs now that the plant is no longer transpiring through its stomata.
It is important to note that the rate of metabolic activity in cells increases with temperature; thus the importance of minimizing temperature is clear. Pathogens and diseases tend to flourish in high temperature and high humidity environments and produce may become affected at any point in the post-harvest pathway to the consumer. In terms of pre-cooling, removing initial field heat is an important first step to limiting the influence of diseases. Moreover, there is a tradeoff to be made with respect to humidity control and it is certainly beneficial to have a high humidity environment for fruits and vegetables with regard to water loss, but leaves the produce more susceptible to disease; another important management decision for producers.
1.3 OBJECTIVES OF THE STUDY
The main objective of this work is to design an indirect evaporative cooling system for preserving perishable farm produce. Other specific objectives include;
1 To design and fabricate an indirect evaporative cooling system using locally available materials.
2 To test the performance of the system using some of the vital performance indices such as Heat Exchanger Effectiveness etc.
3 To evaluate the quality of some perishable agricultural produce stored in the system.
This project is designed mainly for the testing of indirect evaporative cooling system, as a replacement for direct evaporative cooling system for preservation and storage of some agricultural produce such as tomatoes etc. There is need for storage of these perishable farm produce because not all the harvested will be used immediately (Anyanwu, 2004). Also for some produce, if not harvested will rotten in the farm. Some methods of preservation of raw and processed fruits/vegetables include: storage in ventilated shed, storage at low temperatures, use of evaporative coolant system, waxing and chemical treatment (Olosunde, 2006). Most of the peasant farmers cannot afford the cost of purchasing high- tech storage equipments for their harvested crops. Evaporative cooling has been found to be an efficient and economical means of reducing temperatures and increasing humidity in an enclosure where the humidity is comparatively low (Sushmita et al., 2008). Minimizing deteriorative reactions in fruit and vegetables enhances their shelf lives, implying that the produce will be available for longer periods; this would reduce fluctuation in market supply and prices (Dzivama, 2000).
1.5 SCOPE OF THE PROJECT
The work will mainly focus on developing and testing of an indirect evaporative cooling system, using locally available material in order to provide effective means of preserving perishable agriculture products such as tomatoes, vegetables etc. The work involves three stages viz; the design and fabrication of the system, testing of the system to evaluate some performance index and evaluation of the quality of some perishable farm produced to store in the system.
It does not form ice but only cools and preserve.
Evaporative coolers are not effective in the humid regions.
The Cooled air may bring dust and pollen into the space causing the Growth of micro-organisms such as molds on the cooler pad.