ABSTRACT
The ambient temperature of tropical countries like Nigeria is as high as 40oC during the dry season. This ultimately gives rise to increase in demand for ice which is used to reduce the temperature of water, soft-drinks as well as other uses. This increase in demand for ice block makes the design and construction of a machine which can be used for the production of ice within a short period of time in order to save energy and time imperative and a worthwhile venture. This paper presents the design and performance evaluation of a model ice block making machine that can freeze water quickly. In order to achieve quick freezing, certain design consideration, such as the quantity of water to be frozen, the choice of cooling system and the methodology of attaining the desired result. The methodology adopted involves increasing the area of heat transfer and employing a vapour compression refrigerating system to generate the refrigerating effect. The design analysis of the evaporator, compressor and condenser are presented in details. This paper also reports on the material selection, fabrication methods and the experimental procedures and results. A temperature of -140C was achieved in the freezing chamber and a freezing duration of 70 minutes for 1kg of ice block as test results.
Keywords: Design, Refrigeration, Quick Freezing, Ice-Block.
INTRODUCTION
The process of transforming water to ice by cooling below the freezing point of water 00Cis termed ice formation. Since ice formation is a cooling process, it could be said that ice formation is a heat transfer process that result to a phase change. For cooling to occur, the temperature gradient must be brought down by expediting the heat in the water and achieving a temperature in the range of -50C to -10C) in a process called refrigeration [1].The temperature changes in water as a result of reduction in enthalpy due to cooling are shown in the cooling curve in Figure 1.
Point ‘A’ is the nucleation temperature, where a critical nucleus is formed in the liquid phase producing a localized warm up or negative cooling rate as shown on the cooling rate curve in Figure 1. Ice crystal growth will then propagate from this nucleus during small time interval ‘A’ – ‘B’–‘C’.Meryman1966[2,3] further explains that at point C, the cooling rate and accompanying heat flux in this small region will rapidly increase to point D to match the surrounding global heat flux called the critical cooling rate, which controls the extent or limit for ice crystals growth. The cooling rate which controls ice crystals size is some average value over a specific temperature change at any location. And when the crystal starts forming, it is certain that it will grow as long as the temperature of the entire system is below freezing [4]. However, Bald1990 [1] suggested that this approach will not produce an explicit relationship between cooling rate and ice crystals size. In some cases liquid is cooled sufficiently quicker so that nucleation does not occur and avoid ice formation. This process is called nitrification and results in an amorphous solid or glass. Muldrew1997 [4] explains that the liquid is in a detestable unit, it gets below a characteristic temperature, the glass transition temperature which is indicated by a sharp exotherm. Once below this temperature, the system is not merely a viscous liquid, but is a solid and it is in a stable thermodynamic state. Achieving nitrification with pure water requires very small water and incredibly fast cooling. It has been proposed experimentally that the heat conducted through the wall of a convectively cooled cavity equals to the heat given off by water for it to transform to ice. However, Weityet at, 1976[5] proposed that the rate of heat given off by a control volume immersed in a coolant equals to the convective heat entering the coolant. Freezing of water could be said to be a two stage process that is cooling the water to the freezing point and freezing the water for phase transformation to occur. However, analysis shows that the heat evolved in cooling is comparatively small to the heat evolved by the cooling and freezing process, as a result of latent heat of fusion, due to the energy that goes into hydrogen bond formation in the crystal. Muldew 1997 [4] analytically states that each water molecule is hydrogen bounded to four other neighboring molecules, each bond having an energy between 10 and 40 kJ/mol and concluded that 80 calories (350J) of energy is released by the transformation of 1gram of water at 0°Cto 80°C.
The applications of refrigeration include household (domestic) refrigerators, industrial freezers, cryogenics, air conditioning and heat pumps [3]. In developing tropical countries like Nigeria, the use of refrigeration as a domestic refrigerators are the most prevalent due to very high temperatures. Domestic refrigerators are used for chilling beverages at homes, offices, seminars, cocktail parties, general meetings, canteens and conferences. Oladunjoye [6] designed and constructed a two compartment freezing unit to enhance the availability of ice block. The need to have cold beverages during such meetings necessitated the design of an innovative refrigerated cooling table to be used for conference services [7, 8]. The demand for ice block has increased due to high temperature experienced in these tropical countries. Ice block making machine can be accomplished by variety of refrigerating effect generating methods. The time of freezing of the water is of considerable importance when designing an ice block making machine. The coefficient of performance (COP) and the energy used per system is also a vital factor in ice block machine design. In order to increase the heat transfer area, the coil was wound round the evaporator. Several designs were made at different time by different designers using adsorption system but they usually have a drawback of low C.O.P (0.12 to 0.23) [9]. This paper presents the design and evaluation of an ice-block making machine using the principle of a vapour compression system.