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operational costs by reducing the running time in order to decrease cost of electricity utilization or supplementary use. Vegetables and fruits can be made more acceptable to consumers by drying. Comparison of Conventional and Heat Pump Drying Processes Heat pump drying has the ability to recover the latent and sensible heat by condensing moisture from the drying air which may other drying methods cannot. As the use of air conditioning is increasing in this region, so also must the use of heat pump drying be explored. Drying Rate In air drying, the rate of removal of water depends on the conditions of the air, the properties of the food, and the design of the dryer. For a heat pump, the heat transfer from the system to the hot body is desired, and the coefficient of performance is expressed aswhere is the electrical power input of the compressor. However, results from Perera and Rahman 28 indicated that using reduced air temperatures at the onset of drying as in the case of heat pump drying, followed by temperature elevation as drying proceeds, yields a better quality product. The changes in air conditions when air is heated and passed through a bed of moist product are shown in Figure. Finally, the heat pump can also be used as cooling plants, which is a basis for further developments towards the cooling and storing of fruit. It has been used for heating, ventilation, and air-conditioning in domestic and industrial sectors in most developed countries of the world including South Africa. Earlier published works in the area of heat pump assisted grain drying found the concept to be mechanically feasible but not attractive economically due to the low fuel prices prevailing at the time 8,. 14, it can be calculated by integrating the energy used and by calculating the amount of moisture removed as in where is the specific energy consumption of drying; is the energy consumption of drying air; and is the mass of the evaporated water.
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In addition, heat pumps with multiple modes of heat input and intermittent operation allow the use of smaller heat pumps to service more than a single drying chamber for simultaneous drying of different products. Efficiency calculations are useful when assessing the performance of a dryer, looking for improvements, and in making comparisons between the various classes of dryers which may be alternatives for a particular drying operation. In addition, the fact that the major source of electricity in some sub-Saharan regions including Nigeria is hydropower gives confidence that there is safety to the environment as well as reduced energy costs 10,. Other definitions of specific moisture extraction rate with respect to the compressor power are reported. Viability Drying with oxygen-sensitive materials, such as flavor compounds and fatty acids, can undergo oxidation, giving rise to poor flavour, colour, and rehydration properties. From microscopy, it was found that shrinkage of apple slices dried in convection is significantly anisotropic, while less damage to the cell structure during freeze drying leads to more isotropic deformation. This paper tried to divulge the principles and potentials of heat pump drying technology and the conditions for its optimum use. Examples of heat pump assisted drying include heat pump assisted solar drying, microwave drying, infrared drying, fluidized bed drying, atmospheric freeze drying, radiofrequency drying, and chemical heat pump assisted drying 21,. The absorption of moisture by the air would be the difference between the absolute humidities at and, that. Heat pump dryers are able to enhance microbial safety in the foodstuffs by maintaining the relative humidity data at acceptable low level. Also, the operating temperature of heat pump dryers is not limited by the environmental humidity.
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