Heat pump technology has been utilized for heating, ventilation, and air-conditioning in domestic and industrial sectors in most developed countries of the world. Several potential users view heat pump drying technology as fragile, slow, and high capital intensive when differentiated with conventional dryer. This paper tried to divulge the principles and potentials of heat pump drying technology and the conditions for its optimum use. Also, numerous methods of quantifying performances during heat pump drying as well as the quality of the dried products are highlighted. Obligatory factors for maximizing the capacity and efficiency of a heat pump dryer were recognized. Finally, the erroneous view that heat pump drying is not feasible economically was clarified.
Consumers, in a bid to have healthier and additional natural foodstuffs, have been motivated to extend their daily intake of fruits and vegetables because their nutritional values as suppliers of vitamins, minerals, fiber, and low fat are well recognized. However, the water content of most fruits and vegetables is higher than 80%, which limits their shelf-life and makes them more susceptible to storage and transport problems. Vegetables and fruits can be made more acceptable to consumers by drying. In addition, there is market for dehydrated fruits and vegetables which increases the importance of drying for most of the countries worldwide. Although drying is an energy intensive operation, it is highly very indispensable.
Drying is required to increase the shelf-life of foods without the requirement for refrigerated storage; to reduce weight and bulk volumes, for saving in the cost of transportation and storage; to change perishable products (surplus) to stable forms (e.g., milk powder); to manufacture ingredients and additives for industrial transformation (so-called intermediate food products (IFPs), like vegetables for soups, onions for cooked meats, fruits for cakes, binding agents, aroma, food coloring agents, gel-forming and emulsifying proteins, etc.); and to acquire specific convenience foods (potato flakes, instant drinks, breakfast cereals, dried fruits for use as snacks, etc.), with quick reconstitution characteristics and good sensorial qualities, for special use, like in vending machines, or directly for consumers. Also, the loss of product moisture content during drying results in an increasing concentration of nutrients in the remaining mass making proteins, fats, and carbohydrates present in larger amounts per unit weight in the dried food than in the fresh.
In the procedure of drying, heat is needed to evaporate moisture from the product and a flow of air to carry away the evaporated moisture, creating drying a high energy consuming operation. There are various heat sources available for drying and these have been well discussed in many articles. However, due to the extending prices of fossils and electricity and the emission of CO2 in conventional drying technique, green energy saving and other heat recovery techniques for processing and drying of produce become very important. Heat pump technology has been successfully utilized for drying agricultural products as well as for other domestic dehumidification/heating applications. It has been utilized for heating, ventilation, and air-conditioning in domestic and industrial sectors in most developed countries of the world. However, heat pump drying (HPD) of fruits and vegetables has been mostly unexploited.
Heat pump drying has the capacity to recover the latent and sensible heat by condensing moisture from the drying air which may other drying methods cannot do. The recovered heat is recycled back to the dryer through heating of the dehumidified drying air; hence the energy effectiveness is increased substantially as a result of heat recovery which otherwise is lost in the atmosphere in conventional dryers. This enables drying at lower temperatures, lower cost, and operation even under humid ambient conditions.
At the final stage of drying, there will be little difference of the moisture ratios at the inlet and outlet of the drying chamber. The corresponding temperature divergence will also be minimal and these will result in ineffective drying and low thermal productivity. However, with heat pump drying, there is control of the moisture and temperature of the air as well as heat recovery. In this way, heat pump dryer can enhance the product quality while utilizing less energy.
There are many achievable ways of applying heat pump drying. These possibilities contain varying the following: mode of operation, HPD cycle, drying media, supplementary heating, and heat pump dryer operation, number of heat pump stages, and temperature for drying. One improvement that heat pump has over other heat sources for drying is that it can be applied to any kind of dryers. Any dryer that utilizes convection as the primary mode of heat input can be fitted with an appropriately designed heat pump, but dryers that need huge amounts of drying air, for example, flash or spray dryers, are not suited for HP operation. Heat pump drying technology has been mixed with other drying techniques to overcome some problems encountered in those techniques and to obtain enhanced product quality, lessen energy consumption, high coefficient of performance, and high thermal productivity.
Examples of heat pump assisted drying contain heat pump assisted solar drying, microwave drying, infrared drying, fluidized bed drying, atmospheric freeze drying, radiofrequency drying, and chemical heat pump assisted drying. This is in specific with heat sensitive materials like fruits and vegetables that require only low temperature. For example, combining HPD with solar drying enhances the drying and reduces cost. A heat pump is attractive because it can deliver more energy as heat than the electrical energy it consumes. Also it can utilize modified atmospheres to dry sensitive materials like fruits and vegetables. Moreover, the number of stages of heat pump in the dryer and other arrangements can be differ to enhance the performance of the dryer. In addition, chemical heat pump dyer has the advantage of being designed for continuous operation which allows for stable optimum operating conditions.
The quality elements of heat pump dried products are classified below.
- Microbial Safety
- Ascorbic Acid Content (AA), Volatile Compound, and Active Ingredients Retention
- Aroma and Flavor Loss
Factors affecting the drying rate will differ somewhat depending upon the type of drying system utilized.
- Nature of the material: physical and chemical composition, moisture content, and so forth;
- Size, shape, and arrangement of the pieces to be dried;
- Wet-bulb depression or relative humidity or partial pressure of water vapor in the air
- Air temperature;
- Air velocity
In general, the drying rate lessens with moisture content, extends with escalation in air temperature or lessens with escalation in air humidity. At very low air flows, extending the velocity causes faster drying, but at greater velocities the effect is minute indicating that moisture diffusion within the grain is the controlling mechanism.
Heat pump dryers are promising technologies that keep product quality and lessen energy consumption of drying, particularly for high value products like fruits and vegetables. The application of heat pump drying contributes positively to the following fruit and vegetables quality elements including enhanced microbial safety, better color, vitamin C retention, enhanced volatile compound, aroma and flavor compounds, rehydration, and texture. Finally, some aspects that can make heat pump drying cost successful and energy systematic were explained. Adoption of heat pump drying technology for drying of fruits and vegetables will enhance product quality and lessen energy absorbed in the process.
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