Drying: Physical and Structural Changes

Heat pump drying

The heat-pump-drying method is an implementation of a heat pump to recover the evaporation heat energy in the convective-drying method. Drying air is circulated throughout the system, consisting of an evaporator and condenser sections to remove the MC effectively from the herb product at a relatively low temperature.

The difference between convective drying and heat pump drying is that in heat pump drying an active system with a heat pump is used to blow the hot air directly onto or pass the surface of the herb.In heat pump drying, the focus is on the energy efficiency, surface load of herb, drying air velocity, and temperature.

Although HPD is not a very new technology,

There has been continuous improvement in the design and application of HPD technologies for food drying. HPD consists of a refrigeration cycle to recover heat from dryer exhaust air and reheat it to the required temperature before it enters back into the dryer. Detailed discussion on HPD and its operation can be found elsewhere. The HPD system was traditionally used for the recovery of heat; however, HPD is used in food drying to improve product quality by utilizing some of the other advantages it offers.

The primary advantages of employing the HPD system include providing a wide range of operating conditions (humidity and temperature), possibility of using inert gas as drying medium, and combining with other heat modes. For heat-sensitive food products with long drying times, HPD is very useful as drying can be carried out using relatively lower temperatures in combination with dehumidified air. As presented in several published studies, the use of inert gas has resulted in better physical properties of the dried material. A closed-loop HPD system can substantially reduce the operating cost, as the same inert gas can be recycled, with some top-up as necessary. Some of the applications of inert gas food drying include drying of apple, ginger, guava, papaya, lecithin, etc.

The most common HDP system used is mechanical compression with low coefficient of performance. Some research has focused on improving process efficiency, including multistage heat pumps, chemical heat pumps, or use of heated pipes. The chemical heat pump has good commercial potential but needs thorough research.This is useful during the final stages of drying in order to minimize possible damage to the physical structure of the foods. Solar-assisted HPD is extremely useful for agricultural products.