Heat balance of a multistage spray-dryer: principles and example of application

摘要

The authors present the principles of isenthalpic air drying, the use in drying of the enthalpic diagram of air and mass and enthalpy balances. As an application of these principles, a pilot multistage spray-dryer, operating on skim milk, was experimentally studied. Air flow rates were measured by a hot wire anemometer at different positions of the air pipes. It was checked that the mass flow rate of air delivered by a fan was independent of air temperature. A thermohygrometer was used to measure the temperature and relative humidity of the air. The mass flow rate of concentrated skimmilk was estimated from the characteristics of the spraying nozzle. Introduction of air into the internal and external fluid beds was taken into account. From these data, the heat balance showed a difference between inputs and outputs of 2.9% which may be interpreted as heat losses and probable errors. The specific heat consumption was close to 4 900 kJ.kg(-1) of evaporated water. This overall value includes the rather low energetic efficiency of the small flow rates of air used in both fluid beds. This specific heat consumption corresponds to 2.1 times the latent heat of vaporisation of water at 70degreesC, i.e. a thermal efficiency of 48.8%. These figures represent a use of energy less efficient than in industrial dryers, which may be explained by the small size of the plant, operating at only 122 kg of concentrate and 70 kg of evaporated water per hour.