Developments of Heat Pump Drying Technology
1 Ingvald Strommen, 2 Odilio Alves-Filho, 2 Trygve Eikevik
1 Department of Refrigeration and Air Conditioning,
The Norwegian University of Science and Technology
2 SINTEF Energy Research, Division of Refrigeration and Air Conditioning
ABSTRACT
Today, heat pumps are used in industrial drying of materials of biological
origin, like food and wood. Heat sensitive materials with improved product
quality can be achieved due to low drying temperatures and independency of
the outdoor air. A properly designed heat pump dryer operates with reduced
energy consumption due to high coefficient of performance and higher dryer
thermal efficiency. The right choice of working fluid in the heat pump and
closed circuit for the drying air imply that environmental impact is
reduced compared to other traditional dryers. In this way heat pumps used
in drying operations satisfy important demands in industrial drying with
respect to product quality control, reduced energy consumption and reduced
environmental impact.
This paper presents an overview on the design of heat pump dryer,
components sizing, and new developments on working fluids. A new CO2
heat pump dryer was recently built and it operates in conventional and
transcritical cycles for flexible drying conditions. Besides being highly
energy efficient it is an environmentally friendly technology with wide
applications and represents a more complete working fluid than refrigerants
on the market today. The selection of temperature and drying conditions are
discussed with respect to capacity and energy aspects. Built recently in
our laboratory, a non-adiabatic heat pump fluidized bed dryer, with
condenser immersed into the bed, presented 400% capacity increase compared
to the adiabatic dryer. The dried product quality will be influenced to a
large extent by the selection of the drying temperature regimes. Different
fruits and vegetables were dried at temperatures below and above the
freezing point of the material. Quality parameters such as hardness,
porosity, density, rehydration and color can be adjusted according to the
drying conditions. A new cold extrusion process to form a porous frozen
matrix in combination with heat pump drying has been designed and tested.
The heat pump drying step is performed with temperatures below and above
the freezing point of the material. The new dried products present low bulk
density and have highly instant properties.
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