Computer modelled heat recovery could slash processors' energy bills

The Commonwealth Scientific and Industrial Research Organisation (CSIRO), in collaboration with Horticulture Australia Limited and the Australian Prune Industry Association, determined that drying prunes was much more efficient, by up to 50 per cent, if processors recycled exhaust heat from dehydrating tunnels and used solar power.

Lead researcher Dr Henry Sabarez, who is based at CSIRO,​ said that​the Australian prune industry realised a couple of years ago that it needed to improve cost efficiency in the dehydration process to remain competitive and to mitigate the environmental impacts of production.

Drying costs ​

In Australia, the present cost of prune dehydration accounts for a significant portion of the total production costs depending on factors such as dehydrator designs or fuel type.

Sabarez explained that this factor coupled with “considerable competition in the local market from products imported from countries with access to low labour and fuel costs together with the rising consumer demand for a high quality product,”​ triggered the commissioning of the research.

He said that the team set out to establish the critical operating conditions and key design features that minimise energy use and maximise throughput in the drying of prunes:

“This involved undertaking a detailed assessment at the industrial scale of the drying performance and operating costs of existing dehydrator designs,”​ said Sabarez.

Process kinetics ​

Specialised monitoring sensors were used, he said, when testing the dehydrators to gain a more thorough understanding of the systematic variations of the process conditions including temperature, airflow and humidity.

Sabarez said that the data generated by the sensors was then combined with results from laboratory tests investigating the drying kinetics of prunes under controlled drying variables, which was underpinned by a computer modelling method using COMSOL Multiphysics software.

“The computational modelling described the physics involved in the drying process such as heat, mass and momentum transfer and this information was used to subsequently identify major design features and optimal levels of operating conditions for efficient dehydration,”​ continued the lead researcher.

Heat recovery ​

The team developed an integrated heat recovery system to effectively recover and re-use the heat from waste exhaust air utilising automated air recirculation control in conjunction with a heat exchanger, said Sabarez.

He told this publication that: “better understanding and control of the right amount of moisture-laden exhaust hot air being re-circulated through experimental and computer modelling studies are crucial to allow recouping of the heat without significantly affecting the drying process.”​

In addition, said Sabarez, an automated solar-based heating system with improved heat storage capability, using phase change materials, was incorporated to provide supplementary heat to pre-heat the incoming fresh air prior to its heating by a gas burner to the desired level of temperature.

“Both the heat recovery and solar-based heating systems were designed so that they could easily be retrofitted into the existing dehydrators,” ​he added.

Sabarez stresses that energy and throughput gains can be achieved in a wide range of other food processing sectors through their combined approach of identifying and utilising optimal levels of operating conditions and by incorporating heat recovery and solar-based heating systems to provide supplementary heat in a more efficient way.