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IIR document

Heat pump heat recovery options for dryers in the food industry.

Number: pap. ID: 418

Author(s) : WANG J. F., CLELAND D. J.

Summary

Spray dryers commonly used in the food industry are characterized by quite high inlet air temperatures, no air recycle and minimal heat recovery so the thermal efficiency is often less than 50%. Heat recovery using a combined heat exchanger and heat pump was analyzed for a typical spray dryer with inlet ambient air heated to 200°C and exhaust air with dry bulb of 68°C and dewpoint of 38°C. The heat pump design is a tradeoff between greater heat recovery and lower COP as evaporation temperature decreases. With an evaporation temperature of 25-30°C, it was possible to provide up to about 40% of the air heating load with more than 20% lower energy cost, such that the extra capital cost might be justified. The transcritical cycle, with dehumidification of exhaust air at a constant temperature and inlet air heating in the supercritical region, is thermodynamically well-matched to the drying process. The ideal critical temperature is between 70 and 110°C, so a trade-off between efficiency, cost and safety may be needed with respect to refrigerant choice. Refrigerants R134a looks most promising while R32 and R290 also have good energy efficiency but are flammable. Similarly, the balance between air-to-air heat exchange and heat pump heat recovery will depend on the relative capital expense and energy cost reductions for the two options.

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Pages: 8 p.

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Details

  • Original title: Heat pump heat recovery options for dryers in the food industry.
  • Record ID : 30002956
  • Languages: English
  • Source: Proceedings of the 23rd IIR International Congress of Refrigeration: Prague, Czech Republic, August 21-26, 2011. Overarching theme: Refrigeration for Sustainable Development.
  • Publication date: 2011/08/21

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