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Development of Heat Pump Dryer for Heat Sensitive Crops

M A Hossain
FMPE, BARI

N Jahan
FMPE, BARI

M N Amin
FMPE, BARI

M S Hassan
FMPE, BARI

Abstract/ Executive Summary:

A heat pump was designed and fabricated in Farm Machinery and Postharvest Process Engineering Division, Bangladesh Agricultural Research Institute, Gazipur during 2013-14. The heat pump consisted of rotary compressor (2.75 kW), coil type condenser, expansion valve (electronic control) and hydrophilic type evaporator. The average temperature difference between outlet and inlet of heat pump was 10.39 ^oC. At heat pump outlet temperature of 40 ^oC, the outlet relative humidity was 53% that was 10% less than the ambient relative humidity. The average relative humidity difference between outlet and inlet of heat pump was 13%. Air velocity and airflow rate increased with the increase of air temperature of heat pump. Therefore, the heat pump was found suitable for drying heat sensitive crops at lower temperature.

Keywords: Compressor, Condenser, Pump, Dryer

Location: Farm Machinery and Postharvest Process Engineering Division, Bangladesh Agricultural Research Institute, Gazipur

Start Date: 00-00-2013

End Date: 00-00-2014

Program Area: Farm Mechanization

Commodity: Pump

Objectives:

Fabrication of a heat pump dryer suitable for heat sensitive crops
Performance evaluation of a heat pump dryer for heat sensitive crops.

Methodology:

The heat pump dryer was consisted of evaporator, compressor, condenser, expansion valve and drying unit.The heat pump dryer was a combination of two sub-systems: a heat pump and a dryer. In a heat pump system, the working fluid (refrigerant) at low pressure was vaporized in the evaporator by heat drawn from the dryer exhaust air (4-1). The compressor raises the enthalpy of the working fluid of the heat pump and discharges it as superheated vapour at high-pressure (1-2). Heat was removed from the working fluid and returned to the process air at the condenser (2-3). The working fluid was then throttled to the low-pressure line (using an expansion valve) and enters the evaporator to complete the cycle (3-4). The inlet drying air passes through the drying chamber at point 5 and picks up moisture from the product. The moisture carrying air at point 6 was then directed to the evaporator coil. During the dehumidification process from point 6 to 7, the air was first cooled sensibly to its dew point. Further cooling results in water being condensed and dehumidified from the air. Latent heat of vaporization was then absorbed by the evaporator for boiling of the refrigerant. The recovered heat was pumped to the condenser point 7. The air leaving the evaporator was heated in the condenser and then passed to the dryer for drying the product. Farm Machinery and Postharvest Process Engineering Division, Bangladesh Agricultural Research Institute, Gazipur during 2013-14. The power of the heat pump was 2.75 kW with rotary type compressor. The evaporator was hydrophilic type and condenser was coil type. The expansion valve was electronic control. The connected pipe was 25 mm diameter, copper made and insulated with soft rubber. The heat pump was operated with single phase alternating current. The refrigerant was R417A. The dryer part was designed but could not be fabricated due to limitation of time. The heat pump was tested in the laboratory in the month of May 2014. The entry and exit temperature and relative humidity were recorded with a digital hygrometer. The air velocity and airflow rate were measured by an anemometer (TA 430, Airflow, England). The drying unit of heat pump dryer was designed and drawn in Auto CAD software.

Source of Information: Annual Research Report, FMPE, BARI--2013-14

Article Link (Online Journal):

Funding Source:

1.

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Inlet and outlet temperature and relative humidity were measured to observe the efficacy of heat pump. The main function of heat pump was to reduce the air humidity so that it can evaporate more moisture from the product that enhances drying even at low temperature. Here inlet condition means the ambient air condition and outlet condition refers to the air leaving from the condenser i.e. before entering into the dryer. It was observed from the figure that the outlet temperature of heat pump was higher than inlet temperature. The average temperature difference between outlet and inlet of heat pump was 10.39 ^oC. The outlet relative humidity of heat was much higher than that of ambient relative humidity at all the times. At heat pump outlet temperature of 40 ^oC, the outlet relative humidity was 53% that was 10% less than the ambient relative humidity. The average relative humidity difference between outlet and inlet of heat pump was 13%. Therefore, the heat pump was found suitable for drying any product especially heat sensitive crops that to be dried at lower temperature (40 ^oC). Both the air velocity and airflow rate increased with the increase of air temperature of heat pump. The reason was that to increase the air temperature at the outlet of the condenser, more liquid flow as well as airflow were required. This was just opposite to air cooler, to reduce air temperature, it must increase the liquid flow as well as air flow from the air cooler. The overall, dimensions of the dryer was 2 ´ 1.72 ´1.35 m. The numbers of shelves were six and each of the self was divided into two trays. The average temperature difference between outlet and inlet of heat pump was 10.39 ^oC. At heat pump outlet temperature of 40 ^oC, the outlet relative humidity was 53% that was 10% less than the ambient relative humidity. The average relative humidity difference between outlet and inlet of heat pump was 13%. Air velocity and airflow rate increased with the increase of air temperature of heat pump. Therefore, the heat pump was found suitable for drying any product especially heat sensitive crops that to be dried at lower temperature.

Knowledge Management: Report/Proceedings