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CSIR-NATIONAL INSTITUTE FOR INTERDISCIPLINARY SCIENCE AND TECHNOLOGY

CSIR- NIISTTHIRUVANANTHAPURAM

For more informa�on: The Director /Head, RPBD, Na�onal Ins�tute for Interdisciplinary Science & Technology, CSIR,

Thiruvananthapuram 695 019 INDIAPhone 0091 471 22515226/ 2515270, Fax 0091 471 249 1712

Refrigeration Adsorption Dehumidified Drier ( RADD) for drying of Vegetables, Heat sensitive materials

and other food products

ADVANTAGES

Website: h�p//www.niist.res.in

1. Since the air is recycled back into the drier room rather than ven�ng it out to the atmosphere, energy is conserved.

2. The energy required for the removal of moisture (latent heat of condensa�on) together with the sensible heat is transferred to the refrigerant. On compressing the refrigerant heat is generated which is used to heat up the same dehumidified air at the condenser side of the refrigera�on system. Subsequently only refrigerant need to provide only the sensible heat for the cooling as well as the energy lost due to radia�on and convec�on.

3. The heat of compression of the refrigera�on system is used for hea�ng the par�ally dehumidified air which require lesser and lesser energy as it gets dried. Therefore, the system is highly energy efficient.

4. There was a 60% reduc�onin total process �mewith 2400% increase in capacity for drying. 12 tons of nutmeg could now be dried in 20 hours as opposed to 500 kg in 48 hours earlier.

5. Since the recycled air gets saturated with the essen�al oil, further loss of essen�al oil from the material being dried does not happen and the product retains its aroma.

c6. The opera�ng temperature is only 60 C compared to the earlier opera�ng

ctemperature of 80 C. Therefore, cracking of the Nutmeg is prevented.

7. The low humidity in the drier room facilitates higher mass transfer rates into the ambient air.

8. The system operates automa�cally and no manpower is required for the firing of fuel or for hot air genera�on.

9. The cost of energy for the drying opera�on is only Rs. 7 per kg which is only 1.2 % of the price of the raw material (Nutmeg)

10. The resultant product from this system has very low Aflatoxin content since it facilitates faster drying.

11. Reduced Aflatoxin Content (less than 5 ppb) has led to opening up of European Markets, thereby aiding higher value realiza�on.

12. A variety of heat sensi�ve materials like Cardamom (elachi), Onion, medicinal plants and leaves could be dried without affec�ng the quality and these were demonstrated.

Refrigeration Adsorption Dehumidified Drier ( RADD) for drying of Vegetables, Heat sensitive materials and other food products

The high Aflatoxin content caused by high humidity led to subsequent rejec�on of these exo�c spices in foreign markets. Exposure to hot air caused essen�al and fixed oils to ooze out of the kernel as well as cracks to appear on the surface. Because of these inherent defects, the produce was not ge�ng a premium price.

The ins�tute undertook the work of moderniza�on and a�er preliminary laboratory study it was decided to go in for Refrigera�on Adsorp�on Dehumidifica�on Drying (RADD). The idea was to lower the humidity in the drying chamber, thereby facilita�ng higher mass transfer rates from the spice ‐ nutmeg , cardamom ( elachi) ‐ for the drying to

Processing of Nutmeg prior to 2011: (Counter Clockwise from Top): 1. Sun Drying of Nutmeg; 2. Firing of Hot Air Generator 3. Mat over which Nutmeg is spread for drying; 4. Drying in Progress

SOLUTION

The RADDSystem

The major opera�on involved in the processing of heat sensi�ve

exo�c spices like cardamom (elachi), saffron, nutmeg etc are the drying

under adverse clima�c condi�ons prevalent during the rainy season, when

it is harvested. Sun drying cannot be resorted to, thereby necessita�ng

drying using fire wood, electricity or furnace oil fired indirect through flow

driers. Addi�onally, the high humidity necessitated drying at elevated

temperatures and higher air flow rates, which led to loss of flavour as well

as development of surface cracks on the spices. Further, the current

prac�ce of sun drying followed by stacking the material in a Drying

Chamber and intermi�ently sending in hot air, without any provision to

either monitor or control the temperature or for uniform distribu�on of air

inside the chamber, lead to long residence �me and thereby improper

drying, resul�ng in poor quality product and development of mold,

Aflatoxins etc. In most cases, since the resultant product is not uniformly

dried, they had to be manually sorted and dried again with the next lot.

Thus, the product obtained was of inferior quality and the processing

opera�on took 48 – 72 hours.

A system as depicted below was designed, fabricated and installed for the drying opera�on, which could address the above listed issues in the drying opera�on.

c Humid air from the drier room, at about 60 C,is passed through a finned tube heat exchanger, where it is par�ally cooled and sent to the evaporator of a refrigera�on system. Heat exchange between the refrigerant and the par�ally cooled, humid air results in its coolingbelow the dew point, as a result of which, the moisture in the air gets condensed as liquid water. The condensed water is removed from the system. While condensing, the latent heat of condensa�on as well as the sensible heat is transferred to the refrigerant. The dehumidified air, in certain cases, is further dehumidified using a silica gel adsorber and then sent to the shell side of the finned tube (FT) heat exchanger, where it exchanges heat with the hot incoming air from the drier room. Meanwhile, the refrigerant coming out of the evaporator is compressed again, and the heat of compression is removed in the condenser by exchanging heat with the air to be recycled to the drier, thus hea�ng the air up.The resultant dehumidified, hot air is pumped back into the drier room for the drying opera�on.