Case Study

NATURAL CONVECTION DRYER _(Trinidad, West Indies)

Overview:

Status: experimental Type: chanter dryer

Heating mode: mixed

Air circulation: closed cycle natural convection

The main feature of this unique dryer is that it works in a closed cycle by natural convection generated by the mass transfer condensation on the dehuraidifier integrated to the unit, it thus produces both dry cropsand water.

Figure 1. Drying Cabinet Showing Drying Trays. Ripe Bananas are Being Dried in this case.

Characteristics:

The basic principle of operation of this natural convection,air-recirculating unit is as follows:

Air heated in the solar air collector passes into the drying cabinet and removes moisture from the crop. As the air removes the moisture from the crop, it cools and falls in the dehumidifier where it deposits its moisture on the dehumidifier roof. From there, the air re-enters the solar air heater (collector) to be reheated and the cycle starts again. The rectangular air box is sloped at 20° and is covered with .two layers of glass. The collector has a tinned black absorbing surface to maximize the heat transfer to the air stream. The top of the wuilector box is connected to the top of a drying cabinet whose roof acts as a supplementary collector. This cabinet contains five drying trays with wire mesh bottoms. Two apertures connect the bottom of the drying cabinet with the rectangular dehumidifier box which is covered by a layer of glass. Short ducts connect the dehumidifier with the lower end of the flat plate collector.

Dimensions:

Drying cabinet roof:

Dehumidifier box:

Short ducts:

Flat plate collector:

1,22 m X 0,91m (volume 0,5 m3) 1,22 m X 0,91 m x 0,15 m 0,30x0,15 m of cross section 1,83 m x 1,22 m

Materials of Construction: Drying chamber:

Transparent cover: Frames:

Insulation: Trays: Solar Air Heater:

Transparent cover: Absorber: Insulation: Frames:

glass, 2 sheets these were fabricated from rectangular cuts in a 0,457 mm (26 gauge) galvanized mild steel plate using a metal folder and a welding machine

Styrotex

6,4 mm galvanized wire mesh

Glass, 4 sheets iron sheet vermiculite made with 0,457 galvanized steel

Location:

University of West indies St. Augustine, Trinidad West Indies

Latitude : Longitude :

Climatological Data;

Dryers operate during the dry season, approximately January to April,

Sunshine hours 8-10 per day

Ambient temperature 28-32°C

Relative humidity 70-85%

Solar energy received 5,4 to 6,0 Kwhrs/(m2.day)

Practical Operation;

Number of units used in the past: none Nusfoer of units currently in use: one

Periods of Operation:

(a) On experimental basis: January to April 1972

(b) In field operations: none

Drying Data:

Material Quantities (Kg) Drying Times

(sunlight hours)

Yams

0 0

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