During the test period the sunshine hours were recorded with a Heliograph and the solar radiation with an Actinometer. Measurements were taken to find out the efficiency of the drying process. The results are shown in Table 2.

TABLE 2: Efficiency of the Drying Process with Different Layer Thicknesses of coffee Beans

Test number

Thickness of Layer of Coffee beans

Moisture R .oved f om Cof-- se beans Kg/m2


Required to Remove water (kwhr/m?)

Total Solar Heat Received



At the same time that the above tests were performed, the effect of different types of floors or trays was studied. Four types of floors were chosen and installed on a structure 80 cm high and covered with stabilized polyethylene. The materials tested were: (1) screening; (2) bamboo matting; (3) wood and (4) concrete.

A layer of coffee of thickness 33,3 mm was used (12,5 kg of dry parchment cof-fee/m2). The initial moisture content was 50 to 52 percent wet basis and the final moisture content was 11 percent wet basis.

TABLE 3: Drying Coffee under a Polyethylene Covered Roof on _4 Different Drying Surfaces

Surface Moisture Tested Removed from Coffee beans

Number Daylight of Days Hours for during Drying Drying Period (d?ys) (hours)


Hours during




Heat required to remove water

Votai Solar Heat




Screen 10,862 11,44 137,33 49,75 7,3 47,8 15,27

Matting 10,925 11,67 140,06 50,78 7,35 48,6 15,12

Concrete 11,100 12,50 149,95 54,98 7,45 52,3 14,30

Note: The final weight of dried coffee in each one is 12,5 kg/m2 (33,3 mm thick)

It should be pointed out that the outside solar radiation received was recorded by means of an instrument located at the meteorological station of CENICAFE. The radiation transmitted through the polyethylene sheeting was not measured.

Comparing test C/3 of Table 2 (33,3 mm layer thickness) which refers to tests without plastic cover, and any one of the tests of Table 3, it was found that there exists a significant difference between the reported efficiencies. This difference is no doubt due to the polyethylene sheeting energy losses.

The average temperature of the coffee during the tests was 35°C and this figure was used to calculate the energy needed to evaporate the water, assuming .a Intent, heat of vaporization of 0,67 kwhr.

Economic Details;

(1) The costs of the dryers vary depending on the size and regions. On the average, a tray "pasero", Figure 1, is about 2,40 m long x 0,80 m wide and 0,15 m high, and its cost varies between $2.40 to $4.00 U.S.* As the surface of the tray is approximately 1,9 square meters, the cost per square meter of tray area is $1.25 to $2.10 U.S.

The cost of the trays (Figure 2) is approximately $280.00 U.S. and the metal frame end $200.00 U.S. As the roof and floor surfaces total in area 50 m2, the cost per square meter is then $9.60/m2 U.S.

The cost of a concrete court (Figure 3) can vary between $2.00 U.S. to $3.00/m2, without including the roofs to cover the coffee during the night and rainy hours.

A polyethylene marquee costs about $2.00/m2 U.S. when made in the simplest way and with the most economical materials. It has an area of 45 m2, (9 x 5 m). Using better construction techniques and superior materials, the price will increase. The best ones are made with glass and cost about $12.00/m2, with metal frame and concrete floor.

The "Elba" house type is a unit that adapts the drying roof and its cover to a construction type which serves as a house or a storage area. In this case, the cost varies with the type of construction.

(2) The dryer is used during the harvest period, from March to May and from September to December, i.e. 7 months per year.

(3) It is difficult to evaluate the cost of drying per unit of dried materials because it depends on the cost of the unit, the number of workers and the quantity of material dried.

(4) It is very difficult to estimate the useful life of the dryer but it has been found that the trays "paseras" and carts have a life of 5 to 10 years and the larger dryers such as the "Elba" houses can last 20 years or more depending on the maintenance provided.

(5) There are no particular technical problems reportedly associated with the construction and operation of the types of dryer herein described. Economic considerations and regional requirements will determine the type of dryer to be vsed.

Comments on the Dryer;

The described models are those used by the farmers and tests were performed to determine their efficiency. The Colombian coffee farmer has traditionally used a similar method, drying coffee on trays and especially on wooden floors.

Farmers with less economic means usually dry coffee on concrete or wooden terraces . A farmer with medium resources normally usas plastic or glass transparent covers to protect the beans from the rain.

The sun drying of coffee is widely practised in Colombia. Approximately 70 percent of the yearly national production, i.e. 400,000 tons of dry parchment coffee, is dried by means of solar energy.

The sun drying techniques are well suited to the small and medium scale farm holdings in Colombia. It has been reported that there are two limiting factors for the use of sun dryers for coffee processing in Colombia. The first is that for production larger than 12 tons per year, the drying surfaces required become quite .expensive. The second is the weather. The rainy season coincides with the harvest period. Often this tends to lengthen the sun drying process.

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