Studies

ÉTUDES DE CAS

CASE STUDY(A

COFFEE DRYERS (Colombia)

Overview:

Status: operational Heating mode: natural

Type: tray-dryers and Air circulation: natural air flow partial-chamber dryers

A number of natural and solar dryers, employing natural sundrying techniques, have been used for small and medium scale drying operations on Colombian farms for many years. The types of dryers described are utilized throughout the coffee producing regions of Colombia. Maize, beans and cocoa are also dried in these units. Their performance is studied with respect to the drying of coffee.

Bamboo Direct Ground Contact
Figure 1 - The picture shows a "pasera" supported by a bamboo frame to avoid direct contact with ground

Description of the Different Types of Dryers Employed;

The dryers described here are basically very simple to operate, insulation is not used in any of the systems described. All of these dryers work . solely with natural air convection. All the construction materials are locally available. Depending on the farmer's requirements, the dryers have different shapes and dimensions.

The photographs show typical examples of the various types of dryers described below. The names in quotation marks are the local name for each type of dryer.

A. "Paseras" (Fig.l). For the construction of the trays or "paseras", wood is used. During the drying process, the coffee beans are directly exposed to solar radiation, and during rainy periods and at night time they are stored under a cover.

B. Coffee drying carts "carras para secor cafe" (Fig.2a,2b) are again made out of wood, iron wheels fixed to the carts' frames allow them to be moved easily on wood, cement or iron tracks. The roof of the shelter is made of corrugated metal sheets and serves to protect the drying coffee beans against rain and dew.

C. Concrete courts "patios de cementos" (Fig.3,4). The concrete courts are cast 10 cm thick and have a slight slope to drain off the rain water. Fig.3 illustrates how the product can be pushed under the small roof for protection against unfavourable weather, in Fig.4, the principle of operation is the same as in Fig.3, except that a mobile roof can be set over the drying product during the night or in case of bad weather conditions.

D. Marquees of polyethylene and glass "marquesinas de polietileno y vidrio" (Fig.5,6). Fig.5 s hows a transparent cover set permanently over the drying produce spread on a wooden surface which is elevated above the ground. Fig.6 represents a variation of the marquee dryer of Fig.5. The structure is somewhat different, having all the sides open to ambient air and a floor made of concrete.

E. "Elba" house dryer "secadero Elba" (Fig.7) . This is another type of dryer extensively used throughout Colombia. The Elba dryer is usually built on top of a flat roof house or storage shed. It has a concrete or wooden floor and concrete walls. The main feature is its hinged roof made of corrugated metal sheet that can be opened for drying and shut during night and rainy periods.

From the description above, the dryers can be classified as (a) tray type dryers and (b) terrace type dryers. "A" and "B" belong to the first group and "C", "D" and "E" belong to the second group.

Location :

These dryers are spread out over the coffee growing zone of Colombia. This region is located between latitudes 1°N and 11°N and meridian 72°W and 780w, with a height above sea level between 1 200 and 1 800 meters.

Climatological data; Chinchina, Caldas, Colombia

Latitude 75036'N

Longitude 4°53'W

Altitude 1360 m

Annual Precipitation 2 510,4 mm

Rainy Days/Annum 237

Relative Humidity 75%

Maximum Temperature 30,3°C

Minimum Temperature 14,1°C

Average Ambient Temperature 20,6°C

Observation Period 1941 - 1970

Period of operation:

For many years now, in the coffee producing regions, small and medium production farms have used these types of dryers. CENICAFE does not have information about when the dryers began to be used.

The dryers are used during the harvest period, i.e. from March to May and from September to December, although in some regions they are used all year long at the same time that the crop is being harvested.

At CENICAFE the dryers are tested in field operations and also on an experimental basis.

Drying data;

The dryers are mainly used to dry coffee but they are also used to dry other crops such as maize, beans and cocoa.

Operating procedure:

The procedure outlined here is typical for the drying of parchment coffee. Normally, the beans are placed in the dryer or on the drying surface of the dryer with a moisture content, wet basis, of 50 to 52 percent. These coffee beans also contain some superficial water after washing. Some water is naturally drained off and some is evaporated by solar radiation; the drying process takes place until the moisture content.of the beans,wet basis,reachfes 11 percent.

The coffee beans are stirred many times, with hand-operated wooden tools, during the drying process. The loading density in the dryer is normally 24 kg of freshly harvested coffee per square meter. The weight of the final product is 12,5 kg per square meter of dry parchment coffee with 11 percent moisture content, wet basis. The drying time is on the average 8 to 9 days. There are about 100 daylight hours during this period, with about 40 sunny hours.

Drying time:

To study the drying time of coffee in the drying trays ("carros para se-cor cafe") (Fig.2), a test was performed using different quantities of coffee spread on the trays. The results are shown in Table 1.

TABLE 1; Drying Time at Different Layer Thicknesses on Trays

Test number

Thickness of coffee beans on the trays

Tray Loading Density Number of

(kg/m2 of dry coffee at 11 percent moisture content, wet basis)

3,125 6,250 12,500 18,750

Days for Drying

Daylight Hours during Drying Period

(hours)

33,37 48,85 101,45 145,33

Sunshine

Hours during

Drying

Period

(hours)

16,5

25,9

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