Case Study

BATCH SHALLOW BED COFFEE DRYING BINS (Puerto Rico)

Overview;

Status: operational Heating mode: Indirect

Type: chamber dryer Air circulation: electric fan forced convectia

This drying system has been used regularily since 1962 during the coffee harvesting season. The pre-dried product is placed in a shallow drying bin located inside a building. The roof of the building is designed to collect heat for use in the drying process. Some recommendations are made about this type of drying system.

Figure 1. A Diagram Showing How the Roof of a Coffee-Processing Building can be Used as a Solar Heat Collector as well as a Section view Through the Drying Bins Located in the Building.

Figure 1. A Diagram Showing How the Roof of a Coffee-Processing Building can be Used as a Solar Heat Collector as well as a Section view Through the Drying Bins Located in the Building.

Characteristics:

The roof of the coffee processing building is tilted 6,3°C towards the south and acts as a solar collector. The building houses all the equipment needed for processing the coffee from the coffee cherry pulping to the storage of the processed coffee. The drying of the coffee beans is carried out in two bins: the pre-drying bin and the drying bin. Each bin can hold a layer of coffee beans 30 cm thick. Their walls are made of concrete blocks. The bottom of the bins are made of wire mesh supported by expanded metal on steel rods. The pre-drying bin is equipped with an 18 inch axial flow fan operated by a l's HP electric motor. The fan is located beneath the floor of the bin and pulls the air downwards through the coffee bed. The drying bin is provided with four electric heating elements, each rated 1 300 watts for supplemental heating and an 46 cm axial fan operated by a .40 Kw electrical motor.

The solar collector utilizes the whole area of the roof. Plywood ceiling sheeting is fastened on top of 5 cm x 15 cm rafters. Horizontal 5 cm x 5 cm purlins are placed on top of the plywood and galvanized iron sheets roofing nailed onto them. The outside surface of the roof is coated with black asphalt roof paint to enhance absorption of solar radiation. Openings at opposite ends of the roof provide inlets for the ambient air. The heated air is used in the drying bin only. The air is circulated by a fan in the drying bin. This fan draws the air heated by the collector through a duct running down in the center of both the sloping roof and vertical south wall, and pushes it from beneath the drying bin, upwards through the parchment coffee bed. The optimal drying temperature for the coffee beans is 120°F (49°C) and heating elements are used for reaching this temperature in case of unfavourable weather.

Dimension; The overall dimensions of the coffee processing building are: 6, 1 n x 9,1 b of floor area, the south facing wall is 4 m high and the north 4,9 m.

The pre-drying bin and the drying bin each measure 1,7 m x 1,7 m of surface area.

Materials of Construction: Drying Chamber

Transparent cover: none used

Frames: the side walls of the bins were made of concrete blocks

Insulation: the bins are located inside the processing building

Racks: wire mesh supported by expanded metal on steel rods

Solar Air Heater:

Transparent cover: none used, but: a nylon plastic film was tested separetely as a surface for a solar heat collector

Absorber: galvanized iron sheet painted with black asphalt roof paint

Insulation: none used except the plywood ceiling sheeting

Frames: lumber

Forced convection (by means of two fans) provides the driving force during drying.

through the layer of parchment coffee and pushes it out of the building. This downward movement of the air supplements the force of gravity and helps to remove the superficial water from the parchment coffee which becomes skin dry within 2 to 3 hours. The parchment coffee is then transferred to the drying bin where the air - heated by the roof collector - is blown by the dryer/fan upwards through the layer of parchment coffee. The drying is continued in the dryer bin until the product is at the required moisture content.

Economic Details; Cost of Materials:

Equipment $1 356

Materials 965

Labour 500

Total Cost $2 821 (based on 1962 local prices)

The additional cost for incorporating the solar heat collector to the coffee drying facilities is estimated to be about $100.00.

Annual Operating Expenditure:

Additional cost for operating the roof solar heat collector if „»-actically negligible.

Cost of Drying Related to a unit of Material Dried:

Electric energy consumption for all processing operations was approximately 12,5 Kw hour per 45 Kgs of dried coffee produced during the 1963 season when the test data was recorded. Compared to the energy requirements of other installations, the use of solar energy provides a reduction of as much as 66 percent in electricity costs.

Estimated Life of the Dryer: 15 years Comments on the Dryer:

No particular problems were encountered during the construction and operation of the dryer.

After having tested the coffee drying facilities, one can draw some conclusions about coffee drying using this type of dryer.

. A general guide for small and medium farms is to have approximately one square meter of drying area for each 5 000 Kgs of coffee produced yearly. The fans should be able to deliver at least 15 cubic meters per minute per square meter of dryer area against a water gage pressure of 2,5 cm. . The drying bin should contain a layer of coffee beans 30 rm thick when operating full capacity. . The drying air temperature should not exceed 49°C.

. With careful management, much of the coffee can be dried without using supplemental heat.

The additional cost of installing the solar collector was recovered by reductions in operating cost of installing the solar collector was recovered by reductions in operating cost in one season of operation.

The results of this project indicate, nonetheless, that it is not practicable to depend entirely on solar energy for heating the drying air. . Electric heating elements which provide up to 1 100 watts for each 1 000 Kgs processed yearly are recommended during periods with little sunshine as well as during the night time when necessary.

Location : Barrio Anones

Las Marias, Puerto Rico

Latitude: 18°16'N Longitude: 67ow (Mayaguez) Altitude: 244 m above sea level

Climatological Data:

The data presented here was collected during the testing period of September to November 1962 at the location of the dryer. This location has - high rainfall and much cloud cover during this time of the year. A daily pattern of temperature and solar energy, typical for the periods of observation, shows clear sky from sunrise until about 11:00 where clouds begin to form. Rain usually occurs between 12:00 and 14:00, followed by gradual clearing and afternoons with partly cloudy skies.

The average solar energh received daily per square foot of horizontal surface was calculated for three observation periods during the tests:

September 13 to September 30, 1962: 6,12 Kwhr/(m2.day) October 1962: 5,87 Kwhr/m2.day)

November 1962: 5,41 Kwhr/m2.day)

Practical Operation:

Number of units used in the past: only one Number of units currently in use: only one

Periods of Operation:

(a) On experimental basis: September 1962 to November 1962

(b) In field operations: September 1962 to date

The dryer has been field tested throughout the 1962 and 1963 harvest seasons and has been in operation every harvest season since that time.

Drying Data:

Parchment coffee beans were dried. Before being dried, the coffee cherries are first pulped and the pulp discarded to the compost pit. The coffee beans are then put in the fermentation tank for 10 to 12 hours, washed and transferred to the pre-dryer where the superficial water of the product is removed. When the skin is dry (2-3 hours in the pre-drying bin), the beans are transferred into the drying bin where it will reach its final dry state in 24 hours or more depending on the weather conditions. It has been shown that a layer of coffee 30 cm deep can be effectively dried from 55 to 12 percent moisture content (wet basis) within 24 hours, using air temperatures not exceeding 120°F and air flow rates of at least 50 cubic feet per minute per square feet of bin area. The yield was approximately 245 to 360 Kgs of processed coffee per batch. Dvring the 1962 operation, the electric energy consumption amounted to approximatelyIV15 Kw-hour for each Kg of dried coffee. This figure compares advantageously with the electric energy consumption of 0,82 Kw-hr/Kg of dried coffee that was spent when solar energy was not used.

Operating Conditions:

The washed parchment coffee is pumpfed into the pre-dryer and the wash water is drained down through the layer of parchment coffee and collected in a pipe which runs it back to the washing tank. The pre-dryer fan draws unheated air

. However, another way to obtain higher temperatures is to construct the solar heat collector by placing a glass sheet or transparent cover film about two inches above the absorber plate, using the space between for an air passage, a collector of this type would provide a temperature increase of 50%, over the collector described previously.

principal Investigator(s): phillips, A.L.

Rodriguez-Aria's, J.H. Justiniano, Domingo

Agricultural Engineering Department College Station University of Puerto Rico Mayaguez, Puerto Rico 00708

References:

(1) Phillips, A.L., A Solar Energy Method for Reducing Coffee Drying Costs,

The journal of Agriculture of the university of Puerto Rico, Volume XIVII, No. 4, October 1963, pp. 226-235

(2) Phillips, A.L., Drying Coffee with Solar Heated Air, Solar Energy

Volume IX, No. 4, October December 1965, pp. 213-216

(3) Phillips, A.L., Further Observations on the use of Solar Energy for

Reducing Coffee Drying Costs, Journal of Agriculture of the university of Puerto Rico, volume XLIX, April 1965, No 2, p. 272

(4) Phillips, A.L., Research Note: Evaluation of a "Mylar" Plastic.Film as a Surface for a Solar Heat Collector: Journal of Agriculture of the University of Puerto Rico, Volume XLIX, October 1965, -No. 4, pp. 484-486

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