Case Study

LARGE SCALE SOLAR AGRICULTURAL DRYER (Barbados)

Overview; Status

experimental

(partly dismantled since 1969) chamber dryer

Heating mode : indirect

Air Circulation: electric fan-forced ventilation

This large scale agricultural dryer was designed to provide livestock with locally produced feed. Zt was intended to be used primarily for the production of dried corn. The operation was done in two steps: initially, the product was partly dried in a mobile drying cart and then transferred to a room where drying was completed to the required moisture content.

Figure 1. Storage Bin Roof Solar Air Heater and Field Air Collector.

Characteristics:

Essentially the dryer was designed to operate in two stages - handling 0,9 m3 (77oKg) of freshly shelled corn per day. The first part consists of a mobile solar air heated drying cart which should reduce the moisture content of the freshly harvested corn from 30% to 182 in the first day of operation. The corn is then transferred to an 18 cubic meter capacity solar air heated storage bin, where the moisture content is gradually reduced to the equilibrium moisture content of 13%. Particular attention was paid to the air flow design so as to cau:

minimum pressure drop through all parts of the air heaters and drying chambers.

A centrifugal fan blows air through a diffuser duct into the 29 meters long solar field air heated collector. The collector is fabricated from three plastic sheets, the top transparent, the centre a black mesh with 50% openings, and the bottom a layer of insulation, sandwiched between two films, the upperside coloured black and the lower side aluminum. The sheets are sealed along the long edges, and supported by tension straps every meter. The centre mesh rests on a rigid screen which is stretched between posts in the field. The collector is inflated on both sides of the tensioned layer and heats the air blown longitudinally through it. The end of the collector is connected to the mobile drying cart. The latter is insulated to reduce heat losses, and fitted with air flow dividers supporting a perforated drying floor. The fresh corn is loaded into the cart, which is covered by a sloping double-layered plastic roof.

When the moisture content has been reduced to the required level, the corn is fed into a blower and transferred to the storage bin dryer, where its moisture is gradually reduced to 13%. This dryer was part of a farm building, one of the rooms being converted into a drying chamber, with a plenum chamber and perforated drying floor. The roof has been used as the solar air heater collector.

Dimensions: See figures Nos. 2 and 3. Materials of Construction:

The plastic sheets, the fans and the insulation were imported.

Location; Bullens Agricultural Station

St. James, Barbados, west Indies

Climatological Data:

335 days per year with more than 4 sunshine hours per day 312 days per year with more than 6 sunshine hours per day

This data was obtained from a nearby meteorological station and summarizes analyses of records for the years of 1961 to 1965.

Practical Operation:

Number of units used in the past: one

Nunber of units currently in use: only the roof collector of the storage bin is used occasionally.

Periods of Operation:

(a) On experimental basis: October 1966 to November 1969

(b) in field operations: idem

Drying Data:

Ear corn was the main product dried. Por the field drying cart, one test reported that the moisture content of 72 Kg of freshly harvested corn was reduced from 24,5% to 14% in 17 hours of drying. Other tests were carried out drying sweet potatoes, copra and grasses. The products were dried in their natural state.

Operating conditions of the Drying System; First step:

Solar Field Air Heater Collector Air Flow Rate in Collector

Effective Air Flow Rate in Mobile Cart Dryer (at full load)

Average Velocity through Mobile Dryer Maximum Solar Air Heater Exit Temperature

During Fair Days at this Flow Rate Average Solar Field Air Heater Efficiency

Second step:

Storage Bin Roof Solar Air Heater Air Flow Rate in Collector

Effective Air Flow Rate in Storage Bin Drying Chamber (at full load)

Average velocity through Bin Dryer Maximum Solar Air Heater Exit Temperature

During Fair Days at this Flow Rate Average Roof Solar Air Heater Efficiency

Economic Details: Cost of materials:

0 0

Post a comment