Plastic sheet

Plastic sheet

Assembly-side with blind attached

( all internal surfaces of frome and mot finished matt blacK paint)


Figure 31 Assembled drier - Section 3-3 end elevation

Assembly-side with blind attached

( all internal surfaces of frome and mot finished matt blacK paint)


Figure 31 Assembled drier - Section 3-3 end elevation




Status: experimental Operating mode: direct

Type: chamber-tray dryer Air circulation: fan forced convection

The dryer presented here is a direct solar dryer used and tested on an experimental basis. The main conclusions show the improvement that can bring on heat utilization and drying time, the use of a small controlled air flow passing through the depth of the grain during its travel across the tray-dryer box.

Figure 1. This Picture shows the Experimental Set-Up. On the Left, the Downdraught Dryer and on the Right, the Crossdraught Dryer.


This dryer, the downdraught dryer, was used only experimentally for comparison tests against a similar dryer with its inlet and outlet above the level of the grain surface (crossdraught dryer) . Their performance was compared to an Allgate dryer in which material is placed on a plastic sheet directly on the ground.

The dryer (see figure 2) comprises of a sloping tray at about 20o to the horizontal. The tray is fitted with a false floor of perforated metal on which the grain to be dried is laid. The tray is divided into four equal compartments by wooden partitions. Both its ends are blocked, apart from small apertures above the level of the grain surface at the lower end and a single hole at the top end below the perforated metal through which air is extracted by a small exhaust fan (0.25 m3/min). Air entering the bottom just below the transparent cover is drawn through the grain supported on the perforated sheet at a rate determined by the fan damper. This rate is indicated by a float-type airflow meter. The unit is currently not in existence.

Dimensions: Overall dimensions of the dryer are 61 cm x 122 cm.

Materials of construction» Drying Chanter

Transparent cover: cellulose acetate on softwood frame Frames: hardwood body insulation: none used (bottom of trays are damp resistant hard board)

Trays: perforated steel sheet

The drying trays were originally painted black so that in a unit without a fan, the black surface of any unfilled compartment would induce convection of air. This precaution was unnecessary as trays were always covered with grain in the final version and the airflow was regulated by a fan.

Location t National institute of Agricultural Engineering (NIAE) Wrest Park, Silsoe Bedfordshire, England

Climatological Data;

Reported at time of experiments only. Sunshine hours: about 10 per day Shade temperature: 18 to 21°C Relative humidity: 40 to 60%

Practical Operation:

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

Periods of Operation:

(a) On experimental basis: September 1963 without fans; August,

September 1964 (down and crossdraught dryers) with fans

(b) in field operations: Nil

Drying Data : Barley and wheat:

Quantities dried: Drying times:

no specific treatment was used. Germination should not have been affected at temperatures used. The temperatures inside the drying space varied between 32° and 50°C.

from 13 to 30 Kg per tray, corresponding to depths of about 2,5 to 5 cm.

a 23 Kg batch was reduced from 21% to 15% moisture content (w.b.) in about 9 hours.

Operating Conditions :

During experiments: mainly sunny, light wind, light clouds insolation: 0,4 to 0,58 Kwhr/(m2.hr)

Economic Details:

Not known; the experimental unit was for research only and much more costly than a practical dryer of this type would be. The economics would have to be assessed in light of the country in which the dryer would be used as it can be built from locally available materials.

Estimated Life of the Dryer:

Several years, but cellulose acetate sheet would have to be renewed bi-annually.

Comments on the Dryer: . Construction is basically simple.

. Especially useful for the drying needs of a few people. . The results of the series of tests §ive clear indication that heat utilization in a covered solar dryer with uncontrolled air flow is only of the same order as when material is laid on the ground to dry

The heat utilization is considerably improved when air flow is controlled. and directed downwards through the material to be dried (downdraught dryer) by means of a small fan. The downdraught dryer presented here utilizes the solar heat three (3) times more effectively than a sun drying process does and the unit is at least 60% more efficient than the crossdraught system. . The fan could be electric, wind or hand powered.

. A solar air heater could be added for generating a thermally induced draught for application where the use of even a small fan is unpractical.

Principal investigator(s): Bailey, p.H.

Williamson, W.F.

National Institute of Agricultural Engineering

Scottish Station

Bush Estate,Penicuik,Midlothian

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