Air distribution system

The purpose of the air distribution system is to deliver the drying air to the drying zone in the dryer and to remove the moisture that was extracted from the grains. In suction systems they also collect the dust that is created after the air leaves the drying section. For fixed bed dryers usually positive pressure systems are used to blow the air through the grain bulk while re-circulating and continuous-flow dryers usually have negative pressure (suction) based air distribution systems.

Comparison of pressure and suction based air distribution systems. 

  Pressure system Suction system
Type of dryers Fixed bed batch Dryers with moving grain, re-circulating batch and continuous flow
Air tightness of bin Fixed batch can be made airtight easily, large outlet Moving mechanical parts make sealing difficult
Heater Before fan Before dryer inlet
Fan High temperature resistance needed, sometimes exposed to flames Lower temperature resistance
Dust Stays mainly in fixed bed, set free when unloading Sucked out with the drying air

Suction systems have the big advantage of collecting all the air that exits the dryer and thus also collecting the dust which can then be easily separated e.g. in a cyclone. This is getting more important with tighter emission control requirements. There are re-circulating dryers with pressure systems, especially when they have circular bins with radial airflow from inside-out because in this case the air outlet is much larger than the inlet. But these kind of dryers excessively release dust into the environment during operation.

Major elements of the air distribution system are a plenum chamber, air channels, and air ducts or false floors.

Plenum chamber

The plenum chamber is a chamber into which a fan delivers the drying air before it enters the grain bulk. The purpose of the plenum chamber is to let the air calm down before it enters the air distribution system in order to guarantee an equal distribution of pressure and temperature of the drying air throughout the drying section. The contribution of a properly designed plenum chamber to even drying and thus to producing good quality is often not known. The bigger the chamber is the more even the airflow will be. Generally speaking metal sheets are cheap and by providing for a sufficient plenum chamber is often a simple and cost effective way to improve drying air distribution.

Air ducts, false floors and air-sweep floors for fixed-bed drying bins

For fixed-bed batch dryers three different air distribution elements are used: air ducts; false floors; and air-sweep floors.

Comparison of air ducts, perforated false floors and air-sweep floors

System Air ducts Perforated false floor Air-sweep floor
Cost Low Medium High(grill shaped metal plus fan)
Air distribution Uneven Optimal Optimal
Requirements Sealed floorAdditional plenum chamber needed Stable support structure needs withstand walking on it Support structureStrong fan for conveying
Constraints Manual unloading Uneven drying at high MC Manual unloading Dust creation

Air ducts are the cheapest solution for distributing the air in the grain bulk. Because of their uneven air distribution at the inlets they should only be used with low initial grain MC, e.g. for low-temperature drying in second stage dryers. Ducts are simple and easy to manufacture and they can be removed for unloading and cleaning the bin.

In batch dryers perforated false floors offer the most equal air distribution, which is important for high initial moisture contents. They are of simple design and are usually made of perforated metal sheet. The compartment underneath serves as plenum chamber. On the downside perforated metal sheets are more expensive than plain metal sheets that are used for air ducts. A strong support structure is needed for perforated metal sheets because during mixing and unloading laborers are often walking on the false floor.

The unloading of fixed-bed batch dryers can be mechanized by using air-sweep floors which, in combination with a strong fan, can convey the grain in the bin to an outlet opening. Air sweep floors are on principle perforated false floors but with grill shaped holes that give the air velocity a horizontal component. At low air speeds, when the floor is used for drying, the horizontal component of the air velocity is negligible. At higher air speeds, however, the air can convey individual grains horizontally. This effect is used for unloading after the initial grain flow by gravity has ended because the angle of repose of paddy was reached.

Cross-section through a perforated metal sheet used in air-sweep floors (left) and arrangement of fife sheets in a drying bin for automatic unloading (right).

For unloading the fan needs to be able to provide an airflow higher than that for drying. An additional fan can also be used. To reduce the capacity of the fan the floor of the drying bin can be equipped with air-sweep channels which can be operated alternately during unloading. In this case the air distribution is still better than with air ducts.

The metal sheets for air sweep floors are more expensive than perforated sheet metal and they might not be available locally. The major drawback of air-sweep floors, however, is the excessive dust creation during unloading because the high velocity air effectively separates all dust contained in the grain bulk and blows it out of the drying bin.

Air ducts, general recommendations

The air distribution system should impose the least possible resistance to air-flow in order to provide sufficient drying air and to transport evaporated water away. For maximized airflow and even air distribution:

  • Provide generous plenum chambers in pressure systems for even air distribution (metal is cheap, fuel and quality loss is more expensive in the long run);
  • At perforated false floors make sure that the open area (area of the holes) is at least 20% of the total area to avoid pressure drop at the perforated metal sheet. Use sheets with holes of around 2 mm to avoid clogging by fine particles and to prevent grains from falling through the holes;
  • Make sure air ducts have sufficient size. It is better to over-design than to use too little diameters since turbulent flows in small diameter ducts waste a lot of energy;
  • Prevent losses at junctions of ducts by using the same diameters;
  • Make round curves instead of sharp edges when the airflow needs to change direction; and
  • For inlets use nozzle shaped fittings instead of straight cut tubes.