Home >> Buyingguide >> Cassava Drying

Cassava Drying

"......"

The most suitable types of equipment available for drying cassava is a pneumatic dryer, also known as flash dryer. Flash dryers are used to process cassava in many tropical countries.

The high evaporation rate, and the consequent short drying time, provided by this type of dryer, allows using elevated air temperatures, and achieving superior levels of energy efficiency, without overheating the material or jeopardising product quality.

1. How to measure performance

The performance of a dryer depends on how efficiently it can dry the input material. This is measured by specific energy consumption.

Specific energy consumption is defined as the heat input to the dryer per unit mass of water evaporated, and is normally expressed in megajoules per kilogram of water (MJ/kgwater).

Heat input to the dryer can be measured in different ways. Most commonly it is measured as the amount of fuel needed to dry a specific amount of material.

When comparing dryers make sure the conditions at which the tests were performed are very close, especially the starting and finishing moisture of the cassava grits.

2. Parts of a flash dryer

The main components of a pneumatic dryer are: burner, heat exchanger, feeder, blower, drying duct and cyclone separator.

Burner

Burners are devices that burn fuel to produce heat. The heat generated by the combustion is used to warm air via a heat exchanger. For small-scale cassava drying, diesel is the most common fuel type used. In flash dryers, the burner should be thermostat-controlled, to allow adjusting the drying air temperature.

Check before buying

  • Do not buy a flash dryer if the burner is not thermostat-controlled

Heat exchanger

Heat exchangers are devices that transfer heat from the combustion to the drying air. Because the gases from combustion are poisonous they need to be separated from the drying air by solid walls that prevent mixing. The efficiency of heat exchangers depends largely on the surface area between the two gasses.

Several types of heat exchangers are used in food processing, but the most common types are the double-pipe and the shell-and-tube heat exchangers.

Double pipe heat exchanger

Double-pipe heat exchangers are the simplest type and consist of one pipe inside another pipe. They have limited heat transfer surface and consequently lower efficiency.

Shell and tube heat exchanger

Shell-and-tube heat exchangers consist of a bundle of parallel tubes enclosed in a shell. This configuration allows a wide heat transfer surface and consequently higher efficiency.

Check before buying

  • Choose a dryer with a shell-and-tube heat exchanger instead of one with a double-pipe heat exchanger

Feeder

Feeders are devices that introduce materials into the dryer at a controlled and specified, rate. In flash dryers it is important that the feeder promotes good dispersion of the material in the airstream. Wet cassava grits are very sticky and can easily turn into lumps. For this reason, screw feeders are not recommended. Instead, a rotary valve feeder, also known as a star feeder, should be used.

Check before buying

  • Do not buy a dryer that has a screw feeder.
  • Do not buy a dryer if the walls of the feeder’s hopper have an angle less steep than 75°.
  • Do not buy a dryer if the parts of the feeder that come in direct contact with the cassava grits are not made of food-grade stainless steel.

Fan

In flash drying, centrifugal fans are used to induce the air. They are usually driven by an electric motor using belts for power transmission. The fan can be installed at the beginning of the drying duct (positive-pressure conveying) or after the cyclone separator (negative-pressure conveying).

There are three main impeller designs available for centrifugal fans: forward-curved, backward-curved and straight-blade. Straight-blade impellers are more robust, can support high-temperature and can tolerate particulate material in the airstream. For this reason, in dryers that are positive-pressured, it is important that the blower uses a straight-blade impeller.

Check before buying

  • In a dryer that uses positive-pressure conveying system, do not buy it if the impeller of the centrifugal fan is not the straight-blade type.
  • Do not buy a dryer if the parts of the fan that come in direct contact with the cassava grits are not made of food-grade stainless steel.
  • Do not buy a dryer if the belts drive components are not enclosed under a safety guard.

Drying duct

The drying duct is where the wet solid is dried. The heated airstream is responsible for both transporting and drying the solid. It is important that the drying duct has enough length to provide the time needed to achieve the desired moisture content. A drying duct of incorrect length results on low energy efficiency.

Check before buying

  • Do not buy a pneumatic dryer, if the length of the drying duct diverges significantly from the length calculated using the cassavatech.com drying-duct-designer.
  • Do not buy a pneumatic dryer if the drying duct is not thermally insulated.
  • Do not buy a pneumatic dryer if the parts of the drying duct that come in direct contact with the cassava grits are not made of food-grade stainless steel.

Cyclones

Cyclones are devices that separate the material from the drying air using centrifugal sedimentation. The dried grits are collected at the bottom of the cyclone and the air exits from the top. This air may still contain small-sized particles and a second cyclone can help to separate them further. Two cyclones in series are most common, but sometimes three cyclones in series are used.

Check before buying

  • Do not buy a pneumatic dryer that uses a cyclone that does not follow one of the standard designs.
  • Avoid buying a dryer that uses cyclones in parallel.
  • Do not buy a dryer if the parts of the cyclone that come in direct contact with the cassava grits are not made of food-grade stainless steel.

Learn more