Drying with heat integration

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GENERAL INFORMATION

Drying is defined as the application of heat under controlled conditions, to remove the water present in foods by evaporation to yield solid products. It differs from evaporation, which yields concentrated liquid products.

For storage and preserving of crops, fungi and insects feed on poorly stored crops and reduce crop quality. Relative humidity and temperature affect mold and insect growth which is reduced to a minimum if the crop is kept cooler than 10ºC and if the relative humidity of the air in equilibrium with the stored crop is less than 60%. Many of the enzymes, which promote undesired changes in the chemical composition of the food, are unable to grow, multiply or function in the absence of sufficient water. [1]

Many types of heat exchangers can be used. Thermal wheel (75-90%), plate exchangers (70%), tubular heat exchangers (60%), scrubbers (60%) and two-sections exchangers (50%). [2]


TECHNOLOGY DESCRIPTION, TECHNIQUES AND METHODS

Recovery heat and pre-heating can be achieved in two ways:

  • Heat recovery within the dryer system, between hot and cold stream. This techniques depend on the dyer equipment.
  • Heat recovery using heat exchanger between the dryer and the surrounding process.


As it has commented above, there are many heat exchanger that can be used. Thermal wheels are pretty common for this purpose.


Rotary wheel heat exchanger

Rotary wheel heat exchanger is a circular honeycomb matrix that rotates. The cold airflow passes through on part of the wheel, and the hot airflow passes through the other part. Configuration, construction and sealing of the wheel do not allow to mix the flows. As the wheel is rotating, the only thermal contact of the flows occurs in the matrix, which is heat absorbent. This matrix can be coated with hygroscopic coatings or be made of porous synthetic fibre to allow and enhanced the humidity adsorption and release, transferring not only sensible but latent heat to the air flows. This kind of wheels are named enthalpy or desiccant wheels.

In the heat exchange process a little pressure drop occurs. The heat rotary exchangers efficiency is about 60-85% [3]. The higher values correspond to the wheels that also recovers latent heat from air humidity. These machines are quite big, because of the air ducts and the wheel itself, so it is necessary to take into account enough space for its installation.


Rotatory Wheel heat exchanger.jpg

Illustration 1: Rotatory Wheel heat exchanger [4]


Dryers can recovery heat using the recirculation of the exhaust gases. (no heat exchanger). Exhaust air-to-air heat exchangers are used, allowing the makupeair to be preheated before entering the dryer. Depending on the needs it is also possible to use air-to-water, air-to-steam and air-to-oil heat exchangers.


CHANGES IN PROCESS

ENERGY SAVINGS POTENTIALS

CHANGES IN THE ENERGY DISTRIBUTION SYSTEM

REFERENCES

[1] 2011 ASHRAE HANDBOOK. HVAC APPLICATIONS

[2] Handbook of Industrial Drying

[3] Fundamentals of Heating Systems. ASHRAE

[4] Fundamentals of Heating Systems. ASHRAE




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