Chimney cowls have different operating principles, purposes and performances. They are divided into the following groups:
Permanent cowls. They use the kinetic energy of the wind to generate a vacuum in the chimney. They do not change their position relative to the blowing of air. They usually have a cylindrical shape of the letter "H" or star-shaped. They are used at the ends of ventilation ducts and flue gas.
Self-adjusting. Mounted on a rotating mechanism, set up under the direction of the wind, shielding the flue outlet. On the leeward side of a vacuum is created, which strengthens the chimney draft. They can be mounted on ventilation ducts, smoke and gases. They are more efficient than fixed epiphysis. They are characterized by the most fanciful shapes of all the epiphysis (like eg. The plume or sail).
Rotary/Spinning. Their heads are amazed by the wind in rotation. In the chimney arises vacuum (the bigger, the stronger the wind), which increases and stabilizes the string and prevents string reverse. They are very efficient. They can be used on ducts.
The main disadvantage of the above-mentioned chimney cowl types is the dependence of the wind. If there's no wind or wind is weak, the base does not work and does not produce additional negative pressure. Solution is to install the base of a hybrid or mechanical.
Hybrid devices consist of a base of the rotary motor and which sets it in motion in the event of failure of the wind. In contrast, when air traffic is appropriate, base functions as a conventional rotary and current consumption is minimal.
The roots of the fan motor, which is located outside the flue forces air into the manifold, where it is blown through the gap between the nozzle and the construction, to the outlet of the top. The air gap is given a high velocity, so that just behind a vacuum is created which causes sucking the exhaust gas from the nozzle. Mechanical root is used to help exhaust the natural ventilation and the operation of chimneys discharging gases from solid fuel boilers and fireplaces.