Induced draft
As induced draft refers to a device (now mostly a fan ), the combustion exhaust gases from a vacuum - furnace exhausts and the fire dissipates into the atmosphere.
In the case of small furnaces, the natural chimney effect ( natural draft ) is usually sufficient to safely discharge the exhaust gas and maintain the negative pressure in the furnace. Here the chimney itself represents the induced draft and is also called chimney draft. If there are several chimney tubes connected in parallel, one also speaks of a multi-section chimney.
With larger furnaces in the industrial sector, the amount of exhaust gas is usually so large that the chimney effect alone is not sufficient. In this case a fan is necessary to assist. Also this induced draft fan is briefly as induced draft referred. Modern furnaces control the negative pressure using suitable induced draft fans in order to achieve better exhaust gas values.
Function of the negative pressure
Maintaining the underpressure in the furnace is important to ensure that no hot or harmful exhaust gases can penetrate into the installation room in an uncontrolled manner through cracks and gaps in the furnace wall and endanger the operating personnel. Conversely, if small amounts of "false air" flow into the combustion chamber in an uncontrolled manner due to the negative pressure, this is less serious, since these amounts are negligible compared to the total amount for the furnace.
Due to the negative pressure in such firings, the combustion air necessary for combustion is also sucked into the combustion chamber. In the case of large firings, this suction must in turn be supported by fans, the combustion air fans, which blow the combustion air into the combustion chamber.
About druckfeuerungen as opposed to need sub druckfeuerungen no suction. Instead, the combustion air fan creates an overpressure in the combustion chamber, which expels the exhaust gases towards the chimney. In order to ensure that no exhaust gases escape in an uncontrolled manner through cracks and gaps, the combustion chamber walls must be made gas-tight for overpressure firing .
Induced draft types
In industrial furnaces such as waste incineration plants and conventional power plants , the induced draft is designed as a large centrifugal fan or axial fan; they often have a multi-stage rotor with variable blade adjustment or swirl controllers in the radial design. In power plants with downstream flue gas desulphurisation systems (FGD), it is arranged between the E-filter and FGD; another option is vertical arrangement in the chimney. Induced drafts behind large power plant blocks, with a diameter of over 5 meters and outputs of up to 13.5 MW, are among the largest blowers ever built.
The so-called blowpipe , a steam jet blower , was mainly used in steam locomotives .