Heating furnace issues
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It is said that negative pressure is caused by density differences and pressure drops, but I still don’t understand it. Could some friends help me analyze the reasons in detail? And how is the draft force related to the chimney height?
The flue gas inside the chimney is hot and has low density, while the cold air outside is cold and has high density; this creates a pressure difference between them. This pressure difference is the force that drives air into the furnace and out through the chimney; the higher the chimney, the greater the suction force.
There is negative pressure inside the heating furnace; then why doesn’t atmospheric air enter from above? Why can one only enter from below?
The inside of the heating furnace is under negative pressure, but the pressure at the chimney outlet is greater than the atmospheric pressure outside the chimney. So the smoke from the chimney comes out! Because the entrance is at the bottom, under the effect of suction, it enters from below
Why is the pressure at the chimney outlet greater than the atmospheric pressure outside the chimney? Is it because of the exhaust fan? Is the pressure at the outlet of a chimney with natural ventilation also greater than the atmospheric pressure outside the chimney?
The smoke flowing in the chimney does so at a certain velocity; at the outlet, this velocity is converted into pressure energy. As a result, the pressure at the chimney exit is higher than atmospheric pressure. This works on the same principle as a vacuum pump. The same applies whether it is forced ventilation or natural ventilation – the only difference is the velocity of the smoke flow. When designing the heating furnace, due consideration was given to the fact that the design diameter and height of the chimney must ensure safe exhaust of flue gases; the flow velocity of the flue gases must be at least 4 m/s, otherwise air will flow back into the chimney.
It is the upward movement of hot air that creates the negative pressure inside the furnace; first, the hot air rises, and then negative pressure is generated. You need to understand this sequence: it is precisely because the hot air exits through the chimney that an upward pull is created, which in turn allows sufficient air intake at the bottom. Controlling the negative pressure in the furnace helps with air intake through the bottom air ducts, and adjusts the degree of complete combustion of the fuel or gas.
Just think of the chimney in a home where firewood is used for cooking! The main function of the exhaust fan in the furnace is to provide sufficient air for the combustion of fuel, ensuring that the fuel burns as completely as possible in order to improve the efficiency of the furnace
Since the temperature inside the furnace is higher than that of the outside atmosphere, the density of the smoke entering the furnace is lower than that of the atmosphere. This creates a force that allows air to enter the furnace, causing the smoke inside to rise and be discharged into the atmosphere through the chimney. As the smoke is expelled, a negative pressure is generated inside the furnace. The higher the chimney, the less pollution it causes on the ground.
It works on the same principle as buoyancy; a hydrogen balloon can rise in the air because the density of hydrogen is lower than that of air, which creates buoyant force. If we consider the hot air as a single entity, it will rise. There is also some mixing of air with the smoke, but the upward force is primarily caused by the difference in densities.