chimney draft

There will definitely be an effect; it’s just a matter of whether it’s noticeable. At least, with the old chimney diverting some of the smoke flow, the velocity of the smoke stream decreases, and thus the frictional losses along the path also decrease. How tall is that old chimney?

Generally speaking, the higher the height of a chimney, the greater the suction force. However, I think you need to first understand why suction force is generated. If the heights of the chimneys are the same, then the suction force will be identical; it’s just that the resistance from the smoke is distributed more evenly, which results in a slightly lower level of resistance. But when two chimneys are connected, horizontal resistance also comes into play. I think it would be better to install it at a slight angle. If the height of the old chimney is high, the effect will be much better; in other words, the suction force will be greater. The suction force increases due to the larger temperature difference

A chimney is used to exhaust the smoke generated by the combustion of fuel in the furnace and to maintain a certain negative pressure inside the furnace; it is an important component of the furnace. The chimney must have a certain level of suction. If the smoke is not removed in a timely manner, combustion cannot continue properly ； If there is no negative pressure in the combustion chamber, air cannot enter, and combustion cannot proceed properly. For proper combustion to occur, a certain degree of negative pressure must be maintained in the radiation chamber. The principle behind the creation of suction in a chimney is that the flue gas has a high temperature and thus low density, while the ambient air has a lower temperature and higher density. This difference in density between the inside and outside of the chimney creates a pressure difference, which pushes the gas out into the atmosphere. In the furnace, the various resistances present in the entire flue gas flow from the combustion chamber to the chimney outlet consist of the following: 1. The suction force in the combustion chamber, that is, the negative pressure inside the furnace, which is generally between 2 and 4 millimeters of water column. Part of the negative pressure is caused by the overall suction force of the chimney. 2. The resistance in the convection chamber is related to the gas velocity, the arrangement of the tubes, and the number of tubes. The greater the speed, the greater the resistance. Tubes are generally arranged in a triangular pattern; a smaller distance between the tubes results in higher resistance, as does an increased number of tubes. 3. The resistance of the flue is related to the gas velocity; the higher the velocity, the greater the resistance. The gas velocity is generally 5 to 6 meters per second; in addition, local resistance is also generated by changes in the flue cross-section (expansion or contraction) and by elements such as baffles and bends during the flow of flue gas. 4. Resistance of the air preheater. 5. The resistance of the chimney is related to the velocity of the gas. High speed results in high resistance, but too low a speed leads to poor smoke exhaust. The gas velocity can reach 7–8 meters per second, but it should not be less than 5 meters per second to prevent air from flowing back into the chimney. The sum of these resistances is denoted by ∑ΔP. To ensure smooth smoke exhaust and maintain a certain negative pressure in the furnace, a force is needed to overcome the resulting resistance; when this resistance is high, mechanical ventilation is employed. The capacity of the chimney exhaust fan should be determined based on the maximum volume of gas. It is generally overcome by using the chimney effect generated by natural convection. Its chimney draft must be equal to or greater than ∑ΔP. The draft in a chimney is caused by the difference in gas temperatures inside and outside the chimney, which leads to a difference in gas density. This density difference creates a pressure difference that constitutes the chimney’s draft; it overcomes resistance and drives the flow of smoke gases. It can be calculated using the following formula: ΔP = h (r_out – r_in). Where: h is the height of the chimney, in meters; r_out is the density of the atmosphere outside the chimney, in kg/m3; r_in is the density of the gases inside the chimney, in kg/m3. As can be seen from this formula, the suction force ΔP of the chimney is proportional to both the height of the chimney and the difference in gas densities. The higher the chimney, the greater the suction force ； The greater the difference in gas density, the greater the suction force. The difference in gas density is related to the temperatures of the gas inside and outside the chimney as follows: the greater the temperature difference, the greater the density difference, and thus the greater the draft force in the chimney. Due to process constraints, the temperature of the flue gas does not change significantly. Atmospheric temperature, on the other hand, changes with seasonal variations. When designing the chimney height, the most extreme summer conditions should be taken into account; under unchanged process conditions, the flue dampers should be opened more in summer and closed more in winter.

From the perspective of suction force alone, it is certainly feasible to use an old chimney. As mentioned above, it’s better to make it inclined, and the larger the passage, the better. However, from an economic standpoint, it’s better to increase the height of the chimney. Increasing the power of the fan is not recommended; please have those downstairs check this

The buoyant force (suction force) resulting from the difference in specific weight of air – hot air rises while cold air descends, thereby causing air flow; this is the principle behind natural ventilation in chimneys. Any object in the atmosphere (including the air itself) is subject to buoyant force from the air, but why can the air remain in balance (without considering wind) and not rise? That is because the air is subject not only to the upward buoyant force but also to its own gravitational force. Under normal circumstances, the upward buoyant force on the air is equal to its downward gravitational force, which allows for a state of balance. However, the smoke emitted from the chimney is hot air at a high temperature; the higher the temperature, the more it expands, and the less gravity acts on each unit volume of it. Since buoyancy remains unchanged while the gravity of the hot air decreases, the smoke rises upward. As the hot air rises, the pressure inside the furnace decreases; therefore, cold air from outside enters the furnace. Once heated, this air also rises. And so it goes, over and over again. Formula: Buoyant force (suction force) = Unit weight × Height difference. Let F be the buoyant force acting on each unit volume of gas, G be the gravity acting on each unit volume of gas, p be the density of the smoke in the chimney, P’ be the density of the outside atmosphere, f be the cross-sectional area of the gas flow, and H be the height. Under normal conditions, F = G = mg = pfHg. Since the temperature of the smoke in the chimney is high, the gravity acting on each unit volume is less than the buoyant force; as a result, a pressure difference W = (p – p’)gfH is generated. The pressure difference per unit area (suction force) is W’ = (P – P’)gH. It can be seen that the suction force is related to the height H of the chimney, the temperature of the smoke, and the temperature of the atmosphere. The higher the chimney, the higher the flue gas temperature; the lower the atmospheric temperature, the greater the draft force.

There should be an effect! It can be diverted! The higher the chimney, the greater the suction force; if possible, it’s better to make the chimney taller!

The draft force of a chimney is affected by temperature, and it seems difficult to rely on this draft force to meet the process requirements.

The new chimney is 60 meters tall, while the old chimney is 50 meters tall. Using the old chimney again now probably won’t yield much effect, right?

I think it should have some effect, but due to the height issue, the effect won’t be very noticeable.

After diversion, the flow velocity decreases and the resistance of the flue gas is reduced, which is somewhat useful. However, the resistance of the chimney itself is already very low compared to the suction force; since the height is 60 meters, the suction force isn’t that small anyway.