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I am responsible for a brick kiln in the Punjab province of Pakistan. My government requires all brick kilns to be upgraded to match recent new standards, which include the addition of a blower near the chimney. We consulted with local engineers who presented several different blower designs and conflicting arguments regarding which design is best.

Here are the two main designs most of them agreed upon, with the fan being 52 inches in diameter and having 6 blades:

Blower Design #1

Diagram of a centrifugal blower showing a 24-inch outlet diameter, with the following gaps measured between fan and housing every 90 degrees starting immediately before the outlet: 18 inches (top); 12 inches (left); 8 inches (bottom); 2 inches (right).

Blower Design #2

Diagram of a centrifugal blower showing a 24-inch outlet diameter, with the following gaps measured between fan and housing every 90 degrees starting immediately before the outlet: 24 inches (top); 18 inches (left); 10 inches (bottom); no dimension specified (right).

Which design will work better? I also have my doubts about the fan blades being straight. Would straight blades be appropriate for this use? What about forward or backward curved blades?

We will mount the blower at the base of the chimney, pulling hot air from cooking bricks and blowing it into the chimney. The goal is to improve the efficiency of the coal-burning process in the kiln. We refer this as a "zigzag kiln."

Here is a sketch of a similar blower configuration. Our blower will be placed above the walls instead of in front of the walls, but the airflow otherwise should be the same:

Sketch of blower drawing air from an area on the left and blowing into a separate chimney area on the right

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  • $\begingroup$ Can you provide details of the government standards? $\endgroup$ – Jonathan R Swift Feb 18 at 13:18
  • $\begingroup$ government didn't issue any details, only the installation of blower is mentioned, all kiln owners are installing them as suggested by local engineers who don't seem to have much idea of what needs to be done @JonathanRSwift $\endgroup$ – asim Feb 18 at 13:26
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    $\begingroup$ What is the purpose? What is the medium? Temperature? What is the flowrate? What are the pressures? What is upstream and downstream of the blower? My hunch is that this shall guarantee proper draft in the chimney and the the blowers will be mounted on top of the chimney - is this correct? $\endgroup$ – mart Feb 18 at 14:13
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    $\begingroup$ There is also a very detailed design manual for improving brick kilns in Nepal but if you are not located in Nepal it may be less applicable. Some discussion of impeller blade design starting p. 59. As you are not the engineer you may not be performing this design yourself but resources like this may help you to ask good questions of the engineers who bid on your project, and decide whom to hire for this project. $\endgroup$ – Air Mar 1 at 18:13
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    $\begingroup$ I am in Punjab province of Pakistan @Air $\endgroup$ – asim Mar 2 at 13:13
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Normally fans are not custom built by a company that does not build fans. The concept of a centrifugal fan is not particularly complicated, but it is difficult to achieve target flow rate, pressure, and efficiency if your company does not do it on a daily basis. If you are not able to purchase a fan, another option is to copy someone else's simple straight blade design with known fan curves.

Answering your specific question: The top design with the two inch gap on the left is better at handling trash (foreign material in the air stream). The bottom design with the small gap on the right will have a better efficiency because it wont recirculate as much air around the housing. The style of the fan (blade shape) is really a minimal problem compared to knowing the flow rate, and pressure required. This NREL article has some good information on centrifugal fan blade shape, but it basically simplifies to particulate handing reliability vs efficiency.

For clarification of the application (for myself) I attached some references below for a "continuous moving fire zig-zag brick kiln"

brick kiln image from GKSPL introduction to brick kilns

Other than coal fired brick industry rules-of-thumb (unknown to me), here are some ways of estimating specifications. This is potentially how the engineers sized those 52 inch diameter fans above.

I would estimate the airflow requirements based on stoichiometric fuel combustion and excess oxygen requirements. If you know how much fuel is added and how much excess O2 is needed, engineers can calculate the flue exhaust flow rate that this fan will need to provide.

Temperature of the flue of the current operation should be known, but note that forced draft modification may change this temperature. Temperature of the air affects the density of the air. The lower the density, the larger the fan needs to be to transfer the same mass flow.

If the new zig-zag stacking configuration has been used with natural draft, you can estimate the minimum pressure requirements by measuring the pressure at the base of the stack compared to atmospheric; and add in the buoyancy pressure of the hot flue. This is a difficult one to estimate because the flow requirements and duct geometry are unknown.

I would recommend over-sizing the fan and motor by at least 50% (which the engineers have likely already done), then when commissioning, adjust pulley diameters to reduce the speed of the fan to the optimum operating flow rate. This also gives the ability to adjust the pressure/flow if other brick packing and temperature configurations are used in the future. A variable frequency drive, makes this even easier if it is available. I do not recommend using a damper as it greatly reduces the efficiency of the system. Note that if the electric motor is has less than 40% of its rated amps after optimization, it should be replaced with a smaller motor to allow the motor to be fully loaded and operate more efficiently.

Once the engineers have all of that information they just need to select a fan that meets the requirements of airflow and pressure for a given temperature. There are lots of good fan manufactures in every country and they will produce a fan that meets your needs cheaper than you can design and build it. Alibaba is a good resource if you cant find any.

As side note, the forced draft will greatly change the heating and cooling rate of the bricks. The airflow may need to be reduced from optimum fuel use in order to reduce cracking. Also forced draft may structurally load areas of the kilns with vacuum pressure that may not have had any load in the past (baffles, doors, walls, roof, etc). Even 500 pascals can cause significant loads over large surfaces. It would be a good idea to have the engineers evaluate that scenario before commissioning. Also, unintended air (tramp air) may leak through cracks and access doors creating cold zones that were not a problem in the past. Make sure everything is well sealed.

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    $\begingroup$ This answer is an excellent starting point. My educated guess based on the sector, fuel, and apparent location is that the main goals here are black carbon control and cost reduction/mitigation. The kiln may have recently converted to the zig-zag design and the blower may be required to support this change. I doubt there is any FGR here, or that NOx reductions are a significant goal. For some more context from a western source, try searching up EPA report 160R12002. $\endgroup$ – Air Mar 1 at 18:31
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    $\begingroup$ Thanks @Air, I am used to computer controlled boilers and dryers in the US with scrubbers and strict emissions requirements. I read that EPA report as well as some other articles to better understand the process; I will update accordingly. Pretty impressive really; I would like to see one in real life. We are so spoiled with sensors and VFD's it is hard to imagine how to do it without. $\endgroup$ – ericnutsch Mar 2 at 6:29

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