What is the cone on top of some heater stacks for? Fired Heater Features – What’s That For?!

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By Joe Tleimat

The circled portions of the heater stacks are commonly referred to as velocity cones (or tips).  A velocity cone is a cone-shaped stack extension added to some fired heaters to increase stack exit velocity.

Here are three common reasons to increase the exit velocity:

  1. It reduces the downwash of flue gas at the tip which causes stack metal corrosion. Some companies specify a minimum stack exit velocity (of 10-15 fps) at turndown operation for this reason.  Using a stainless-steel tip instead of carbon steel can solve this issue.
  2. A side benefit of higher stack exit velocity is that it provides some dampening of the effect that wind gusts have on heater draft variation.
  3. Occasionally the stack flue gas dispersion analysis uses 50 feet per second as the exit velocity to determine the minimum stack height required. This is a common practice in the boiler industry.
    • For boilers, flue gas dispersion analysis typically assumes 50 feet per second (fps) stack exit velocity as the basis. The stack height is then set to provide the desired flue gas dispersion.  This does not have a significant impact on a boiler design or cost since boilers typically have fans that move flue gas.  The stack pressure drop is typically insignificant compared to the entire boiler system pressure drop.
    • On the other hand, natural draft heater stacks are sized differently. Natural draft stacks are typically most cost-effective with 20-30 fps stack exit velocity (compared to 50 fps for boilers).  If designed for 50 fps exit velocity, the pressure drop (incremental vs an exit velocity of 20-30 fps) would be in the range of 0.15 – 0.25 inH2  This is a sizable portion of the draft generated by the stack.  Unfortunately, personnel responsible for the flue gas dispersion analysis typically do not have this appreciation. Therefore, a minimum 50 fps exit velocity is specified and the dispersion analysis is carried out to establish a minimum stack height.  Frequently, the height required from a draft perspective is considerably higher than the height for dispersion requirements.  Overcoming this extra pressure drop can add 30-60’ to the stack height of a natural draft heater.  Not only does the stack cost more, but the heater structure and foundation become more substantial and costly.  To minimize the cost and size impact, the stack designer typically only reduces the diameter of the very top of the stack to achieve the desired exit velocity instead of the entire length of the stack.  This avoids additional pressure drop associated with higher stack frictional losses. A simplified dispersion calculation suggests that a 100’ tall stack with 50 fps exit has the same dispersion characteristics as a stack at 25 fps and 105-110’ tall.

If you have a heater with a velocity cone sized for >40 fps and is draft limited, consider redoing the dispersion analysis with a lower exit velocity.  You may discover you can remove the cone and gain 0.15-0.25 inH2O of draft.

Have questions about velocity cones?  Contact us at info@xrgtechnologies.com!

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ERWIN PLATVOET
As CTO of XRG, Erwin is a true innovator, whose career spans more than three decades in heat transfer and combustion industries. Erwin is a graduate of Twente University in the Netherlands with a MS in Chemical Engineering. Erwin has served the industry around the globe in a variety of roles including Research and Development Engineer, Cracking Furnace Specialist, and Director of Engineering, and now CTO. Erwin holds eight patents in fired heat transfer and emissions control technology, has published numerous papers, and co-authored the John Zink Combustion handbook and Industrial Combustion Testing book. Erwin has been an active member of the API 560 and API 535 subcommittees and taken an active role in revising these standards.
BAILEY HENDRIX
Bailey graduated from Oklahoma State University with a Bachelor of Science in Mechanical Engineering. Upon graduation, she joined the private sector as an Applications Engineer in Tulsa, OK at a local combustion company where she managed the sales activities for the process burner refining market. She quickly accelerated her career, becoming the Refining Account Manager responsible for all business development and sales of process burners in North and South America. Her strong leadership skills and interpersonal qualities led her to a position as the Western Hemisphere Sales Director for the process burner business, leading a group of sales engineers in the areas of new equipment, retrofits and burner management systems. Her financial and commercial acumen drives the success of XRG Technologies’ business development.
ALLEN BURRIS
Allen’s background includes 10 years of experience in designing and selling process burners. Allen is a graduate of Oklahoma State University with a BS in Mechanical Engineering and is a licensed professional mechanical engineer in the State of Oklahoma. His knowledge and superior customer focus led him to a career change to process design, custom-engineered fired heater sales, and associated sub-systems for the petrochemical, refining and NGL industries. With more than two decades of experience in the combustion and fired heater industry, Allen has what it takes to overcome challenges associated with complex projects and possesses.
TIM WEBSTER
With over 25 years of experience in the combustion industry, Tim brings a wealth of industry experience and technical expertise to XRG. Tim graduated with a Bachelor of Science in Mechanical Engineering from San Jose State University and received a Master of Engineering from the University of Wisconsin. Tim began his career engineering custom combustion systems for a wide range of applications including boilers, heaters, furnaces, kilns, and incinerators. Tim is a licensed professional mechanical engineer in the states of California, Texas, Louisiana and Oklahoma, has authored numerous articles and papers, and has co-authored several combustion handbooks.
matt martin
As the Lead Scientist at XRG, Matt has over 30 years of experience in the combustion industry. He specializes in CFD of fired equipment, including UOP platforming heaters, burners in process heaters, thermal oxidizers and flares with over 300 simulations of installed, field-proven equipment. Matt received a Bachelor of Science in Computer Science with a minor in Mathematics from the University of Tulsa. He has written numerous publications, is listed as inventor or co-inventor on 27 patents and was awarded the title of Honeywell Fellow in 2011 for technical excellence and leadership.
gina briggs
Gina is a native Oklahoman and attended the University of Tulsa, graduating with a BSBA in Accounting. She is a Certified Public Accountant and Chartered Global Management Accountant. Gina began her career with the Tulsa office of Deloitte Haskins and Sells, providing audit and tax services. Since leaving Deloitte, she has held CFO positions with privately held companies in the manufacturing, construction and distribution industries. In 2013, she began a consulting practice providing contract CFO services to companies, one of which was XRG and joined XRG as CFO in 2019. Gina has always enjoyed working in the small business arena, helping business owners to profitably grow and manage their businesses.