Common Plenums: 5 reasons fired process heaters need them

Posted on

Make Common Plenums More Common!  

by Erwin Platvoet

Fired heaters need air for the burners that supply heat to the process tubes. Air is supplied through individual burner plenums or a common plenum. Most burners come with an individual plenum which means each burner has an individual air supply duct (called plenum or windbox) using forced or natural draft and is controlled by a damper. This damper is almost always a louver-type design with one or two opposed blades.

Far less common is the common plenum (pun intended). A common plenum is essentially a big box around the burner inlets. Air is still supplied by either natural or forced draft, and burners still have individual dampers, but of a different design.

Examples of common plenumsExamples of common plenums

                   Figure 1: Examples of common plenums

Cost is the main reason why common plenums are not used as much anymore. Furthermore, the additional box around the burners requires more space because the air velocities cannot be too high. With the plenum walls in the way, burner maintenance and operation can be more difficult.

However, common plenums offer advantages that are often downplayed but can be worthwhile.

  1. Common plenums can be designed with just one main inlet damper

Having a damper on each burner seems like a method to achieve more control flexibility, but in reality, each one needs to be set the same as the others for proper air/fuel ratios. Instead, after a while they end up all over the place, creating combustion problems from uneven air distribution. A properly designed common plenum provides internal air distribution without interference from an operator.

The air inlet can be designed with a single damper, this is actuated electrically or pneumatically for optimal O2 balancing in conjunction with the arch draft. Such an air delivery system would rarely, if ever, need manual intervention by the operator.

Especially for natural draft heaters that are operated in extreme ambient temperatures, blizzards or desert sandstorms, a single remotely controlled damper is much easier to manipulate than sending an operator out.

Common plenum with a single inlet damper

Figure 2: Common plenum with a single inlet damper

 

2. Common plenums provide superior air distribution

A common plenum can be internally fitted with flow correction devices such as baffles, guide vanes, and perforated plates to ensure even air distribution among the burners. Proper design ensures a maximum difference of +/- 2.5% excess air in natural draft applications, which is superior to individual plenum designs. This is because natural draft burners with individual plenum are very sensitive to wind. In short natural draft heaters, the available draft for the burners can be less than 0.2 inH2O (5 mmH2O), so any variation in ambient conditions like wind pressure and velocity can cause great variations in air flow.

3. A common plenum is insensitive to wind

This is an area where the benefits of a common plenum are most obvious. Wind can play an outsized role in the air control of a natural draft heater. The air flow into the burners depends on the difference between the firebox pressure and the exterior pressure. Even at modest wind speeds, these exterior pressures vary substantially around the heater and the burners.

Consider the following example:

A customer experienced natural draft burners flaming out during gusts of wind. To tackle the problem, the burner inlets were extended and aimed upwards. However, CFD analysis of the wind speed of 20 km/h indicates high gradients of velocity around the burner inlets, resulting in ambient pressure variations that were high enough to cause burners to run out of air.

Velocity gradients due to wind speed of 20 km/h

Figure 3: Velocity gradients due to wind speed of 20 km/h

 

A well-designed common plenum can eliminate these variations and ensures a good air distribution.

Common plenum design and flow distributionCommon plenum design and flow distribution

               Figure 4: Common plenum design and flow distribution

 

4. Common plenums offer freeze protection

In cold climates, operational problems occur that are non-issues everywhere else. Water can form hydrates in natural gas lines and cause plugging, especially around high pressure drop zones like flow orifices, where the Joule-Thomson effect causes additional drops in temperature. Hoar frost may form on pilot inlets, perforated plates, and burner inlets. This can be mitigated or prevented by running the pilots and fuel lines inside the common plenum and shielding them from the wind. The internal plenum temperature is also higher due to radiation from the floor casing.

5. Noise reduction

Most combustion noise comes from the firebox through the burner plenum. Stringent noise requirements (such as Sound Pressure Levels below 80 dB(A)) can be difficult to achieve with individual burner plenums. In some cases, they must be 5 – 10 ft long to cause sufficient dampening of the noise. However, by design, a common plenum offers a natural barrier against the transport of noise to the environment which makes it far simpler to achieve low to extremely low SPL levels.

Conclusions

While common plenums are more complex to design than individual burner plenums, they offer many advantages that can make the heater easier to operate and more robust against the impact of ambient conditions.

Ask an expert
© 2026
Please fill out the form below and we will be in contact as soon as possible.
GET IN TOUCH
This field is for validation purposes and should be left unchanged.
Name(Required)
Please fill out the form below and we will be in contact as soon as possible.
GET IN TOUCH
This field is for validation purposes and should be left unchanged.
Name(Required)
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.