Direct-fired burners are often evaluated based on heating capacity, efficiency, and operating range. But for today’s industrial and commercial HVAC equipment, combustion performance and emissions are also important considerations from the beginning of the design process.

OEMs and equipment engineers need burners that can deliver the required heat while maintaining a stable flame across changing airflow and operating conditions. They also need combustion systems designed to help control emissions and support the temperature rise required by the application.

This is where two-stage combustion can provide an important advantage in direct-fired make-up air burner design.

Midco International’s HMA burner platform uses a two-stage combustion approach designed to promote a shorter, more stable flame while supporting clean combustion and flexible equipment integration.

What Is Two-Stage Combustion in a Direct-Fired Burner?

Two-stage combustion manages how the combustion process develops within the burner and flame zone.

Rather than treating combustion as a single uncontrolled process, the burner design helps establish conditions that promote a more controlled flame pattern.

In Midco’s HMA burner platform, two-stage combustion contributes to a shorter and more stable flame.

This is important because flame characteristics can directly affect emissions, temperature rise, burner profile requirements, equipment dimensions, and overall system performance.

Why Does Flame Stability Matter?

A stable flame is critical to reliable direct-fired burner operation.

Airflow through make-up air equipment can change based on operating conditions, system design, outdoor temperature, and application requirements. The burner needs to maintain acceptable combustion throughout its intended operating range.

An unstable or excessively long flame can create integration and performance challenges.

A shorter, controlled flame can provide OEM engineers with greater flexibility when designing the burner into make-up air equipment, process heaters, and other direct-fired applications.

How Can Two-Stage Combustion Help Reduce Emissions?

Combustion conditions influence the formation of emissions such as nitrogen dioxide (NO2) and carbon monoxide (CO).

Midco’s HMA burner design uses two-stage combustion to help promote cleaner combustion and reduced NO2 and CO emissions when properly applied.

This is particularly important for direct-fired systems because combustion products enter the heated airstream.

For OEMs, selecting a burner designed around stable, clean combustion can provide a stronger foundation for meeting the requirements of the complete heating system.

Why Are NO2 and CO Important in Direct-Fired Applications?

Because direct-fired burners introduce combustion products directly into the supply airstream, emissions performance is an important design consideration.

NO2 and CO levels can be influenced by several factors, including burner design, firing rate, airflow, pressure drop, profile configuration, gas input, and overall equipment setup.

Standards applicable to certain direct-fired equipment can include ANSI Z83.4, Z83.18, and Z83.25, depending on the specific application and equipment category.

Burner selection should therefore be considered as one part of a complete system designed and tested for its intended use.

Two-Stage Combustion Can Support Higher Temperature Rise

Temperature rise is a major consideration when designing direct-fired make-up air equipment.

The system needs enough capacity to bring incoming outdoor air to the desired discharge temperature while maintaining appropriate combustion conditions.

Emissions can become one of the factors that limits how aggressively a direct-fired system can operate.

A burner designed to maintain cleaner and more stable combustion can provide OEMs with greater flexibility when developing equipment for higher temperature-rise applications.

This can be valuable in make-up air units, process heating equipment, spray booth applications, and other systems that need to heat large volumes of outdoor air.

Burner Design Is Only Part of Emissions Performance

A low-emissions burner doesn’t guarantee low emissions after it has been installed in an improperly designed system.

The surrounding equipment plays an important role.

Airflow distribution, burner profile design, differential pressure, firing rate, burner placement, gas pressure, and equipment geometry can all influence combustion results.

For this reason, Midco’s technical information emphasizes that emissions and combustion performance are application specific.

Actual combustion testing is an important part of confirming that the complete system performs as intended.

Why Profile Design Matters

The profile surrounding a direct-fired burner helps establish the airflow conditions necessary for combustion.

If the profile opening is incorrectly sized or airflow is uneven across the burner, flame characteristics and emissions can be affected.

Burner length, firing rate per foot, airflow velocity, differential pressure, and profile opening should be evaluated together during equipment design.

This is another reason OEM engineers should approach the burner and make-up air unit as a complete combustion system rather than treating the burner as an independent component.

Airflow Can Affect Combustion Performance

Direct-fired burners operate directly within the moving airstream.

Turbulence, obstructions, poor airflow distribution, incorrect pressure conditions, or improperly positioned equipment components can affect flame stability.

Maintaining relatively uniform airflow across the burner helps create predictable combustion conditions.

Whether the equipment uses a push-through or pull-through arrangement, engineers need to consider how air approaches, crosses, and leaves the burner.

What Applications Benefit From Two-Stage Combustion?

Two-stage combustion can be particularly valuable in applications requiring a combination of high heating capacity, clean combustion, stable flame characteristics, and flexible operating range.

These may include:

Make-up air units, rooftop heating equipment, process air heaters, spray booth systems, industrial ventilation equipment, and other direct-fired heating applications.

The appropriate burner and configuration ultimately depend on the airflow, temperature rise, equipment dimensions, emissions requirements, and operating conditions of the specific application.

Cleaner Combustion Gives OEMs More Design Flexibility

For equipment manufacturers, combustion performance affects much more than emissions.

A shorter and more stable flame can influence cabinet dimensions, burner placement, profile configuration, operating range, and the ability to achieve the required temperature rise.

This means two-stage combustion isn’t simply an emissions feature. It can contribute to the overall flexibility and performance of the equipment.

When burner design, airflow, controls, and equipment configuration work together, OEMs have a stronger foundation for developing reliable direct-fired systems.

Midco International HMA Direct-Fired Burners

Midco International’s HMA burner platform is designed for OEMs and equipment manufacturers developing direct-fired make-up air and process heating systems.

Two-stage combustion helps the HMA platform provide stable flame characteristics while supporting reduced NO2 and CO emissions and higher-performance direct-fired applications when properly designed and applied.

Ultimately, emissions performance depends on the complete system. Burner selection, airflow, profile design, differential pressure, controls, firing rate, and commissioning all need to work together.

For OEM engineers, this system-level approach helps deliver what today’s direct-fired equipment requires: dependable combustion, strong heating performance, controlled emissions, and greater confidence across real-world operating conditions.