November 2025 Volume 7

EQUIPMENT & TECHNOLOGY

MODERNIZING TUS TUNING ON HEAT TREAT FURNACES By Ben Witoff

The Manual Uniformity Tuning Model Heat treat furnaces require precise combustion system tuning to produce high-end parts for the aerospace, automotive, and construction industries. Temperature uniformity surveys (TUS) are the accepted industry standard for verifying the quality of these metal processing furnaces. Current standards such as AMS2750H specify a temperature uniformity that must be maintained inside the furnace work zone. End users like Boeing, GE, and Pratt & Whitney have mandated these temperature uniformity quality standards on their suppliers. Today’s furnace and combustion system TUS tuning methods are slow, inefficient, and outdated. These methods require skilled technicians to make precise, manual adjustments to single components in an iterative fashion. Adjustments require this recursive approach because the system of equations governing an industrial furnace’s heat distribution is nonlinear. In the typical case of a multi-burner furnace with more than one temperature measurement point, the temperature distribution across a measurement array is not directly proportional to the change in a single burner’s firing rate. Due to the system’s nonlinearity, each independent tuning adjustment has incidental, cascading downstream effects on the rest of the system. Every attempt to resolve temperature disparity in one area of the furnace can consequently bring another area out of compliance. Reinvention of the Tuning Process Fives North American Combustion, Inc. (FivesNA) has developed a solution that when used before each TUS, shortens the time of the temperature uniformity tuning process and optimizes the furnace temperature uniformity. The North American® CertiFire™ panel implements a patented1 temperature mapping algorithm that creates a linear approximation of any furnace’s system of equations, regardless of its geometry or complexity. Once linearized, the temperature distribution can be resolved through simultaneous adjustments. The temperature mapping algorithm creates a response matrix that correlates changes to the furnace’s heat inputs with changes in the steady-state distribution of heat throughout the furnace’s work zone. A thermocouple array is used to measure the work zone’s 3-dimensional temperature distribution while the furnace’s burners are modulated. It is critical to the accuracy of this response matrix that the burner modulations are precise and repeatable. To accomplish this, actuated gas valves are inserted in the gas line to individual burners taking the place of a manually adjusted limiting orifice valves. Each individual burner modulation has its own characteristic effect on the entire work zone’s temperature distribution. Figure 1 shows two different burner modulations and Figure 2 shows the resulting furnace temperature distribution over the same period. Nine thermocouples were placed on a rack within a furnace in accordance with the AMS2750H standard for the furnace volume and class, with eight thermocouples at each of the cubic work zone’s vertices and one in its center2.

Figure 1: Two Different Burner Modulations

Figure 2: Temperature Response to Two Different Burner Modulations The firing rate of each burner was increased to a fixed amount for a set number of minutes. The second burner was not adjusted until the work zone’s bulk temperature returned to the baseline average temperature. The two burners noted in Figure 1 were firing in the same plane, several feet from one another. Despite the burners’ close proximity and similar adjustments, their effects on the temperature distribution shown in Figure 2 are uniquely different. Not only does the overall rate of temperature change differ between the curves, but so do the individual thermocouple reactions. Thermocouple 5 (shown in orange), for example, shows the largest change in temperature for the first burner’s modulation, but experiences a much weaker response during the second burner’s modulation. The linear approximation of the furnace’s system of equations can be written as shown on the left in Figure 3. Where the vector T represents the temperatures of q thermocouples, the vector B represents the bleed valve modulations of r burners, and the response matrix K represents their relationship. By compiling each of these burner modulations and their resulting temperature effects, the furnace’s unique response matrix can be calculated using the formula shown on the right in Figure 3.

FIA MAGAZINE | NOVEMBER 2025 12