Super System T4420 User Manual





Protective Atmospheres, Measurement Technologies and Troubleshooting Tools



Furnace atmospheres are critical to meet metallurgical specifications defined by control processes. The
makeup of a furnace’s atmosphere in the heat treating process varies based upon the application. This
paper will cover the different types of atmospheres, the sensors used to measure the atmosphere, and
troubleshooting procedures for validation of the environment.

Endothermic Atmosphere

Typical endothermic gas generators supply an atmosphere using air and hydrocarbon gas which are mixed
and passed over nickel bearing catalyst at about 1900°F. Using methane (CH4) mixed at an air-to-gas ratio
of 2.77:1; a properly functioning generator will theoretically produce an endothermic gas consisting of 20%
carbon monoxide (CO), 40% hydrogen (H2), and 40% nitrogen (N2). Using propane (C3H8) in lieu of CH4
and an air to gas ratio of 7.16 to 1, the resulting endothermic gas composition will be approximately 24%
CO, 32% H2 and 44% N2. . The gas is then cooled to maintain the integrity of the gas composition. Correct
cooling of the gas is critical to avoid carbon monoxide from reversing into carbon (soot) and CO2. This is
the base atmosphere used in the carburizing process. A few assumptions must be made when
determining carbon potential in the furnace with a supply of endothermic gas. Measuring this atmosphere
can be accomplished in a number of ways; the focus of this article is use of oxygen sensors, dew point, and
infrared measurement.

Nitrogen Methanol

When used in a furnace at the typical operating temperatures, the methanol immediately dissociates into
carbon monoxide and hydrogen. When mixed, 60% methanol and 40% nitrogen and endothermic
equivalent atmosphere are formed in the furnace.

2 CH

OH 33% CO + 66% H2

3

19.8% CO + 39.6% H

+ 40.6% N2 = Endothermic Equivalent Gas

2

Methanol (Methyl alcohol) has a boiling point of 149F (65C). One gallon of MeOH = 237 CFH of H2 and CO.
To calculate the correct flow of Methanol and Nitrogen required, divide the total furnace flow required by

1.6, then multiply by 66% for your N2 and 33% for your Methanol.

For example, if you require 400 SCFH the results would be the following:

N2 = 400/1.6 = 250 *.66 = 165 SCFH.

MeOH = 400/1.6 = 250 * .33 = 82.5 SCFH.

SUPER SYSTEMS INC – TECHNICAL DATA SHEET

7205 EDINGTON DRIVE, CINCINNATI OH 45249

Phone: [phone] Fax: [phone]

www.supersystems.com

T4420





Assumptions are made when determining carbon potential. Using sensors and the base atmosphere in the
furnace under equilibrium conditions, these assumptions must be consistent, known, and repeatable.

Exothermic Atmosphere

Exothermic gas is the byproduct of combustion, and is widely used in the Annealing process. Hydrocarbon
gases such as natural gas, propane, or butane are burned in an exothermic gas generator with air to
create a rich or lean mixture based on the air-to-gas ratio. Rich, medium, and lean mixtures are
described in the chart. The mixtures do contain high moisture content.

RICH EXOGAS MEDIUM RICH EXOGAS LEAN EXOGAS

CO 11% 8% 5%

CO2 5% 7% 9%

CH4 5% 5% 5%

H2 14% 10% 5%

N2 Balance Balance Balance

Measuring an Endothermic Atmosphere in a Generator

The most common measurement of this atmosphere is confirmed by using a dew point measuring device
or an oxygen sensor. Dew point is the temperature and pressure at which gas begins to condense into a
liquid. The dew point measurement is accomplished by taking a sample of the atmosphere after the
cooling section of the generator.

Using an oxygen sensor to measure and control the dew point in an endothermic generator is done via a
calculation in the dew point control instrument. This calculation uses the oxygen millivolts generated by
the sensor, the hydrogen factor of the controlling instrument, and the temperature of the oxygen sensor.
The temperature is required for the calculation but the dew

point of the gas is not temperature dependent.
Often oxygen sensors are installed at 1900°F although they will provide the same reading when operating
at 1500°F. The generator gas exiting the retort is sent through a heat exchanger to freeze the
composition. As long as the sensor is accurately measuring the oxygen millivolts of the gas, the
temperature of the sensor can be as low as 1100°F. Changes in sensor location have occurred over the
years. Initially a sensor would be mounted on the top of a retort in an air cooled fabricated fixture to
measure the oxygen. Then a ceramic reheat well mounted through the sidewall of the generator was used.
Now a modified sheath with an integral reheat well makes the installation much easier. The sheath and
the integral well are aluminized prior to assembly. (See Figure 1) The nickel in the RA330 sheath material
does not react with the endothermic gas. This is especially important between the 900°F to 1300°F where,
over time, the endothermic reaction will reverse if nickel is available. This type of design has the ability to
run at lower temperatures, thus allowing many years of operation.

SUPER SYSTEMS INC – TECHNICAL DATA SHEET

7205 EDINGTON DRIVE, CINCINNATI OH 45249

Phone: [phone] Fax: [phone]

www.supersystems.com

T4420