FEATURES

APPLICATION

Mixtures of hydrogen and nitrogen are often required in the heat treating industry to create a special atmosphere in furnaces. The Thermco gas mixer can create gas mixtures from bulk sources to create a system that is economical and reliable. The Thermco surge tank based system has infinite flow rate turndown capability able to supply a single furnace or a multiple number of furnaces. Some of the common processes used with the gas mixer are:

The Thermco furnace atmosphere gas mixer is designed to meet the applicable requirements of NFPA 86, Standard for Industrial Furnaces Using a Special Processing Atmosphere.

Gas mixers are available for mixing hydrogen and argon for welding purposes (usually plasma welding). The typical range of these gas mixers is 0 - 10% H₂ in Ar, which can produce a mixture of 5% H₂ / 95% Ar. The H₂ / Ar gas mixers are designed the same as the furnace atmosphere gas mixers, except the argon flow is shut off during a power failure and manual reset is not required to restart hydrogen flow after an alarm.

PRINCIPLES OF OPERATION

Within the gas mixer, the major and minor gas streams are regulated to the same pressure. Downstream of the regulators, the major gas flows through a fixed orifice, and the minor gas flow through a metering valve, which acts as an adjustable orifice. The two gas streams are mixed together under turbulent flow conditions and fed into a surge tank. Once the pressure in the surge tank reaches the upper set point on a pressure switch, the gas supplies are automatically shut off with solenoid valves. As mixed gas is required the pressure in the surge tank falls until it reaches its lower set point. At this point, the solenoid valves are opened and the cycle repeats.

The gas analyzer continuously monitors the mixture in the surge tank. If a mixture adjustment is to be made, the operator turns the minor gas-metering valve accordingly, and the gas analyzer reads out the resulting mixture. Once the desired mixture is obtained, the mixing system will maintain a constant mixture ratio.

With changing mixed gas flow rate, only the cycling frequency changes; the pressure drops across the orifices remaining the same, producing a consistent mixture.

This design is simple and rugged; controllers or flowmeters on the process gas are not required.

Model 9120 for Hydrogen / Nitrogen

THERMCO GAS ANALYZER / ALARM SYSTEM

The Thermco gas analyzer and the safety alarm system have been incorporated into a single unit.

The gas analyzer is a thermal conductivity type manufactured by Thermco. Thermal conductivity is the ideal method for measuring hydrogen/ nitrogen or hydrogen / argon gas mixtures due to high thermal conductivity difference between hydrogen and nitrogen /argon. The thermal conductivity gas analyzer does not contain any depleting chemicals. The Thermco hydrogen analyzer is exceptionally stable, with zero adjustment recommended once a month. A calibration check recommended once per year. The hydrogen percent readout is digital, with a resolution of 0.1% hydrogen.

The gas analyzer uses a positive pressure purge to meet the explosion proofing code. Nitrogen or argon from the gas mixer is used as the purging gas.

Special Designs

FURNACE ATMOSPHERE APPLICATION

Gas mixers for the furnace atmosphere application will contain the following alarms:

• High hydrogen alarm . If the hydrogen level rises above the high alarm value on the gas analyzer, the hydrogen flow will be shut off and light and a horn will be energized.

• Low hydrogen alarm. If the hydrogen level falls below the low alarm value on the gas analyzer, a light and a horn will be energized.

• Low Nitrogen Pressure. If the inlet nitrogen pressure falls below the minimum necessary for proper gas mixing, the hydrogen flow will be shut off and a light and a horn will be energized.

• Power failure. If a power failure should occur, the hydrogen will be shut off and the nitrogen will continue to flow. A light and a horn will be energized when power resumes.

If high hydrogen percentage, low nitrogen pressure or the power failure alarms continue for more than approximately 5 seconds, manual reset by the operator is required to restart hydrogen flow.

The gas mixing system constitutes part flow control system for furnace atmospheres; such aid meets the applicable sections of NFPA 86, chapter 7. The gas mixing system should be installed in an overall system that meets NFPA 86 requirements.

WELDING GAS APPLICATION

For welding applications, the alarm functions are the same, except hydrogen flow will restart without manual reset. There is on argon flow on a power failure.

General Specifications

±3.0% full range over 50ºF to 90 ºF (10ºC to 32ºC) temperature range

± 5.0% of full range over 10ºF to 104ºF ( 12ºC to 40ºC) temperature range

For the full 50-80% H₂ in N₂ range, accuracy is ±3.0% H₂ over 50 ºF to 90 ºF (10ºC to 32ºC) and ±5.0% H₂ over 10ºF to 104ºF(-12ºC to 40ºC)

Stated accuracy assumes that the input temperatures of the gases are equal. Gas mixer accuracies depend upon many variables, including gas mixer maintenance, and environmental conditions around the gas miner. These accuracies are presented as typical performance for these systems.

The gas analyzer accuracy is separate. Contact the Thermco sales engineer for details.

FLOW CAPACITY

Rated capacity of 0 - 2,000, 0 - 5,000, and 0 - 10,000 SCFH is at mid-range setting. Consult Thermco for details on available capacity. Mid-range setting is the middle of the gas mixer adjustment range, i.e., a gas mixer with a range of 0 - 20 % Hydrogen in Nitrogen has a mid-range setting of 10 % Hydrogen in Nitrogen.

TEMPERATURE RANGE

Ambient and Process Gas 10º to 104ºF ( 12ºC to 40ºC)

NORMAL SUPPLY GAS INLET PRESSURE RANGE

100-125 PSIG(6.9 - 8.6 BARG) for major and minor gases for all models. Input pressures do not have to be equal.

NORMAL MIXED GAS OUTPUT PRESSURE RANGE

10-40 PSIG / G (0.7 - 2.8 BARG) for models 8300, 8400, 8600

10-50 PSIG / G (0.7 - 3.4 BARG) for models 8350, 8450. Mixed gas pressures are adjustable with a regulator on the gas mixer.

GAS CONNECTIONS AND PIPING

For all models the nitrogen / argon input connection is 1 in. NPT female pipe and the hydrogen input connection is ½ in. NPT female pipe. For all models the mixed gas connections 1 in. NPT female pipe, except the model 8600 which has a 1 ½ in. NPT female connection.

SURGE TANK

30 gallon for models 8300, 8350; 60 gallon for models 8400, 8450; 120 gallon for model 8600; carbon steel ASME coded, and CRN registered; pressure relief valve provided.

POWER REQUIREMENTS

115 VAC ±10 VAC 50/60 Hz., 0.9 Amps

(220 VAC, 50 / 60 Hz. Available on request)

ELECTRICAL CONSTRUCTION

The gas mixer is designed to meet the National Electric Code for a hazardous area, classified as Class I, Division 2 Group B. No other code requirements or approvals are claimed by Thermco.

READOUT

SIGNAL OUTPUT

ZERO, CALIBRATION FREQUENCY

Zeroing of the analyzer is recommended once per month. Zeroing is accomplished with the major gas supplied to the gas mixer with controls provided on the gas mixer panel. Checking the calibration is recommended once per year. Calibration requires a certified gas mixture. Large ambient temperature changes may require more frequent zeroing and calibration.

NOTICE CONCERNING SUPPLY SYSTEMS

Because these gas mixers operate by intermittently filling a surge tank in the gas mixer, the gas mixer will demand the supply gases at full gas mixer capacity for some period of time, even if the mixed gas demand is small. For this reason, bulk gas supply systems (not portable liquid cylinders) should be used for major gas. Please contact a Thermco sales engineer for guidance.

INSTALLATION LOCATION

The mixing system must be located indoors. If the gas mixer can not be placed in the main building, it must be placed in a separate building designed to protect the gas mixer from rain, snow, and heated to maintain the temperature between 10ºF and 104ºF.

WARNING

Improper use of this product can cause death, serious injury, or property damage. Personnel dealing with this equipment should read and understand the warning labels and instruction manuals provided by Thermco. Only personnel familiar with industrial gases should attempt to install or service this equipment. Gases used in this equipment are flammable and therefore constitute a fire and explosion hazard; gases from high pressure and therefore the hazards associated with pressurized systems exist. Gases from high pressure cylinders must be reduced to the specified pressure before entering the mixing system.

DOCUMENTATION

Each gas mixer is supplied with two instruction manuals, which include complete wiring and flow diagrams. A complete data sheet is prepared for each gas mixer specifying major parts of the instruction manual is kept on file at Thermco, and engineering assistance is provide if required. Thermco has been providing service on gas mixers since 1964.

ORDERING INFORMATION

Thermco gas mixers are available through many local industrial gas suppliers. Thermco gas mixers may also be ordered directly from the factory. Before ordering please generate the proper model number by using the system on the rear page of this conditions for the application. If there are questions, please contact the Thermco sales engineer.