The gases will not separate in the pipeline downstream of the gas mixer. Once gases are mixed in the mixing system they will continue to be mixed. Gases remain mixed because the gas molecules move around at high speed, causing collisions that keep the gases mixed. Because of this property, there is no limit to the length of a mixed gas pipeline in regards to mixed gas stratification.
When cylinders are filled it is typical to initiate filling with one gas constituent and topping off the cylinder with the remaining gas constituents. Therefore there is stratification at the time of cylinder filling. This problem can be corrected by using a dip tube inside the cylinder or rolling the cylinder after filling. When gases are mixed in a Thermco gas mixer, the gas constituents flow together at right angles, causing turbulence at the mixing point. This creates a homogenous mixture. The gases remain mixed after this point in the mixing process.
Flow meter accuracy is subject to many variables, including pressure, temperature and the physical characteristics of each gas, such as specific gravity. Rotometer type flow meters are especially likely to yield inaccurate readings because the gas in the flow meter tube is not at the pressure or temperature the tube was designed for. The Thermco gas mixer with a gas analyzer yields a result that is more accurate than all but the most accurate flow meters.
The gas pressures into the Thermco gas mixer do not have to be equal. There is an inlet pressure envelope that is specified for each gas mixer. The standard inlet pressure for Thermco gas mixers is 100-125 PSIG (6.9-8.6 BARG). The user should maintain the inlet pressures within the specified range in order for the gas mixer to meet its accuracy specification.
The total required flow rate will depend upon several factors. These include the number of welders, shield gas flow per welder and the “on” time for the welders (duty cycle). The gas mixer should be sized to be able to supply the peak flow of shield gas required. For example, if a user has 50 welding stations and each station uses shield gas at 35 SCFH, the potential flow rate is 50 x 35 = 1750 SCFH. If the welders are “on” for 50% of the time, the required usage is 1750 x .5= 875 SCFH. Some safety margin should always be considered. At times more than 50% of the welders may be welding, and some welders may turn up the flow on their flow meters. Also, robotic welders may use a greater flow rate than manual welders. In the above example, if a safety margin of 25% is desired, the needed flow rate would be 875 x 1.25=1093.25 SCFH.
Standard Thermco gas mixers are available in increments of 750, 2000, 5000 and 10000 SCFH. In this case the correct choice would be the 2000 SCFH model.
Gas analyzers based on the thermal conductivity principle use the thermal conductivity properties of gases to analyze gas mixtures. Different gases have different thermal conductivities. When measuring a two gas mixture, the thermal conductivity of the mixture can be compared to the thermal conductivity of a pure gas, usually the base gas in the mixture such as nitrogen or argon. By experimentation the output of the thermal conductivity detector can be determined for various two gas mixtures; this data is input into a linearizer that produces a linear signal proportional to the gas mixture %. If three or more gases are in the mixture it is unpredictable what combination of the three gases is creating the output from the thermal conductivity cell. Therefore this technique is normally used only for two gas mixtures.
The only three gas mixture that Thermco measures using thermal conductivity is the CO2/O2/Ar mixture if the user has an independent method of measuring the O2. Consult with Thermco for more details.
When gas mixers are manufactured for food service, all the wetted surfaces are cleaned to remove particulates and other contaminants that could be carried downstream of the gas mixer and eventually contaminate food being packaged. Also, thread sealants and lubricants are used that are authorized for use in food plants.
Only gas mixers manufactured for oxygen service can be used in oxygen service. Because the amount of cleaning and changes in materials required, Thermco does not recommend attempting to convert a non-oxygen service gas mixer to an oxygen service gas mixer.
Since oxygen service parts, thread sealants and lubricants can be used with most non-oxygen gases, it may be possible to convert an oxygen service gas mixer to non-oxygen service. Older gas mixers may be uneconomical to convert. Consult Thermco for details.
Each gas mixer manual contains a data sheet. The data sheet lists the recommended spare parts. At a minimum these parts consist of a repair kit for the cycling solenoid valve, repair kit for the major gas regulator and repair kit for the minor gas regulator. For some systems additional spare parts may be recommended.
Many spare parts are stocked by Thermco, but the best practice is to have the recommended spare parts on hand to prevent a prolonged shutdown. For special gas mixing systems (examples are high pressure or high flow) some parts are not stocked by Thermco and may have lead times of several weeks. In these cases it is very important for the user to stock the recommended spare parts.
Thermco gas mixers and gas analyzers are manufactured with the highest quality components, but over time performance will deteriorate. The life span of a product will be dependent on many factors, including location (indoors is better than outdoors), amount of preventative maintenance performed, temperature variations and air quality. On average the expected life of a Thermco gas mixer or analyzer is 15 years. At a certain age replacement of a gas mixer or analyzer with a new unit will be more economical than the cost of repairing an old unit, especially when considering the cost of downtime to production.
The normal calibration period for Thermco gas analyzers is 1 year. For some special high accuracy analyzers the calibration period will be less than 1 year; for these situations refer to the manual for special instructions.
Gas mixers will have improved accuracy performance under stable temperature conditions. Therefore installation indoors is preferable to outdoors if possible. Variations in ambient and gas temperature will affect many of the gas mixer components, especially the accuracy of regulators in the gas mixer. The regulator diaphragms become stiffer in cold temperatures. If the gas mixer contains a gas analyzer this will also be affected by changing temperatures. The Thermco literature contains information about accuracy at different temperature conditions.
Some Thermco gas mixers are not designed to be installed outdoors because they are not weatherproof. Consult the Thermco literature for details.
Thermco maintains records of gas mixers going back to 1964. Electronic copies of the manuals can be created and sent to the user for a modest fee. Consult Thermco for details.
Gas mixers are made for specific pressure conditions at the factory. Sometimes users will want to raise or lower the original pressures. It is usually possible to make small to moderate changes in the pressure conditions by adjusting the gas mixer in the field. This may involve some hardware changes such as new pressure gauges, orifices or gas regulators. Large pressure changes may require hardware changes that made modification impractical. An example would be raising the pressure conditions above the MAWP of the surge tank. For questions about changing the pressure conditions consult with the Thermco technician.
The user should consult with the Thermco technician before changing the gases used in the gas mixer. Some gas changes are possible and others changes are not possible. Oxygen service and food packaging gas mixer have special requirements. Thermco gas analyzers are setup for specific gas pairs and changing the gases will affect the gas analyzer. Consult with the Thermco technician to see if the analyzer can be modified for the new gases.