Which is the better system
This will be very dependent on the circumstances of the heat treatment operation as to which best suites the operation. However, if you are operating with a single endothermic gas generator, and if the system is all that you have for atmosphere generation, then it behooves the company management to ensure that the generator is well maintained. You can liken the endo generator to the human heart. If your heart stops beating, then it is checkout time!! The only thing that is likely to save you is a heart transplant or some heart resuscitation method.
The endo generator can be likened to the heart. If the generator goes down, then you are not producing a protective atmosphere for the heat treatment furnaces. So it is necessary to have either a good preventative maintenance program or a reserve generator. In a nutshell, the generator is then the ‘heart’ of the heat treatment operation. Preventative maintenance is much more superior to catastrophic maintenance and is more productive.
A good maintenance program would include a planned ‘burnout’ procedure on a regular basis. The generator furnace builders recommend a burnout frequency of one to two weeks. The accurate costing for the cost per cubic foot of generated process gas can only be determined by the cost of city gas in your particular area.
A reason that some companies have gone over to a pre-mix gas is the problems that occur during the winter months of gas spiking. The gas companies find it difficult to guarantee a stable and consistent gas analysis. This combined with atmospheric pressure and variable humidity levels make it difficult to ensure consistent levels of gas chemistry.
Alternatively, some companies have opted to take the route for the pre-mixed gas with Nitrogen/Methanol. There are very distinct advantages and disadvantages with this system of atmosphere generation:
- No equipment maintenance, other than the furnace into which the atmosphere is delivered to
- No spares to be carried
- The system does not require regeneration or burnout
- It is usually an outside location for the storage tanks so it does not contribute to any heat losses into the heat treatment shop
- The systems chemistry is not influenced by atmospheric pressure and humidity
- It is a very simple system to operate
- Base cost of pre mixed gases which make higher operating costs
- The user company has to make a secure storage area, which involves either the manufacture of concrete plinths, or a separate storage area.
- External source (gas supplier) controls the nitrogen purity and the methanol quality
- Occasional ‘burps’ occur from the methanol tank
So which system do you choose? If you are generating furnace atmosphere with an endo gas generator, the generator becomes either redundant or kept as a reserve backup.
If it is a new startup heat treatment operation, then one has a choice of route to follow for the most economic and consistent gas chemistry.