Salt baths are often used instead of conventional atmospheric furnaces for many heat treatment applications. The heating medium is a hot liquid compound, normally referred to as ‘salt’, and is contained in a metal pot, heated by external electrical currents or submerged heaters. The items to be treated are immersed and heated rapidly and uniformly to temperatures of up to 700°C. One of the advantages of this process is that air is excluded from the piece being treated. Unfortunately, the baths need to be regularly inspected and this means the removal of the salt, which solidifies as it cools down.
Dealing with Used Salt by Conventional Methods
Until now it has been common practice to pump the hot liquid salt into drums and allow it to cool and harden before cutting open the drum and breaking up the salt for disposal. When the bath is refilled, new granular salt is needed, which needs to be gradually melted. The only way to save the old salt is to grind it down to its original consistency and reuse it. However, this is a time-consuming task, and can lead to hot spots around the heating elements in the bath if not done properly. If not reused, the salt must be properly disposed of, so increasing the costs of the inspection process.
New technology for Dealing with Used Salt
However, Southport-based Mannings has come up with an answer to this problem. It has developed technology to pump out the hot liquid, keep it hot and pump it back into the bath after it has been inspected or repaired. The whole process takes less than two days.
One customer for this service was Flight Refuelling, a company founded to develop air-to-air refuelling equipment that has supplied the Royal Air Force for more than 30 years. It specialises in the design and manufacture of structural aircraft parts and has a reputation for innovation and high-quality engineering. It also uses one of the largest salt baths in the UK, and consequently spends a lot of time and money on inspection. The company turned to Mannings to see if the cost and time could be reduced.
For this project, Mannings used a large 12-tonne capacity holding tank fitted with electric heating elements. The power was provided by 50kVA transformers and the required temperature regulated in zones within the tank. The hot liquid salt was pumped into the holding tank using a special pump designed and built by Mannings. The client was impressed - Andy Wills, Facilities and Safety Manager at Flight Refuelling, said, ‘We were very impressed by the time and cost savings that resulted from this new method and our next inspection will be carried out in the same way.’
Scalability of Technology
Mannings has developed a range of specialised equipment to cater for all sizes of salt baths. Alan Bannister, Site Superintendent at Mannings, said, ‘Our thermal and environmental expertise is combined by this technology and we believe that there is considerable potential for this application.’
The reduced down-time and cost savings offered by this procedure have been welcomed in the industry and regular contracts are already being carried out. Mannings can also make any necessary repairs to salt bath heating systems during a system’s downtime.