Hot Zones - Choosing the Right Hot Zone for Your Application

Interview conducted by Jake WilkinsonNov 14 2016

Hot zones are arguably the most crucial component of any heat treatment furnace. With a multitude of different designs on offer, determining what hot zone is best for your application is becoming increasingly difficult. To address this problem, Plansee has developed a range of three different, customizable, hot-zones.

AZoM spoke to Bernd Kleinpass and Peter Mallaun of Plansee about the important differences between hot zones that purchasers should be aware of, the new range of hot zones they have developed, and which will best suit your needs.

Why is the correct design of hot zones important in the heat treatment industry?

The hot zone is one of the key components of a vacuum furnace or other furnaces as well.

It contributes to the performance of the whole system. It has an impact on the process quality, the temperature uniformity, the heating and cooling rates and the furnaces energy consumption.

The hot zone design also influences the total cost of ownership because it determines the service time, spare parts demand, and maintenance costs.

What are the different components of a typical hot zone and what roles do they play?

The main component of hot zones are the heating elements  together with the element support, the shield pack which works like a heat reflector, and the load support. Products are placed on the load support, which keeps the goods in place in the work zone.

There can be other features as well such as a gas quenching system or other customized components. We provide a questionnaire which allows our customers to select specific options which are frequently found in hot zones but that not everybody needs. For example, gas quenching systems are only required for situations where rapid cooling is essential.  

What is the difference between a full metal hot zone and a hybrid hot zone. What processes are carried out in each type?

The clue is in the name. The all metal hot zone is built up using parts which are made from metal. All metal hot zones do use a small amount of ceramics for electrical insulation.

For high temperatures the dominating metal used is molybdenum, and for even higher temperatures, of up to 2,500 °C (4,532 F), we use tungsten. Steel is suitable if the hot zone temperature is going to stay below 1,000 °C (1,832 F).

Hybrid hot zones are built using a variety of different material families. Metals still play an important role but graphite and ceramics are also used. The graphite and ceramics provide thermal insulation as they can be introduced as fibers, which display great insulation. The higher insulation and the lower construction costs means hybrid hot zones require less investment to purchase and operate.

However, all metal hot zones function better in terms of product quality as they create the best furnace conditions. The vacuum level is higher, the heat transfer is faster and there is no contamination. Hybrid hot zones can contaminate the product due to reactive elements, such as carbon, being leached out of the graphite. This is problematic when heat-treating sensitive products such as titanium or stainless steel.

All metal hot zones are used in high demand industries, such as aerospace, electronics and medical, or for processes such as metal injection molding. Whereas hybrid or graphite hot zones are more commonly used in heat treat shops, where less sensitive materials are processed.

How do the different metals used in hot zones affect the hot zones function?

As said before, in all metal hot zones different metals are applied, and for the design of molybdenum hot zones (1,000 – 2,500 °C / 1,832 – 4,532 F) different qualities of molybdenum materials are available, which differ in  their properties and performance.

Doping changes the qualities of the metal, for example doped lanthanated Molybdenum (ML) enhances  high temperature strength, ductility and durability.

Molybdenum is fantastic for use in hot zones as it has a high melting point, low vapour pressure and is extremely effective at reflecting heat. In fact, molybdenum is seven times more effective than steel at reflecting heat, so when you have one layer of molybdenum it has the same effect as seven layers of steel.

What different hot zones do Plansee provide?

Plansee is fully dedicated to producing hot zones and not complete furnaces. Our hot zones can be small or large, or square, round, horizontal, vertical, and function at temperatures between 1,000 °C (1,832 F) and 2,500 °C (4,532 F). In addition to this, as I said earlier, you can also have a vacuum or gas pressure in the furnace, so the variety is very high.

This complexity is not a problem for our customers. All they have to do is tell us their demands, whether its specific details of a furnace such as type, size, shape and just what processes they plan to use the furnace for and what conditions they require. We’ll then provide the correct, customised solution for that individual case.

We have also now introduced three hot zone packages - the basic hot zone, the premium hot zone, and the so called Enerzone. These all have the same  size, but offer different features, qualities and prices to fit each different customers needs and budget.

What are the key differences between these three different hot zones?

The basic hot zone has a reliable design and it works very well. It is engineered to be an industry standard and mainly uses pure molybdenum and some lanthanum dope. High quality materials are used for the heating elements. The basic hot zone is the most cost effective solution for less demanding conditions.        

The premium hot zone is a more advanced hot zone. It uses high performance materials such as molybdenum-lanthanum and titanium-zirconium-molybdenum (TZM) and has additional design features for the hot face and  the hearth construction, , meaning you can put more load into the furnace. You will see less wear in the furnace, and better performance which helps save money over time. The premium hot zone is very universal and it can be used in nearly any operation.

Our third version is the Enerzone, which is developed for reduced energy consumption. We use more layers of molybdenum in the shield pack and additional shielding for all feed throughs. This significantly reduces the energy loss that occurs due to  heat radiation.

Whilst we have increased the insulation of the furnace we have also reduced its weight, which means the furnace has a 20% lower power demand. This helps reduce the cost of running the furnace and also provides a better temperature uniformity, improving the quality of heat treatments.

There are also other differences between the furnaces. On our website we have a detailed table where we list all of the features and differences between the three types of furnaces, and the individual versions that we provide.

How have Plansee used computer simulation to optimize its hot zone designs?

We believe a lot in computer simulation. These days it's an extremely important and powerful tool and we’re fortunate to have a very strong simulation team.

We apply these tools mostly in the engineering and development phase of our hot zones. You can play around with the design ideas, and materials, and we can see immediately how they influence the furnace conditions and the load on the different hot zone components.

This allows us to optimize at the very beginning and to develop lasting and cost effective solutions. We determine design rules and engineering principles which we then apply to each individual project.

What led Plansee to develop a range of hot zones rather than sticking to a one size fits all model?

As I explained earlier, the furnace market is so diverse, as there are so many different industries and processes that require a furnace. As a result, most customers have specific requirements and they look for a good fit for their needs at a reasonable price.

We believe that with these three packages, we can satisfy the majority of our customer’s needs. The three versions were developed after carefully listening to the needs of market players.

In developing the furnaces, we wanted to make purchasing decisions more easy.  But we also offer fully individual solutions, however, these add time and associated costs to the project meaning they are not always the best solution for all customers.

Do you offer advice to your customers on what hot zone would be most appropriate for their application?

Absolutely. Again due to the fact that we serve so many different applications and industries we have a large range of experience and offer the expertise and advice.

Ultimately, the customer has the final choice and must make his selection. We will help guide them through the entire process. We support from the very beginning into what hot zone would work best for their needs and budget, and we continue like this throughout the entire duration of the project.

Do you offer hot zone retrofitting to customers who already have a furnace or do you allow other furnace manufactures to supply your hot zones?

Yes, we have cooperated with several leading OEMs for many years and they use our hot zone solutions in their original furnaces. The end users can order a furnace with a Plansee hot zone and then this is of course the best solution from the very beginning.

Also, there are many older furnaces in the field already, and often their hot zone design is not updated or the hot zone design cannot fulfil new requirements for a process. In these cases we also offer retrofits and upgrades of existing solutions for end users.

In such cases we often work together with OEMs because sometimes not only a hot zone has to be retrofitted but maybe other components as well. But we also work directly with the end user to  replace the hot zone only.

Where can our readers find out more about your new hot zone range?

The easiest and fastest way to have a look on our homepage. There you find a lot of information about our products and the materials we use.

You'll also be able to find some interesting and simple tools, so for example you can estimate energy and cost saving when you use the Enerzone by just putting in some basic data. You can then find out the first price indication for the hot zone and get in touch with one of our technical experts if you want to know more.

You can also reach out to our local sales team who would then provide you with more information and the detailed quote, whatever your request is.

We believe we make it easy for the customer to get in touch with us and we are happy to help and serve them as well as possible.

About Bernd Kleinpass

Bernd Kleinpass has more than 20 years of engineering and sales experience in thermal processes.

His team of product managers and design engineers have totally dedicated themselves to build the ideal hot-zone for every customer.

Bernd holds an PhD in engineering and has an extensive background on high melting metals and their application in high-temperature furnaces.

About Peter Mallaun

Peter Mallaun is involved in designing new and custom products, working closely with customers to better understand their requirements and delivery top customer satisfaction.

He is a skilled engineer, working in the field of thermal process engineering for 16 years.