Process Temperature Control Rings PTCR Part 2/6 – Application. Soak time. Atmosphere.

In this and in the next newsletters, we would like to give you useful tips and recommendations for the use of PTCR.

Did not read part 1 of our newsletter? Here you can find all details about ring types, right choice and advantages of PTCR.

5. How do I use Process Temperature Control Rings PTCR control rings?

The rings have to be placed at any location in the kiln where a determination of the thermal performance is desired. Rings do not have to be measured before they are put into place in the kiln because differences are statistically corrected and integrated in the temperature conversion chart. Sticking of the rings is prevented by placing a small amount of aluminium oxide powder/plate or silica powder/plate underneath the ring.

After exact positioning at the desired position in the furnace, the firing process can take place.

After termination of the firing cycle, the ring diameter(s) has to be measured. Make sure that the rings are identifiable regarding their position in the kiln for mapping. Every ring only has to be measured once, the special PTCR Micrometer ensures an operator independent, reliable and accurate reading of the ring diameter. In order to assure the reliability the diameters have always to be measured at a fixed position e.g. at the middle numbers of the batch number or the two T’s impressed in the ring. (see figure above)

The outer ring diameter can be converted into a Ring Temperature (RT) using the temperature conversion table delivered with every new batch of PTCR. The RT-value thus obtained is an accurate representation of the total (cumulative) amount of heat transferred to the PTCR and your products during the firing cycle.

The first time the PTCR is applied, different (trial) runs with different parameter settings have to be made and the final products have to be compared to the quality standard. In this way the optimal process settings and RT values are coupled. In a new run, if the RT values are the same, the firing conditions have been the same and it is assured that the same amount of heat was transferred to the ring and your product.

To video

6. How is the performance of PTCR affected by the soak time?

The PTCR will continue to contract as long as it is exposed to heat. Effectively this means that at a constant set temperature, the RT value will be higher, the longer the soak time is.

It has to be emphasized, that RT values do not correspond to the absolute temperatures, because the heat transferred to the ring is measured, a longer soak time at the same top temperature will cause the ring to shrink further, resulting in a higher RT-value. However the PTCR allow the accurate determination of temperature differences within and between certain kiln loads. The best approximations are obtained by applying a ramp up speed of 120°C/hour and a top temperature hold time (soak time) of 60 minutes. PTCR can be used up to 10 hours at maximum temperature. Please ask for the correction graph for hold times of more than 60 minutes.

7. Can I use PTCR under any atmosphere?

The PTCR originally has been developed for use in heating processes in air. However the ring may be applied under different atmospheric conditions as well, provided some rules are taken into account. We advise clients to contact our customer service desk in case of using the rings for firing processes under reductive or vacuum circumstances. In this way the specific advice can be given in order to get optimal results. 

As a general rule it can be stated that, under vacuum conditions and reductive atmospheres like N2/H2 mixtures, the rings have to be pre treated to burn out the organic binder system of the ring completely. If this is not done properly, the organic material will not disappear as CO₂, but will decompose to carbon.

This carbon, left behind in the ring, will inhibit the shrinkage of the ring, resulting in a lower RT-reading than expected. Under vacuum conditions, the carbon may even precipitate on the kiln wall and contaminate the kiln. In addition to that, under vacuum the RT-reading will be lower than expected, because the heat transfer mechanism lacks the contribution of convection.

Would you like to pre treat PTCR yourself? Ring-specific pre-firing recommendations are available. Please contact us!

In the next part you will read:

• The application areas for PTCR
• The reference temperature table