Liquid Coatings Q&A: Low- and High-Temperature Oven Control

Q. Why is it difficult for an oven to operate at both low and high temperatures?

A. When purchasing an oven, it is best to put all temperature requirements in the specifications.

Low temperature is classified as about 200–500°F and above. Between 750–800°F is considered high temperature. Consistent temperatures under 200°F can be difficult depending on a few factors:

• Original oven design: Is this a high-temperature oven that needs to be decreased?
• Adjustability: Is the oven gas or electric? Gas ovens have a burner turn-down ratio, while electric ovens have adjustment capabilities using the controller and the electrical element.
• The types of recirculation and exhaust fans and the design temperature range and limits.

 

Low-temperature ovens that need to be increased pose the biggest problem due to the construction of the heater box and oven shell. To protect a facility’s combustible structures—such as the floors and ceiling—the maximum external temperature of an oven is 160°F, although some companies have rules that require a lower temperature. Moreover, a larger burner or hotter element can’t simply be added to an existing structure. Can the fan handle hotter air? Hotter air requires sufficient air to keep the heater box cool enough.

In the case of a high-temperature oven that needs to be decreased, the limitation is whether or not the mechanical elements can be turned down. Can the heater source, burner or element be turned down? Some items just don’t have the turn-down ratio. And even if they do, you may have to do some reprogramming.

We recently had this issue with some new ovens (set to run at 300–500°F) that needed to run at 115°F. We had to examine the burner capability and several other factors. We were lucky because of our equipment versatility and the fact that the oven could be reprogrammed. However, this is not always the case.

Other roadblocks: The recirculation fan may put off heat and many high-static fans will raise the oven temperature. On a continuous run and a 30°F rise in temperature, an oven may get much hotter. You can solve this by speeding up the exhaust fan to exhaust and bring in more air to cool the recirculation air and oven interior, but this may require a new exhaust fan.

And, like I mentioned, some burners don’t have the turn down ratio. The flame pulsing on/off can cause spikes in temperature and irregular heating. (Electrical ovens have some of the same issues.) In addition, occasionally thermocouples may need to be changed or added.

A forward-thinking approach to these issues is best. Quality oven suppliers optimize oven design to meet specifications.

 

Marty Powell is a territory manager for Global Finishing Solutions. Contact him at mpowell@globalfinishing.com or at 800-848-8738.

 

Originally published in the May 2016 issue.