An insider’s guide to identifying process heating problems that aren’t revealed by your temperature control.
Walk onto a manufacturing floor dealing with quality issues, and chances are good you’ll hear something like:
“It’s not a heating problem. I have the temperature spot on.”
And in many cases, that’s correct. The controller indicates the right temperature. The heater is running properly. There are no current alarms on your machine.
But the process continues to generate defectives, reject rates rise, process times go out of spec, or the quality of the product begins varying from batch to batch.
This is a very difficult problem to address for many manufacturers. When product problems occur in a process, and the temperature control is displaying an acceptable reading, it means that the problem exists somewhere else.
More specifically, in a great majority of situations, there is some issue with heat distribution and/or heat transfer.
In many cases, the challenge isn’t generating heat. It’s delivering that heat efficiently and uniformly throughout the process. Understanding the fundamentals of industrial heat transfer can help uncover issues that temperature readings alone may not reveal.
Perhaps one of the biggest confusions when it comes to thermal processes is regarding the terms “heat” and “temperature.”
While we can certainly relate to both concepts, heat and temperature are very different from each other.
Temperature is just a measure of heat present in an object. A heater may indicate a certain number of degrees Celsius or Fahrenheit, but that’s all that number says: that there is a certain level of heat present in the specific spot where your sensor was installed.
There are cases where the same exact temperature produces completely different results in multiple production lines.
Think about an extruder where the temperature control shows exactly what you need in the barrel.
Everything seems to be working perfectly well. However, you start getting problems such as:
Again, the temperature reading provided by your temperature control is just that, and it doesn’t tell you how evenly heat has been distributed in your process.
This is the source of the problem.
A manufacturing process doesn’t react to average heat. It reacts to the minimum and maximum values that it receives.
That means that in case the heat distribution in your process isn’t good enough, your controller may be showing the correct reading. At the same time, hot and cold zones may appear in:
In case of an extruder, lack of temperature uniformity will probably never come as an obvious heater problem.
Instead, signs will appear in the form of:
And it can take months to identify the issue.
Another possible reason for problems occurring in a manufacturing process is related to improper sensor placement.
Many operators and engineers are under the misconception that temperature readings taken via a thermocouple represent actual process temperature.
However, it should be clear by now that a thermocouple provides temperature readings at a certain spot, which means that it may not necessarily reflect the overall process temperature correctly.
The accuracy of your process data depends heavily on sensor selection, placement, and installation. Learn more about our industrial thermocouples and temperature sensing solutions. To learn more about our products here.
As an example, you may have your thermocouple placed closer to a heater than to the material being processed.
This means that you will:
In short, your controller was telling you the truth; it just told you a very incomplete story.
Thermal lag describes the difference between heat generation and absorption.
This difference arises due to the fact that heaters warm up much faster than whatever process they were supposed to provide heat to.
Therefore, while you may receive instant readings from your process control that everything is fine, the process may still be experiencing issues.
Typical reaction to decreased production output is to increase the temperature setpoint in order to make the machine work faster.
Sometimes, it might actually solve the problem.
But most likely, you just increased the temperatures to compensate for heat transfer and/or heat distribution issues.
Which resulted in higher energy usage.
Watt density describes how many watts of power are produced by a heating element in each square inch of its surface.
A heater with too high of watt density may create hot zones while showing perfectly normal temperatures in the process.
Consequences include:
As a general rule of thumb, high watt density isn’t automatically a bad thing.
However, it needs to be carefully considered together with all aspects of your process — including type of material, its conductivity, required cycle times, operating temperature, etc.
A heater can produce plenty of thermal energy. But if it cannot properly transfer that energy to your process, your production will suffer.
Several factors determine efficiency of heat transfer. These include:
For instance, a cartridge heater may be working just fine with regards to temperature control, but unable to properly distribute heat because of its improper fit.
To overcome this problem, operators tend to raise the temperature, which leads to more energy consumed and more strain put on your system.
Sometimes, improving heat transfer can achieve better results than increasing process temperature.
Here is another interesting question: how much time do suppliers dedicate to discussions about your processes before they recommend their products?
In case you deal with manufacturers who specialize exclusively in catalog products, your conversation will start immediately after the introduction.
On the contrary, experienced heating engineers ask questions such as:
Answers to these questions can lead to far more relevant product recommendations.
So, how can you fix your processes and improve their performance?
In case your temperature control is showing acceptable temperatures, but there are defects in production, chances are there is some issue with your heating system.
Possible reasons include:
The good news is that if you understand this list of issues, you’ll be able to diagnose problems, lower waste rates, and increase your process reliability. To get an opinion from us, get in touch with us here.
Of course, it’s important to keep an eye on the right temperature, but what really matters is how consistently you can get your process heated up.
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