President of EPS, Ken Klein, has been sharing tips about what to look for when considering a large industrial oven purchase. Last week he told us about the importance of air flow and rate of rise. This week, we’ll hear from him about temperature uniformity and the challenge of temperature control:
Uniformity – What temperature uniformity do you need? If there isn’t a spec we will assume it’s not so critical we need to do something special to our normal design. Many spec writers confuse uniformity with accuracy. Accuracy refers to the capability of a control instrument to achieve desired temperature that is stable and repeatable to within a certain tolerance. Uniformity, on the other hand, is largely a function of airflow. If airflow is managed properly, the air temperature uniformity throughout the chamber will be tight, assuming you don’t block the airflow completely with your load.
Most builders will state the oven zone in which uniformity can be expected, i.e. to within what distance of the walls, ceiling and floor. This defines the “uniform zone”. Your builder should advise you that if he has to test the uniformity before the unit leaves his shop he will state all uniformity statements and certifications apply to an empty oven at steady state conditions.
One more note on uniformity. If you specify, for example, + 10F at 300F, can you expect the readings you get during a survey to be 290F to 310F throughout the “uniform zone”? Only if you have done one of two things:
1) By experimentation you have been able to physically locate the sensor (the control thermocouple) at a point that represents the mean of all temperature readings in the chamber, or…
2) You have taken all readings, found the control thermocouple to be closer to one end of the range (say 305F when readings range from 290F to 310F) and put an “offset” into the controller, which “adjusts the reading of the sensor so that it appears to read the mean of the range.
Control – this often proves the biggest challenge for a spec writer, but it has pretty simple rules. The simpler your operation the simpler the controls should be. Let’s go from one extreme to another. Say you are leaving the temperature in the unit at one point all day, and taking your parts in and out several times a shift. A single set point controller will suffice, teamed up with a door switch that shuts down the heat and circulation when the door is opened for loading and unloading. At the other end of the spectrum is the application where perhaps you’re composite curing and you need to:
• ramp up and down at a controlled rate
• acquire data from vacuum transducers and part thermocouples
• Make data available to your computer system for archiving and print out. This most often requires a graphical human-machine interface (HMI) and a PLC (programmable logic controller).
In between these two extremes are the programmable controllers that will allow the cycle to run automatically, ramping up, holding, cooling down and shutting off.
Next up: Ken Klein concludes the Oven Selection and Specifications series.