So then a question about the power output modulation. If reducing the power output using the controller just reduces the number of times per cycle that the element is fired (presumably at full power) then is there still a chance of scorching during these short bursts of full power, if solids have settled on the element?
Said another way, is there any practical difference between running a 9000w element at 50% modulated power compared to running a 4500w element of the same size at full power. Trying to understand the value of finding an ULWD element vs just using a higher watt density element throttled down to lower power.
Thanks
An important consideration when addressing this question is the mode of power modulation used. Most low cost controllers use a pulse width modulation mode. In this mode you have a fixed cycle time, which can usually be adjusted while not in operation, but when controlling stays constant. Then the controller turns on full power for a percentage of the cycle time that matches the percent power setting. For example, if the cycle time is 2 seconds (a common minimum cycle time for PIDs), and the power setting is 50%, then the controller will turn on full power for 1 second, and then be off for another second. For 25% power, you will get full power for 0.5 seconds, and then off for 1.5 seconds. Whether or not full power for 1 second is going to be a problem or not depends on how fast the element surface heats up when power is applied. If the element surface can reach max temp in 0.1 seconds, then you might have a scorching issue, but if it takes 2 seconds for the element surface to reach maximum temperature, then it wouldn't reach max temp if only on for a second or less, and you have much less chance of scorching.
The max surface temp an element can reach is going to depend on the watt density of the element, the fluid flow over the element (and whether it is laminar or turbulent), and whether or not there is any surface "contamination" on the element which would slow heat transfer from the element to the fluid.
All else being equal, the max surface temp of a 9000W element PW modulated to 50% is going to be higher than the max surface temp of a 4500W element running at full power, unless the heat up time of the element is longer than the cycle time of the PWM.
The Auber EZBoil DSPR controllers use something different than PWM. They turn on and off for individual cycles of the AC current/voltage waveform. So, that when calling for 50% power, they will be on for one AC cycle, and then off for the next. Thus when calling for 50% power or less, the maximum on time for the element will be 16.7 millisecond (0.0167 sec.) This short max on time will keep the element surface temp from reaching what it would with PWM. So, you are less likely to have scorching issues with this type of power modulation than with simple PWM.
Learn more about power modulation modes in the Appendix (starts on page 12) of the attached .pdf.
In any case, you don't want to use an element with more power available than you ever actually intend to use, as you will have better granularity of power control if you choose an element that you can run from 0 to 100%, rather than 0 to 50%.
The above discussion just scratches the surface of this complex topic.
Brew on
