I need a sanity check. I have a 200L drum with a 6kW Immersion heater installed horizontally about ~5" above drum bottom and a temperature transmitter installed horizontally about 3.5" from drum bottom. When we turn on the heater (as verified by current readings), we repeateably get no TT response for about 30 minutes, then a slower than expected temperature rise for several hours, and then a significant increase in temperature rise until we hit boiling a few minutes later. What explains this behavior?
Latest theory: There's very little convective heat transfer beneath the heater until the bulk temperature is significantly higher, so the first several hours have a very low temperature increase, and then during the last few minutes where the bulk temperature is closer to boiling you have far more convection so the TT sees more agitation and the bulk temperature averages out, resulting in a rapid temperature increase. I'm surprised the effect would be this dramatic, but I could believe it.
Any other thoughts?
Theories ruled out:
- TT is not immersed in water but exposed to headspace: this was my first guess, thought maybe we don't see significant temp jump until steam present and in contact with TT in an otherwise air headspace but ruled this out, the TT is definitely installed horizontally and submerged in water.
- bad TT: sensor is a dual thermocouple with both reading the same temperature.
- mis-ranged instrument or PLC: confirmed it's ranged fine and ultimately it gets to the expected boiling temperature so the readings are real. Also, did a sanity check on kJ delivered vs overall temp rise and math works out.
- Filter / Delay of some kind on PLC: Checked, everything is normal, no filtering.