Duct Heater Problem 2

[QualityTime]

I have a 92 kW electric duct heater. In one general location, the straight portion of the coil termination on all 3 phases has "broken off and disappeared". We shorted to ground and the fuses went. The coils are of uniform color so I don't think we are starved for air...manufacturing defect, bad crimping,????? Suggestions please. I have appended a photo

 

[willard3]

You vaporized the wires before the fuses tripped. Find the problem on your end.

 

[QualityTime]

Willard3, thanks for the comment. What happended was the controls on the BAS which controls the heater was in essence put in a manual mode without knowing that was done and the heater output increased to a point where the fire damper in the hot deck closed. Is it possible that the wires melted....? I would have thought that the wires could take a higher temperature than that....and why just there and not in the coiled part.

 

[Compositepro]

Resistance wire often fails at the terminations. The contact resistance at the crimp increases with time and fatigue occurs in the wires due to the coils vibrating. At 480 vac you will have a very long and stable arc when the wire breaks. This will vaporize the wire without causing any excess heating in the rest of the coil. I don't know why it is not done more often, but doubling over the resistance wire at the termination can make a more durable termination. The current density in the wire is reduced and the contact area increases. (Probably less income from selling replacement coils).

 

[QualityTime]

Hi Compositepro...thanks

 

[ChasBean1]

Your coil at 92 KW can heat almost 10,000 cfm about 30°F. Pretty manly for an electric heating coil. Can you give the mechanical specs? I.e., cfm and desired heat?

 

[QualityTime]

Hi ChasBean1....yes it is a big heater. It services a couple of floors in a building and it is installed in an existing air handling unit and it is a one for one replacement of the old heater which had been in place for 30 years. The old heater was a step feed and the new one is 4-20 mA controlled. It is the workhorse. There is a temperature sensor downstream of the duct heater (TT-0106) and it measures the temperature of the hot deck airflow. The BAS controls the output of the heater based on a programmed temperature reset curve and uses TT-0106 reading to vary the heater output. The system has been under BAS control for two winters with no problems

What I did not tell everyone was that TT-0106 was inadvertently put into “in service mode” and it was set at temperature of 70 oF (as opposed to letting it read the actual discharge temperature). Well, if the BAS says the set point temperature should be 73 oF and TT-0106 continually reads 70 oF …you can guess what the heater was eventually told to output….Things got pretty hot. Regardless there was no damage. I don’t want to get into too much detail but when I trouble shot the problem and got TT-0106 back to normal “out of service mode” I found the fuses blown and the heater wire leads “gone”.

The heater wires have no gray color anywhere so we have never been starved of air. So I am trying to figure out what could have happened because it all seems to have been triggered by this overheating incident. Without going into details about the various integral duct heater safeties and the external safeties I programmed into the BAS on top of those safeties to ensure there is no overheating of the ductwork it happened. It is too long to explain why, but for our purposed her just assume the duct discharge temperature had to have hit at least 165 oF because the fire damper fusible link went and the duct heater was cycling even though there was no airflow across the duct heater. I am trying to do some detective work to understand the problem:

• Did one wire break…did it then vaporize that straight run of wire? Did the flopping wire then touch the frame or did it touch the other two coils causing the fuses to blow…..but how does one explain the missing straight runs of the other two wires.

• When I did a cursory inspection of the good wires I did feel an “indentation” on one of the straight run leads. This kind of leads me to think there might have been a manufacturing defect in coiling of the resistance wires in those locations and the high heat (high amperage) due to this incident put it over the edge and caused the simultaneous break of all three wires….all three wires leads then vaporized and then they eventually touched the frame and then the fuses blew. To me, if there was an indentation in the wiring we have less area in the wire and too high a current…. I will be inspecting the coils under better lighting this week

• Only on one of the ceramic insulators do you see a black mark

 

[ChasBean1]

Great response and well detailed!

You know the root cause: someone set the AI in-duct reading to 70°F such that the heater went full bore to maintain 73°F when the real value was actually masked.

Full electrical heat in a non-flowing air duct means trouble, unless safeties suit the application. A high temperature limit kill switch might be necessary (separate from the in-duct temperature sensor), set for about 130°F near the heater, below the temperature of the FD fusible link.

The proving switch that enables the heater to operate could also look at an airflow value versus a pressure value. There can be plenty of static in a dead duct while having a no-flow condition enabling this to happen.

As far as forensics after the fact… I had a dryer with an electrical element that fried; turned out it was a drum bearing failure causing unusual contact points resulting in short circuits. It’s difficult to tell what happened exactly after an electrical failure, much like trying to diagnose a lightning strike. For now, consider instead the safety elements that prevent the failure in the first place.

One thing: “The heater wires have no gray color anywhere so we have never been starved of air” and “without going into details about the various integral duct heater safeties and the external safeties I programmed into the BAS on top of those safeties to ensure there is no overheating of the ductwork” could hold some answers. There’s complexity built into these statements and one small detail could hold the key…