heater power loop - break SSR control signal or power

[tjagiello]

Hi guys,

I've got a 2kW heater which is controlled by a PID done using a suitably rated SSR (actually a bit over-the-top one as it's 50A, but I didn't want to buy another lower-rated one). This is inside a machine which has other PID controllers, two solenoid valves and some electronics, all powered up from 230VAC. I want to install two switches, one to energize everything but the heater and another one to turn the heater on as well (in addition the heater won't turn on before the first switch is turned on). I don't want to use a big hefty 15A switch to turn on the heater and decided to use a smaller rocker switch and do the heater engage via SSR (AC control signal). So here's the question:

Do I put the rocker-switch-controlled SSR on the power supply line to the heater, or do I put it between the PID and the PID SSR (so effectively on the DC control line)? Is there's a rule that says you should always break the power supply line instead of the control line? I'm asking because the heater SSR gets a bit hot during start up (the heater is inside a boiler) and I would like to eliminate the need to put another SSR right next to it.

Of course I can do it both ways but I was curious which is the preferred one? Would appreciate any suggestions.

Regards,
Tom

 

[itsmoked]

Without a doubt you should use a measly 15A switch and break the actual heater power - NOT the SSR control lines.

SSR's fail ON with some regularity! You want to be able to kill the heat with a switch meant to "kill the heat". Most switches of any quality at all can do 15A.

Also the SSR turn ON signal from most PID controllers is just barely 5VDC on a good day. This low voltage demands special switches that have gold contacts for reliable operation.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

It is common safety practice that heaters have a mechanical contact that is controlled by an over-temperature limit controller. A manual operated switch does not really satisfy this requirement. So if the only purpose of your switch is to be able to turn-off the heater without affecting power to anything else you can do it either way you suggest. However, you do need to consider what will happen when your ssr or controller fail to full power mode. Eventually, they will fail. Sometimes the consequences can be nasty.

 

[itsmoked]

Very good point Comp!

If this is a system that runs unattended then you should have a manual reset overtemp switch monitoring the chamber temp. I too have seen the results of stuck ON heaters in an oven situation. I was amazed the facility didn't burn entirely down.

tjagiello; I your case of a boiler the risks are cranked up several fold. Hopefully you have a blow-off in case your heat runs away for any reason.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[amptramp]

Think electric water heater controls.

Ditto: DO NOT rely on a SSR for power isolation.

 

[tjagiello]

Hi guys,

thanks for the tips!

Ok so to sum up:

- the second switch will be a 16A sealed switch (just for additional protection as it might operate in a wet environment) and will be used to trigger the heater power supply loop.

- there is an over pressure valve and a check valve between the pump (that fills the boiler) and the boiler, so if the heater gets stuck in the ON position, the pressure will rise and the OPV will open. Do you think this is enough or should I put a blow-out valve as well?

- for additional protection (as mentioned already) a thermal switch would be a good idea. I was thinking about using a thermostat but that will simply turn the heater off for a while, until the temperature goes down below the thermostat setting, so it's a bit useless. Is there anything I can use that's more like a thermal fuse? so blows above a certain temperature and stays broken, until you change it (like a fuse)?

Regards,
Tom

 

[wayne440]

" Is there anything I can use that's more like a thermal fuse?"

A Google seach using the phrase "thermal fuse" should yield myriad results. Common applications are coffeemakers, clothes dryers and the HVAC industry.

 

[waross]

You need a "High limit" switch. This is similar to a thermostat however the temperature is usually fixed and the unit must be reset after it operates on high temperature. If you are applying heat to a closed vessel a high pressure safety valve is needed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

You might peruse this site to get a feel for the subject:

http://www.inspectapedia.com/heat/Aquastats.htm

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

One more tip: a pressure relief valve on any boiler must be placed so that it vents vapor - not liquid.

 

[tjagiello]

Hi guys,

the boiler is going to be fully filled with water as it's going to be sitting at around 93*C-95*C, so I can't really put a relief valve on it, other than an OPV. Water under 9bar of pressure will be moved through the boiler, so it's mostly going to act as a heating device (to disperse water at around 90*C).

Regards,
Tom

 

[amptramp]

Based on your last post which states the water temperature in the "boiler" will be within 5C - 7C of boiling and already under pressure, I would suggest posting your question in the Boiler and Pressure Vessel Engineering forum. I am sure the ASME codes to which you must adhere will be quickly pointed out.

Obviously, your "boiler" is a potential bomb. Check out the video in the link below to observe what a domestic water heater without proper "control" can do.

http://www.videosift.com/video/Mythbusters-Water-Heater-Rocket

 

[tjagiello]

Hi amptramp,

I understand the damage it can cause, but with a fitted thermal protection cut-out and an over pressure valve set to 12bar it should fail safe, right? Sure those protection devices can fail and it will blow anyway, but anything an fail really. In idle mode it will be sitting at 93-95*C, the 9bar pressure will be applied from time to time for around 30-40s not more, so it won't be at full 9bar pressure all the time.

Thanks for pointing out the forum though, I will ask my question there as well as I want this to be safe.

Regards,
Tom