100kW Heater With Thyristor Controller

So why don't you just order a strategic spare in time for the next campaign start-up date?

Element life is significantly decreased by by the thermal shock (dtemp/dt) caused by the initial high current due to low cold resistance and this obviously applies only at the first seconds after energising, and from a more predictable life decrease by prolonged final high element temperature. For the second factor gradual HTF temp rise is irrelevant, but initial current IS very important for element life. Now the time constant we are talking about on applying "soft-start" must be seconds to minutes, not hours, so a solution resembling constant RMS current (using variable duty cycle on your SCR) is universally best. S.M.

"Please help us to decide the best way to operate the heater."

Employ a consultant that knows the process.

Where are you based? I'm not offering to help, I want to be sure I'm at a safe distance!

you can bring the heaters to the 20% of temp as recommended procedure which will prevent the thermal shock and after that you can decrease the temperature rising curve,s time .i don't think there will be any risk of life span decrease because every manufacturer keep the minimum 10% tolerance in heating products .

secondly the fluid in your system is heat exchanger which will never let the heater body to reach on highest temperature in short time .

I would be concerned about thermal shock not only to the heaters, but the rest of the process. Perhaps the slow warm up is limited by the thermal constraints of other machinery parts and the process itself.

I know in the paper industry the dryer cans are warmed up slowly to minimize thermal stress, and allow condensate in the system to be discharged and dried out. You just cannot apply full steam to a cold cast iron drum without many undesirable consequences.

Perhaps there are other mitigating factors in your process.

There's reasons why this guy posted as an AP. The lack of knowledge and the resulting catastrophic disaster because of the lack of knowledge, lessens the chance of his identity being found out

And, your reasons?