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Multistage heater management

We have a two stage heating (2x 22.5kW) system for an oil centrifugal purification process. The regulation needs to be maintained within one degree to allow the liquid interface to be stabilized, but the heater only is currently piloted by ON/OFF thermostats for each of the stages. The oil inlet temperature is varying between 30 and 60 degree depending on other part of the plant (diesel engines), and outlet needs to be 80 degrees sharp.

We would like to improve the system and use a PID regulator such as the Piccolo 108 or 3208 with 3 outputs, each piloting 3 stages of 15kW in a single heater enclosure. Two stages are operated with traditional relays, and one stage with a SSR in burst mode.
We would like the heater output to be pseudo linear as follow:



While we have a good experience with traditional PLCs, I would like to come up with a cost effective minimalist solution involving one regulator only with 3 control outputs, one alarm output, one logic input, and the usual benefits of using an autotune PID regulation.

1/If I setup a module such as the P108 with 3 outputs (1 logic, 2 relay) as heater, I assume that all 3 outputs are going high at the same time if the heating demand increases?
2/ Is there a way of setting the internal logic to have outputs relays going high at different levels of demand ?
3/If none of the above is possible with the P108 or 3008 series, what would be the minimal range of product that I can use to achive the above result (through logic internal wiring I suspect)?

Thanks in advance.
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