Single phase and 3 phase supply to same equipment

[Stan80]

I have a single phase stainless steel pasteuriser that has two control boxes attached to it. One box houses the control for the electronics and motor for the mixing paddle. The other box houses the supply to the water jacket immersion heater. The immersion heater is 1.5kw single phase and is very slow to heat the water jacket. At the moment, the power cable enters the control box and a tail feeds into the immersion box.
There is single and 3 phase in the building. Is it safe to replace the single phase immersion with a 6kw 3 phase one and have a separate supply to that box?
It will then have both a single phase cable to the control box and a three phase cable to the immersion box.

 

[westward10]

Can't you use three phase for all of it including the control side.

 

[Stan80]

Can't you use three phase for all of it including the control side.

I had thought about that, but the controller box has a single phase to three phase inverter speed controller for the three phase motor and I'm not sure about the supply requirements. Rather than mess around with the supply to that, I thought it would be simpler to just run a separate 3 phase supply to the immersion box, that way it would be much simpler to work out the power requirements for the new immersion.