The heat exchanger is a good idea. It makes me remind the Lava Boiler in GT5.
It can be designed as a special kind of boiler.(hot coolant–coolant : water–steam) But the normal boiler (HU : water–steam) have already existed.It seems only can produce HU… 
Therefore,we have already had liquid burning box.
The HU can drive machines.But using its HU to drive machines is a little unreasonable. In reality, heat mainly produce by the directly burning of fuel (coal or oil) or electric heat(eddy current or electric resistant).Heat exchangers are mainly used in nuclear plants to make the coolant re-generate.