The diathermic oil boilers are manufactured as having spiral oil pipes,
which are wrapped as to keep the speed of oil at optimum rate. According to
the capacities, pipes are wrapped as one, two or three screw-steps.
The regions that has no heat transfer to thermal oil are coated with
refractory material. Boilers body is insulated against to the heat losses
with 10 cm glass wool and 1 mm thickness of galvanized sheet metal.
Appropriate usage up to 350C.