Why heater before operation will be into the air in the device?
The first, air expansion coefficient is big, the residual air is heated in the airtight container is in danger of explosion overpressure. Second, low thermal conductivity, air pipes in air part of can not get effective cooling during heating, may cause damage of partial wall overtemperature. Third, in line with air may form the "water hammer", a strong shock.
At high temperature, heat conduction oil and oxygen reaction, coking carbide which affect service life.
General way of heater is a heating medium for the required material heating, heat conduction oil heater, for example: air does not do, unstable pump pressure, the air blocking in the pipe, heat conduction oil can't circulation, quantity of heat cannot be passed on to the material.
