Method for internally coating ducts with synthetic resin

A method of internally coating an elongated iron or steel pipe, comprising the steps of:

• a. clamping said pipe between seals and incorporating it in a closed gas-circulating system;
• b. relatively displacing said pipe in a first direction through an induction coil to heat said pipe along the inner surface thereof;
• c. sweeping a gas stream along said heated surface;
• d. charging said gas stream with particles of a synthetic resin flowable at an elevated temperature, said particles having a particle size of 5 to 300 microns and being charged into said gas stream at a rate of up to 2000 g/m.sup.3 (STP) thereof whereby said gas stream entrains said particles and said particles are fritted to said inner heated surface;
• e. cooling said gas stream upon its emergence from said inner heated surface;
• f. recharging said cooled gas stream by metering additional synthetic-resin particles at substantially the same rate at which said particles are fritted to said inner heated surface;
• g. recirculating said recharged cooled gas stream into contact with said inner heated surface to frit additional synthetic-resin particles onto said surface;
• h. separating excess resin particles nonadherent to said surface from said gas stream upon its emergence from contact with said inner heated surface to form a particle-free gas stream;
• i. cooling said particle-free gas stream;
• j. recirculating said cooled particle-free gas stream while simultaneously induction-heating said pipe with said coil to effect coalescence of said resin particles fritted to said inner surface by relatively displacing said pipe and said coil in the opposite direction to the movement in step (b);
• and
• k. cooling said resin-coated pipe to room temperature.