An auxiliary multi-pass tube bundle heat exchanger for improving the melting efficiency of melter kettles used to melt thermoplastic pavement marking materials. The auxiliary multi-pass tube bundle heat exchanger includes a heat transfer tube bundle having a plurality of heat transfer tubes in which the flow of molten thermoplastic material reverses directions at least once. Hot heat transfer oils flows around the plurality of heat transfer tubes. Molten thermoplastic material is pumped from the bottom of a melter kettle, through the auxiliary multi-pass tube bundle heat exchanger and to the top of the melter kettle. A drainage system is provided to drain molten thermoplastic material and any settled glass beads form the auxiliary multi-pass tube bundle heat exchanger and a purging system is provided to purge molten thermoplastic material form the auxiliary multi-pass tube bundle heat exchanger using compressed air.