When you purchase a heater your choice may be between a two-pass design and a three-pass design. Both have their merits. A two-pass heater has a single coil. Burner gases make two passes through the heater. They first pass through the inside of the coil, then around its outside.
Two-pass thermal fluid heaterA three-pass heater has two coils, a smaller coil inside a larger coil, separated by a small gap. Burner gases make three passes through the heater. They first pass through the inside of the smaller coil, then between the two coils. And finally they pass around the outside of the larger coil.
Heatec’s three-pass heaters are noteworthy for their compact size and high thermal efficiency. If limited space is an over-riding consideration, the three-pass heater is your best choice. Otherwise, a two-pass heater like the one at the top of this page is a better choice. Heatec built this heater for a well-known manufacturer of roofing materials. Its input is one million Btu/hour. Except for its expansion tank, the heater is fully assembled on a single skid.Its convection section with finned tubing increases thermal efficiency by three percent. Actually, the convection section can be designed to increase thermal efficiency up to eight percent, depending mainly on operating temperatures. Such two pass heaters with a convection section can equal or surpass the thermal efficiency of our three-pass heaters. And, unlike a typical three-pass heater, the two pass heater has easy access to the coil for maintenance. Moreover, its single helical coil is much longer than those in comparable three-pass heaters. So it has much more coil surface area exposed to radiant energy of the burner flame. Consequently, it has a much lower radiant flux rate than the shorter coil of a three-pass heater. Lower radiant flux rate results in lower film temperature, longer life for thermal fluid, and longer coil life. Heatec builds both two-pass and three-pass heaters and recommends the one best suited for the particular application.
