The choice of a heating system for heating fuel gas is often narrowed down to one of three systems: a process-bath heater, a combination of a thermal fluid heater and heat exchanger, or an electric process heater. A typical process-bath heater consists of a shell that contains a fire tube and process heating bundle. A burner is mounted on one end of the fire tube, which doubles back to an exhaust stack. The shell is filled with a solution of water-glycol, molten salt or oil, which is heated by the fire tube. The heated solution then heats the process bundle and the fuel gas that flows through it.
A typical thermal fluid heater consists of a shell that contains a helical coil. A burner is mounted on one end of the shell and fires through the helical coil. The burner heats the coil directly with its hot gases, which double back to an exhaust stack. Thermal fluid (water-glycol) is pumped through the coil and carries the heat to the heating tubes inside a nearby heat exchanger. The fuel gas flows through the shell of the heat exchanger, where it is heated by its heating tubes. The thermal fluid circulates continuously through the heater and heat exchanger tubes.
Electric process heaters consist of a shell that contains electric heating elements. The heating elements come in direct contact with the heat of the gas as it flows through the heating chamber. The heating elements are bundled together to make them easy to remove if necessary. Since the product is heated directly, there is no need for thermal fluid with electric process heaters.
Initially, a bath heater will usually cost less than a combination thermal fluid heater and heat exchanger of comparable heating capacity. The bath heater is usually a much simpler heater. It doesn't need a pump or a three-way control valve, and it may use either a natural draft burner or a forced-draft burner. In remote areas, a natural draft bath heater can be built to operate without electricity or with a small generator or solar power.
A combination thermal fluid heater and heat exchanger has many important advantages over a bath heater. The thermal efficiency of a thermal fluid heater is usually about 15-percent higher than that of a bath heater. The higher efficiency results in significantly less fuel usage and lower emissions. The thermal fluid heater system uses less liquid and has faster fluid velocity than a bath heater. Consequently, reaction time is faster and provides better temperature control, and there is less liquid to maintain. The heating bundle in the heat exchanger used with the thermal fluid heater is much easier to replace than the coil inside a bath heater, and it also costs less. The helical coil in the thermal fluid heater rarely requires maintenance. Components of the thermal fluid heater and the heat exchanger are subjected to somewhat lower temperatures than the fire tube of a bath heater and last longer as a result.
The bath heater is an open system that is subject to evaporation. Accordingly, it requires routine additions of bath solution. The thermal fluid system is a closed system that rarely requires additions of fluid. An electric heater is also a closed system, but unlike the other two, it doesn't require any thermal fluid.
Electric heaters are extremely efficient since an exhaust stack does not exist. They also emit zero emissions. But they could cost more to operate than the other two because the cost of electricity is currently more expensive than natural gas. Larger units may require more electricity than is available.
Despite some of the drawbacks, electric heaters have some advantages over the other two. Some of those have been covered already, like zero emissions, high efficiency and no need for thermal fluid. They also have a smaller footprint for when space is limited. They have a turndown ratio of 100:1, whereas a forced draft bath heater and thermal fluid heater are limited to an 8:1 turndown ratio. This can be an issue when there are extreme variants in flow.
Many factors come into play when choosing the appropriate heater for a gas system. Seek a manufacturer that will ask the right questions and consider all those factors to help you choose the right solution for your application.
For more information, visit www.heatec.com or call (423) 821-5200.
