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The choice of a heating system for heating fuel gas often narrows down to one of three systems:
- A process bath heater
- A combination of a thermal fluid heater and heat exchanger
- An electric process heater
A typical process bath heater consists of a shell that contains a fire tube and a 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 heats the process bundle and the fuel gas that flows through it.
Heatec's Aquatec bath heaters at a power plant.
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 element comes in direct contact with the heat 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.
Electric heaters and bath heaters are usually limited in size. Bath heater sizes range from about 0.5 to 15 million Btu/hour and electric heaters from 100 kilowatts to about 3 megawatts. (A kilowatt is equal to 3,412 Btu.) But thermal fluid heaters range from 0.5 million to 80 million Btu/hour. So heating capacity could be the deciding factor. However, when all systems offer suitable sizes, it is appropriate to look closely at the advantages of each system.
Heatec’s electric process heater.
Initially, a bath heater will usually cost less than a combination of thermal fluid heat and a 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:
- Higher thermal efficiency
- Lower emissions
- Lower liquid volume
- Faster reaction time
- Better temperature control
- Easier to maintain
- Longer heater life
- Closed system eliminates frequent refilling
The thermal efficiency of a thermal fluid heater is usually about 15 percent higher than a bath heater. The higher efficiency results in significantly less fuel usage.
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. It also costs less than the coil in the bath heater. 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. Consequently, they will last longer.
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.
A Heatec thermal fluid heater and heat exchanger combination for fuel gas conditioning.
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 currently, the cost of electricity is 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 is extreme variants inflow.
There are a lot of factors that come into play when choosing the appropriate heater for a gas system. The people at Heatec will ask the right questions and consider all those factors. They will help you choose the right solution for your application.
For more information on fuel gas heating systems or other heating systems visit www.heatec.com, or give Heatec a call at 423-821-5200.
