When you purchase a thermal fluid heater, the cost of fuel to operate the heater is a major concern. The amount of fuel needed is related to the thermal efficiency of the heater. Increasing its thermal efficiency reduces the amount of fuel needed as well as operating costs.

An effective way to increase a heater’s thermal efficiency and reduce fuel costs is to install a special heat exchanger in the heater’s exhaust stack. The heat exchanger recovers heat from the exhaust stack that would otherwise be lost into the atmosphere.

This special type of heat exchanger is known generically as an economizer or convection section. It consists of an enclosure that contains coils of pipe, and the enclosure fits between the heater and the bottom of the exhaust stack.

Enlarge this picture The heat exchanger is installed between heater and exhaust stack.

The exhaust gases flow around the outer surfaces of the pipe coils. Thermal fluid from the heating system circulates through the inside of the pipe coils. Consequently, the hot gases preheat the thermal fluid before it enters the heating coils inside the heater. Preheated thermal fluid needs less heat from the coils inside the heater and from the burner that produces the heat. Consequently, the burner uses less fuel.

Figure 1 shows a thermal fluid heater that uses an economizer. It has an output of 13 million BTU/hr. The economizer is expected to boost thermal efficiency by 9 percent, thereby reducing fuel usage by the same amount, which should achieve a significant fuel savings for the user.

The estimated savings are calculated in the following way: If the heater operates at its rated output for 60 hr/wk, and the economizer boosts its thermal efficiency from 81 percent to 90 percent, over a one-year period, the economizer would reduce total heat usage from 50,074 million BTU to 45,067 million BTU. This would achieve a savings of 5,007 million BTU. If the price of natural gas is $9 per million BTU, the savings would amount to $45,063.

Enlarge this picture The pipes are arranged to form coils. The exhaust gases flow around the outer surfaces of the pipe coils.

Economizers, which often have the ability to boost thermal efficiency from 3 percent to 9 percent, depending on operating temperatures, generally are offered as option on thermal fluid heaters. If not, many companies that specialize in economizers can retrofit a unit to most existing thermal fluid heaters, even those from other manufacturers.

Keep in mind though that not all economizers are alike. For instance, in one design, the coils in the heat exchanger are designed to float in the enclosure to allow for free expansion. This is done to reduce stress on the coils. In this design, the use of corbelling (baffles or projections to help control the flow of hot gases), which often creates maintenance issues, is avoided.

Most economizers of this type of design use coils that have serrated fins, which help increase the surface area of the pipes, enabling them to capture more heat for a given length of pipe. This allows the unit to be smaller than a comparable unit containing bare pipe.

While adding an economizer to a heater is an extra cost, it may be a worthwhile investment for many applications. Given the current high fuel prices, a unit could pay for itself in a short period of time.