Efficiency factors
Helical coil heaters are inherently efficient for a number of reasons. The spiral path of a helical coil imparts much higher turbulence to oil flowing through it than the straight sections of serpentine coils. Higher turbulence causes better heat transfer and lower film temperatures. Moreover, the oil always fully contacts its surfaces thereby eliminating stagnant zones, which can reduce efficiency and are common in other types of heaters.
But not all helical coil heaters have the same efficiency. Subtle design differences make significant differences in efficiency. Critical design factors include the flame pattern, combustion gas velocity, heat transfer surface area, thermal fluid turbulence, positioning of the helical coil, effectiveness of the insulation and how well the unit is sealed.
A secondary heat exchanger can be installed in the exhaust stack of a helical coil heater to recover heat from exhaust gases and use that heat to increase efficiency. Increases up to 4 percent can be expected. The choice is between two types of heat exchangers: a combustion-air preheater or an oil pre-heater.
The combustion-air preheater is an air-to-air heat exchanger (Figure 24). It uses the heat of the exhaust gases to preheat combustion air supplied to the burner to temperatures of 150–200 degrees F. This reduces the amount of heat that the burner has to produce. Thus, the burner operates at a lower firing rate.
Figure 24. Combustion Air Pre-heater On Helical Coil Heater.
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The thermal fluid pre-heater is an air-to-oil heat exchanger and is commonly known as an economizer (Figure 25). It uses the heat of the exhaust gases to pre-heat the oil as it returns to the helical coil in the heater. This reduces the heat load on the burner allowing it to operate at a lower firing rate.
Figure 25. Thermal Fluid Pre-heater On Helical Coil Heater.
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The actual effectiveness of both types of secondary heat exchangers depends on temperature differences. With the combustion air preheater it’s the difference between the exhaust gases and the ambient air temperature, about 525 degrees F. With the economizer it’s the difference between the exhaust gases and the temperature of the return oil, about 300 degrees F. The greater the difference the greater the increase in efficiency.
The combustion-air preheater usually has a somewhat higher temperature difference. Hence, it usually increases efficiency a little more than the economizer if sized properly. However, there are a couple of drawbacks for the air-preheater. It increases the back-pressure on the burner exhaust and that can make burner adjustments more sensitive and temperamental. And an air-preheater increases NOX emissions to some extent. Apart from those issues the combustion-air preheater is usually more cost effective.
The economizer does not increase NOX emissions and does not usually affect burner adjustments. But it is likely to cost more than an air pre-heater, despite being less efficient. The size of the economizer is governed mostly by the difference in temperature between the exhaust gases and return oil. Smaller temperature differences necessitate larger economizers. As you would expect, larger economizers cost more than smaller ones.
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