In the first part of this two-part article series (January 2016, p. 18), I noted that a 5 percent difference in efficiency can mean a difference of thousands of dollars in annual fuel costs. In this article, I’ll offer advice on how to make the best choices relating to boiler or thermal fluid heater efficiency.
Two major components of a boiler or thermal fluid heater impact overall efficiency of the unit. The first is the basic design of the boiler or the heater pressure vessel. (The pressure vessel contains the water in a boiler or the thermal fluid in the oil heater.) The second is the burner.
Boilers and oil heaters are limited by the efficiency rating that will drop the stack temperature below dewpoint. That number is typically 84 to 85 percent. To exceed 85 percent efficiency, the boiler or oil heater can be fitted with an economizer, which is a heat-exchange device in a boiler or thermal fluid heater that improves efficiency and saves fuel. They typically are made of stainless steel and can operate in wet conditions. The boiler feedwater, which typically is well below the boiler operating temperature, can be heated in the stainless steel stack economizer. Using the economizer can increase the overall efficiency from the typical 83 to 84 percent up to 87 to 90 percent, depending upon the inlet temperature of the water feeding the economizer.
Pressure Vessel Efficiency
For the boiler or oil heater, the goal is to remove as much heat from the combustion process (the burner) as is possible before the exhaust gases exit the boiler or heater. The longer the dwell time in the unit, the more efficiently the heat is transferred into the pressure vessel and then into the hot water or hot oil. Therefore, the lower the temperature of the exhaust gases, the better the efficiency. Likewise, the better the efficiency, the less fuel that is used to produce the same heat to the process.
Unlike the Environmental Protection Agency’s testing of automobiles, there is no agency testing boilers or heaters for efficiency. For this reason, it is important to be cautious about manufacturers’ efficiency claims. Instead, ask prospective suppliers for a guaranteed stack temperature at maximum firing rate. Knowing the stack temperature and the amount of excess oxygen allows you to determine the overall efficiency using existing efficiency charts. (See table 1 for a sample chart or perform an Internet search for “gas efficiency charts.”)
For steam boilers, an efficient and durable design is a four-pass, water-backed, scotch-marine boiler. “Four pass” means that the hot gases from the burner travel the length of the boiler four times before exiting. “Water backed” means there is little refractory (high-temperature concrete) at the rear of the boiler. This style can achieve 83 to 84 percent efficiency depending upon operating pressure and the burner. Of course, heating the boiler feedwater with a stack economizer can bring that efficiency up even higher — to roughly 86 to 90 percent, depending upon the temperature of the feedwater.
For oil heaters, one efficient style employs two sets of coils that are designed to allow three passes of the hot gases over the heating surfaces. Some manufactures have additional methods of scrubbing the last bit of efficiency from the hot gases such as fins on the coils. And, of course, most manufacturers offer stack economizers. These are particularly helpful if the plant needs water heated. The waste heat leaving the thermal fluid heater can be used to heat the water. Without the need for hot water, an economizer may or may not add to the overall efficiency. Ask you heater supplier for a study on your particular application.
The primary function of the burner is to mix the gas and air for combustion and to safely control that combustion so that the firing rate matches the load. (Think of it like a cruise control option on a car.)
How well the gas and the air are mixed and the ability to provide a smooth burn close to the burner head are what determines the burner efficiency. To have enough oxygen to complete the gas combustion process, the burner needs to deliver 15 ft3 of air for every 1 ft3 of natural gas. This will provide all the oxygen the fire needs with 3 to 5 percent excess oxygen.
Because the atmosphere is 20.7 percent oxygen, every percentage point of oxygen contains four times that much inert gas (mostly nitrogen). Excess oxygen of 5 percent results in 25 percent excess air that is not needed or used in the combustion process, but it will need to be heated anyway. This excess air is wasting heat and lowering the efficiency.
For this reason, the most efficient burner is the one that burns clean — with little or no carbon monoxide (CO) and with the least amount of excess air. Too little air will result in a burner with unburned fuel, which translates to carbon monoxide.
The most efficient burners can achieve complete combustion with 2 to 3 percent excess oxygen and without making CO. Do not be distracted by the carbon dioxide (CO2). CO2 is naturally produced in the combustion process. The higher the CO2 percentage, the lower the oxygen percentage, which results in the greater the efficiency. If it is not acceptable to generate CO2, an electric heat source will be necessary.
The last consideration when discussing efficiency is the burner tune, or the gas/air mixture. Keeping the burner in perfect tune during all firing rates means changes in tuning between low fire and high fire.
Today, almost all automobiles have onboard computers with exhaust-gas analyzers that keep the motor in perfect tune, up and down mountains and during any weather and air temperature changes. Likewise, boiler and heater burners have a similar option typically called an oxygen trim system. (This feature is not standard on most burners.) If the burner produces 8 MMBTU/hr or more, the energy savings achieved using computer control with an exhaust gas analyzer will pay for the upgrade in one to two years. The density of the air and, hence, the oxygen per cubic foot of air, changes with temperature and weather. The oxygen trim system continuously monitors the exhaust gases for the ideal fuel/air mixture, fine tuning for the best efficiency continuously.
You can have a technician tune the burner and use an exhaust-gas analyzer to accomplish at one time, at one temperature, what the oxygen trim system does continuously. Most burners offer computerized controls (also called micro-modulation) along with oxygen trim systems. Ask your boiler/heater supplier for options with predicted payback times.
To sum it up, if you want the best bang for your buck in your boiler or heater purchase:
- Buy the correct size — not too small and not too large.
- Buy a boiler or thermal fluid heater with a proven track record of efficiency and durability.
- Whether or not an economizer will help depends upon your operating parameters. Ask your heater sales representative.
- Choose a burner that will burn clean with a minimum of excess air.
- Computerized modulation will help, but make sure you have support in your local area for whatever burner and burner controls the heating system uses. A high efficiency burner with no local support may be more trouble than it is worth.
Finally, to achieve the best bang for your boiler or thermal fluid heater, the boiler or thermal heater must be properly designed for the given application. Look for the highest efficiency for the unit along with dependable operation and readily available service.