Boiler Retrofits Help Increase Efficiency, Lower Energy Bills
- By Dan Willems
- August 1st, 2008
Schools across America are facing tight budgets. Escalating energy bills are a substantial factor in those tight budgets, and boilers account for one-third of those energy bills.
Therein lies an opportunity. Almost 80 percent of boilers in the United States are more than 30 years old, according to the U.S. Department of Energy. These older boilers typically have efficiency rates up to 77 percent. But with the right retrofit, aging boilers can achieve efficiency rates in the high 80s to lower 90s.
It may be hard for school administrators in cash-strapped districts to justify capital improvements. However, upgrading boiler systems can save schools hundreds of thousands of dollars in energy costs — allowing them to redirect funds to the classroom.
Let’s take a look at the retrofitting process, so you can get on the road toward these savings.
Locate the Inefficiency
Before upgrading a boiler system, your first step should be having an energy audit conducted by a qualified auditor who can review your entire system, including the boiler, transmission, and process.
Energy losses will be identified during the audit. Some inefficiencies could include:
- control systems that do not accurately predict or control the system;
- not recovering all of the boiler exhaust gas energy;
- poor combustion due to inadequate controls;
- motors without variable speed drives; and
- improper system design.
Whatever the problem, the energy analysis will help identify a solution.
Start With Boiler Controls
The first target for retrofits is in the control system. New developments in boiler controls create opportunities for substantial efficiency gains.
Boilers must operate with an excess supply of oxygen in the combustion gases to ensure complete combustion of the fuel, thereby yielding maximum heat energy. However, too much oxygen cools the flame, and too little leads to incomplete combustion. Therefore, control of the air and fuel levels is paramount to optimal efficiency.
Many boiler burners are controlled by a single modulating motor with jackshafts to the fuel valve and air damper (commonly referred to as “jackshaft control”). This arrangement, set during startup, fixes the air-to-fuel ratio over the firing range.
Unfortunately, environmental changes, such as temperature, pressure, and relative humidity, alter the fixed air-to-fuel ratio, making combustion inefficient. To account for these conditions, boilers with jackshaft systems are typically set up with four percent to seven percent oxygen in the stack. This oxygen level reduces boiler efficiency and, over time, linkages wear — making repeatability impossible.
To fix this problem, incorporate parallel positioning into the boiler’s control system, which uses dedicated actuators for the fuel and air valves. Air and fuel position curves are programmed into the programmable logic controller (PLC) for each actuator, and repeatability is excellent. Boilers that incorporate parallel positioning can operate at lower oxygen levels, thereby reducing energy consumption. This equates to $10,000-$50,000 annually in fuel savings for a 600 HP boiler, and will only go higher as fuel prices escalate.
Another way to make a boiler system more efficient is to use an oxygen trim system in the exhaust gas. This system continuously senses oxygen content and provides a signal to the controller, which “trims” the air damper and/or gas valve, maintaining a consistent oxygen concentration. This minimizes excess air while optimizing the air-to-fuel ratio.
O2 trim systems typically increase efficiency up to two percent. This equates to $5,000-$25,000 annually in fuel savings for a 600 HP boiler. As fuel prices continue to rise, these savings will increase.
A variable speed drive allows a motor to operate only at its required speed rather than a constant 3,600 RPM, at which the motor would typically run. This speed variance eliminates unnecessary energy consumption. A variable speed drive can be used on any motor but is most common on pumps and combustion air motors above five HP. These drives also produce quieter operation compared to standard motors and reduce maintenance costs by decreasing the stress on the impeller and bearings.
The energy saved by retrofitting a boiler system with variable frequencies translates easily to dollars. For example, a 50 HP motor operating at a slower speed and utilizing only 40 HP, 12 hours per day, 365 days per year, with a load factor of one and motor efficiency of 86 percent, will save $3,360 per year (based on $0.10/kWh).
Incorporating Heat Recovery
Another option for retrofitting projects is to incorporate heat recovery into a boiler system. Heat is often allowed to escape through the stack, resulting in a loss of energy. There are “post-combustion” opportunities to recover heat loss, resulting in further energy savings.
Blowdown heat recovery transfers the blowdown steam energy to the boiler feedwater, recuperating about 90 percent of this energy. Many boiler rooms route blowdown to a flash tank, which allows safe discharge of the steam by reducing (flashing) the steam pressure in an enclosed tank. Low-pressure steam is vented from the tank, and condensate is discharged to the drain.
In many cases, these tanks are not insulated nor do they allow recovery of the lost heat. A blowdown heat recovery system transfers the blowdown steam energy to the boiler feedwater, recuperating about 90 percent of this energy.
Feedwater economizers transfer energy from the boiler exhaust gas to the boiler feedwater in the form of “sensible heat.” Sensible heat is extracted by the transfer of the heat energy of one body (in this case exhaust gas) to another, cooler body (the boiler feedwater). This reduces the boiler exhaust temperature while preheating the boiler feedwater, increasing overall efficiency two percent to five percent ($5,000 - $60,000/yr fuel savings for a 600 HP boiler). Savings are even higher if a condensing economizer can be used to recover both the latent and sensible heat. In that case, savings can exceed $100,000/yr in this 600 HP boiler example.
The most effective way to reduce emissions is by upgrading or enhancing the burner. The largest emissions improvement is seen by incorporating flue gas recirculation (FGR), which reduces peak flame temperatures where thermal NOx is formed. The temperature reduction minimizes harmful greenhouse gas emissions.
Increasing the burner’s turndown rate will increase efficiency, reduce maintenance, and save fuel cost because it reduces the number of on/off cycles for the burner. For each of these cycles, a pre-and post-purge is dictated by code. During these purges, large volumes of air pass through the boiler, resulting in heat blown out the stack. A high-turndown burner minimizes the number of cycles, saving significant amounts of energy and reducing emissions.
Clearly, there are numerous benefits to making your school’s boiler system more efficient. But it could take some schools years to save enough money for a boiler system upgrade. Fortunately, some suppliers offer programs to help schools upgrade their boiler systems without an initial investment.
Some suppliers offer lease programs, which use the energy savings to make lease payments on the new equipment, thereby resulting in no initial capital expense. In addition, local environmental groups and some utility companies offer grants for boiler upgrades, which help to offset the capital expense.
Dan Willems is vice president of Product Development at Cleaver-Brooks. He can be reached at email@example.com.