Efficiency and Cost Benefits of Controls Retrofits

January 12, 2012

One-third of a facility’s energy bill can be directly attributed to its boiler room, and system inefficiency leads to higher energy costs.  Replacing an older boiler system is one way to achieve energy savings, but it is not the only option.  Retrofitting an existing boiler is another way to increase efficiency, reducing both operating costs and emissions.

When deciding whether to retrofit or replace a boiler, its overall condition should be the determining factor, not the boiler’s age.  If a boiler’s water-side and fire-side surfaces show minimal signs of heavy scaling, pitting, cracking, or stress, its pressure vessel is likely in good shape and retrofitting is a viable option.

If improving efficiency is the goal, then it is important to identify the problem.  The main cause of energy inefficiency is system heat loss.  If retrofitting is recommended, the first area to upgrade is the controls system.  New developments in boiler controls create opportunities for substantial efficiency gains.  Following are options to help an existing boiler system produce measurable efficiency increases and fuel cost decreases.

Parallel Positioning -- Many boiler burners are controlled by a single modulating motor with jackshafts to the fuel valve and air damper. 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 a high amount of excess air. This higher excess air level reduces boiler efficiency and, over time, linkages wear -- making repeatability impossible.

To solve this problem, consider incorporating parallel positioning into the controls system. It’s a process using dedicated actuators for the fuel and air valves. Burners that incorporate parallel positioning can be set with lower excess air levels. Energy savings of up to 5 percent can be realized by introducing a parallel positioning system.

O₂ trim -- Another way to ensure peak efficiency is to use an oxygen sensor/transmitter in the exhaust gas. The sensor/transmitter continuously senses oxygen content and provides a signal to the controller that “trims” the air damper and/or fuel valve, maintaining a consistent oxygen concentration. This minimizes excess air while optimizing the air-to-fuel ratio.

Variable speed drive -- Variable speed drives enable a motor to operate only at the speed needed at a given moment, rather than a constant 3600 RPM as a drive runs. This speed variance results in the elimination of unnecessary electrical energy consumption. A variable speed drive can be used on any motor but is most common on pumps and combustion air motors of greater than 5 horsepower. These drives also produce quieter operation compared to a standard motor, and they reduce maintenance costs by decreasing the stress on the impeller and bearings.

Lead lag – Lead lag sequences the operation of multiple boilers, matching system load.  Lead lag enables the boilers to operate at peak efficiency, reduces cycling and decreases maintenance and downtime.

Facility Increases Efficiency by 44%  

Troy Jensen, controls systems specialist for 23-unit Intermountain Healthcare, knew he could cut operating costs at Utah-based Logan Hospital by optimizing the boiler system at the 400,000-square-foot, 225-bed facility.  In 2010, he received funds to install advanced Hawk integrated controls systems on the hospital’s three 300-horsepower CBLE boilers.  This gave Jensen the ability to continuously monitor and control the boilers’ 0₂ level, stack temperature, lead/lag sequencing and water level to increase efficiencies and reduce operating costs.

Because he is not always on site, and an emergency in the middle of the night could necessitate a quick increase in the amount of steam required, Jensen integrated the controls into the hospital’s building automation system.  He then created a schedule to reduce boiler operating pressure in accordance with the hospital’s needs.  Jensen also gave the Central Sterile Supply Department access to the controls at their workstations so when emergencies arise they can override the system.  

Through his analysis of 24-hour trend reports, generated by his newly automated systems, Jensen learned that the boilers were inordinately oversized and cycling during low-load conditions, even with a 6:1 turndown capability on their integrated burners.  

Knowing that excessive cycling was substantially increasing fuel costs due to repeated pre- and post-purge losses, Jensen decided to explore his options.  Instead of starting with the current 300-horsepower boilers that turn down to 60 horsepower, Jensen wanted to start with a boiler of about 60 horsepower and go down from there. 

Jensen contacted Servco Industrial Equipment Representative Jason Hansen in Salt Lake City to discuss the best options.  In June 2011, Jensen had a ClearFire-V boiler installed at Logan Hospital.  The highly efficient boiler can modulate from 60 horsepower down to 12 horsepower. The flexibility of pairing the smaller ClearFire-V with the existing boiler eliminated the issue of the boilers cycling off and on at night, and shared the majority of summer load conditions.

Even if a boiler performs well throughout an efficiency analysis, incremental savings may be attainable because of the boiler's original sizing to its load. Inefficiency often occurs during summer, when an oversized boiler remains mostly in low fire, cycling several times an hour. This drives up radiation and convection losses as a percent of input while increasing excess-air levels and reducing combustion efficiency. Excessive cycling and poor combustion efficiency can reduce a boiler's efficiency from a normal full-capacity rating of 83 percent to a percentage in the low 70s.

Purchasing a small “summer boiler” properly sized for reduced-load conditions is one recommended solution. Taking into account the hourly cost of plant downtime per unit, annual savings can be substantial with this strategy, especially during times of fuel-price volatility. This solution can be applied to any efficient boiler with “shoulder” operating months. Even on new projects, this can be an effective energy solution.

“With the Hawk controls and ClearFire integrated into my building automation system, I run on 56 percent of the gas that I used to run on,” said Jensen.  “That’s a 44 percent reduction in natural gas.”   Jensen said he recouped his expenses in less than one year.  He adds that an optimized boiler system with Hawk controls can easily be replicated in other facilities, resulting in a 30 percent to 40 percent decrease in operating costs, which in his facility amounts to an annual savings of $700,000 to $1.3 million.

In addition, the new system has environmental benefits as well.  Jensen compared CO samples from July 2009 and July 2011.  “The savings was equivalent to taking 2,000 cars off the road,” he declared.  “When you hit numbers like this -- talk about environmental impact.  Natural gas isn’t always going to be around.  It’s a finite resource.  Wouldn’t it be responsible for us to use less if we can?” 

Jensen is eager to share his knowledge with industry colleagues. He added, “The healthcare industry is unique because we always have to be prepared for the worst-case scenario, which would be a major emergency in the middle of a blizzard.  Many facilities purchase equipment based on this kind of scenario, and the boilers are left at the same set point all the time, even during the summer.  This is inefficient and is wasting our natural resources.”

Many facility managers today are looking at how to cut costs, and Jensen explains that if they try to reduce the pressure on an oversized boiler, it cycles a lot, which results in excess fuel losses and maintenance issues.  In addition to controls solutions such as parallel positioning, Jensen said there is still room for improvement. Another retrofit solution to consider is heat recovery, which further increases efficiency and reduces operating costs.  Here are some options to consider:

Economizers -- Economizers transfer energy from the boiler exhaust gas to the boiler feed water in the form of “sensible heat.” Sensible heat is created by the transfer of the heat energy of one body, in this case exhaust gas, to another, cooler body -- the boiler feed water. This reduces the boiler exhaust temperature while preheating the boiler feed water, increasing overall efficiency.  Economizers typically increase energy savings by 2.5 to 4 percent.

Two-stage condensing economizers -- This type of economizer combines the functions of both a standard non-condensing economizer and a condensing economizer.  The first section of the economizer recovers energy by preheating boiler feed water.  The second section recovers energy by preheating a cool liquid stream such as make-up water.  Sensible and latent energy is captured from the flue gases that leave the boiler.  Condensing economizers can increase energy savings by up to 10 percent, depending on design and operating conditions. 

High turndown burner -- Increasing burner turndown rate will increase energy savings and reduce maintenance.  Energy savings is realized due to a reduction in on-off cycles.  Each on-off cycle is followed by purge cycles.  During a purge cycle, large volumes of room air pass through boiler, resulting in heat being blown out the stack.

Blowdown heat recovery -- All boilers must remove dissolved solids from the boiler to maintain water purity and ensure a long boiler life.  Many boiler rooms route blowdown to a flash tank that 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 feed water, recuperating about 90 percent of this energy.

Proper Maintenance is Mandatory 

Installing the proper equipment is the first step to achieving fuel savings over a long period of time.  To keep the boiler operating at peak performance, it must be regularly maintained and tuned.  Technicians should keep boiler logs of operating parameters and evaluate the findings regularly to detect any issues or trends.  On an annual basis, a technician should test all controls for proper operation and calibration as well as inspect, clean and lubricate mechanical linkages according to the manufacturer’s instructions. 

In addition to prolonging equipment life, regular boiler tune-ups are required under the EPA’s new Area Source Rule.  To learn more about the new EPA rules for boilers, visit www.cleaverbrooks.com/epa.

Featured in the January issue of HPAC Magazine.