Performance and the Efficiency Side of Upgrades
A boiler system that has been operating for 20 years or more is likely to be operating far less than the nominal efficiency of 80 percent. If the boiler pressure vessel is in good shape, consider upgrading the system to restore its efficiency and reduce operating costs.
New developments in boiler accessories, controls and burners will enhance a system’s performance and produce measurable efficiency gains. Following are options to consider:
Fuel-Air Ratio Control
Many boiler burners are controlled by a single-modulating motor with a single jackshaft to which are connected linkages/arms to the fuel valve and air damper. Over time these linkages tend to wear, stretch and loosen, causing the fuel-to-air ratio to drift into either very inefficient excess-air conditions or very unsafe fuel-rich conditions, which is also very inefficient because of the heat transfer limitations caused by the soot fouling.
To alleviate this problem, consider incorporating parallel positioning into the control system, replacing the single-jackshaft system. It is a control scheme that uses dedicated motorized actuators for the fuel and air valves, electrically tying them into the electronic firing-rate control on the boiler. Burners that incorporate parallel positioning can eliminate this unwanted and inefficient “drift” between the optimum fuel-to-air ratio, consistently keeping the burner in sync as it modulates between low and high fire. Fuel savings of 2 to 5 percent are not uncommon with those boilers that modulate considerably during a given operational day.
Another way to ensure peak efficiency is to use an oxygen trim sensor/transmitter in the exhaust gas stream. The sensor/transmitter continuously senses oxygen content and provides a signal to the controller that “trims” the air damper or fuel valve, maintaining a consistent oxygen concentration when environmental conditions such as ambient temperature, relative humidity and barometric pressure change. This system then minimizes excess air while optimizing the fuel-to-air ratio and at the same time, helps to keep the burner from going excessively rich, which is not only inefficient, but very dangerous too. It should be noted as well that a 2 percent increase in oxygen over the optimum set point is a 1 percent loss in efficiency.
Adding variable-speed drive enables a motor to operate only at the speed needed at a given moment, rather than a constant RPM regardless of load demand. This speed variance results in the elimination of unnecessary electrical energy consumption because as the speed lowers, the power consumed decreases as a cube function of the speed. A VSD can be used on any motor but is very commonly found on boiler feed and circulating pumps and combustion air motors of more than 5 HP. These drives also produce quieter operation and softer starts compared to a standard motor, thereby reducing maintenance costs by decreasing the stress on the motor shaft and associated bearings.
Incorporating lead-lag control of multiple boilers that are incrementally sized for the load can substantially reduce energy costs due to a reduction of boiler cycling. Remember, every time a boiler cycles off, it goes through a pre- and post-purging cycle before reigniting. This means that ambient air is blowing through and across the heating surfaces of the boiler for approximately two minutes, purging valuable BTUs into the atmosphere.
Advanced-control systems often referred to as SCADA Systems (System Control and Diagnostic Annunciation) are PLC-based platforms incorporating burner management, combustion control and monitoring in one integrated package. By installing an advanced-control system, a facility can expect to see better performance, enhanced safety, lower maintenance and reduced operating costs.
A key benefit of an advanced control is that a boiler can be remotely monitored and controlled within its safety parameters while at the same time gathering/trending valuable information relating to safety, energy consumption and maintenance trigger points. Operators can monitor boiler status via email, text messaging, voice mail, the Internet or an internal communication system. In the event of a boiler-system alarm or malfunction, the control system can automatically alert an operator in a nearby or remote location, permitting the more efficient use of an operator’s time.
Upgrading the burner after about 20 years of service is an excellent idea because typically the features it contains are not up to today’s standards. The solution is to replace the burner with a high-efficiency one that may also include lower NOx features in addition to a high-turndown feature. Increasing the burner turndown rate will increase energy savings and reduce maintenance. Energy savings increase when on-off cycles are reduced as indicated in the lead lag control comments above. Often, these high-turndown burners can work in conjunction with a multiple-boiler lead lag control system, delivering even more energy savings due to an even further reduction in cycling.
Incorporating heat recovery is another way to improve energy efficiency. For instance, a non-condensing vent-stack economizer can be added, which transfers energy from the boiler exhaust gas to the boiler feed water. This normally wasted heat increases the temperature of boiler feed water before it enters the boiler, and for every 10-degree rise in feed water temperature there is a 1 percent increase in efficiency.
A condensing economizer can also be incorporated with a non-condensing economizer in a single package, capturing both sensible and latent energy from the flue gases leaving the boiler. In this case, the non-condensing unit is the first stage heating the boiler feed water, and the second stage is the condensing portion that can be used to heat any other cold water demand, thereby allowing the flue gas to condense and capturing the latent BTU energy before the products of combustion exit the stack. This combination of non-condensing and condensing economizers has been found to save as much as 4 to 8 percent of the normal fuel consumed.
A blowdown-heat-recovery unit is the most effective method of capturing otherwise wasted energy from the effluent leaving the boiler as a result of continuous blowdown of Totally Dissolved Solids (TDS) in the boiler. The unit transfers the heat to the boiler’s feed water supply before it returns to the feed tank or deaerator, and just like the stack economizer, for every 10-degree rise there is a 1-percent improvement in efficiency. The payback in fuel savings for a blowdown-heat-recovery unit is typically less than a year.
A flash tank with economizer takes the trapped condensate from a high-pressure source and allows it to flash before entering a lower-pressure condensate return line. This flash steam (if continuous) can then be used for any number of low-pressure steam uses, including unit heaters, humidification or deaeration, and as such, save energy otherwise wasted that has to be made up by the boiler. Once the flash has been removed, the hot condensate passes across an integrated heat exchanger for heating boiler feed water, saving fuel in the process just as mentioned above.
To find out which upgrades will restore your boiler system to its peak efficiency, visit cleaverbrooks.com or contact your local Cleaver-Brooks representative.
Remember, even with all of the latest technology, regular maintenance is still necessary. Properly servicing a boiler system on a consistent basis will maintain its efficiency and lower fuel costs for years to come.
Article courtesy of Today's Boiler.
Download: Today's Boiler November 2016