It should come as no surprise that the typical facilities manager rarely is in the position to purchase a new boiler and, thus, is interested in exploring opportunities to significantly decrease fixed operating costs by improving boiler efficiency—preferably at minimal cost and with a high return.
If you are just such a facilities manager, take a moment to consider the turndown performance and low-fire combustion efficiency of your burner, especially in relation to the expense of its primary fuel.
What follows should help you decide what might yield the highest return for the lowest investment. For some, the most value might come from a linkagelesscontrols upgrade. For others, it might come from a complete burner retrofit.
On a hospital campus, boiler stacks located approximately 50 yards from the hospital and protruding from the roof of a two-story mechanical building were in full view of all of the windows on the rear side of the 15-story main building. There was little doubt that the No. 1 commissioning criterion was to avoid making smoke when operating on oil, as it is easy to imagine a patient calling the nurse's desk at the sight of the slightest amount of smoke.
Installed in 2001, the system featured two oversized 400-hp firetube boilers fitted with dual-fuel gas/oil burners. The burners were cam-andlinkage type and fitted with a common HVAC modulating actuator driving the cam and linkage-arm assemblies connecting the air-control damper, gas butterfly valve, and oilmetering valve.
The HVAC modulating actuator was equipped with only one low-fire end switch. That meant low-fire turndown on gas operation, which likely occurred 360 days a year, was only 3-to-1, despite the fact the burner was capable of operating on gas at a turndown ratio of 8-to-1 or better.
The commissioning report from the annual inspection performed only a few weeks earlier revealed that turndown on oil was limited to 3-to-1. The excess-oxygen (O2) and efficiency numbers were very good, but the turndown was limited to 3-to-1. Why? If low-fire oil operation had been pushed any lower on this burner, smoke (soot) would have started to appear in the products of combustion exiting the stack.
Over the course of three hours in the hospital boiler room, the steadystate load was estimated to be 60 to 100 hp. With low fire delivering 150 hp, the boiler continuously cycled on and off—approximately 15 times an hour. That meant 15 purge cycles an hour were blowing ambient air through the hot boiler and wasting money.
Had the stack location of the boilers not been so visible, the oil turndown ratio might have been a little better, perhaps 4-to-1. Regardless, significant turndown is sacrificed when most dual-fuel boilers operate on gas. Probably more than 80 percent of the dual-fuel burners installed in North America face the problem presented by this one-lowfire-switch actuator situation.
If this sounds familiar, consider contacting the burner/boiler/combustion-controls manufacturer's representative in your area. Several electronic linkageless control systems are available, with all incorporating the ability to have different turndown settings for oil and gas operation.
In the case of the hospital with the visible stacks, the burner spent most of its time operating at or near low fire. Consequently, burner turn-down and excess O2 performance at low fire, along with the accuracy and repeatability of the linkageless control system, become the key evaluation criteria for efficiency and performance.
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Linkageless-control upgrades on smaller burners usually are not costeffective. Often, it is too costly to rebuild a control panel, create an electrical drawing, and field-retrofit new control actuators with flexible couplings, custom brackets, and more. Given these circumstances, a new high-turndown burner with a linkageless-controls package probably is the best, most cost-effective choice.
How much can you expect to save? It depends on the individual application, but suppose a facility is spending $60,000 a month on natural gas to fire boilers. Losses associated with reduced turndown easily could reach 10 percent. Regardless, many engineers report having saved as much as 5 to 15 percent or more installing a linkageless-control upgrade on an existing burner package. For the hospital with the visible stacks, that savings could add up to more than $100,000 annually. However, one cannot attribute all of these savings to tighter, more accurate linkageless control. The reality is that larger boilers usually are dual-fuel and have burners fitted with a one-low-fire-switch HVAC actuator, which tends to sacrifice everyday gas performance.
Increasing turndown and eliminating excessive cycling are, in most cases, the little-understood, hidden opportunities contributing substantially to greater efficiency and lower costs. Do not get sidetracked by individuals trying to convince you to improve your excess-air performance at high fire because boilers almost never operate at high fire. The key to uncovering potential savings is turndown, low-fire excess-air performance, and accurate, repeatable control.
Once you have decided to perform either a linkageless-controls upgrade or install a new burner fitted with a linkageless control system, it is time to consider the potential benefit of an O2-trim package. Have the burner/boiler/ combustion-controls representative you trust most present a cost/benefit/payback report.
Typically, O2-trim systems make the most sense on heavily used larger boilers. Not all linkageless systems are capable of integrating an O2-trim system. Consider the possibility of a future upgrade when making your controls selection.
Another feature available on many linkageless burner-control systems is variable-frequency-drive control of combustion-air-blower motors. Burners are noisy, frequently reaching levels of 90-plus decibels. In addition to reducing noise, which can go a long way toward making your boiler plant a better work environment, adding this feature to your upgrade will produce electricity savings. Be sure to inquire about this feature because not all linkageless systems offer it.
About the author
The director of the Combustion Control Products Group, Group Region Americas for Siemens Building Technologies Inc., Bertram Leng, PE, has 23 years of industry experience. He can be contacted at [email protected].