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Part 3: Short-Cycling and Energy-Efficient Boilers
By Ryan T. Evans, P.E.
June 2012 -
The final optimization strategy is to prevent short-cycling. For safety reasons, a boiler undergoes a purge cycle when it receives a call for heating. This purge might last 60 seconds, and it removes any remaining gases in the boiler before ignition.
Likewise, when the boiler cycles off, the fan remains on to purge remaining unused gases remaining. Though the process removes dangerous gases, it also removes precious heat. Five cycles per hour can result in a 2-3 percent increase in fuel use.
The solution generally is load-matching the boilers, but a recent trend involves building-envelope upgrades that often reduce the need for steam for comfort-heating applications.
If replacing the plant is not an option, then it might make more sense to retrofit the burner with one that offers a good turndown — high-fire to low-fire capability — or to add a smaller, so-called summer boiler for the low-load conditions.
Managers also should provide manually adjusted stack dampers to assist with boiler retrofits. Managers also should ensure each boiler is fine-tuned at the time of its initial commissioning. If the boiler calculations and design were complete and accurate, and it was properly commissioned, the boiler should be optimized.
But what happens when retrofitting a new burner? The process changes a variable in the boiler calculations, which can lead to a less-than-optimal setting. Stack dampers allow a boiler operator to make small adjustments to optimize draft by removing the products of combustion without robbing the boiler of precious heat.
Managers still will want to keep stack temperatures 50-100 degrees higher than the water temperature to avoid condensation in the stack. To do so, managers might want to implement stack-temperature monitors. These monitors not only allow technicians to check for low temperatures — condensation — but also indicate when temperatures are too high, wasting fuel and energy.
Managers have access to a great deal of good information on boiler optimization from credible sources. Much of it applies to steam boiler systems. But managers searching for quick benefits and low-hanging fruit or for optimization projects that have quick paybacks and less downtime should consider looking at the burner and its controls first. These solutions often have the biggest impact with the lowest cost.
Ryan T. Evans, P.E., LEED AP BD+C is a mechanical engineer and sustainability consultant for Henderson Engineers in Kansas City. He specializes in energy and durability issues, and assists his clients with financial analyses of capital improvement projects.
Part 1: Energy-Efficient Boilers Require Proper Maintenance
Part 2: Oxygen Issues With Energy-Efficient Boilers