When a steam or hot water boiler satisfies the high limits of the pressure or temperature demands that have been set on its controls, it will cycle off. The problem is that when this occurs while demand is still present, the boiler will quickly be needed back online; this is known as “short-cycling.” Most process and heating systems do not have a steady load--the load will actually vary based on production schedule, changes in outside air temperatures, or other variable factors. The inefficiencies associated with this cycling process are costly.
A boiler's furnace area typically sees a large volume of fuel introduced during normal operation, and there is always a risk of combustible gases accumulating in the furnace even after the burner has stopped firing. A large amount of fuel in an enclosed space is a recipe for disaster when the burner comes back on, so as a safety precaution, a boiler's operation cycle includes "purge" stages that use the blower to push a large amount of air through the boiler immediately after the burner turns off (post-purge) and then again before it turns back on (pre-purge), just to make sure that any accumulated fuel has been pushed out before a flame is reintroduced. Herein lies the efficiency issue: the air being forced through the boiler to clean out excess fuel is relatively cold, and it forces hot gases out of the boiler. When the boiler fires up again, it will have to make up for that heat loss by firing for a longer time at a higher rate, which means that your fuel cost will be higher, not to mention the time lost waiting for the purge cycles to complete. If you have a batch process that needs constant heat, a short-cycling boiler could put you at risk of losing quality in that batch of product due to the delayed delivery of heat. It can also hurt the lifetime of your equipment by putting more stress on the boiler vessel and any moving parts, causing these components to require maintenance (and therefore boiler down-time) more frequently.
There are several things that can be done to avoid short-cycling. First, rather than using one large boiler that would short-cycle, multiple smaller boilers can be used. They can be configured to run in a lead-lag control system, where one boiler can carry the smaller loads at low fire, and other boilers will be brought online as needed when the load swings and demand increases. Re-engineering an existing system may have a high initial cost, but it can pay for itself several times over during the life of the boilers system.
Set points can be adjusted to help prevent frequent cycling as well. If the set points are so tight that the temperature has to remain within a few degrees or the pressure must stay within just a couple psi, the boiler will likely cycle very often. However, if the heating load or production process can tolerate a wider temperature swing or a larger pressure differential, then the boiler can run longer during operation and stay off longer during low-load conditions. In some cases, it may be critical to hold on a specific value, but if there is room to allow the set point to fluctuate, it will increase your system’s efficiency.
Next, burner design can play a huge factor in increasing efficiency—in fact, many facilities with existing boilers have opted to just replace the burner instead of buying and installing a new boiler. A burner with a higher turn-down ratio can drop the boiler’s low-fire rate and carry a smaller load without cycling off, and opting for a fully modulating burner will allow more flexibility in the firing rate to handle a wide range of loads in between the typical low and high fire points. Equally important is the transition to a linkage-less system; this type of burner setup allows for tighter combustion control, which gives you added efficiency to reduce fuel costs, but more important is the eliminated problem of hysteresis. This is when the system is unable to repeat a set performance, and it often occurs on a linkage system after combustion is set on low-fire. When the jackshaft drives to high fire and comes back down, the O2 levels can be off by 2% or more. This causes technicians to set the O2 levels too high throughout the firing range, which results in too much air in the combustion and high flue gas losses. By moving to a linkage-less burner control system, O2 levels will stay below 3% throughout the firing range, which can achieve fuel savings of 12-15% over a linkage burner operating at an average 6% O2 level.
Finally, new burner controls can help save time and money by eliminating the need for pre and post-purge cycles. Some of the latest control designs have implemented a “revert to pilot” function. Earlier, it was stated that the accumulation of combustible gas in the furnace while the burner is not firing can cause a safety hazard when the burner reignites, which is why the potential accumulated fuel is flushed out with large amounts of air from the blower fan. However, if the burner simply drops from low-fire to pilot instead of turning completely off, the continued presence of a small flame will prevent any unwanted build-up of fuel from a leaking valve. Any problems should be apparent if there is still a large flame in the furnace when it is supposed to be running on pilot. Without the need to purge the system, the heat loss from expelling hot flue gases can be avoided, and the burner can ramp back up when needed.
Better system design can save thousands of dollars per year in fuel cost alone. If the goal is to save the company money, make it easier and more efficient to run production or heating processes, prolong the life of your equipment and reduce down-time, then eliminating short-cycling should definitely be on the priority list.