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Hybrid Boilers and Intelligent Controls Eliminate Short Cycling

Key Points
  • Short cycling, a major problem in many facilities, represents up to 50 percent of fuel costs.
  • Intelligent control systems are available on the market for reducing or eliminating short cycling.
  • Energy savings range from 10 to 30 percent; fuel costs can be drastically reduced.

A boiler cycle involves firing the burners, shutting off the burners, post-purging/flushing the combustion chamber, an idle period, a prepurge and a return to burner firing. Purging ensures that residual combustible gases are non-existent before firing and it removes a lot of heat from the boiler. If a boiler cycles frequently, perhaps because it is oversized, this is known as short cycling.

Short cycling is a major problem in many facilities and represents up to 50 percent of fuel costs; cycles as high as 10 to 40 per hour are possible. At low load conditions, boiler efficiency is often reduced by 20 to 30 percent even with a condensing boiler at low return temperatures. If excess cycling isn't eliminated, maintenance increases and frequent downtime occurs.

Boilers become smarter

There are several intelligent control systems on the market for reducing or eliminating short cycling. Greffen Systems offers the M2G, which is designed for closed-loop hot water boilers fueled by natural gas or oil. The system monitors the temperature of the water every 10 seconds and determines the heat transfer rates at the first and second stages of firing. It then determines if it should continue the first stage firing or begin a second stage, selecting the most economical operation to accommodate low or heavy loads. Over time, the M2G teaches itself when and how long a boiler should fire.

The manufacturer claims energy savings range from 10 to 30 percent, with a payback of 6 to 24 months. Installation is simple and no calibration is required for the life of the system. An independent, two-year test by the Gas Technology Institute resulted in savings between 10 and 17 percent with cycle reductions of up to 55 percent. The Cypress Fairbanks Independent School District in Houston, Texas also measured energy savings of 20.6 percent over a 12-month period.

Cleaver-Brooks has patented a thermodynamic process control system (TPC-FI), which measures the exact energy a hydronic system is using by measuring outdoor air, the change in system temperature and the hot water flow (in gallons per minute). The system documents real-time loads by measuring supply, return and system flow; run hours are also monitored. To achieve maximum thermal efficiency, the system adjusts the flow rate within the boiler's required limits to maintain a constant change in temperature across the unit regardless of the firing rate. Boiler shutdown is based on load, size, type, priority and hours of operation; the appropriate output is selected based on the range allowed. One distributed network controls all equipment (boilers, pumps, control dampers, mechanical draft devices and automatic isolation valves), with up to 64 devices possible. The manufacturer claims fuel consumption can be reduced by up to 72 percent with a payback within months.

Such a control system allows you to design a hybrid plant combining both condensing and non-condensing boilers, with a possible installed cost savings of one-third to one-half of the cost compared to an all-condensing plant. The ideal combination can also reduce fuel consumption in excess of 40 percent when compared to existing plants or new ones with all non-condensing systems. With load sharing, the appropriate boilers are chosen for current conditions, leading to a significant reduction or elimination of boiler short cycling.

A 192,000 square-foot correctional facility in Michigan took advantage of the thermodynamic process control system when redesigning its boiler plant. The heating system was reconfigured to include two smaller vertical condensing boilers and dual redundant heat exchangers. The two existing boilers and water heater were connected to the control system to act as back-ups. More than $150,000 was saved in upfront costs and the heating and water heating capacity was reduced by 82 percent.