Hybrid boiler plants also may include alternative-fuel boilers, such as electric/electrode units. The use of one alternative fuel over another can be driven by the instantaneous cost of the fuel. Cost monitoring of fuel can drive a switchover between operating boilers and should be part of operating procedures.

Where to Use Hybrid Systems

Higher return-water-temperature requirements are one of the main stumbling blocks to using non-condensing boilers for higher-efficiency designs. As mentioned previously, in most applications, water has to be returned to a boiler at or above 130°F to 140°F to prevent flue-gas condensation. Normally, the dew point of exhaust gases is approximately 135°F.

Hybrid systems can be utilized in legacy/existing plants or new designs. Older non-condensing boilers could reduce overall project cost significantly, provided their pressure vessels and burners are in acceptable shape.

An energy solution that combines the ideal number of condensing and non-condensing units can reduce fuel consumption by more than 40 percent relative to existing systems or new non-condensing systems. Even though this savings is caused by the reduction of boiler cycling, all of the design requirements for comfort, domestic water, snow melting, etc. are satisfied.

Figure 1 shows a heating profile typical of many commercial buildings. Within this profile, maximum heating load occurs during winter months. However, when fuel consumption is normalized against a unit of consumption, such as heating degree days, more heat is consumed in off-peak months, such as October and April (Figure 1). This occurs when a proportional-integral-derivative (PID) loop cannot be maintained within acceptable parameters, resulting in what often is referred to as “short cycling.”

Two control concepts lead to a boiler system's ultimate savings, represented by the theoretical curve in Figure 2. The first concept to consider is intelligent flow. Current control schemes are based on PID temperature controls. Although these controls are an improvement over older systems they have not caught up with advances in processing software. British-thermal-unit heating-load consumption now can be calculated to match a heat profile exactly. In other words, British thermal units that are lost in a system can be recovered immediately through mass-flow balancing.

By utilizing this applied control, needless boiler cycling is reduced greatly, if not eliminated. Under current control scenarios — on-off boiler cycling at low-load conditions and the pursuit of PID-loop temperatures — boiler efficiency can be reduced by 20 to 30 percent. Even when utilizing a condensing boiler at low return temperatures, theoretical efficiencies of 95 percent can drop to as low as 65 percent under high cycling conditions. Through the use of system delta-T and flow rate, actual consumed heating load can be calculated. Therefore, theoretical energy savings is the difference between the curves in figures 1 and 2.

The second concept to consider is intelligent load sharing. With a properly sized boiler, run cycles can be limited to two or fewer per hour under no-load conditions. To accomplish this, a small boiler is sized to allow 30 min of run time under no-load conditions (delta-T multiplied by 500 [8.3 lb per gallon of water multiplied by 60 min]). Given system volume and the delta-T of a boiler's operating set point, minimum firing rate can be calculated. With this minimum firing rate, a boiler with an appropriate turndown can be selected to track heating load, picking up minimum losses as they occur. During most evaluations, this outcome usually is achieved by a boiler smaller than the rest of the units attached to the heating plant. The use of a smaller boiler is similar to the “summer-boiler” concept used in steam plants. In these cases, a small steam boiler is used to carry light loads, leaving only a small process load. The steam used for sterilization and/or humidification in a hospital is one example.

To accomplish intelligent load sharing, a heating-plant control must be able to calculate the load consumed and recognize the minimum and maximum capacity of each boiler. The controller also must be able to turn modulating boilers on and off to match the load exactly. Sizing and control schemes using only temperature variation (without mass-flow calculation/selection) usually employ multiple equally sized boilers, resulting in considerable on-off cycling as the load drops below minimum turndown. This is extremely inefficient because of frequent pre- and post-purge losses and stresses mechanical equipment, leading to higher incidences of costly repairs and downtime.

How Hybrid Systems Work

Condensing boilers are used in hybrid systems when heating loads drop to an outside-air temperature of about 32°F to 35°F. In northern climates, this accounts for approximately 75 to 80 percent of the heating season and about one-third of the heating load. Actual loads will need to be verified using load-calculation software or existing load profiles.

As heating load increases with a drop in outside-air temperature, switching to non-condensing boilers provides heat for the incremental increase in demand. Built-in algorithms enable the transition from condensing to non-condensing units. When utilizing the piping configuration shown in Figure 3, condensing-boiler outputs are driven up to or above 140°F. This configuration ensures that the inlet to non-condensing boilers can prevent condensation.

As load increases (increasing heat loss), the non-condensing boilers take charge. If the non-condensing units are sized for two-thirds of the load, condensing boilers can supplement when a smaller load is needed or during the most severe conditions. If more than one non-condensing unit is used, the control can change or sequence the operating units in a lead/lag setting to equalize run time. The use of non-condensing boilers allows for higher temperatures (more British thermal units) on colder design days. This concept supports higher supply temperatures, keeping the heating coil surface in reheat boxes to a minimum, and accommodates the use of indirect domestic-hot-water heating.

Hybrid-System Candidates

A burner-management system, sometimes referred to as a flame safeguard, can help determine good hybrid-system candidates. Many burner-management systems keep track of the number of cycles and run hours. If boilers are cycling excessively, the boiler system is a good hybrid-design candidate. Many boiler rooms experience 10 to 40 cycles per hour, which can indicate a heating plant that is oversized for small loads. Other indicators include excessive maintenance requirements, frequent downtime, and general customer frustration with fuel bills and/or system performance.

As previously mentioned, Figure 2 shows potential energy savings as the difference between the trend lines. The captured savings will be greater during off-peak months, such as October and April. Systems in moderate or mid-range climates generally will produce greater overall savings because they experience additional operating hours during off-peak months. Warmer climates still can achieve potential savings if a summer reheat schedule is used.

A traditional non-condensing boiler plant has an ASE of 65 to 70 percent because of on-off cycling during off-peak design seasons. A boiler plant using condensing boilers exclusively could reach an ASE of up to 93 percent. However, a properly designed hybrid system could reach these levels at a lower installation cost than that of an all-condensing-boiler plant.

Finally, a minimum savings of 20 percent can be expected with hybrid systems, though savings as high as 40 to 50 percent can be realized. Additionally, hybrid systems can reach these savings levels without changing high supply temperatures during design-day conditions. The final result is retrofit applications that are more affordable with shorter payback periods. Combining condensing boilers with new (or existing) non-condensing units can help achieve the best of both worlds.

Steve Connor is the director of marketing/communications for Cleaver-Brooks Inc. A frequent speaker and author on boiler design, efficiency, and safe operation, his background includes more than 40 years of experience in steam generation, including engineering, service training, and field-application sales.

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