Condensing boilers: the final step in boiler savings
Using a counterflow heat exchanger arrangement to take the exhaust below the point of condensation results in improvements in efficiency in the magnitude of 10% to 15% over an annual heating cycle.
In recent decades, high priority has been given to improved fuel efficiency in equipment of all types. One approach that has proven very successful is extracting nearly all the heat of combustion in a unit’s exhaust by cooling it below the point of condensation, thus capturing the latent heat of vaporization. An example that many are familiar with is the condensing-type home furnace, which uses a counterflow heat exchanger arrangement to take the exhaust below the point of condensation. The result of this feature is improvements in efficiency in the magnitude of 10% to 15% over an annual heating cycle.
The same strategy is increasingly being deployed in commercial and industrial hydronic boilers. Manufacturers today offer fully condensing hydronic boilers, often used for space or process heating. These use advanced heat exchangers, usually in series, to reduce the temperature of boiler exhaust below the condensing point, typically around 130°F (55°C). The cooler the return water, the higher will be the unit efficiency.
The U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) performed an evaluation of condensing boiler installations for guidance for the General Services Administration (GSA) Rocky Mountain Region. The report concluded that condensing boilers in space heating applications can increase the steady-state heating efficiency from 6% to 18% over standard boiler systems. The report indicated that the level of efficiency improvement will depend on how the condensing boilers are designed and operated, and on the temperature of the return water.
Return water temperature is important
A condensing boiler will be most attractive in certain hydronic applications with relatively low return water temperatures, such as with under-floor heating, cast iron radiator systems, process heating applications, or any application where there is a significant amount of cool makeup water added. The cooler the return water is when entering the boiler, the higher the potential efficiency. In a situation where the water is 100°F (38°C) and the unit is sized appropriately, unit efficiencies may range from 95 to 98%.
Kyle Bottorff is a product manager for Fulton Heating Solutions, a major manufacturer of condensing boilers and related equipment. Regarding potential markets for condensing boilers, he explains, “The largest fuel saving benefits occur when boiler return water temperatures are under approximately 135°F. However, even when in higher temperature operation, a condensing boiler is generally more efficient by several percentage points over a non-condensing alternative. Retrofit applications with 160-180°F water temperatures can still reap the efficiency benefits of a condensing boiler by implementing an outdoor reset schedule. The highest heating water temperatures are only required on the coldest days of the year, so return water temperatures are automatically lowered (into fully condensing) during the mild weather shoulder loads which make up the majority of the season.”
Thriving at part-load
Non-condensing boilers tend to be at their greatest efficiency when at 80% to 90% of full load. However, condensing boilers tend to be at their most efficient at lower part-load levels. Because many boilers operate at these lower levels, this contributes to their having overall higher annual efficiency.
Bottorff notes that in addition to space heating, certain other applications make sense for condensing boilers. This might include food processing, dairy plants, or other industrial applications where the boiler water is consumed. He adds, “Many closed-loop applications using water or a water/glycol solution as the heat transfer fluid operating between 40°F and 200°F can also be great fits.”
Many candidates for condensing systems
Bottorff adds, “Payback on a condensing boiler will be quicker as water temperature requirements decrease. Some areas where condensing boilers are growing in popularity include automotive and aerospace painting facilities, indirect pool heating, indirect domestic hot water, and wastewater treatment plants.”
According to Bottorff, Fulton manufactures four condensing boiler lines – Endura, Endura+, Vantage and Pulse. These range in size from 750,000 Btu/Hr to 6,000,000 Btu/Hr. All are firetube heat exchangers built with Duplex stainless steel, a robust alloy for long-term service and reliability. The Fulton products are designed to save on installation costs by eliminating the requirement for dedicated boiler pumps and primary-secondary piping arrangements.
Unique design elements
Exhaust condensate from a natural gas-fired condensing boiler typically has a pH ranging from 3.0 to 5.0, or mildly acidic. This acidity would be damaging to a cast iron or mild steel heat exchanger, so in a condensing boiler, heat exchangers are usually either an aluminum alloy, or stainless steel. Units are equipped with drains that collect condensate for transfer to a floor drain or sump. In some locations, the condensate also needs to be neutralized, which can be accomplished with a collector filled with limestone chips or magnesium oxide.
Owners should check with local wastewater treatment specification for possible pH limitations. In a pure condensing boiler, the combustion exhaust temperature is low enough that an exhaust blower is needed. Low temperature exhaust means the exhaust stack can be PVC construction, although an existing non-corrosive flue is entirely satisfactory.
Opportunity for major savings
Cleaver Brooks is another manufacturer of a range of boilers and related products that offers its ClearFire® line of condensing boilers. Catie VanWormer, PE, is a product manager with the ClearFire line who offers her views on condensing boiler potential. “For the majority of new construction projects, it makes sense to use condensing boilers for a number of reasons; operating efficiency gains, controls, reduced footprint, and a modular design approach with efficiency.” However, they require the right situation.
VanWormer explains, “When evaluating a retrofit project to convert from non-condensing to condensing there are items that must be researched before determining the best solution; system operating temperatures, the ability to reduce those temperatures to reach condensing performance, project budget, existing system piping design, venting, and combustion air routing.” She offers, “Cleaver Brooks representatives have the ability to help potential customers in their areas — engineers, contractors, or end users – to determine the best solution for their specific system.”
In some situations, the condensing boiler may be combined with new or existing conventional non-condensing boilers. Because a non-condensing boiler is typically less expensive, this approach allows an owner to take advantage of condensing technology without the expense of an entire fleet of condensing boilers. In this situation, the initial cool return/makeup water is routed first through the condensing boiler, then through the non-condensing boiler/s once the water temperature is well above the condensing point.
Van Wormer points out that this can be the best solution where there are legacy non-condensing boilers that are still in good operating condition. “Adding a condensing boiler and controls in an appropriate piping arrangement can drastically improve the overall plant efficiency without replacing all the equipment.”
The Cleaver Brooks ClearFire condensing boilers feature the AluFer® tube which greatly increases heat transfer and the effective heating surface. The AluFer tube has an aluminum internal heat exchange surface within an outer stainless shell. These boilers feature a true counterflow heat exchanger with dual return connections to maximize boiler efficiency. They also feature a highly efficient pre-mix burner and outside air reset capabilities.
Condensing economizer another approach
If your application is not suitable for a condensing boiler because of space considerations, another alternative is to add a condensing economizer in the boiler exhaust stream. These can be used with cool boiler makeup water streams or possibly domestic hot water streams to achieve the benefits of condensing heat recovery.
When you are using a condensing boiler or a condensing economizer, you are maximizing the efficiency of your steam plant. Ask for help from your engineer or a manufacturer’s representative in evaluating your savings potential.