Modulating Economizers: Recovering Heat, Recovering Profit

By Steen Hagensen

In most boilers today, heat exchange devices, or economizers, pre-heat the incoming feed water with the boiler’s own exhaust gases to maximize efficiency. Over the years, improvements have been made to economizers that now have better heat transfer rates than ever, but it is still only very few boilers that can benefit from their installation. Atmospheric and fan-assisted boilers don’t create a powerful enough draft to force flue gases through the economizer, which makes them unsuitable for economizer installations.

This changes with the introduction of the modulating, fan-powered economizer. With this concept it is now possible to look at a total boiler plant as the load rather than that of the individual boiler.

Traditional Economizers
Traditional economizers are primarily used with forced draft boilers and steam boilers. The design consists of a round or a cube-shaped housing installed at the outlet of the boiler. It has single or double row tube coils with fins in either copper or stainless steel. The flue gas passes over the coils, which transfers the heat to the water that flows inside the coil. The material depends on the application, but if condensation is likely to occur, or if the flue gas temperature exceeds 400° F, 316 grade stainless steel is normally used. One drawback is that the design creates flow resistance on the air side of 0.2 to 0.5 water column inches at full boiler load. Very few boilers are able push against a higher resistance.

Another disadvantage is that the traditional economizer has a constant water flow. Constant flow works well when the boiler is on full fire, but if the burner has a turn-down ratio and the boiler operates or modulates at less than full capacity, then the economizer and the flue gas no longer have enough heat to maintain the water temperature. What was water in at 170° and water out at 180° at 100% load, may now become water in at 180° and water out at 192° at 75% load. A 50% load might see water in at 180° and out at 186° while one can expect a mere two degree difference—180° in and 182° out—with a 25% load. The numbers are all based on a constant water flow.

Modulating Powered Economizers
A modulating fan-powered economizer consists of a boiler fan and an integrated economizer. The boiler fan has variable speed and is controlled by the boiler draft requirement. It is designed for high flue gas temperatures and for condensing applications. The economizer can be of a finned coil design, just as a traditional economizer, or a radiator finned-tube design. In either case, the coil or tube design is compact with tight coils and tubes. The radiator style economizer is extremely efficient but a drawback is that the radiator design creates a high pressure resistance, which creates a need for the boiler fan. In fact, a radiator-style, finned-tube design can only be used because a fan is powering the system.

Further, a variable speed pump controlled by the water temperature circulates the water through the economizer. The fan and pump are managed by a single controller that governs the air and water flows based on a pre-determined draft set-point and temperature set-point.

The concept allows the designer to look at a “boiler plant” as the load to work with, instead of one per boiler or appliance. The boiler plant could even consist of a combination of boiler types such as forced draft boilers and atmospheric water heaters.

Economizers are usually rated at the maximum load, but in reality, this load is rarely achieved. A boiler heating system may operate 3,500 hours per year, but only 10-15% of those hours will be a 100% load, while the remaining hours will average a 40-50% load.

In a typical 5,000 MBH atmospheric heating boiler system used in schools, hospitals, hotels and resorts, the radiator design is far more efficient than the traditional economizer over the entire range of load. Over 40% more BTU recovery can be expected at full load. A 60% load and 40% load can each recover about half and a third, respectively, of the BTUs in the system.

The modulating fan-powered economizer is also significantly more efficient than a traditional economizer in the mid-range load levels. This makes it extremely suitable for most systems and particularly for boiler systems with varying loads.

Steen Hagensen, President of ENERVEX Inc., has been in the boiler and venting industry for over 25 years. He has served on NFPA 54/Z 223.1 National Fuel Gas Code Technical Committee for more than 10 years. Hagensen has a number of articles published in industry magazines and frequently speaks for ASHRAE Chapters in the US and Middle East and other industry groups.

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