Boilers: Types, applications, and efficiencies

03/22/2013


Applying condensing hot water boilers

Condensing water boilers are increasingly desired by many building owners. This is due to the very high efficiency of these devices. Typical thermal efficiencies range from 90% to 98%. Many building owners are asking their consultants to use these devices for new, replacement, and retrofit applications. However, this can present a challenge for all of these applications unless the consulting engineer/designer and the owner fully understand the limits of using a boiler designed for condensing temperatures.

Condensing boilers are low water temperature devices when applied in the condensing temperature range. (Note: There is at least one boiler manufacturer that is currently advertising a condensing boiler with 160 F minimum return water temperature.) In order for the flue gases to be in the condensing temperature range to achieve the maximum heat transfer of the fuel energy to the water, the supply water temperature is ≤130 F with return water temperatures as low as 80 F. As with any counter-flow heat exchange device, the leaving temperature of the heated fluid (water in this case) cannot be greater than the leaving temperature of the hot side or heating medium (combusted fuel in this case). The lower the fuel gas temperature, the more efficient the boiler becomes as more heat is extracted from the combustion gases.

Figure 4: Cast iron, straight water tube, and fire tube boilers require special consideration to avoid damaging the boiler. A high-pressure 180,000 lbs/hr steam plant uses a multiple fire tube boiler.The drawback of this low water temperature is applying these in a replacement or retrofit application. Due to the low supply water temperature, most heating coils that were selected at a higher entering water temperature will not perform well at this lower water temperature. This is the case with many replacement and retrofit projects. Therefore, thoughtful consideration must be given at a minimum to determine the performance of these older existing coils with the lower water temperature. Replacement of at least some of the coils will be necessary. This is especially true of heating coils in small zone devices such as variable volume terminal boxes, cabinet unit heaters, finned tube radiation, and convectors as these devices are usually designed for 180 to 200 F entering water temperature.

However, condensing boilers can be applied in existing systems designed for higher water temperatures if the boilers are applied in a hybrid condensing/noncondensing design. This allows the boilers to function in the condensing range during the partial heating load periods (when condensing water temperatures can be used) while allowing the boiler to produce noncondensing supply water temperatures (typically ˃130 F) during the higher heating load periods. The boilers will then function at a lower efficiency, but greater water temperature at a reduced overall heating output. A word of caution is required related to boiler flue material: The boiler flue material must be capable of withstanding the corrosive effects of condensed flue gases as well has flue gas temperatures produced when operating out of the condensing temperature range.

New heating water systems using condensing boilers must take into account the requirement for larger than “normal” heating coils required in all air handling equipment. Variable air terminal boxes and finned tube radiation must have the lower water temperature accounted for in the sizing selection process. Hot water coils in terminal units, in this writer’s experience, are a minimum of two rows and typically 3 to 4 rows to provide more surface area for heat exchange.

Modular condensing boilers are almost always applied in a primary-secondary piping arrangement with each boiler requiring a dedicated return water pump to ensure proper flow within the boiler. As previously mentioned, the use of automatic flow balancing valves should be used for each boiler in the supply water (leaving side) piping to further help ensure proper flow throughout the operating flow and pressure range of the entire heating water system.



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