Boilers: Types, applications, and efficiencies

Engineers should understand which boiler is appropriate for the application, and then know how to maximize its use.

03/22/2013


Learning objectives

  1. Understand the various types of fuel-fired boilers.
  2. Learn about specific boiler types and applications.
  3. Know how to maximize heating water systems efficiency.

Boilers are the basic foundation of heating and domestic hot water in many commercial, industrial, institutional, and education facilities. The term “boiler” can be a misleading because in many applications, the boiler does not produce water at boiling temperatures of 212 F or above. This article will begin with the various types of fuel-fired boilers for a general description then focus on specific types and applications.

Figure 1: Traditional hot water applications typically consist of a medium water temperature system with a minimum of two boilers piped in parallel in a primary heating water loop configuration, as shown here. All images courtesy: Michael E. MyersThere are two types of efficiencies with fuel-fired boilers: combustion efficiency and thermal efficiency. Combustion efficiency is the percentage of chemical potential energy of the fuel that is converted during the combustion process to produce thermal energy. Thermal efficiency is simply stated as the percentage of potential fuel energy that is converted to thermal energy leaving the boiler in the form of heated water or steam. It is thermal efficiency that the consulting-specifying engineer should be most concerned with in the equipment selection process. Please reference the BTS-2000 test standard from The Hydronics Institute Division of AHRI for heating boilers for additional boiler efficiency testing information, or the 2012 ASHRAE Handbook—HVAC Systems and Equipment for more information.  

Boilers, in general terms, fall into two main categories with each main category having several types based on type and purpose for each design. The main categories are hot water and steam. Table 1 includes most types of boilers, applications, and range of typical efficiencies.

Boiler controllability and system efficiency

To maximize boiler and ultimately the heating system efficiency, the boiler controllability or “turn-down” ratio must be carefully considered for the individual project application. Boiler systems should be selected and sized to allow for a wide fluctuation in the heating load of the building, thus allowing the system to closely match the building heating requirements at any given time.

Traditional training of many HVAC engineers and designers is to provide two boilers each sized at two-thirds of the total heating load. The thought behind this method considers the fact that if the heating load calculation was performed properly, one boiler will be sufficient during a typical winter’s day. The second boiler will effectively serve as a 100% backup on a typical day. This type of design also allows for proper heating of the building during the most extreme cold weather (beyond ASHRAE winter design temperature).

Figure 2: Flexible water tube boilers typically consist of an upper and lower water drum with a series of bent steel tubes designed to absorb the stresses of thermal expansion.While this is a good conservative approach and has served the industry well over the years, it may not be the most efficient application due to the controllability of the boilers selected. Many smaller boilers have high- and low-fire settings that do not allow for matching of the boiler capacity to the actual building load. In many cases, if the boiler has “modulating” firing controls, the turn-down ratio is not great enough to meet lower heating demands of the system. Unless multiple boilers are used to allow for smaller or tighter control of the heating water supply temperature, this system will not be as efficient as it could be.

Hot water applications

Traditional hot water boiler primary piping systems: Traditional hot water applications typically consist of a medium water temperature system (180 to 210 F) with a minimum of two boilers piped in parallel in a primary heating water loop configuration (see Figure 1). Heating water supply temperature is set based upon the outside ambient air temperature and “reset” to a certain temperature in a straight-line fashion with a minimum water temperature at a predetermined outside ambient temperature.

Cast iron, straight water tube, and fire tube boilers require special consideration to avoid damaging the boiler (commonly known as thermal shock) with return water that is more than 20 F less than the water leaving the boiler. The reset range for supply water temperature is 210 to 160 F. Return water temperature must remain at a maximum of 20 F lower than the leaving water temperature of the boiler, and flow through the boiler must remain within close range of the boiler manufacturer’s requirements to ensure that flash steam (a knocking sound within the boiler is indicative of flash steam and low water flow) is not produced.

Figure 3: A basic primary-secondary boiler loop piping shows non-equal flow in two loops that are connected by a common “decoupler” section of piping.Flexible water tube boilers cause much less concern about thermal shock than their cast iron or straight tube counterparts. Flexible water tube boilers typically consist of an upper and lower water drum with a series of bent steel tubes designed to absorb the stresses of thermal expansion (see Figure 2).

The overall efficiency of this application can be relatively low when compared to other types of boilers and piping configurations. Inefficiencies include maintaining the temperatures of the boiler and water mass along with maintaining the temperature of the distribution piping system based on limitation of the boilers. Modular boiler types, which will be discussed later, reduce these overall losses.

Another disadvantage of the traditional primary-only boiler loop piping is the constant flow or partially variable flow required in the supply loop piping system. Constant flow primary-only systems do not maintain water temperature differential (supply minus return water temperature) during the full range of load conditions. Maintaining the design water temperature differential, over the full range of load conditions, is a major efficiency objective.


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