Boilers: Types, applications, and efficiencies
Engineers should understand which boiler is appropriate for the application, and then know how to maximize its use.
- Understand the various types of fuel-fired boilers.
- Learn about specific boiler types and applications.
- 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.
There 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).
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.
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.
Primary/secondary boiler piping systems: Decoupling the boiler piping flow loop from the building flow loop has become more common in the past several years. The concept of primary/secondary pumping has been around since the 1950s. It has been primarily used in chilled water systems. The advantages include the ability to have non-equal flow in two loops that are connected by a common “decoupler” section of piping (see Figure 3). This is a similar arrangement to a primary-secondary chilled water piping system. The primary loop allows for constant flow through the operating boiler while allowing the secondary building loop to act semi-independently of the primary loop. The helps avoid many of the flow issues with primary-only piping arrangements (such as low flow, flash steam in tubes, and increased fouling of the tubes due to low tube velocities).
All of the hot water boilers listed in Table 1 can be applied successfully in a primary-secondary piping arrangement. Special care, as previously stated, must be used when applying cast iron, straight water tube, or fire tube boilers to ensure the maximum water temperature difference (between supply water and return water to and from the boiler) is not exceeded throughout the boilers’ operating range.
Caption: All efficiencies are typical and based on natural gas as the fuel. The efficiency ranges are typical based on a multiple manufacturers published data.
Maximizing efficiency in fuel-fired hot water boiler systems is dependent upon a few key design points. These points, for consideration by the system design engineer/designer, are as follows:
- Accurate heating load for the building based upon conduction loss without credit for lighting, people, and equipment.
- Determining the minimum heating load for the system during the “shoulder” months (late winter through spring, late fall) for the particular location. This can be accomplished by modifying the computer based heating load calculation to provide a typical energy model for the building or by specifying the month to determine the lowest heating requirement. This is relatively easy to accomplish using Trane TRACE 700, Carrier HAP, or similar software that allows for typical-year or full-year energy modeling.
- Selection of high-efficiency or ultra-high-efficiency boilers to match the full range of the calculated heating load. In many cases, the use of more than two boilers is suggested to allow for a good range of boiler “turn-down” capacity to meet the instantaneous heating load of the building. An example is to use three boilers each at one-third of the required system capacity. This will allow for the traditional two-thirds capacity “reserve” or back-up while allowing for greater controllability of the heating water system. Additional boiler(s) may be required for full redundancy to meet the building owner’s requirements for N+1 or greater reliability.
- Consider the modular boiler approach (noncondensing type or condensing type) with lower boiler mass and minimum of 5:1 turn-down ratio.
- Use nontraditional controls to manage staging of multiple boilers to match the instantaneous heating load. Boiler staging should be accomplished by trending and matching the supply water temperature required in the secondary loop at any given time.
- Monitoring heating coil control valve position versus required supply air temperature. Use this information to determine the secondary loop supply water temperature setpoint.
- Enable and disable each boiler based on the building energy management system calculation of the required system heating load. Once enabled, allow each boiler to fire and modulate on its own controls. Many packaged or modular boiler manufacturers have optimization hardware/software to maximize the efficiency of their equipment and that can be provided with energy management system interfaces or nonproprietary communications protocols such as BACnet. This writer suggests the use of optimization programming provided by the boiler manufacturer in multiple boiler design installations with the use of “rolling” sequence control (1,2,3,4; 2,3,4,1; 3,4,1,2; etc.) to maximize/exercise the use of each boiler in the system. If this control/programming is not available from the boiler manufacturer, it can be readily accomplished by the temperature control contractor.
- Use flow meters and temperature difference in the secondary supply and return main piping to calculate the current boiler capacity required by the heating system. This is a simple yet extremely effective method of matching the boilers to the instantaneous building heating demands.
- Provide automatic boiler isolation valves to allow non-required boilers to be isolated from the primary boiler loop. This works only for boilers capable of withstanding thermal shock. Do not use this on cast iron, straight water tube, or fire tube boilers without using the previously mentioned thermal shock prevention techniques. (Individual boiler circulation pumps with blending valve to allow for slow warm-up.)
- Provide boiler stack dampers to reduce heat loss of non-firing boilers.
- Use individual primary pumps dedicated to each boiler. The use of automatic balancing valves may help ensure proper flow through each boiler over the entire load operating range of the system.
- Use variable flow secondary system to reduce system pumping requirements.
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.
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.
Steam boilers are applied in many applications for building heating and many forms of process heating and humidification systems. The use of steam boilers has dropped in recent years, but they still remain the choice method of distribution energy for heating in large facilities such as hospitals, campuses, and some downtown areas of major cities.
Steam boilers can be classified in several ways; however, they are either low pressure (15 psig or less) or high pressure (greater than 15 psig). They can be fire-tube Scotch marine with wet- or dry-back design, cast iron, or water tube design. Steam generating boilers require large volumes for the phase change of water to steam to reduce operational issues related to small water to steam interface area.
Steam boilers also require proper water chemistry for proper operation. Boiler feed water/makeup must be low in hardness (typically 2 grains/lb or less) with low total dissolve solids in order to reduce water surface tension. Water surface tension is a primary cause of water spouting within the boiler’s water to steam interface area. Increased “water spouting” can result in rapid fluctuation of the water level in the boiler, which is indicative of water carryover from the steam generation volume of the boiler into the steam piping header. Water carryover from the boiler to the steam header usually leads to the boiler shutting down on its low water safety. If the steam header becomes partially or fully water-logged, complete shutdown and drainage of the system is required.
Steam boilers and steam piping systems are large thermal flywheels. The system requires a substantial start-up time for boiler and piping system warmup. Large piping system usually require warmup in multiple sections to avoid or minimize vacuum (sub-atmospheric) pressure forming during the warmup process due to steam condensing back to water. Steam systems cannot react to rapidly changing system demands if the boilers are staged on from a cold state. Therefore, steam boiler staging is based upon weather, steam header pressure, and the boiler operator’s experience rather than the staging controls used in hot-water boiler systems.
While steam is an extremely effective method of transporting thermal energy (considering the latent heat of vaporization) and requires no pumping on the vapor side of the system, steam boiler systems are inherently inefficient. Recent experience indicates the natural gas to steam plant output (thermal efficiency) to range from 55% to 65% based on measured usage data for an 180,000 lb/hr plant.
Boilers and water-based heating systems are available in a wide variety of types and configurations. Determining the boiler type for a specific application is the responsibility of the consulting engineer in conjunction with the owner or operator of the facility. Applying the boiler in most efficient configuration is the responsibility of the consulting engineer.
The table of boiler types included in this article is not intended to be complete, but only a reference to basic types and configurations available. Fabrication materials vary between manufacturers along with patented designs.
The most efficiently designed boiler-based system uses the most efficient boiler and system configuration for the application. Controlling the system to meet the system demands is the major key to overall efficiency. Using the simplest but most effective boiler plant/system staging controls with the feedback of building heating requirements on a minute-by-minute basis is the key to optimizing any hydronic heating system’s efficiency. System feedback input includes continuous monitoring of flow requirements and supply-return water temperature differential, using this data to calculate real-time requirements of the facility, and then making decisions on the staging of entire heating plant.
Variable primary-only boiler systems can be accomplished. However, this must be achieved with the input of the boiler manufacturer’s engineering/applications group. Special attention must be given to maintain minimum tube velocity required by the boiler manufacturer.
Condensing boilers offer the greatest energy efficiency if properly applied. Small terminal coils (such as terminal boxes, finned tube radiation, cabinet heater, etc.) are not designed for low temperature water. Therefore, special care must be used in selecting these coils. Condensing boilers can be applied to existing systems if proper precautions are realized by the consulting engineer and the facility owner/operator. If the existing system is to take full advantage of the efficiency potential of these boilers, every coil must be evaluated for performance at the lower water temperatures. The alternative is to apply these boilers with the proper controls to allow for noncondensing water temperatures during peak heating periods.
Michael E. Myers is senior mechanical engineering manager at WD Partners in Dublin, Ohio, where he is responsible for managing and directing the mechanical engineering division. He has more than 33 years of experience in HVAC, plumbing, and fire protection engineering. He is a former ASHRAE distinguished lecturer, former ASHRAE chapter president and a previously published co-author on HVAC design.