Boilers: Types, applications, and efficiencies
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.