Back to basics: Boilers and hot water systems

Courtesy: WSP USA Buildings

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Learning Objectives

• Know and understand the applicable codes and standards for boilers.
• Learn about water distribution, pumping options and hot water system accessories.
• Understand boiler applications for comfort heating and improving boiler efficiency.

Boiler insights

• Boilers, frequently utilized in commercial properties, have the ability to heat air and various fluids.
• The International Mechanical Code outlines the design and specification requirements for boilers and their integration into hot water systems.

Boilers are essential mechanical systems equipped with heat exchangers that raise fluid temperatures to satisfy building demands or processes. In designing comfort heating systems, a common temperature differential is maintained between 20°F and 30°F between the inlet and outlet water temperatures. Boilers operate using a fuel source such as natural gas, diesel, or propane.

Through a combustion process that draws in fresh air for the boiler burner, energy is produced from the fuel and transmitted to the working fluid. These boilers are constructed in accordance with the regulations established by the American Society of Mechanical Engineers as part of the Boiler and Pressure Vessel Code.

The initial Boiler and Pressure Vessel Code was released in and has since evolved into a comprehensive document regarded as essential for the construction and handling of boilers. It encompasses codes applicable not just for heating systems meant for human comfort but also for applications such as power generation and nuclear energy.

The BPVC consists of more than 10 sections; Section I focuses on the construction guidelines for power boilers, while Section VII emphasizes care, maintenance, and operational practices for these systems. Section VIII pertains to pressure vessels functioning above 15 pounds per square inch (psig).

Heating boilers are categorized under Section IV, which includes steam generation, potable water heaters, and other hot water applications. This section comprises installation and inspection standards, particularly significant for low-pressure settings. Low-pressure steam boilers operate at pressures below 15 psig, while hot water boilers function at below 160 psig. These boilers can be powered by various fuels, including natural gas, fuel oil, electricity, or coal. Furthermore, Section VI stipulates operational guidelines that include controls for heating water boilers.

Aside from the BPVC, design engineers are guided by numerous standards when specifying boilers for comfort heating. Under the International Code Council, multiple codes are available, such as the International Building Code, International Mechanical Code, International Plumbing Code, and the International Fuel Gas Code. These codes provide design engineers with foundational criteria for creating functional, safe systems.

Boilers find extensive application in heating, ventilation, and air conditioning systems, particularly for comfort heating. Water heaters cater to domestic systems providing potable water, with specific standards detailed in the IPC.

The IMC's edition is comprehensive, consisting of 15 chapters and two appendices. Chapter 7 talks about guidelines for combustion air. Generally, the configurations involve two openings—one at a higher elevation and another at a lower level—or a single large opening. The necessary opening size is calculated by assessing the fuel-burning equipment within the mechanical room, utilizing a formula that requires a minimum of 1 square inch for each 1,000 British thermal units per hour or 3,000 Btu/hour, depending upon the selected design.

Chapter 10 of the IMC outlines specifications pertaining to boilers, water heaters, and pressure vessels. Notable points from this chapter include:

• Compliance with the ASME BPVC is mandatory for all boilers.
• Design of fuel-fired rated appliances must comply with ASME-CSD-1, the standard concerning Controls and Safety Devices for Automatically Fired Boilers. Under the IMC, boilers with input ratings below 12.5 million Btu/hour observe the ASME BPVC's guidance in Section I or IV, while those exceeding this threshold must adhere to NFPA 85: Boiler and Combustion Systems Hazards Code to mitigate explosion risks.
• Installation must align with manufacturer's guidelines.
• Clearance specifications are crucial. Design engineers must account for adequate serviceable space around the boilers, as well as easy access for removal or replacement. Minimum clearance is generally 18 inches, but may exceed this depending on boiler type, size, and application. A few considerations during boiler room layout include:
  • Validating the spatial requirements for substantial equipment pieces to ensure cost efficiency.
  • Factoring in variations in equipment dimensions from proposals during the award phase, as these can differ among vendors.
  • Utilizing 3D modeling techniques is currently essential for designers to ensure appropriate clearances for equipment maintenance.
• Steam boilers need additional overhead clearance, especially vital in retrofit scenarios where existing structures may limit available space.
  • Under subsection .3.1, the IMC provides detailed clearance requirements for boiler tops, which should be reviewed during new boiler room design.
• Heating water boilers require dedicated room enclosures with fire-rated walls as per IMC, IBC, and NFPA requirements. Previous discussions illustrated these code sections relevant to hot water systems.

Below are key sections concerning boiler connections:

• All boilers are to be equipped with a makeup water supply and shut-off valves positioned at both the supply and return lines. An automatic low-water cutoff is necessary to shut down combustion in instances of insufficient water levels.

Another important aspect is ensuring the installation of safety and relief valves. These components are essential for both hot water and steam boilers, which must be rated appropriately for the respective systems they serve. Typically, safety valves are spring-loaded, allowing them to open when system pressure exceeds a designated setpoint.

Boiler Water Distribution

Historically, piping systems can be categorized into one-pipe, two-pipe or four-pipe designs. One-pipe systems are used exclusively for heating via a dedicated loop, linking the return line following the equipment. While effective, this method can lead to challenges including increased head pressure and temperature losses throughout the pipeline, thereby escalating operational costs. A prevalent example of such a system is perimeter radiation, often used in colder regions, identified as a monoflow system.

Two-pipe systems are established for alternating supply and return piping for heating and cooling, necessitating seasonal changeovers. Whereas four-pipe systems maintain separate dedicated loops—one for chilled water and another for hot water—allowing for varied operational cost assessments based on design requirements.

Proper pipe sizing is crucial in the distribution loop. Water moving through pipes must counter friction losses; hence, flow velocity is a critical factor. Engineers must grasp the concept of Reynolds number, a dimensionless figure that discerns laminar flow from turbulent flow within a pipe.

The calculation for Reynolds number considers the fluid's density and velocity, characteristic dimensions, and dynamic viscosity. The formula typically yields lower Reynolds numbers indicative of laminar flow, whereas greater figures indicate turbulent flow. Engineers utilize charts like the Moody Diagram to analyze flow type based on known parameters and pipe roughness. Furthermore, codes provide baseline guidelines for recommended velocities suited to specific systems, as demonstrated within ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings, Chapter 6, which presents suggested flow rates and maximum velocities.

Pumping Options

Pumping solutions within hot water central plants generally fall into two categories. A prevalent design is the variable primary system, where primary pumps serve the building, leading to advantages such as lower operational costs and streamlined maintenance.

Alternatively, a primary/secondary setup is beneficial when the central plant's geographic distance from the served building is considerable or when catering to multiple buildings with diverse requirements. Design engineers must analyze the return on investment to achieve the most economically viable design for the owners, who typically desire returns of five to seven years for investment decisions.

Hot Water System Accessories

Closed-loop heating systems are complemented by accessories that enhance performance, including expansion tanks, air separators, pressure gauges, and temperature sensors. As water heats, its volume expands relative to system temperature and pressure, necessitating these safety accessories.

Contact us to discuss your requirements for an Industrial Steam Boiler. Our knowledgeable sales team is available to assist you in identifying the most suitable options for your needs.

There are multiple types of expansion tanks available, with bladder and diaphragm tanks being the most commonly used. Bladder expansion tanks, designed as closed systems, have a bladder that expands with water volume as system temperature rises, containing both air and water within the tank.

Diaphragm tanks contain a diaphragm positioned at midrange, wherein the lower segment holds water and the upper section contains compressed air. The diaphragm adjusts as the water expands, functioning like a sine wave. Selection of the suitable expansion tank depends on the specifics of the design, with recommendations for placement on the return line to avoid any impediments to pump operation.

Air separators are vital accessories in closed systems that eliminate the risk of air buildup. It is common for air to be entrapped during system startup; however, adequate circulation helps expel air effectively, protecting equipment and optimizing system performance.

Additional necessary accessories comprise pressure gauges, Pete plugs, and thermometers, which are consistently specified for all equipment within the system.

Temperature sensors and pressure gauges are essential at connections with equipment and heating or cooling coils.

Boiler Load

Two types of boilers are generally identified in comfort heating water systems: condensing and non-condensing types. Due to their heightened efficiency at lower return water temperatures, condensing boilers have gained popularity. These units facilitate the condensation of flue gases to reclaim the latent heat of vaporization, with generated condensate conveyed to drains equipped with neutralization kits to protect against corrosive effects.

Conversely, non-condensing boilers do not permit flue gas condensation, necessitating that design engineers ensure temperature specifications align with the boiler type to avoid damaging components over time. Non-condensing boilers typically operate at elevated temperature ranges, such as 180°F to 150°F, and generally feature a larger footprint compared to their condensing counterparts.

Boilers are designed to produce hot water for various applications, including comfort heating, domestic heating, or process heating. A building load calculation is the initial step a design engineer undertakes to size the equipment appropriately. Factors influencing building load encompass aspects such as building type, geographic location, orientation, and the envelope's glazing type and layout.

Software tools enable load calculations, with an emphasis on deriving total peak load measured in British thermal units per hour and gallons per minute (gpm) required for user comfort. Utilizing peak condition data, engineers can design systems adhering to defined temperature differentials.

The set temperature difference holds significance as it directly affects the system's hot water flow rate. As a practical example, consider a 50,000-square-foot medical office with a calculated total peak load of 1.5 million Btu/hour and a defined temperature difference of 30°F, resulting in a flow rate of 100 gpm. This flow rate is determined through an analytical formula considering the specified temperature difference.

It's crucial to remember that increased temperature differences will lead to reduced gpm, consequently saving on pump energy requirements. However, this requires balancing benefits and drawbacks during the design phase.

Understanding the relevant formulas for calculating and monitoring systems is essential. For deeper insights, refer to the ANSI/ASHRAE Handbook on Fundamentals.

A critical concept in heating systems is the sensible load, calculated as:

The constant factor derives from air density (in pounds mass/cubic feet) and its specific volume (in cubic feet/pounds mass) under standard conditions. The psychrometric chart depicts how sensible loads move horizontally from point A to point B. There are two pivotal stages in heating system designs: preheating for treating outdoor air and reheating for localized heat application.

A practical illustration can be observed in the variable air volume box. These design parameters play a significant role in determining the sizing of boiler plants, hence necessitating the assessment of not only the building envelope load but also the system's operational requirements. Since condensing boilers benefit from decreased return water temperatures, the design for VAV reheat coils becomes instrumental in effectively transferring heat. Systems with design temperatures below 130°F should consider deploying three-row coils to meet the desired leaving air temperature effectively.

In conclusion, the applications of boilers are extensive, necessitating that engineers possess a comprehensive understanding of relevant codes and standards. Moreover, it is acknowledged across industries that condensing boilers perform better with lower return water temperatures, thus requiring precise design parameters aligned with a building system's objectives to achieve optimal functionality. Careful consideration of design parameters is vital before progressing to drafting specifications.

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