Selecting a boiler for heating, process loads

Boilers for heating and domestic hot water systems are used in many nonresidential buildings and across educational, hospital, and industrial campuses. This article reviews the codes and standards that regulate boiler system specification and design, plus energy efficiency and efficacy of these boiler systems.

04/22/2015


This article has been peer-reviewed.Learning objectives

  • Learn about the different types of boilers available, system design, and available system options.
  • Understand the codes, standards, and permitting considerations that boiler systems installation and design need to consider.

Packaged boilers for heating and domestic hot water systems are used in many nonresidential buildings and across many university, hospital, and industrial campuses. Packaged boilers normally fall into the category of factory assembled boilers, which are then shipped via, truck, rail,or barge to the end user's site. In recent years some boiler manufacturers have begun selling packaged boilers as modules that are assembled in the field-which are still considered packaged boilers.

When designing a boiler system many options are available, but not all options are appropriate for every system. In some cases a modular boiler with high- and low-fire (two-stage burner) may be an acceptable solution where in another case a fully modulating boiler with 10:1 turndown ratio may be more appropriate. Cost; operator capabilities; available space; international, national, state, and local codes as adopted; and steam or hot water requirements will set the system parameters used in selecting each boiler system.

In boiler systems, steam, high-temperature hot water, and hot water are used for comfort heating, humidification, and process heating media across a wide range of building environments. Boilers can be installed in individual buildings or, as in the case of many campus environments, as part of a central plant boiler system that provides heating and process media to the campus through direct buried or tunnel distribution systems. Care needs to be taken in evaluating central boiler plants versus building-specific boiler plants. Each system type carries pros and cons with redundant capacity, operator availability, capital cost, lifecycle costs, and existing system configuration weighing the new design.

In either scenario, the boilers specified to generate steam, high-temperature hot water, and hot water vary in construction and size depending on the amount and temperature of media required. Packaged boiler steam systems vary in pressure from 15 to 1000 psig. Piping for high-, medium-,and low-pressure steam systems in industrial settings is governed by ASME B31.3: Process Piping Design. In addition, ASME B31.9: Building Services Piping may be followed depending on system pressure and temperature.

Table 1: This outlines the industrial packaged boiler types, sizes, and system component options. Courtesy: Stanley Consultants

Boilers are designed to ASME Boiler and Pressure Vessel Code requirements. Piping and boiler system materials are dictated by the design pressures and temperatures desired for the system. High-temperature and pressure systems may require the use of alloy steel piping to distribute steam between the boiler and processes. Piping systems of boilers that will be used to generate power will be designed to ASME B31.1: Power Piping. Each of the respective codes will identify the allowable operating and design stresses for materials based on temperature. Steam and hot water distribution at lower temperatures for industrial and process systems can use both carbon and stainless steel piping depending on final service requirements.

Boiler types

Boilers come in two primary designs: fire tube and water tube. In a fire tube boiler, the boiler gases and heat are within the tubes in the boiler; while in water tube boilers, boiler feed water is within the tubes and drums of the boiler. Water tube boilers primarily are used to generate steam, while fire tube boilers are used for both steam and hot water (see Table 1). Fire tube boilers are most commonly found in single building applications where steam is used for small process systems and building heat. 

Figure 1: A fire tube boiler cutaway shows the burner throat, fire tubes, and boiler tube sheet. Courtesy: Stanley Consultants

These boilers are typically about 50% to 60% of the cost of a water tube boiler due to the smaller size and less steel content for a given boiler capacity. Steam generating fire tube boilers have less water in the boiler than a water tube boiler, which limits the amount of load swing that the boiler is able to handle. Yearly cleaning and inspection on the fire tube boiler is easier than on a water tube boiler, and tube repair can be performed from the boiler exterior with adequate tube pull space rather than from within the boiler (see Figure 1).

Fire tube boilers come in 2-, 3-, and 4-pass flue gas designs. Increasing the number of passes in the boiler increases boiler efficiency while also increasing cost. Steam capacity for fire tube boilers generally ranges between 5,000 and 75,000 lb/h. Hot water boilers are sized in million Btus and range from 2 to 3 MMBtu up to 100 MMBtu.


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