Energy management and water heating systems

Engineers should work to design the most efficient water heating system or boiler possible.

05/14/2014


Learning objectives:

  1. Understand the codes and standards that guide water heating system design.
  2. Learn how to design water heating systems to meet requirements.
  3. Understand key equipment functions of boilers and similar systems.

This article has been peer-reviewed.Over the years, HVAC engineers have been challenged to provide more efficient and sustainable designs that meet societal demands to reduce the overall carbon footprint of a facility. Performance guidelines provided by programs such as the U.S. Green Building Council, Green Globes, and Energy Star provide guidelines for energy efficiency, while agencies such as the Environmental Protection Agency (EPA) and the Dept. of Energy (DOE) establish compliance guidelines for commercial HVAC systems.

Generally speaking, manufacturers developed equipment with improved operating characteristics and efficiencies driven by DOE regulations and the addition of ASHRAE Standard 90.1 or, in some instances such as chillers, the need to change due to the climatological impact of the refrigerant as regulated by Section 608 of the Clean Air Act. While equipment efficiencies improved, the proper installation and application of this equipment once configured as an operating system was often misapplied. The natural assumption was that by using highly efficient components, the overall system operation would follow suit as it was integrated with other highly efficient components.

The failure of this mind-set became apparent with chilled water systems and the variety of pumping configurations available. This created a more analytical approach to not only chiller static full-load efficiency ratings, but also how single and multiple chillers operated together at part-load, the efficiencies at those conditions, and the hours operating at these specific conditions. This, along with cooling tower and variable speed pumping strategies, produce highly efficient systems.

As time went on, hot water heaters/boilers also became more efficient out of necessity to be competitive and to meet new regulations. Unfortunately, the application of the variety of boiler types, properly integrated with system pumping and loading configurations along with control strategies, was overlooked to some extent. A boiler was a boiler. It provided hot water and it was 86% efficient, at best. In general, boiler efficiencies range from 80% to 96%. In some cases, several smaller boilers may have been configured to operate in sequence, which provided a certain level of redundancy and operational steps while attempting to keep the boilers at maximum firing rates to maintain them at a higher efficiency.

Limitations affecting a hot water system’s overall efficiencies were based on two fundamental factors. First, the misapplication of boiler type/size, and second, the ability of the boiler’s burner to completely modulate in response to the demand for hot water. In most cases, burners were stepped down or up in a number of stages, or what is known as turndown ratio. The most common was usually four steps, or 25% increments of modulation. Some boilers have higher turndown ratios or, in some instances, infinite modulation with variable speed blowers.

Newer equipment burners now may have the ability to provide infinite modulation and very high efficiencies. While this may seem like great news, there are still limitations and operational pitfalls if proper application and control strategies are not considered. The 2012 ASHRAE Handbook--HVAC Systems and Equipment provides a thorough discussion of boiler types, configurations, and general applications for new installations.

Selecting the proper boiler

A boiler can provide either hot water or steam, which makes it a pressure vessel and subject to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. In the spectrum of available heating media, boilers can provide steam from 5 to 160 psig and higher. Conversely, hot water can be provided from as low as 85 F to in excess of 230 F in high temperatures/pressure water systems. These conditions are produced by a variety of boiler types grouped into general classes. Working temperature, pressure, fuel type, type of draft (mechanical or natural), and construction type are just a few of the identifying properties of boilers. There are other characteristics such as the general configuration of the heating surfaces (tubes) and condensing/noncondensing types. Selecting the proper type of boiler to meet the application requirements has a direct impact on system performance.

In the case of existing boilers, Energy Star has a software package called Portfolio Manager that allows an owner to track consumption and any changes to overall facility operations. Consumption data from utility bills is input and used to benchmark the performance of a building. The tool can be useful to establish system efficiency and operational costs.

The most common types of boilers used today for commercial applications are a high-efficiency noncondensing modular type, high-efficiency condensing type, or a Scotch marine type for larger applications. The Scotch marine boiler can be identified by a central fluid backed combustion chamber, which is surrounded by fire tubes with two or more hot gas paths, passing through the heating surface area. These are not a condensing type and are limited in overall efficiency, mostly due to burner turndown capabilities and heat exchanger material. Water temperatures below 140 F usually cause the flue gas to condense, which will corrode the cast iron or steel heat exchanger components. In most situations, the boiler plant will be configured as shown in the hot water piping schematic. Noncondensing boilers must have return water temperatures above 140 F to avoid thermal shock.

Conversely, higher boiler efficiencies can be maintained with new condensing type boilers, which are designed to allow the flue gas water vapor to approach or pass through the condensate point, thereby releasing more heat. These units traditionally have a stainless steel heat exchanger. Condensing boilers provide high overall efficiencies due to the high turndown or modulating characteristics of the burners. Additional efficiencies are achieved by using lower return water temperatures or a larger temperature difference between the supply and return water temperature.

Keep in mind that the heat exchangers of condensing boilers are designed to operate with the corrosive condensate of the flue gas vapor. In most cases, there will be a flue gas condensate collection point and drain at the base of the boiler, usually rated at 88% efficient or higher. This condensate is mildly corrosive and may have to be treated with a neutralizing agent prior to discharging it to a municipal drain (check the local codes).

In addition to collecting the condensate, the boiler exhaust stacks must be made of a corrosion-resistant material such as stainless steel, aluminized steel, or PVC. Most manufacturers provide directives with respect to recommended flue type.


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