Analysis and Design of Heating, Ventilating, and Air-Conditioning Systems, Second Edition

Herbert W. Stanford III, Adam F. Spach

• Editorial: CRC Press
• Año: 2019
• ISBN:
  9780429890864
• Edición: 2
• Categoría: Scienze fisiche
• Categoría: Termodinamica

Analysis and Design of Heating, Ventilating, and Air-Conditioning Systems, Second Edition, provides a thorough and modern overview of HVAC for commercial and industrial buildings, emphasizing energy efficiency. This text combines coverage of heating and air conditioning systems design with detailed information on the latest controls technologies. It also addresses the art of HVAC design along with carefully explained scientific and technical content, reflecting the extensive experience of the authors. Modern HVAC topics are addressed, including sustainability, IAQ, water treatment and risk management, vibration and noise mitigation, and maintainability from a practical point of view.

Section II
The Design Method

4

HVAC Systems Design

4.1 Introduction

The design of an effective air-conditioning system must proceed in a systematic sequence involving the following basic steps:

1. Obtain a detailed understanding of the owner’s criteria and performance requirements for the project: In addition, the designer must determine whether there are design constraints, such as a limited construction budget or limited ability of the owner to understand and/or maintain sophisticated heating, ventilating, and air-conditioning (HVAC) systems or equipment. The specific functional requirements for each area of the building must be identified (proposed use, number of people, hours of use, owner equipment to be provided, etc.). Limitations imposed by building codes and/or regulatory agencies must also be considered. Determine whether the project must meet specific sustainability design requirements, such as Leadership in Energy and Environmental Design (LEED).
2. Evaluate alternative systems concepts to satisfy project requirements: Most building types can be provided with more than one type of air-conditioning system that will meet the design criteria defined in Step 1. Each alternative system concept must be identified and evaluated, formally or informally, by the designer to determine the best system for the particular project. The use of Building Information Modeling (BIM), discussed in Chapter 16, has made the comparison and evaluation of the life-cycle costs of alternative systems significantly simpler.
3. Define control zones and compute heating/cooling loads: Typically, a “zone” is defined as a space or a group of spaces with similar requirements that is “controlled” from a single temperature sensing point. To determine whether a group of spaces can be combined, three criteria must be examined for each space:
   1. Heating/cooling criteria: inside design conditions, envelop load variations, internal heat gains, required degree of control, and so on
   2. Operational criteria: daily and/or weekly use of spaces (e.g., hours per day of use, changes in occupancy, and partial use)
   3. Ventilation criteria: ventilation rates and space relationships required for odor/contaminant control
      Only if spaces have similar criteria in all of these categories can they be combined into a single control zone.
4. Locate major mechanical equipment elements, and define building space requirements for the air-conditioning system: Space for air-conditioning systems must be provided in any building in four locations:
   1. Space for primary equipment such as boilers, chillers, and pumps. Typically, one central mechanical equipment room is required to house these systems. The designer must provide adequate space for the primary equipment, space for the additional support equipment, and space to allow for proper maintenance of all types of the equipment.
   2. Space for air-handling equipment. Whereas smaller building may use outdoor rooftop air-handling units, larger buildings will require “fan rooms” to house this equipment.
   3. Sufficient ceiling space for air distribution (ductwork); HVAC, plumbing, and fire protection piping; and electrical raceway is required. These space requirements are in addition to the minimum space required for the ceiling itself and recessed light fixtures.
   4. Outdoor space for condensing units, cooling towers, exhaust fans, and so on. Some of this space may be located on the ground, whereas roof space is required for certain equipment.
   5. When locating and arranging HVAC system elements, take into account required access and clearances for effective maintenance, all of which is easier when using BIM (See Chapter 16) rather than manual or computer-aided drafting methods.
5. Design air distribution systems and effective air diffusion: Once the air diffusion devices are located and the required airflows to each device determined on the basis of the heating and cooling load calculations, the duct systems for distributing supply air and returning air to an air-handling unit can be designed. Exhaust systems and special duct systems for fume hoods, etc., can then be designed.
6. Design piping systems: Piping design starts with determining configuration and routing of the different piping systems, moves on to pipe sizing, and finishes with defining the details of piping installation and equipment connections.
7. Define equipment performance criteria: The designer should define all specific equipment requirements but should not select a specific equipment item (brand, model, etc.) to meet these requirements only for “basis of design.” By defining performance requirements (capacity, size, weight, electrical characteristics, etc.), the designer places the burden of performance compliance on the contractor and equipment vendor. Then, if the equipment item does not perform satisfactorily, the designer is in the position of requiring that the contractor and equipment vendor be responsible for any remediation that is required.
8. Design system operating controls: Even though this item is listed lastly, it is, in fact, the most important part of any HVAC design and must be ongoing throughout the design sequence.

Each of these design steps is described in more detail in the following chapters of this book.

4.2 Criteria for HVAC Design

Over many years of HVAC design experience, the authors have learned that some things either simply do not work or, even if they do at least work, they may be more trouble than they are worth for both the owner and the designer. The following systems and design practices are not acceptable unless their application has been discussed with the owner and the designer is satisfied that the project requires their use:

1. Rooftop HVAC systems or equipment on buildings more than one story in height and/or for which reasonable roof access for maintenance and repair has not been provided. Without adequate access, rooftop HVAC equipment tends to be ignored and maintenance suffers.
2. Rooftop systems or equipment on any building in “snowy” climates
3. Air-cooled chiller systems larger than 150–200 tons, unless required by the owner, air-cooled chillers consume 40%–50% more energy than water-cooled chillers.
4. Direct resistance electric heat, except for terminal unit heating/reheating coils when dictated by owner requirements
5. Two-pipe, changeover heating/cooling systems of any type, except in Climate Zones 6 and 7
6. Unit ventilators: These devices are relatively complex as terminal units, provide poor humidity control, and are noisy.
7. Window air-conditioning units, packaged terminal air conditioners or packaged terminal heat pumps, except for residential occupancies
8. Variable air volume (VAV) systems using direct expansion (DX) cooling, unless a manufacturer’s packaged system with integral original equipment manufacturer controls is used
9. Parallel-configured fan-powered air terminal units unless dictated by the owner or for special applications
10. Fiber glass duct liner or fiberboard duct. Historically, this material has proved to be a breeding ground for mold.
11. Di...