scope:

Introduction

In general, people are creatures of comfort. Many conversations start out with the topic of weather and how it affects an individual's perception of comfort. On the National Academy of Engineering's list of engineering achievements "that had the greatest impact on the quality of life in the 20th century," air conditioning and refrigeration came in tenth, indicating the great significance of this field in the world. With many people in the United States spending nearly 90% of their time indoors, it is hardly surprising that providing a comfortable and healthy indoor environment is a major factor in life today.

Air-conditioning systems usually provide year-round control of several air conditions, namely, temperature, humidity, cleanliness, and air motion. These systems may also be referred to as environmental control systems, although today they are usually called heating, ventilating, and air-conditioning (HVAC) systems.

The primary function of an HVAC system is either (1) the generation and maintenance of comfort for occupants in a conditioned space; or (2) the supplying of a set of environmental conditions (high temperature and high humidity, low temperature and high humidity, etc.) for a process or product within a space. Human comfort design conditions are quite different from conditions required in textile mills or for grain storage and vary with factors such as time of year, activity type, and clothing levels of the occupants.

If improperly sized equipment or the wrong type of equipment is used, the desired environmental conditions usually will not be met. Furthermore, improperly selected and/or sized equipment normally requires excess power and/or energy and may have a higher initial cost. The design of an HVAC system includes calculation of the maximum heating and cooling loads for the spaces to be served, selection of the type of system to be used, calculation of piping and/or duct sizes, selection of the type and size of equipment (heat exchangers, boilers, chillers, fans, etc.), and a layout of the system, with cost, indoor air quality, and energy conservation being considered along the way. Some criteria to be considered are

• Temperature, humidity, and space pressure requirements

• Capacity requirements

• Equipment redundancy

• Spatial requirements

• Carbon emissions

• First cost

• Operating cost

• Maintenance cost

• Reliability

• Flexibility

• Life-cycle cost analysis

The following details should be considered to properly design an air-conditioning system and are covered in greater detail in subsequent chapters of this text:

• The location, elevation, and orientation of the structure so that the effects of the weather (wind, sun, and precipitation) on the building heating and cooling loads can be anticipated.

• The building size (wall area, roof area, glass area, floor area, and so forth).

• The building shape (L-shaped, A-shaped, rectangular, etc.), which influences equipment location, type of heating and cooling system used, and duct or piping locations.

• The space use characteristics. Will there be different users (office, bank, school, dance studios, etc.) of the space from year to year? Will there be different concurrent requirements from the tenants? Will there be night setback of the temperature controller or intermittent use of the building's facilities?

• The type of material (wood, masonry, metal, and so forth) used in the construction of the building. What is the expected quality of the construction?

• The type of fenestration (light transmitting partition) used, its location in the building, and how it might be shaded. Is glass heat absorbing, reflective, colored, etc.?

• The types of doors (sliding, swinging, revolving) and windows (sealed, wood or metal frames, etc.) used. What is their expected use? This will affect the amount of infiltration air.

• The expected occupancy for the space and the time schedule of this occupancy.

• Type and location of lighting. Types of appliances and electrical machinery in the space and their expected use.

• Location of electric, gas, and water services. These services should be integrated with the locations of the heating and air-conditioning duct, piping, and equipment.

• Ventilation requirements for the structure. Does it require 100% outdoor air, a given number of CFM per person, or a given number of CFM per square foot of floor area?

• Local and/or national codes relating to ventilation, gas, plumbing, and/or electrical services.

• The environmental conditions that are maintained. Will fluctuations of these conditions with load be detrimental to the purpose served by the structure?

• The heating and cooling loads (also consider the moisture load, air contaminants, and noise).

• The type of heating and cooling system to be used in the structure. Is it forced air, circulated water, or direct expansion? Will it be a multizone, single-zone, reheat, variable-airvolume, or another type of system? What method of control will be used? Will a dedicated outdoor air system be considered?

• The heating and cooling equipment size that will maintain the inside design conditions for the selected outside design condition. Electric heat or fossil fuel? Mechanical vapor compression or absorption chiller?

• The challenges of over/undersizing the equipment as applied to the structure. Survey any economic trade-offs to be made. Should a different type of unit be installed in order to reduce operating costs? Should a more sophisticated control system be used to give more exact control of humidity and temperature or should an on-off cycle be used? Fuel economy as related to design will become an even more important factor in system selection and operation.

• The estimated annual energy usage and/or carbon emissions.

In general, no absolute rules dictate correct selections or specifications for each of the above items, so only engineering estimates or educated guesses can be made. However, estimates must be based on sound fundamental principles and concepts. This book presents a basic philosophy of environmental control as well as the basic concepts of design. These ideas relate directly to the ASHRAE Handbook volumes: 2018 Refrigeration, 2019 HVAC Applications, 2020 HVAC Systems and Equipment, and most directly to 2021 Fundamentals.