INTRODUCTION TO HVAC. Concepts of Air Conditioning

Transcription

1 INTRODUCTION TO HVAC Concepts of Air Conditioning 1

2 Technical Development Programs (TDP) are modules of technical training on HVAC theory, system design, equipment selection and application topics. They are targeted at engineers and designers who wish to develop their knowledge in this field to effectively design, specify, sell or apply HVAC equipment in commercial applications. Although TDP topics have been developed as stand-alone modules, there are logical groupings of topics. The modules within each group begin at an introductory level and progress to advanced levels. The breadth of this offering allows for customization into a complete HVAC curriculum from a complete HVAC design course at an introductory-level or to an advancedlevel design course. Advanced-level modules assume prerequisite knowledge and do not review basic concepts. This module deals with the functions an air-conditioning system must perform to provide comfort air conditioning. Elementary air-conditioning definitions are explained and the fundamental classification of systems is described. The types of systems, with their components and how they control multiple building zones are discussed. It is intended for people new to the industry or who may not be familiar with the many types of HVAC systems available. At the end of this module, a novice should have a general understanding of air-conditioning systems and how they deal with building zoning considerations Carrier Corporation. All rights reserved. The information in this manual is offered as a general guide for the use of industry and consulting engineers in designing system Judgment is required for application of this information to specific installations and design applications. Carrier is not responsibl any uses made of this information and assumes no responsibility for the performance or desirability of any resulting system des The information in this publication is subject to change without notice. No part of this publication may be reproduced or transmit any form or by any means, electronic or mechanical, for any purpose, without the express written permission of Carrier Corporat Printed in Syracuse, NY CARRIER CORPORATION Carrier Parkway Syracuse, NY 13221, U.S.A.

3 Table of Contents Introduction...1 System Types...4 All-Air Systems...5 All-Water Systems...22 Air-Water Systems...25 Direct-Refrigerant Systems...30 The Refrigeration Cycle...34 Systems Selection...36 Summary...37 Work Session...39 Appendix...41 System Comparison Chart...41 Glossary...43 References...49 Work Session Answers

4 Introduction The struggle to control man s environment began long ago when prehistoric people learned to use fire to produce warmth. For centuries, comfort was defined only as keeping warm. Later, people tried to find ways to conquer the discomfort caused by excessive heat and humidity. Many attempts were made to find cooling comfort. Most of these attempts dealt only with the temperature. Then, in 1902, Willis H. Carrier introduced scientific air conditioning to the world. At last both temperature and humidity could be controlled. Since then, air conditioning has been put to use in a variety of places and almost everyone has experienced it. Many of the benefits we enjoy in Figure 1 modern living would not be possible Willis H. Carrier, the inventor of air conditioning without air conditioning. We often think that air conditioning is only providing cool air, but that process is only part of what comfort air conditioning is about. A complete air-conditioning system needs to address a number of other environmental criteria if comfort is to be maintained. Cool air is used in a variety of ways, to keep food cold, for factory process applications, and to keep us comfortable on hot days. In addition, comfort air-conditioning systems need to control more than just temperature. Humidity, airflow, Figure 2 ventilation, and air quality influence Comfort applications in offices, stores and homes our perception of comfort. There many of ways of providing comfort to a space. This module will look at what the airconditioning system needs to provide and how comfort is maintained with a number of different system types. While many of the principles discussed here apply to refrigeration and process applications, we will discuss comfort-cooling applications. The types of systems described in this TDP relate primarily to commercial buildings where without air conditioning it would be hard to work, shop, or play. The term air conditioning has become synonymous with total comfort control. The business that provides comfort conditioning is also called HVAC, for heating, ventilating, and air conditioning. We will use both terms in the TDP series to describe the comfort control business. 1

5 Comfort We will begin with a brief look at comfort and how the human body perceives it. The body generates heat and then attempts to maintain a constant temperature while coping with a changing environment. The body perceives itself to be comfortable when heat being dissipated from its surface equals the heat being generated within. If too much heat is being removed, we feel cold; if not enough heat is being removed, we feel hot. The amount of heat produced varies according to the activity being performed by the body. We measure all heat energy by a unit called a Btu (British thermal unit). Btu The British thermal unit is a unit of measure of heat energy. One Btu equals the amount of heat required to raise one pound of water one degree Fahrenheit (F). Figure 3 Comfort a balance of heat Human heat generation varies from a base rate of about 300 Btuh when sleeping, to 400 Btuh for office tasks, to approximately 800 Btuh when exercising. Then, for comfort to be maintained, the air surrounding the body must remove heat at a rate equal to that generated by the activity. Heat is removed by one of three forms of heat transfer and by evaporation. Three conditions of the ambient air affect the amount of heat transferred. The Figure 4 ambient air temperature surrounding the Heat generation rates depend on activity level. body affects the heat gain or loss by contact (conduction) with the skin. The greater the temperature differences between the ambient air and the skin, the greater the rate of heat loss or gain. This heat gain is called sensible heat. The ambient air temperature is referred to as the dry-bulb temperature. The relative humidity, or the moisture content of the air, affects heat being dissipated by evaporation. The energy that changed the moisture from a liquid to a vapor is called latent heat. 2

6 Air motion affects both the evaporation rate and the heat dissipated by air motion (convection) and contributes to an individual s sense of comfort. Additionally, the temperature of surrounding surfaces (walls, windows, floors, etc.) affects the feelings of comfort through radiant heat losses or gains. The perception of comfort is also influenced by the quality of the air. Indoor air quality is defined as having adequate oxygen, low perception of odor, and free of irritating particulates and gases. Therefore, comfort air conditioning has come to be defined as the maintenance of six atmospheric factors within well-defined limits. Refer to TDP-102, ABCs of Comfort, for further definition of these limits and Figure 5 the impact these factors have on the Heat transfer from the body perception of comfort. In addition, systems also must not contribute objectionable amounts of noise (unwanted sound). Comfort air conditioning controls: Dry-bulb Temperature Controlling these six factors simultaneously can Relative Humidity be a complex task. Numerous air-conditioning system Air Motion types have been developed to meet the requirements for different building applications. The typical office Radiant Temperature space is the most familiar, and we will use it as an example of how systems meet each of the six comfort Air Cleanliness Dilution or Removal of Odors criteria. The concepts apply equally well to other types of buildings. In an office, heat is generated from people, equipment (like computers, printers and copy machines), lights, and from solar heat gain. The outside walls, roof, floor, adjoining spaces, and ventilation air introduced to the space may either be a gain or a loss and contribute to the total heat load. A system needs to be able to remove from the space an amount of heat equal to that added or the space temperature will begin to increase. Sometimes heat is lost at a rate greater than is generated in the space, and the system must add heat at a sufficient rate to maintain comfort. Air-conditioning systems remove or add heat to maintain the desired conditions. The air-conditioning cooling process involves providing a medium or fluid that will absorb heat gain from the space and use the fluid to transfer the absorbed heat to a place where it can be rejected. Systems are described based on the fluid used for cooling. Figure 6 Spaces typical office loads 3

7 Systems must also provide heat, ventilation air, humidification, and filtration as required. Air-conditioning systems accomplish this in each space or remotely. Systems may also serve only one space or many spaces. The appropriate system selection depends on the building application and how well the system meets all the owners comfort requirements. System Types Air-conditioning systems can be classified into one of four major categories, each type having certain functional and economic advantages. The generic classifications for these systems are: all-air, all-water, airwater, and direct-refrigerant. The identification indicates the controllable medium(s) or fluid(s) supplied to the conditioned areas for cooling air, water, or refrigerant. Each type of system must cool and circulate the air within the space. During the cooling mode of operation, the system supplies cooled and dehumidified air to the space to absorb sensible heat and latent heat. The cooled air acts like a sponge, absorbing the sensible energy and heat energy stored in the water vapor in the air. To maintain comfort, the space temperature is usually controlled at around 75 F and 50 percent relative humidity. Air entering the space is cooler than this and typically at less moisture content than the space, at a temperature normally between 55 and 60 F. The relative humidity is actually higher than the space, near 90 percent. However, the total amount of moisture in a given volume of the supply air is less than the moisture in the same volume of space air. The actual moisture content, or specific humidity, of the supply air is less than the space. Figure 7 System types are defined by the fluid providing cooling. Figure 8 Air absorbs heat. 4