Vice President and Executive Group Publisher: Richard Swadley
Vice President and Executive Publisher: Robert Ipsen
Vice President and Publisher: Joseph B. Wikert
Executive Editorial Director: Mary Bednarek
Editorial Manager: Kathryn A. Malm
Executive Editor: Carol A. Long
Acquisitions Editor: Katie Mohr
Senior Production Manager: Fred Bernardi
Development Editor: Kenyon Brown
Production Editor: Vincent Kunkemueller
Text Design & Composition: TechBooks
Copyright © 2004 by Wiley Publishing, Inc., Indianapolis, Indiana. All rights reserved.
Published simultaneously in Canada
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 646-8600. Requests to the Publisher for permission should be addressed to the Legal Department, Wiley Publishing, Inc., 10475 Crosspoint Blvd., Indianapolis, IN 46256, (317) 572-3447, fax (317) 572-4447, E-mail: .
The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation warranties of fitness for a particular purpose. No warranty may be created or extended by sales or promotional materials. The advice and strategies contained herein may not be suitable for every situation. This work is sold with the understanding that the publisher is not engaged in rendering legal, accounting, or other professional services. If professional assistance is required, the services of a competent professional person should be sought. Neither the publisher nor the author shall be liable for damages arising herefrom. The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read.
For general information on our other products and services please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002.
Trademarks: Wiley, the Wiley Publishing logo, Audel are trademarks or registered trademarks of John Wiley & Sons, Inc. and/or its affiliates. All other trademarks are the property of their respective owners. Wiley Publishing, Inc., is not associated with any product or vendor mentioned in this book.
Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.
Library of Congress Cataloging-in-Publication Data:
ISBN: 0-764-54206-0
For Laura, my friend, my daughter.
Contents
Introduction
The purpose of this series is to provide the layman with an introduction to the fundamentals of installing, servicing, troubleshooting, and repairing the various types of equipment used in residential and light-commercial heating, ventilating, and air conditioning (HVAC) systems. Consequently, it was written not only for the HVAC technician and others with the required experience and skills to do this type of work but also for the homeowner interested in maintaining an efficient and trouble-free HVAC system. A special effort was made to remain consistent with the terminology, definitions, and practices of the various professional and trade associations involved in the heating, ventilating, and air conditioning fields.
Volume 1 begins with a description of the principles of thermal dynamics and ventilation, and proceeds from there to a general description of the various heating systems used in residences and light-commercial structures. Volume 2 contains descriptions of the working principles of various types of equipment and other components used in these systems. Following a similar format, Volume 3 includes detailed instructions for installing, servicing, and repairing these different types of equipment and components.
The author wishes to acknowledge the cooperation of the many organizations and manufacturers for their assistance in supplying valuable data in the preparation of this series. Every effort was made to give appropriate credit and courtesy lines for materials and illustrations used in each volume.
Special thanks is due to Greg Gyorda and Paul Blanchard (Watts Industries, Inc.), Christi Drum (Lennox Industries, Inc.), Dave Cheswald and Keith Nelson (Yukon/Eagle), Bob Rathke (ITT Bell & Gossett), John Spuller (ITT Hoffman Specialty), Matt Kleszezynski (Hydrotherm), and Stephanie DePugh (Thermo Pride).
Last, but certainly not least, I would like to thank Katie Feltman, Kathryn Malm, Carol Long, Ken Brown, and Vincent Kunkemueller, my editors at John Wiley & Sons, whose constant support and encouragement made this project possible.
James E. Brumbaugh
About the Author
James E. Brumbaugh is a technical writer with many years of experience working in the HVAC and building construction industries. He is the author of the Welders Guide, The Complete Roofing Guide, and The Complete Siding Guide.
This series is an introduction to the basic principles of heating, ventilating, and air conditioning (HVAC). Each represents a systematic attempt to control the various aspects of the environment within an enclosure, whether it is a room, a group of rooms, or a building.
Among those aspects of the immediate environment that people first sought to control were heat and ventilation. Attempts at controlling heat date from prehistoric times and probably first developed in colder climates, where it was necessary to produce temperatures sufficient for both comfort and health. Over the years the technology of heating advanced from simple attempts to keep the body warm to very sophisticated systems of maintaining stabilized environments in order to reduce heat loss from the body or the structural surfaces of the room.
Ventilation also dates back to very early periods in history. Certainly the use of slaves to wave large fans or fanlike devices over the heads of rulers was a crude early attempt to solve a ventilating problem. Situating a room or a building so that it took advantage of prevailing breezes and winds was a more sophisticated attempt. Nevertheless, it was not until the nineteenth century that any really significant advances were made in ventilating. During that period, particularly in the early stages of the Industrial Revolution, ventilating acquired increased importance. Work efficiency and the health of the workers necessitated the creation of ventilation systems to remove contaminants from the air. Eventually, the interrelationship of heating and ventilating became such that it is now regarded as a single subject.
Air conditioning is a comparatively recent development and encompasses all aspects of environmental control. In addition to the control of temperature, both humidity (i.e., the moisture content of the air) and air cleanliness are also regulated by air conditioning. The earliest attempts at air conditioning involved the placing of wet cloths over air passages (window openings, entrances, etc.) to cool the air. Developments in air conditioning technology did not progress much further than this until the nineteenth century. From about 1840 on, several systems were devised for both cooling and humidifying rooms. These were first developed by textile manufacturers in order to reduce the static electricity in the air. Later, adaptations were made by other industries.
Developments in air conditioning technology increased rapidly in the first four decades of the nineteenth century, but widespread use of air conditioning in buildings is a phenomenon of the post-World War II period (i.e., 1945 to the present). Today, air conditioning is found not only in commercial and industrial buildings but in residential dwellings as well. Unlike early forms of air conditioning, which were designed to cool the air or add moisture to it, modern air conditioning systems can control temperature, air moisture content, air cleanliness, and air movement. That is, modern systems condition the air rather than simply cool it.
Many different methods have been devised for heating buildings. Each has its own characteristics, and most methods have at least one objectionable aspect (e.g., high cost of fuel, expensive equipment, or inefficient heating characteristics). Most of these heating methods can be classified according to one of the following four criteria:
The term heat-conveying medium means the substance or combination of substances that carries the heat from its point of origin to the area being heated. There are basically four mediums for conveying heat. These four mediums are:
Different types of wood, coal, oil, and gas have been used as fuels for producing heat. You may consider electricity as both a fuel and a heat-conveying medium. Each heating fuel has its own characteristics; the advantage of one type over another depends upon such variables as availability, efficiency of the heating equipment (which, in turn, is dependent upon design, maintenance, and other factors), and cost. A detailed analysis of the use and effectiveness of the various heating fuels is found in Chapter 5 (“Heating Fuels”).
Heating methods can also be classified with respect to the nature of the heat applied. For example, the heat may be of the exhaust steam variety or it may consist of exhaust gases from internal combustion engines. The nature of the heat applied is inherent to the heat system and can be determined by reading the various chapters that deal with each type of heating system (Chapters 6 through 9) or with heat-producing equipment (e.g., Chapter 11, “Gas Furnaces”).
The various heating methods differ considerably in efficiency and desirability. This is due to a number of different but often interrelated factors, such as energy cost, conveying medium employed, and type of heating unit. The integration of these interrelated components into a single operating unit is referred to as a heating system.
Because of the different conditions met within practice, there is a great variety in heating systems, but most of them fall into one of the following broad classifications:
You will note that these classifications of heating systems are based on the heat-conveying method used. This is a convenient method of classification because it includes the vast majority of heating systems used today.
As mentioned, ventilating is so closely related to heating in its various applications that the two are very frequently approached as a single subject. In this series, specific aspects of ventilating are considered in Chapter 6 (“Ventilation Principles”) and Chapter 7 (“Ventilation and Exhaust Fans”) of Volume 3.
The type and design of ventilating system employed depends on a number of different factors, including:
A residence will have a different ventilating system from a building used for commercial or industrial purposes. Moreover, the requirements of a ventilating system used to provide fresh air result in fundamental design differences from a ventilating system that must remove noxious gases or other dangerous contaminants from the enclosure.
The size of a building is a factor that also must be considered. For example, a large building presents certain ventilating problems if the internal areas are far from the points where outside air would initially gain access. Giving special attention to the overall design of the ventilating system can usually solve these problems.
Buildings located in the tropics or semitropics present different ventilating problems from those found in temperature zones. The differences are so great that they often result in different architectural forms. At least this was the case before the advent of widespread use of air conditioning. The typical southern house of the nineteenth century was constructed with high ceilings (heat tends to rise); large porches that sheltered sections of the house from the hot, direct rays of the sun; and large window areas to admit the maximum amount of air. They were also usually situated so that halls, major doors, and sleeping areas faced the direction of the prevailing winds. Today, with air conditioning so widely used, these considerations are not as important—at least not until the power fails or the equipment breaks down.
Although the major emphasis in this series has been placed on the various aspects of heating and ventilating, some attention has also been given to air conditioning. The reason for this, of course, is the increasing use of year-round air conditioning systems that provide heating, ventilating, and cooling. These systems condition the air by controlling its temperature (warming or cooling it), cleanliness, moisture content, and movement. This is the true meaning of the term air conditioning. Unfortunately, it has become almost synonymous with the idea of cooling, which is becoming less and less representative of the true function of an air conditioning system. Air conditioning, particularly the year-round air conditioning systems, is examined in detail in Chapters 8, 9, and 10 of Volume 3.
There are a number of different types of heating, ventilating, and air conditioning equipment and systems available for installation in the home. The problem is choosing the most efficient one in terms of the installation and operating costs. These factors, in turn, are directly related to one’s particular heating and cooling requirements. The system must be the correct size for the home. Any reputable building contractor or heating and air conditioning firm should be able to advise you in this matter.
If you are having a heating and ventilating or air conditioning system installed in an older house, be sure to check the construction. Weather stripping is the easiest place to start. All doors and windows should be weather-stripped to prevent heat loss. Adequate weather stripping can cut heating costs by as much as 15 to 20 percent. If the windows provide suitable protection (they should be double- or triple-glazed) from the winter cold, check the caulking around the edge of the glass. If it is cracking or crumbling, replace it with fresh caulking. You may even want to go to the expense of insulating the ceilings and outside walls. This is where a great deal of heat loss and air leakage occurs.
You have several advantages when you are building your own house. For example, you may be able to determine the location of your house on the lot. This should enable you to establish the direction in which the main rooms and largest windows face. If you position your house so that these rooms and windows face south, you will gain maximum sunlight and heat from the sun during the cold winter months. This will reduce the heat requirement and heating costs. The quality of construction depends on how much you wish to spend and the reliability of the contractor. It is advisable to purchase the best insulation you can afford. Your reduced heating costs will eventually pay for the added cost of the insulation. If you suspect that your building contractor cannot be trusted, you can reduce opportunities for cheating and careless work by making frequent and unexpected visits to the construction site.
Many career opportunities are available in heating, ventilating, and air conditioning fields, and they extend over several levels of education and training. Accordingly, the career opportunities open to an individual seeking employment in these fields can be divided roughly into four categories, each dependent upon a different type or degree of education and/or training. This relationship is shown in .
Relationships between Career Category and Type of Work or Education and/or Training Required
| Career Category | Type of Work | Education/Training |
| Engineer | Design and development | 4 years or more of college |
| Technician | Practical application | Technical training school and/or college |
| Skilled worker | Installation, maintenance, and repair | Apprentice program or on-the-job training (OJT) |
| Apprentice or OJT worker | Training for skilled-worker position | High school degree or equivalency |
Among workers in these fields, engineers receive the highest pay, but they also undergo the longest periods of education and training. Engineers are usually employed by laboratories, universities, and colleges or, frequently, by the manufacturers of materials and equipment used in heating, ventilating, air conditioning, and related industries. Their primary responsibility is designing, developing, and testing the equipment and materials used in these fields. In some cases, particularly when large buildings or district heating to several buildings is employed, they also supervise the installation of the entire system. Moreover, industry codes and standards are usually the results of research conducted by engineers.
Technicians obtain their skills through technical training schools, some college, or both. Many assist engineers in the practical application of what the latter have designed. Technicians are particularly necessary during the developmental stages. Other technicians are found in the field working for contractors in the larger companies. Their pay often approximates that of engineers, depending on the size of the company for which they work.
Skilled workers are involved in the installation, maintenance, and repair of heating, ventilating, and air conditioning equipment. Apprentices and OJT (on-the-job training) workers are in training for the skilled positions and are generally expected to complete at least a 2- to 5-year training program. Local firms that install or repair equipment in residential, commercial, and industrial buildings employ most skilled workers and trainees. Some also work on the assembly lines of factories that manufacture such equipment. Their pay varies, depending on the area, their seniority, and the nature of the work. Most employers require that both skilled workers and trainees have at least a high school diploma or its equivalent (e.g., the GED). The requirement for a high school diploma may be waived if the individual has already acquired the necessary skills on a previous job. The pay for skilled workers and trainees is lower than that earned by engineers and technicians but compares favorably to salaries received by skilled workers or equivalent trainees in other occupations.
Pipe fitters, plumbers, steam fitters, and sheet-metal workers may occasionally do some work with heating, ventilating, and air conditioning equipment. Both pipe fitters and plumbers (especially the former) are frequently called upon to assemble and install pipes and pipe systems that carry the heating or cooling conveying medium from the source. Both are also involved in repair work, and some pipe fitters can install heating and air conditioning units.
Steam fitters can assemble and install hot-water or steam heating systems. Many steam fitters can also do the installation of boilers, stokers, oil and gas burners, radiators, radiant heating systems, and air conditioning systems.
Sheet-metal workers can also assemble and install heating, ventilating, and air conditioning systems. Their skills are particularly necessary in assembling sheet-metal ducts and duct systems.
Some special occupations, such as those performed by air conditioning and refrigeration mechanics or stationary engineers, are limited to certain functions in the heating, ventilating, and air conditioning fields. Mechanics are primarily involved with assembling, installing, and maintaining both air conditioning and refrigeration equipment. Stationary engineers maintain and operate heating, ventilating, and air conditioning equipment in large buildings and factories. Workers in both occupations require greater skills and longer training periods than most skilled workers.
It should be readily apparent by now that the heating, ventilating, and air conditioning fields offer a variety of career opportunities. The pay is generally good, and the nature of the work provides considerable job security. Both the type of work an individual does and the level at which it is done depend solely on the amount and type of education and training acquired by the individual.
A number of professional organizations have been established for those who work in the heating, ventilating, and air conditioning industries or who handle their products. These organizations (frequently referred to as associations, societies, or institutes) provide a number of different services to members and nonmembers.
Some professional and trade organizations have established permanent libraries as resource centers for those seeking to improve their skills or wishing to keep abreast of current developments in their fields. In many instances, research programs are conducted in cooperation with laboratories, colleges, and universities.
Many of these organizations address themselves to the problems and interests of specific groups. For example, there are professional organizations for manufacturers, wholesalers, jobbers, distributors, and journeymen. Some organizations represent an entire industry, while others restrict their scope to only a segment of it. Every aspect of heating, ventilating, and air conditioning is covered by one or more of these professional organizations.
Anyone, member or not, can write to these professional organizations for information or assistance. Most seem very willing to comply with any reasonable request. The only difficulty that may be encountered is determining the current name of the particular organization and obtaining its address. Unfortunately, these professional organizations have shown a strong proclivity toward mergers over the years, with resulting changes of names and addresses.
The best and most current guide to the names and addresses of professional organizations is The Encyclopedia of Associations, which can be found in the reference departments of most public libraries. It is published in three volumes, but everything you will need can be found in the first one. At the back of this volume is a section called the “Alphabetical & Key Word Index.” By looking up the key word (e.g., heating or ventilating) of the subject that interests you, you can find the page number and full name of the professional organization (or organizations) concerned with the particular area. See Appendix A in this volume for a partial listing of these professional and trade associations.
Some professional organizations of long standing have been merged with others or have been disbanded. For example, the Steel Boiler Institute (formerly the Steel Heating Boiler Institute), which maintained standards in the heating industry with its SBI Rating Code, is now defunct. The Institute of Boiler and Radiator Manufacturers (source of the old IBR Code) merged with the Better Heating-Cooling Council to form the Hydronics Institute. A recent attempt to contact the Steam Heating Equipment Manufacturers Association has resulted in the return of a letter marked “no forwarding address.” It seems very likely that it, too, has joined the list of defunct professional organizations.
Appendix A (Professional and Trade Associations) at the end of this book gives a listing of professional organizations. It also contains their present addresses, the names of some of their publications, and a brief synopsis of their backgrounds and whom they represent.
There is still considerable disagreement about the exact nature of heat, but most authorities agree that it is a particular form of energy. Specifically, heat is a form of energy not associated with matter and in transit between its source and destination point. Furthermore, heat energy exists as such only between these two points. In other words, it exists as heat energy only while flowing between the source and destination.
So far this description of heat energy has been practically identical to that of work energy, the other form of energy in transit not associated with matter. The distinguishing difference between the two is that heat energy is energy in transit as a result of temperature differences between its source and destination point, whereas work energy in transit is due to other, nontemperature factors.
Heat energy is measured by the British thermal unit (Btu). Each thermal unit is regarded as equivalent to one unit of heat (heat energy).
Since 1929, British thermal units have been defined on the basis of 1 Btu being equal to 251.996 IT (International Steam Table) calories, or 778.26 foot-pounds of mechanical energy units (work). Taking into consideration that one IT calorie equals 1/860 of a watt-hour, 1 Btu is then equivalent to about 1/3 watt-hour.
Prior to its 1929 redefinition, a Btu was defined as the amount of heat necessary to raise the temperature of one pound of water by one degree Fahrenheit. Because of the difficulty in determining the exact value of a Btu, it was later redefined in terms of the more fundamental physical unit.
Energy is the ability to do work or move against a resistance. Conversely, work is the overcoming of resistance through a certain distance by the expenditure of energy.
Work is measured by a standard unit called the foot-pound, which may be defined as the amount of work done in raising one pound the distance of one foot, or in overcoming a pressure of one pound through a distance of one foot ().
Man raising 1 pound 1 foot to illustrate the foot-pound standard unit.
The relationship between work and heat is referred to as the mechanical equivalent of heat; one unit of heat is equal to 778.26 ft-lb. This relationship (i.e., the mechanical equivalent of heat) was first established by experiments conducted in the nineteenth century. In 1843 Dr. James Prescott Joule (1818–1889) of Manchester, England, determined by numerous experiments that when 772 ft-lb of energy had been expended on 1 lb of water, the temperature of water had risen 1°F and the relationship between heat and mechanical work was found (). The value 772 ft-lb is known as Joule’s equivalent. More recent experiments give higher figures and the value 778 (1 Btu = 778.26 ft-lb). (See the preceding section.)
The mechanical equivalent of heat.
When bodies of unequal temperatures are placed near each other, heat leaves the hotter body and is absorbed by the colder one until the temperatures are equal to each other. The rate by which the heat is absorbed by the colder body is proportional to the difference of temperature between the two bodies—the greater the difference in temperature, the greater the rate of flow of the heat.
Heat is transferred from one body to another at lower temperature by any one of the following means ():
The transfer of heat by radiation, conduction, and convection.
Radiation, insofar as heat loss is concerned, refers to the throwing out of heat in rays. The heat rays proceed in straight lines, and the intensity of the heat radiated from any one source becomes less as the distance from the source increases.
The amount of heat loss from a body within a room or building through radiation depends upon the temperature of the floor, ceiling, and walls. The colder these surfaces are, the faster and greater will be the heat loss from a human body standing within the enclosure. If the wall, ceiling, and floor surfaces are warmer than the human body within the enclosure they form, heat will be radiated from these surfaces to the body. In these situations a person may complain that the room is too hot.
Knowledge of the mean radiant temperature of the surfaces of an enclosure is important when dealing with heat loss by radiation. The mean radiant temperature (MRT) is the weighted average temperature of the floor, ceiling, and walls. The significance of the mean radiant temperature is determined when compared with the clothed body of an adult (80°F, or 26.7°C). If the MRT is below 80°F, the human body will lose heat by radiation to the surfaces of the enclosure. If the MRT is higher than 80°F, the opposite effect will occur.
Conduction is the transfer of heat through substances, for instance, from a boiler plate to another substance in contact with it (). Conductivity may be defined as the relative value of a material, compared with a standard, in affording a passage through itself or over its surface for heat. A poor conductor is usually referred to as a nonconductor or insulator. Copper is an example of a good conductor. illustrates the comparative heat conductivity rates of three frequently used metals. The various materials used to insulate buildings are poor conductors. It should be pointed out that any substance that is a good conductor of electricity is also a good conductor of heat.
Radiation, conduction, and convection in boiler operation.
Conductivities of various metals.
Convection is the transfer of heat by the motion of the heated matter itself. Because motion is a required aspect of the definition of convection, it can take place only in liquids and gases.
illustrates how radiation, conduction, and convection are often interrelated. Heat from the burning fuel passes to the metal of the heating surface by radiation, passes through the metal by conduction, and is transferred to the water by convection (i.e., circulation). Circulation is caused by a variation in the weight of the water due to temperature differences. That is, the water next to the heating surface receives heat, expands (becomes lighter), and immediately rises as a result of displacement by the colder and heavier water above.
Proper circulation is very important, because its absence will cause a liquid, such as water, to reach the spheroidal state. This, in turn, causes the metal of the boiler to become dangerously overheated. A liquid that has reached the spheroidal state is easy to recognize by its appearance. When liquid is dropped upon the surface of a highly heated metal, it rolls about in spheroidal drops () or masses without actual contact with the heated metal. This phenomenon is caused by the repelling force of heat and the intervention of a cushion of steam.
Drop of water on a hot plate illustrating the spheroidal state.
The specific heat of a substance is the ratio of the quantity of heat required to raise its temperature one degree Fahrenheit to the amount required to raise the temperature of the same weight of water one degree Fahrenheit (). This may be expressed in the following formula:
The principle of specific heat.
The standard used in water at approximately 62 to 63°F receives a rating of 1.00 on the specific heat scale. Simply stated, specific heat represents the Btu required to raise the temperature of one pound of a substance one degree Fahrenheit.
Sensible heat is the part of heat that provides temperature change and that can be measured by a thermometer. It is referred to as such because it can be sensed by instruments or touch.
Latent heat is the quantity of heat that disappears or becomes concealed in a body while producing some change in it other than a rise of temperature. Changing a liquid to a gas and a gas to a liquid are both activities involving latent heat. The two types of latent heat are:
These are explained in detail in the next section under Steam.
As mentioned in Chapter 1, several methods are used to classify heating systems. One method is based on the medium that conveys the heat from its source to the point being heated. When the majority of heating systems in use today are examined closely, it can be seen that there are only four basic heat-conveying mediums involved:
Air is a gas consisting of a mechanical mixture of 23.2% oxygen (by weight), 75.5% nitrogen, and 1.3% argon with small amounts of other gases. It functions as the heat-conveying medium for warm-air heating systems.
Atmospheric pressure may be defined as the force exerted by the weight of the atmosphere in every point with which it is in contact (), and is measured in inches of mercury or the corresponding pressure in pounds per square-inch (psi).
Atmospheric pressure.
The pressure of the atmosphere is approximately 14.7 psi at sea level. The standard atmosphere is 29.921 inches of mercury (in Hg) at 14.696 psi. “Inches of mercury” refers to the height to which the column of mercury in a barometer will remain suspended to balance the pressure caused by the weight of the atmosphere.
Atmospheric pressure varies due to elevation by decreasing approximately 1/2 lb for every 1000 ft ascent above sea level. Atmospheric pressure in pounds per square-inch is obtained from a barometer reading by multiplying the barometer reading in inches by 0.49116. Examples are given in .
Atmospheric Pressure per Square-Inch for Various Barometer Readings
Gauge pressure is pressure whose scale starts at atmospheric pressure. Absolute pressure, on the other hand, is pressure measured from true zero or the point of no pressure. When the hand of a steam gauge is at zero, the absolute pressure existing in the boiler is approximately 14.7 psi. Thus, by way of example, 5 lb pressure measured by a steam gauge (i.e., gauge pressure) is equal to 5 lb plus 14.7 lb, or 19.7 psi of absolute pressure.
When air is compressed, both its pressure and temperature are changed in accordance with Boyle’s and Charles’ laws. According to Robert Boyle (1627–1691), the English philosopher and founder of modern chemistry, the absolute pressure of a gas at constant temperature varies inversely as its volume. Jacques Charles (1746–1823) established that the volume of a gas is proportional to its absolute temperature when the volume is kept at constant pressure.
If the cylinder in is filled with air at atmospheric pressure (14.7 psi absolute), represented by volume A, and the piston B moved to reduce the volume to, say, 1/3 A, as represented by B, then according to Boyle’s law, the pressure will be tripled (14.7 × 3 = 44.1 lb absolute, or 44.1 − 14.7 = 29.4 gauge pressure). According to Charles’ law, a pressure gauge on the cylinder would at this point indicate a higher pressure than 29.4 gauge pressure because of the increase in temperature produced by compressing the air. This is called adiabatic compression if no heat is lost or is received externally.
Elementary air compressor illustrating the phenomenon of compression as stated in Boyle’s and Charles’ laws.
Those who design, install, or have charge of steam heating plants certainly should have some knowledge of steam and its formation and behavior under various conditions.
Steam is a colorless, expansive, and invisible gas resulting from the vaporization of water. The white cloud associated with steam is a fog of minute liquid particles formed by condensation, that is to say, finely divided condensation. This white cloud is caused by the exposure of the steam to a temperature lower than that corresponding to its pressure.
If the inside of a steam heating main were visible, it would be filled partway with a white cloud; in traversing the main, the little particles combine, forming drops of condensation too heavy to remain in suspension, which accordingly drop to the bottom of the main and drain off as condensation. This condensation flows into a drop leg of the system and finally back into the boiler, together with additional condensation draining from the radiators.
Although the word “steam” should be applied only to saturated gas, the five following classes of steam are recognized:
Three of these classes of steam (wet, saturated, and superheated) are shown in the illustration of a safety valve blowing in . It should be pointed out that neither saturated steam nor superheated steam can be seen by the naked eye.
Three types of steam.
Saturated steam may be defined as steam of a temperature due to its pressure. Steam containing intermingled moisture, mist, or spray is referred to as wet steam. Dry steam is steam containing no moisture. It may be either saturated or superheated. Finally, superheated steam is steam having a temperature higher than that corresponding to its pressure.
The various changes that take place in the making of steam are known as vaporization and are shown in . For the sake of illustration, only one bubble is shown in each receptacle. In actuality there is a continuous procession upward of a great multiplicity of bubbles.
The phenomenon of vaporization.
The amount of heat necessary to cause the generation of steam is the sum of the sensible heat, the internal latent heat, and the external latent heat. As mentioned elsewhere in this chapter, sensible heat is the part of the heat that produces a rise in temperature as indicated by the thermometer. The internal latent heat is the amount of heat that water will absorb at the boiling point without a change in temperature—that is, before vaporization begins. External latent heat is the amount of heat required when vaporization begins to push back the atmosphere and make room for the steam.
Another important factor to consider when dealing with steam is the boiling point of liquids. By definition, the boiling point is the temperature at which a liquid begins to boil (), and it depends upon both the pressure and the nature of the liquid. For instance, water boils at 212°F, ether at 9°F, under atmospheric pressure of 14.7 psi.
Variation of the boiling point when pressure changes.
The relationship between boiling point and pressure is such that there is a definite temperature or boiling point corresponding to each value of pressure. When vaporization occurs in a closed vessel and there is a temperature rise, the pressure will rise until the equilibrium between temperature and pressure is reestablished.
One’s knowledge of the fundamentals of steam heating should also include an understanding of the role that condensation plays. By definition, condensation is the change of a substance from the gaseous to the liquid (or condensate) form. This change is caused by a reduction in temperature of the steam below that corresponding to its pressure.
The condensation of steam can cause certain problems for steam heating systems unless they are designed to allow for it. The water from which the steam was originally formed contained, mechanically mixed with it, 1/20, or 5 percent, of air by volume (at atmospheric pressure). This air is liberated during vaporization and does not recombine with the condensation. As a result, trouble is experienced in heating systems when one attempts to get the air out and keep it out. Suitable air valves are necessary to correct the problem.
Water is a chemical compound of two gases, oxygen and hydrogen, in the proportion of two parts by weight of hydrogen to 16 parts by weight of oxygen, having mixed with it about 5 percent of air by volume at 14.7 lb absolute pressure. It may exist as ice, water, or steam due to changes in temperature (water freezes at 32°F and boils at 212°F when the barometer reads 29.921 in).
One cubic foot of water weighs 62.41 lb at 32°F and 59.82 lb at 212°F. One U.S. gallon of water (231 in3) weighs 8.33111 lb (ordinarily expressed as 81/3 lb) at a temperature of 62°F. At any other temperature, of course, the weight will be different ().
Weight of Water per Cubic Foot at Different Temperatures
Water changes in weight with changes of temperature. That is, the higher the temperature of the water, the less it weighs. It is this property of water that causes circulation in boilers and in hot-water heating systems. The change in weight is due to expansion and a reduction in water volume. As the temperature rises, the water expands, resulting in a unit volume of water containing less water at higher temperature than lower temperature.
Fill a vessel with cold water and heat it to the boiling point. Note that boiling causes it to overflow due to expansion. Now let the water cool. You will note that when the water is cold, the vessel will not be as full because the water will have contracted.
The point of maximum density of water is 39.1°F. The most remarkable characteristic of water is its expansion below and above its point of maximum density. Imagine 1 lb of water at 39.1°F placed in a cylinder having a cross-sectional area of 1 in2 (). The water having a volume of 27.68 in3 will fill the cylinder to a height of 27.68 in. If the water is cooled, it will expand, and at, say, 32°F (the freezing point) will rise in the tube to a height of 27.7 in before freezing. If the water is heated, it will also expand and rise in the tube; and at the boiling point (for atmospheric pressure 212°F) it will occupy the tube to a height of 28.88 in.
The point of maximum density.
The elementary hot-water heating system in illustrates the principle of thermal circulation. The weight of the hot and expanded water in the upflow column C, being less than that of the cold and contracted water in the downflow column C′, upsets the equilibrium of the system and results in a continuous circulation of water as indicated by the arrows. In other words, the heavy, low-temperature water sinks to the lowest point in the boiler (or system) and displaces the light, high-temperature water, thus causing continuous circulation as long as there is a temperature difference in different parts of the boiler (or system). This is referred to as thermal circulation.
The principle of thermal circulation.
Electricity differs from air, steam, or water in that it does not actually convey heat from one point to another; therefore, including it in a list of heat-conveying mediums can be misleading at first glance.
Electricity can best be defined as a quantity of electrons either in motion or in a state of rest. When these electrons are at rest, they are referred to as being static (hence the term static electricity). Electrons in motion move from one atom to another, creating an electrical current and thereby a medium for conveying energy from one point to another. Many different devices have been created to change the energy conveyed by an electric current into heat, light, and other forms of energy. Electric-fired furnaces and boilers are examples of devices used to produce heat.
This chapter is not concerned with the theoretical aspects of heat loss or gain, or with heating and cooling calculations per se. This information is covered in Chapter 2 (“Heating Fundamentals”) and Chapter 4 (“Sizing Residential Heating and Air Conditioning Systems”). The purpose of this chapter is to describe the various types of insulation used in construction, the methods of applying insulation, and the thermal properties of building materials.
An uninsulated structure or one that is poorly insulated will experience a heat gain in the summer (or a heat loss in the winter) as high as 50 percent. The exact percentage will depend upon the materials used in its construction, because all construction material will have some insulating effect. In any event, it has been observed that approximately 30 percent of heat gain (or loss) is experienced through ceilings and about 70 percent occurs through walls, glass, and window and door cracks or as a result of air ventilation.
Any attempt to heat or cool an uninsulated structure will be terribly inefficient and expensive. Obviously, the possibility of anyone being so shortsighted as to attempt to install a heating or cooling system in such a structure is fairly remote, but inadequate insulation in existing structures or the failure to provide for proper insulation in proposed construction is a common occurrence. The provision for some form of barrier to reduce the rate of heat flow to an acceptable level is therefore of prime importance. Insulation materials serve this purpose with varying degrees of effectiveness. As much as 80 to 90 percent of heat loss (or gain) through ceilings and 60 percent of heat loss (or gain) through walls can be prevented by properly insulating these areas.
The inefficiency of ordinary building materials in resisting the passage of heat brought about the need for the development of materials specifically designed for insulation. A good insulating material should be lightweight, contain numerous air pockets, and exhibit a high degree of resistance to heat transmission. The material should also be specifically treated to resist fire and the attacks of rodents and insects. Finally, a good insulating material should react well to excess moisture by drying out and retaining its resistance to heat flow, instead of disintegrating.
Many inexperienced workers seem to feel that the more insulation one uses, the better the insulating effect. Unfortunately, this does not hold true on a one-to-one basis. For example, twice as much insulation does not insulate twice as well. Apparently a law of diminishing returns operates here. The first layer of insulation is the most effective, with successive layers decreasing in the effectiveness to impede the rate of heat flow.
The capacity of a heating or cooling system is determined by the amount of heat that must be supplied (heating applications) or removed (cooling applications) to maintain the desired temperature within the structure or space. In heating applications, the amount of heat supplied to a space at constant temperature should roughly equal the amount of heat lost. In cooling applications, the heat removed from a space should be roughly equal to the amount of heat gained. Both heat loss and heat gain will be controlled by the way in which the structure is insulated. Good insulation will reduce the rate of heat flow through construction materials to an acceptable level, and this will mean that a heating or cooling system will be able to operate much less expensively and with greater efficiency. It will also mean that the capacity of the heating or cooling unit you wish to install will be less than the one required for a poorly insulated structure. This, in turn, will reduce the initial equipment and installation costs. As you can see, this type and quality of insulation in a structure is an important factor to be considered when designing a heating or a cooling system.
The roof or attic floor of any structure can be insulated, as can the walls of any frame building, whether of stucco, clapboard, shingles, brick, or stone veneer. The form in which the insulation is applied depends on whether the structure is already built or is being constructed.
Heat flows through a barrier of building materials (e.g., walls, ceilings, and floors) by means of conduction and radiation. When it has passed to the other side of this barrier, it continues its movement away from the source by means of conduction, radiation, and convection. The direction of heat flow depends on the relationship of outdoor and indoor temperatures because warmed air will always move toward a colder space. Thus, in the summer months, the warmer outdoor air will move toward the cooler indoor spaces. In the winter months, on the other hand, the reverse is true: The warmer indoor air moves to the cooler outdoors. The purpose of insulation is to reduce the rate of heat flow to an acceptable level. This, in turn, will reduce the amount of energy required to heat or cool the structure.
U1960 ASHRAE Guide and