HVAC instant answers

HVAC instant answers

INSTANT ANSWERS

This page intentionally left blank.

HVAC INSTANT ANSWERS

Copyright © 2002 by The McGraw-Hill Companies, Inc All rights reserved Manufactured in the United States of America Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher

0-07-140949-1

The material in this eBook also appears in the print version of this title: 0-07-138701-3

All trademarks are trademarks of their respective owners Rather than put a trademark symbol after every occurrence of a trademarked name, we use names in an editorial fashion only, and to the benefit

of the trademark owner, with no intention of infringement of the trademark Where such designations appear in this book, they have been printed with initial caps

McGraw-Hill eBooks are available at special quantity discounts to use as premiums and sales tions, or for use in corporate training programs For more information, please contact George Hoare, Special Sales, at george_hoare@mcgraw-hill.com or (212) 904-4069

promo-TERMS OF USE

This is a copyrighted work and The McGraw-Hill Companies, Inc (“McGraw-Hill”) and its licensors reserve all rights in and to the work Use of this work is subject to these terms Except as permitted under the Copyright Act of 1976 and the right to store and retrieve one copy of the work, you may not decompile, disassemble, reverse engineer, reproduce, modify, create derivative works based upon, transmit, distribute, disseminate, sell, publish or sublicense the work or any part of it without McGraw- Hill’s prior consent You may use the work for your own noncommercial and personal use; any other use of the work is strictly prohibited Your right to use the work may be terminated if you fail to com- ply with these terms

THE WORK IS PROVIDED “AS IS” McGRAW-HILL AND ITS LICENSORS MAKE NO ANTEES OR WARRANTIES AS TO THE ACCURACY, ADEQUACY OR COMPLETENESS OF

GUAR-OR RESULTS TO BE OBTAINED FROM USING THE WGUAR-ORK, INCLUDING ANY INFGUAR-ORMA- TION THAT CAN BE ACCESSED THROUGH THE WORK VIA HYPERLINK OR OTHERWISE, AND EXPRESSLY DISCLAIM ANY WARRANTY, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE McGraw-Hill and its licensors do not warrant or guarantee that the func- tions contained in the work will meet your requirements or that its operation will be uninterrupted or error free Neither McGraw-Hill nor its licensors shall be liable to you or anyone else for any inaccu- racy, error or omission, regardless of cause, in the work or for any damages resulting therefrom McGraw-Hill has no responsibility for the content of any information accessed through the work Under no circumstances shall McGraw-Hill and/or its licensors be liable for any indirect, incidental, special, punitive, consequential or similar damages that result from the use of or inability to use the work, even if any of them has been advised of the possibility of such damages This limitation of lia- bility shall apply to any claim or cause whatsoever whether such claim or cause arises in contract, tort

INFORMA-or otherwise.

abc

McGraw-Hill

We hope you enjoy this McGraw-Hill eBook! If you’d like more information about this book, its author, or related books and websites, please click here

Chapter 1: Introduction and Overview 1

Chapter 2: Air and Ducts 7

Chapter 3: Water, Steam, and Pipes 29

Chapter 4: Electricity and Wiring 47

Chapter 5: Measurement and Control 81

Chapter 6: Pumps and Valves 121

Chapter 7: Water Distribution Systems 171

Chapter 8: Chillers 203

Chapter 9: Air-Cooled Condensers and Cooling Towers 235

Chapter 10: Thermal Energy Storage Systems 259

Chapter 11: Boilers 271

Chapter 12: Steam Distribution Systems 307

Chapter 13: Fans and Dampers 349

CONTENTS

Acknowledgments vii

About the Authors ix

Copyright 2002 The McGraw-Hill Companies, Inc Click Here for Terms of Use

For more information about this book, click here.

Chapter 14: Air-Handling Units 371

Chapter 15: Air Distribution Systems 401

Chapter 16: Zone Terminal Systems 425

Chapter 17: Evaporative Cooling 453

Chapter 18: Residential Systems 473

Appendix A: Tools for the Technician 491

Appendix B: Using Multimeters 495

Nomenclature 499

Bibliography 503

Index 505

ebooksdownloadrace.blogspot.in

Many of the photographs in this book were taken on the

Boulder campus of the University of Colorado The ties Management department at the University graciouslygave permission to access the various mechanical spaces and to usethe resulting images In doing so, they have helped provide a clarity

Facili-to the descriptions that would have been impossible otherwise.The final manuscript was reviewed and edited by Stuart Water-bury and Heidi Crimmin Their comments and suggestions werevital in making a book that is hopefully easy to read and use Thanksalso to Elizabeth Gehring, who contributed the section on condens-ing and non-condensing furnaces

Finally, the authors dedicate this book to their wives, Arlene Brethand Heidi Crimmin, who provided immeasurable support and keptthem going and in good health during the writing process

viiACKNOWLEDGMENTS

Copyright 2002 The McGraw-Hill Companies, Inc Click Here for Terms of Use

Peter Curtiss is a consulting engineer in Boulder, Colorado Hereceived his Ph.D in Civil Engineering at the University of Colorado,where he was one the first to investigate the use of neural networks

in the control and optimization of large HVAC systems

Newton Breth has worked as an Instrument Technician at the versity of Colorado since 1977 Prior to that, he spent time in theU.S Army Signal Corps and the Denver Water Board He is certified

Uni-by the Instrument Society of America and holds several U.S patentsrelated to automatic control devices

ixABOUT THE AUTHORS

Copyright 2002 The McGraw-Hill Companies, Inc Click Here for Terms of Use

This book is aimed at the field technician who has the of task

solving problems rapidly and efficiently It is the unfortunatetruth that most facility management teams are understaffed.The technicians spend a lot of time satisfying occupant comfortcomplaints and often do not have the time to spend tracking downthe root of many problems

This guide is meant to provide both theoretical background andprinciples of operation of equipment, while at the same time givingthe reader a practical means for quickly solving problems The basicphilosophy of this guidebook is to work backwards from the effects

of a problem, hopefully eliminating many false leads along the way

We have tried to reach a compromise between solutions that are toosimplistic or too complicated It is emphasized that more often thannot the cause of a control or equipment problem may not be in theobvious place The key point is to identify what is causing the build-ing occupants to express concern

HOW BUILDINGS WORK

Every building is different Even those that appear similar from theoutside will certainly have differences in their structure, their use,and the quirks of their behaviors Like people, no two are alike In

INTRODUCTION AND OVERVIEW

Copyright 2002 The McGraw-Hill Companies, Inc Click Here for Terms of Use

fact, the idea of a building as a person has some merit: most ings have a need to take in fresh air and expel contaminated air.Many buildings provide a constant circulation of fluids around thebuilding, and practically all buildings must maintain an internal tem-perature setpoint for the comfort of the occupants We even nowrefer to problems as if we were describing people: a structure may

build-have sick building syndrome Some buildings are naturally relaxing to

the occupants, others are disconcerting for one reason or another

To the HVAC engineers and technicians, the goal is to make thebuilding a comfortable and safe environment and to do so at a min-imum cost for energy and maintenance This is a noble, yet occa-sionally difficult task As the building size increases and the functionbecomes more varied, the complexity of the HVAC systems cangrow Eventually, it can get to the point where a technician may feellike he is constantly just putting out small fires rather than getting ahandle on the entire building

Figure 1.1 shows a cross-section of a typical residence Here theHVAC is quite simple: a furnace provides heating in the winter and

an air-conditioning unit provides cooling in the summer Just aboutthe only required maintenance is the annual replacement of the fur-nace filter and an occasional inspection of the air conditioning unit.Even then, the occupants of many houses pay almost no attention tothese systems This is to be expected, since for most people the resi-dential HVAC is more or less invisible until something goes wrong

In contrast, Figure 1.2 shows a cross section of a typical officebuilding Here there are many more components to the HVAC sys-tem A mechanical room in the basement sports a boiler, chiller, andcentral air-handling unit A series of pipes and ductwork allow thewater and air to flow through the building to equipment where itmay help keep the building at the desired indoor temperature andhumidity Each occupant will have a different tolerance for heat andcold and humidity, and it is best to try to maintain the building atthe setpoint conditions to avoid occupant complaints In somecases, the physical layout of the building or the original HVAC designcan make it very difficult to provide quick solutions to comfort prob-lems However, the sum of annual salaries in a commercial building

is often several hundred times the sum of annual energy costs, sothe argument can be made that occupant comfort is worth theinvestment in HVAC repair and upgrades Also, in commercial retailbuildings the volume of sales is related to the comfort of the shop-pers Some large market chains accept relatively large energy andlighting bills precisely because these add to the ability to get mer-chandise out the door

Introduction and Overview 3

FIGURE 1.1 Typical residential HVAC system components

FIGURE 1.2 Typical commercial building HVAC system components

A third category of building is the industrial site as illustrated inFigure 1.3 These kinds of buildings come in all shapes and sizes and

it is almost impossible to make generalizations about what even stitutes an industrial building The figure here assumes a large man-ufacturing space with rooftop units used to condition the space But

con-in some manufacturcon-ing buildcon-ings, especially those that create tronic devices or use chemical reactions, the indoor conditions can

elec-be extremely important to produce a high yield of products thatpass quality assurance tests off the assembly line Certain zones ofindustrial buildings may require exact temperature and humidityconditions When one considers that the value of products comingoff the assembly line may be measured in millions of dollars perhour, the need to maintain the environmental conditions is obvious

It is also important to note that the line between commercial andindustrial buildings can be thin Many industrial facilities also have asignificant amount of attached office space, while some commercialbuildings are combined with light industrial use

Chapters 2 through 4 in this book provide general information onthe basic materials used in building HVAC systems This information

is not critical to the quick solution of HVAC problems but is includedhere as a reference for the technician who wants to dive deeper intothe details of a specific issue Chapter 2 describes the properties of airand ductwork, including psychrometrics Chapter 3 gives basic infor-mation about water and piping, and Chapter 4 discusses the funda-mentals of electricity and wiring Chapter 5 presents the differenttypes of sensors used for control and measurement of processes, andalso discusses the actuators and controllers used for these systems.The chapter is not meant to replace a comprehensive guide to pneu-matic or electronic control—it is included so that the technician canquickly answer questions regarding these components without hav-ing to know a lot of detail about them

The remaining chapters in the book discuss various components

of HVAC systems These components are

• Pumps and Valves

• Water Distribution Systems

• Chillers

• Condensers and Cooling Towers

• Thermal Energy Storage Systems

• Boilers

• Steam Distribution Systems

• Fans and Dampers

FIGURE 1.3 Typical industrial building HVAC system components

• Air Handling Units

• Air Distribution Systems

• Zone Terminal Systems

• Evaporative Cooling

• Residential Systems

Each of these chapters contains five sections describing the ponent and its operation The chapter sections appear in each chap-ter in the same order so that you can quickly find the informationyou are looking for

com-Principles of Operation presents the theoretical background and

function of the component This is not an exhaustive coverage

of every single different type of system, but is instead an duction to the essential elements that the technician wouldneed in order to quickly identify the component

intro-Controls describes the different control algorithms that would be

expected with this particular component The positive andnegatives of many of the different control algorithms are dis-cussed

Safety gives tips on how to ensure the personal safety of the

individual working on these systems Since many HVAC

processes incorporate high voltages, dangerous pressures andtemperatures, and moving machinery, there must be consider-able emphasis on the preservation of life and limb

Troubleshooting provides a series of explanations and flow charts

on how to approach problems involving the component Themethod takes a top-down approach where we try to solve theproblem simply at first and then move to more sophisticatedsolutions as necessary

Practical Considerations gives the reader an idea about how to

make the equipment operate more dependably and at lower cost

2 chapter

7

The flow of fresh air through a building is essential to

provid-ing a safe and comfortable environment to the occupants.Maintaining the proper airflow through a building provides aconstant supply of oxygen, removes indoor pollutants, and helpskeep the temperature and humidity at comfortable levels

Air is typically moved through commercial and industrial ings by a series of fans and ductworks This chapter discusses theproperties of air and the mechanics of ductwork See Chapters 13,

build-14, and 15 for more details on fans and air distribution systems

AIR AND DUCTS

Copyright 2002 The McGraw-Hill Companies, Inc Click Here for Terms of Use

in places like Denver Table 2.1 shows how the density of air changes

as a function of temperature and relative humidity for cities at sealevel and at 5000 feet

It takes a certain amount of heat to raise the temperature of air.This amount of heat needed to warm a given amount of air is calledthe specific heat For most HVAC applications, the specific heat of air

at one atmosphere can be taken as 0.24 Btu/lb·°F, meaning it takes0.24 BTU to raise the temperature of one pound of air by onedegree Fahrenheit

Moving air can produce surprisingly strong forces on a body or adoor The force of an airstream is given by pressure = 1⁄2ρ v2where

ρ is the air density and v is the air velocity To find the pressurecaused by a moving airstream at sea level, use the equation:

Lbs / square foot = (feet / min)2÷ 3,093,000 (2.1)Figure 2.1 shows the velocity pressure of an air stream at differ-ent velocities The charts in this figure can be used to determine thepressure exerted on an object in the air stream For example, at anair velocity of 2500 feet per minute (about 28 mph) the air exerts apressure of about two pounds per square foot This may not soundlike much, but on a standard 3 foot x 6 foot door the resulting pres-sure would be almost 40 pounds

Psychrometrics

We never experience truly dry air The ambient air around us is amixture of air and water vapor The standard explanation is that the

TABLE 2.1 Air Density Lookup Tables

FIGURE 2.1 Velocity pressure of air at low velocities (top) and high ties (bottom).

veloci-air “absorbs” the water vapor, although this is not really true; the veloci-airand water vapor exist together but are not linked For a given tem-perature and pressure, the water vapor would exist even if therewere no air However, the absorption analogy is convenient because

it fits the observed behavior The total atmospheric pressure is thesum of the partial pressure of the water vapor, the partial pressure

of the air, and the partial pressure of all the other gases present Asthe temperature of the air increases, the partial pressure of the watervapor increases

The amount of water vapor mixed with the air makes a big ference in the comfort level of people Too little water vapor andpeople complain because it is too dry—their sinuses and eyes dryout Too much water vapor and it is considered too muggy It doesnot take much water to make people uncomfortable For example,

dif-at 75°F only about one fiftieth of an ounce of wdif-ater vapor is enough

to saturate a cubic foot of air That is, to have the maximum amount

of water vapor that can exist at that temperature This is equivalent

to 100% relative humidity As the temperature goes up, it takesmore water to saturate the air and at lower temperatures it takes lesswater Figure 2.2 shows how the saturation amount changes as theambient air temperature changes If you start with saturated air at ahigh temperature and then lower the temperature, the water con-denses out of the air

Air and Ducts 11

FIGURE 2.2 Amount of water needed to saturate room-temperature air atsea level

The study of moist air is called psychrometrics, and the metric chart (Figure 2.3) is often used to quickly find the various prop-

psychro-erties of moist air The psychrometric chart has the air temperature

on the horizontal axis and the amount of water vapor in the air onthe vertical axis The values on these charts are described here

• The humidity ratio is the mass of water per mass of dry air; for

example, lb H20 per lb dry air This is sometimes expressed asgrains of water per pound of dry air (there are 7000 grains perpound.) Figure 2.4 shows how the humidity ratio varies on thepsychrometric chart, and Table 2.2 gives the humidity ratio forsea level and high altitude cities

• The specific humidity is the ratio of the water vapor to the total

mass of the moist air sample This is very similar to the humidityratio except that the reference is the total mass of the air sam-ple with units lb H20 per lb of air + water vapor mixture

• The relative humidity is the amount of water vapor in the air

divided by the amount of water in the air at saturation at thesame temperature and pressure Figure 2.5 shows how the rela-tive humidity changes in the psychrometric chart

• The dew point temperature is the temperature at saturation for a

given humidity ratio and a given pressure If moist air ters a surface at the dew point, water begins to condense out ofthe air This is how a cooling coil is able to remove water fromthe supply air stream to a building

encoun-• As water evaporates, it cools off This is the basic principle

behind evaporative cooling The wet bulb temperature is the

temperature caused by the evaporation of water at a givenpressure Figure 2.6 shows how the wet bulb temperaturechanges in the psychrometric chart

• The enthalpy of the moist air is the sum of the enthalpies of the

air and the water vapor The enthalpy, with units of Btu perpound of air, is used to determine how much energy needs to

be added or removed from air to heat or cool the air Figure 2.7shows how the enthalpy changes in the psychrometric chart.Note that the lines of constant enthalpy are parallel to those ofconstant wet-bulb, so take care not to confuse the two

The psychrometric chart is used when designing and bleshooting HVAC systems that heat, cool, or humidify the air Byidentifying the starting and ending points of a process on the psy-chrometric chart, you can determine where problems might beoccurring For example, Figure 2.8 shows example psychrometric

trou-Air and Ducts 13

FIGURE 2.3 Psychrometric chart for sea level

FIGURE 2.4 Psychrometric chart showing lines of constant humidity ratio

FIGURE 2.5 Psychrometric chart showing lines of constant relative humidity.

FIGURE 2.6 Psychrometric chart showing lines of constant dry-bulb andwet-bulb temperatures

TABLE 2.2 Humidity Ratio Look-up Tables

FIGURE 2.7 Psychrometric chart showing lines of constant enthalpy.

TABLE 2.3 Dew Point Lookup Tables

Air and Ducts 17

TABLE 2.4 Wet Bulb Temperature Look-up Tables

FIGURE 2.8 Example cooling (left) and heating (right) psychrometricprocesses

18

Air and Ducts 19

“paths” for cooling processes and heating processes In this figure,the temperature decrease across the cooling coil is shown, includingthe moisture removal once water begins condensing from the air Ifyour supply air is too dry you may be able to use psychrometrics tounderstand why so much water is being removed from the airstream The evaporative cooling process path shows how the airtemperature goes down as more water is evaporated into the airstream Psychrometrics are used when investigating occupant com-fort problems Figure 2.9 shows the acceptable range of tempera-tures and humidities for the summer months, while Figure 2.10shows the range for winter The summer temperature range ishigher than the winter because people are not as heavily dressed

FIGURE 2.9 Acceptable ranges of temperature and humidity for buildings

in summer (Adapted from ASHRAE, 1981)

Ducts are aluminum or fiberglass conduits that transport cold andhot air into the building, remove stale air, or eliminate smoke frombuildings In commercial buildings, the air distribution system iscomposed of many sections of ductwork and dampers The SheetMetal and Air Conditioning Contractors’ National Association hasissued a number of standards for residential, commercial, and indus-trial duct construction The ASHRAE Systems Handbook (2000) pro-vides an excellent summary of the different duct standards andconstruction techniques

FIGURE 2.10 Acceptable ranges of temperature and humidity for buildings

in winter (Adapted from ASHRAE, 1981)

Ducts come in a variety of shapes and sizes, along with differentfittings Figure 2.11 shows some different types of duct elbows.These are typically manufactured in 22.5°, 45°, and 90° turns andcome in both rectangular and circular sections The difference in theelbows comes mostly from the manufacturing technique and mate-rials used Figure 2.12 shows mitered rectangular elbows These sec-tions often do not present as smooth a turning radius for the air as

do the circular elbows For this reason, turning vanes are often porated into the elbows

incor-Ducts can be connected by a variety of means Flanges, frictionfits, duct tape, sheet metal screws, and metal clips are all used,sometimes together If you are having problems taking duct sectionsapart, check to make sure that there are no hidden screws or clipsholding the sections together Any ductwork that is attached to

Air and Ducts 21

FIGURE 2.11 Die-stamped duct elbow (left), pleated duct elbow (middle),and five-piece gore elbow (right)

FIGURE 2.12 Mitered duct elbow (a) simple duct section; (b) elbow withturning vanes

moving or vibrating equipment (such as fan units) should use a ible duct connector This isolates the ductwork from the vibrationsource and can help prevent subsequent damage to duct connec-tions and sensors mounted on the duct, as well as reduce noisetransmitted from the fans to the zones.

flex-When using electronic sensors to measure air temperatures, flow,and humidity, it can be useful to use wires to electrically connectductwork sections that are separated by flexible connections It ispossible for different duct sections to be at surprisingly differentelectronic potentials with respect to ground If the sensor has a chas-sis ground reference that is in good electrical contact with the duct,

it is possible to set up a current loop through the sensor leads andadd noise to your readings

Sometimes it is necessary to get inside ductwork for repairs, sor installation, or cleaning A number of companies manufacturespecial doors that can be installed in holes cut into the side of duct-work If you use these types of doors, make sure that the door is in

sen-a relsen-atively sen-accessible locsen-ation Remember thsen-at those who mighthave to rescue you must also pass through this opening Also be sure

to place the door such that it does in fact provide access to the spot

in the duct you want to get to Despite their appearance in actionmovies, the inside of ductwork can be a very dangerous place, sothere are a few rules to follow:

• The inside of ducts is completely dark Be sure to bring a worklamp and preferably a backup flashlight Turning vanes andother duct obstructions can be located where you least expectthem, as can vertical duct take-offs that may drop down formany feet These are the kinds of things you want to knowabout before you encounter them

• Always let someone know when you are entering the duct and

inform them when you are out again They should be instructed

to look for you if you do not report back within a reasonableamount of time

• Screws, sharp edges, and other protrusions exist everywhere inducts Often these are hidden by duct insulation Wear headand eye protection if possible Pay close attention to what isaround you in the ductwork so you don’t accidentally put allyour weight down onto the sharp end of a nail or screw

• Remember that moving air can exert a strong pressure, to saynothing of any drag forces A worker in a duct can have theequivalent area of 5 to 10 square feet, and even a low velocityair stream can knock you over If you are anywhere near a verti-

cal duct take-off, you might end up taking a bad fall Always beprepared for sudden changes in the duct air velocity.

• Never approach an operating fan from inside the duct The

tur-bulence near a fan intake can be surprisingly strong Also, fanscan create a significant amount of noise in the confined spaces

of a duct Wear ear protection if you plan to be in a particularlynoisy area longer than ten minutes

Due to the constant presence of moist air in ducts, some air tribution systems can develop serious problems due to the growth

dis-of molds, fungi, and microorganisms Sick building syndrome andLegionnaire’s disease are two of the better-known issues that canarise from dirty or contaminated ducts It is therefore important toperiodically inspect ductwork and filters for dirt, excessive humidity

or standing water, and any kind of bacterial or organic growth Boththe National Air Duct Cleaners Association and the North AmericanInsulation Contractors Association offer guidelines on how to prop-erly clean commercial duct systems

Duct air leakage wastes energy, makes duct pressure control ficult, and can lead to occupant comfort complaints Begin yourtroubleshooting of duct systems by initially checking the duct forunnecessary leakage A few strips from a roll of duct tape are cheapand can help solve and prevent problems

dif-AIR COMPRESSORS

Air compressors are used in HVAC systems for the operation of matic sensors and actuators (see Chapter 5 for more informationabout pneumatic control systems) The air supply in a compressedair system must be clean and free from oil, dirt, and water Figure2.13 shows a simple schematic of the major components in a com-pressed air system A compressor keeps a pressurized storage tank at

pneu-Air and Ducts 23

fastfacts

Ducts should be sized so that the air velocity is around 500 feet per minute through the duct.

some pressure usually between 50 and 150 psi The air then passesthrough a dryer and some filters before it arrives at a pressure regu-lation valve (PRV) The PRV maintains a downstream pressure ofabout 18 to 25 psi that is used by the sensors and actuators Figure2.14 shows what the compressor and storage tank look like in anactual installation

HVAC Instant Answers

24

FIGURE 2.14 Combined air compressor and storage tank

FIGURE 2.13 Typical compressed air supply system

The compressor motor is sized to match the amount of air andthe amount of compression required Table 2.6 shows the theoreti-cal horsepower required by an air compressor for a given outletpressure and air use rate These numbers should be increased byabout 15 percent for high altitude applications.

The storage tank is sized so that the motor and compressor do notcycle too much A compressor that is running all the time indicatesthat there is a leak somewhere in the system, quite possibly upstream

of the PRV In practice, the motor should cycle no more than about 10times per hour It is also important to periodically check the pressurerelief valve on the storage tank to make sure it is working properly

If the air compressor is used to provide pressurized air to a tive large system (for example, multiple buildings), a large stand-alone compressor may be used in conjunction with a separatestorage tank Figure 2.15 shows such a storage tank, which standsaround 12 feet tall and is about 5 feet around Care must be takenaround tanks this large so that they do not get damaged by forklifts,welding torches, etc Failure of a tank of this size and pressure canput a big damper on your day

rela-Once the air has been compressed, it must be dried and cleaned.The air dryer works by either increasing the air pressure (to force thewater vapor out of the air), by decreasing the air temperature (tocondense water vapor), or through desiccant absorption No matterwhat the drying method, there is usually a water drain coming fromthe air storage tank, the dryer, and the filters Check this line peri-odically to make sure it is draining properly It is much cheaper andtakes a lot less time to ensure that your air supply is clean than it is

to replace fouled pneumatic relays and sensors

Air and Ducts 25

TABLE 2.6 Theoretical Air Compressor Horsepower

FIGURE 2.15 100 psi compressed air tank.

Compressor oil can become entrained in the compressed air andtravel into the pipes, where it is removed by the filters and dryers.Most local codes now require that compressed air systems haveequipment in place that prevents this compressor oil from enteringthe water drains Figure 2.16 shows a water/oil separator that is used

to float the oil so that it can be removed before the water is released.Note also that this device sits on an oil catch pad This, too, is oftenrequired by code

After leaving the compressed air tank, the air travels to trollers, actuators, and other compressed air processes throughoutthe building In cases where the compressor serves a campus ofbuildings, there can be compressed air lines that travel relativelylarge distances between buildings Copper tubing and pipe in the 1⁄4inch to 1⁄2inch range is used for this Obviously, a leak in the coppertubing results in a failure of the downstream devices If you havepneumatic controls that don’t seem to be behaving properly, one ofthe first things to check is the air pressure

con-Air and Ducts 27

FIGURE 2.16 Water/oil separator

Also keep in mind that long runs of compressed air tubing ally share a space with other utilities such as electrical lines, steampipes, and water distribution systems You often find that the pipesand wires have not exactly been laid out with maintenance in mind.Scrambling over high voltage cables to fix leaky compressed air lines

usu-is not fun, so when designing systems try to place the runs such thatthey are easily accessible

If the runs are over a hundred feet or so, you should also bly include expansion joints in the tubing The air temperature inthe utility corridor may not always be the same (particularly if steam

proba-or water systems are shut down) As the temperature changes, thecopper tubing expands or contracts and can kink the pipe or evenpull sweated fittings apart Figure 2.17 shows the expansioncharacteristics of copper tubing as a function of temperature Forexample, if you have a 200-foot length of copper tubing and thatdecreases in temperature by 20°F, the pipe shrinks by almost half aninch If the pipe is rigidly fixed to the wall and supports, the shrink-age could easily cause cracks to develop, particularly around elbowsand tees Include expansion joints or fittings as appropriate, even ifyou don’t think it will be a problem It costs very little in additionaltubing and may save a bundle on repair costs

FIGURE 2.17 Elongation of copper tubing as a function of temperaturechange