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FUNDAMENTALS

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HEATING, VENTILATION, AND AIR CONDmONING TABLE OF CONTENTS

CHAYfER ONE: BASIC LA WS AND APPLICATIONS

INTRODUCTION . . . • . . . 1·1 BASIC AIR LAWS . . . • . . . • . . . ' . . . .. • . . . • . . . . .. . •... 1-1 PULLEY LAWS . . . .. . . 1-7 FINDING RPM INCREASE OR DECREASE BY AMPERAGE . .. . . . .• . . . 1-12 FORMULAS FOR ADJUSTING SHEAVES . . . •.. , . . . . 1-14 PERFECT GAS LAWS . . . , ., . . . 1-15 Pascal's Principle . . . .. . . 1-15 Charles's Law . .. . . . .. . . 1-15

Pressure Varies Directly with Absolute Temperature For a Constant

Volume . . . . .. . . 1-15 Gay-Lussac's Law . .. . . 1-17

Volume Varies Directly with Absolute TemperabJre For a Constant

Pressure . . . . . . . . . . . . . . . . . . . • ... . . .. 1-17 , . Boyle's l..aw . . . 1-18

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Pressure Varies Inversely with Volume if the Temperature Remains

Constant . . . 1-18 Effects of Changing Temperature, Pressure, and Volume at Same Time . ... 1-19 HEAT TRANSFER . . . .. . . .. . . 1-20 Conservation Of Energy . . • .. • • . .• . . . •... ,. . . . .. . . 1-20 Heat Flow . . . , •. . . , . . . , . . . , . • . . • . . , . . .. 1-20 Conduction . . . , . . . 1-21 Convection . . . • . . . • . . . • .. .. .. . . . • . .. " .. . . 1-24 Radiation . . . • , .. • . . . • . . . , . . . • . . . . , . . . 1-27 Insulation . . . • . . . .. , . . .. 1-28 PSYCHROMETRIC PROPERTIES OF AIR .. . . , • • . . . " . . . • . . . • . . , .. 1-29 Psychrometric Chart . . . • • . . . . , . . . 1-30 SUMMARY . . . , ... , ' • . .. • .. . • . . . • . . . 1-39

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HEATING, VENTILATION, AND AIR CONDmONING TABLE OF CONTENTS

CHAPTER TWO: HVAC SYSTEMS

INTRODUCTION .. . . .. . . •. . . • . . . 2-1 PURPOSE OF HV AC ..• • . . . . .. • .. . . .. . . • . . . • . . . 2-1 Temperature .. .. . . • . . . • . . • . . .. . . . 2-1 Humidity .. . . .. . . • . . . 2-1 Suspended Particulates (Dust and Gases) . . . • . . . . 2-2 AIR SYSTEMS . . . .. . . .. . . 2-2 Single Zone System . . . • . . . • . . . . • . . • . . • . . . • . . . . 2-3 Variable Air Volume System . . . • . . . • . . . • .. • .. . . • . . . . 2-4 Terminal Reheat System . . . • •. . . • . . . • . . • . . . • . . . 2-5 Induction System . . . . . . . . . . . . . . . . . . . . . • . . • . . • . . . . . . . • . . . 2-6· Dual Duct System . . . .•. .• . . . • . . . . 2-7

Dual Duct System (Low Velocity) . . . • . . . .. .. • . . . 2-7 Dual Duct System (High Velocity) . . . . .. . . . .. • .. . . • . .. . . 2-8 Multizone System . . . .. . . . .. . . • . . . . .. • . . . • . . . . 2-9 FILTRATION SYSTEMS . . . .. . . ... . . 2-10 Fibrous Media Filters . . . • . . . •. .• . . . 2-10 Electronic Air Cleaners . . . . . . . . . . . • . . • . . . . . . • . . . • . .. 2-12 High Efficiency Particulate Air Filter . . . •. .• . .• . . . • . . • . . . . 2-13 HYDRONIC SYSTEMS . . . . . . . . . . . . . . . . . • • . • . . . • . . • . . . • . . . 2-16 Low Water Temperature System (L TW) . . . . . . . . • . . . • . . • . . • . . . . . 2-16 Medium Temperature Water System (MTW) ... . . • . . • . . . 2-16 High Temperature Water System (HTW) .. .. . .. . . .. .. . . 2-17 Chilled Water System (CW) . . . • . . . • . . . . .. . . 2-17 Dual-Temperature Water System (DTW) . . . .. . . • . . . 2-17 Series Loop System . . . .• •. . .• . . •. . . 2-18 One-Pipe System (Diverting Fitting) . . . • . . . • . . . • . . . 2-19 Two-Pipe Systems . . . • . .. • .. .. . .. . .• . .. . 2-20 Combination Piping System . .. . . . .. .. . . • . . • . . . .. 2-22 Three-Pipe System . . . . . . . • . . . . . • . . . • . . . . . . • . . • . . . . 2-22 Four-Pipe System . . . . . . . . . . . . • . . . • . . . • . . . • . . • . . . . . . 2-24 Hydronic Piping . . . .. .. . .. . .•• . . . • . . • • .. . . .. . 2-26 Air Control and Venting .. . . • . . . • . . . .. . . • . . . 2-26 Drains and Shutoffs . . . . . • • . . . . • . . • . . • • . . . • . . . . . 2-27 Balance Fittings . . . • . . . • . . . . •. . . • .. . . 2-27 Pitch . . . .• • . .•. . .• •. . . •.. . . 2-27 Strainers . . . . . . . . • . . . • . . . • . . . . . . . • . . . • . . . . . 2-27

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HEATING, VENTILATION, AND AIR CONDmONING TABLE OF CONTENTS Thermometers . . . • . . . 2-27 Flexible Connectors . . . . . . . . . • • . . . . • . . • . . . • . . . . . . . . . 2-28 Gauges . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . • . . . . . . .. 2-28 Pump Location . . . . . . . • . . . . • . . . • . . . . . . . . . . . . 2-28 SUMMARY

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CHAPTER THREE: HVAC EQUIPMENT

INTRODUCTION . . . .•. . .•. .. . .. .. CRITERIA FOR EQUIPMENT SELECTION . . . • •• .. •. . .• . . .

Demand of Comfort or Process .. . . . . . . . . . . . . . . . . . . . . . . .

Energy Conservation . . . . . ... . . ... . ... . First CostlLife Cost . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . • . . Desires of Owner, Architect, or Design Office . . . .. . .. . . . Space Limitations . . . • . . . •. .. Maintainability . . . .. . . • . . . • . . Central Plant Versus Distributed Systems . ..•. .. • . . . • . . . • .. • .. .. Simplicity and Controllability . . . • . . . • . . . HEATING . . . .. . . • . . . • . . . • . . . • . . . • . . BOILERS . . . • . . . • . . . • . . . • . . . • . . . • . . . • . . Hot Water Boilers . . . .. . . .. . .. . .. .

Steam Boilers . . . • . . . • . . . . • • . . . • . . . • . . . . .. . . ELECTRIC HEATERS . .. . . • . . . . • . . . • . . . • . . . •.. . 2-30 3·1 3·1 3·2 3·2 3-3 3-3 3-3 3-4 3-4 3-4 3·5 3·7 3·7 3·7 3-7 TERMINAL HEATING EQUIPMENT . . . • . . . • .. . •.. • . . . . 3·9 Radiators and Convectors . . . . . . . . • . . . . • • . . . . . .. 3-10 Radiant Panels . . . .. •• . . . • . . . • . . • . . . 3-12 HEAT PUMPS . . . • . . . . • . . . • . . . • . . . • . . . 3·13 Packaged Heat Pumps . . . • . . . . • . . . . . . • . . . • . . . . .. 3·13 COOLING . . . •.. . •. . . .. . . • . . . . •.. .. . . • . . 3·18 Refrigeration . . . • . . • . . . • . . . • . . . •. .. • •.. • . . . • . .. 3·18 Steam Jet . . . • . . . • .. . • .. . .. • .. . . • . .. • . . . 3·19 Heat Sink . . . • . . . • . . .. • . . . 3-20 Absorption . . . ... . .. .. • . . . • . . . .. 3-20

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HEATING, VENTILATION, AND AIR CONDmONING TABLE OF CONTENfS Compressed Gas . . . .. . . • . . . 3·23 Compressor . . . . . . . . . . • . . . • . . . . . . . . . . . . . • . . . .. 3·23 CondenserlReceiver . . . . . . . . . . . . . . . . . . . . . .. 3·23 Metering Device . . . . . . . • . . . • . . . . . . . . . • . . . . . . . .. 3·24 Evaporators . . . . . . . . . . . . • . . . • . . . . . . . . . • . . . 3·25 Chillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . 3·28 Flooded Chillers . . . . . . . . . . . . . . . • . . • . . . . .. 3·29 Direct Expansion (DX) Chillers . . . .. . .. . . • . . . . .. . • . . 3·29 Package Chillers . . . . . . . . . . . . . . . . . . .. 3·29 Cooling Towers. . . . . . . . . . . . . . . . . . . . • . • . . . . . . • . . . . .. 3·31 Open·Circuit Cooling Towers . . . . . . . . • . . . • . . . • . . . • .. 3·33 Closed·Circuit Towers . . . . .. . . .. •.. • . . . • . . . • . . 3·34 Cooling Coils . . . • .. . . • . . . • . . . 3·36 Plpmg . . . .. • . . . 3·36 Pumps . . .. . . • . . . • .. . . .. 3·36 Pump Configurations and Types . . . .. .. • . . . .. 3·37 Performance Curves. . . . . . . . . . . . . . • . . . . . • . . • . . . .. 3·38 Pump Selection . . . . . . . . . . . . ." . . . . • . . . . . . . . . • . . . • .. 3-40 AIR· HANDLING . . . . . . . . . . . • . . . . . . • . . . . . . . . .. 3·40 FANS . . . .. . . .. . . . . . . . . . . . . • . . . . . . . . . . . . . . . . .. 3-41

Classifications of Fans. . . • . . . • . . . . • . . . • . . . . . . • . . • . . . .. 3·42 Fan Control . . . • • . . . • . . • . . .. . .• . . . 3-48 Fan Drives . . . . . . . . . . . . . . • . . . . • . . . • . . . . . . • . . . . . . . • .. 3·50 Fan Laws . . . . . . . . . . . . . . . . • . . . . . . • . . . . . . . • . . . • .. 3·50 Fan Characteristic Curves . . . • . . . . • . . . • . . . • . . . •. . . . 3·51 DUCTWORK . . . • . . . • . . . . • . . . • . . . 3·59

Classification . . . .• . . . . • . . . • . . . .• . . . . .. . . 3·59 Duct System Accessories .. . • • . . . . . • . . . . . . . . . . • . . . • . . .. 3·60

SUMMARY

Turning Vanes . . . . . . . . . . . • . . . • . . . • . • • • . • . . .. 3·60 Dampers . . . . . . . . . . . . . . . . • . . . . . . . • . . . . . . • • . .. 3·64 Louvers . .. . . • . . . •. .• .. . •. .. • . . . 3·66 Grilles. Registers and Diffusers . . . . . . • . . • . . • . . . . . .. 3·67 Silencers . . . . . . . . . . . . . . . . . . • . . . .. 3·71

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3·72

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HEATING, VENTILATION, AND AIR CONDmONING TABLE OF CONTENTS

CHAPTER FOUR: FIELD INSTRUMENT OVERVIEW

INTRODUCTION . . . 4-\ AIRFLOW MEASUREMENT DEVICES . . . . . . . • • . . • . . . • . . . • . . . • . .. 4-\ U-Tube Manometer . . . • • . . . • .. . . 4-\ InclinedNertical Manometer. . . . • • . . • . . . • • . . . . . . • . . . 4-2 Micro-Manometer . . . • . .• . .. . •. .• . ..•. . 4-3 Pitot Tube . . .. . . .. .. . . • . . . •. . • . . . • . . . • . . . • . . 4-3 Construction .. . . •. ..• . .. . . •. . .•. . .•.. . •. . 4-3 Pitot Tube Use . . . . . . . . . . • • . . . • . . . • . . . • . . . . . . . • . .. 4-5 Use of Readings . . . . . . . . • . . . . • . . . • . . . • . . . • . . . . . . 4·9

Pitot Tube Duct Traverses . . . . . . . . . . . . . . . . . . . . . . . .. 4-11

Round Duct Traverses . . . •... • . . . • . . . 4-\3 SquarelRectangular Duct Traverses . . . ... 4·15 Correcting For Non-Standard Conditions. . . . . . • . . . • . . • . . .. 4·16 Pressure Gauge (Magnehelic) . . .. . . • ...• . .. • . . . • . . 4-2\ Rotating Vane Anemometer . . . • • . . • . . . 4-22 Bridled Vane Anemometer . . . . . . . . . • . . . • . . . • . . . • .. 4-24 Deflecting Vane Anemometer . . . . • . . . • • . . . • . . . ..•.. . 4-25 Hot Wire Anometer . . .. ... . . • . . . • . ..• . . . • . . 4-27 Smoke Devices ... . . . . . . . . . . . . . . • . . . . . . . . . • . . . • . . • . .. 4-3\ HYDRONIC MEASURING EQUIPMENT . . . • . . . . . • . . • . . . . . . . • . . .. 4-31 U-Tube Manometer . . . • .. .. . ...• . . . . .. . ... 4-32 Pressure Gauge . . . . . . . . . . . . . . . . . . . . • . . . • . . . . . .. 4-33 Differential Pressure Gauge . . . .. . . . .. . . .. . . 4-34 Venturi Tube and Orifice Plate (Flow Devices) . . . . • . ... •.. . . . ... 4-36 Annubar Flow Indicator. . . . . . . . . . . • . . . • . . . . . .. 4-38 Calibrated Balancing Valve . . . . . . . . . . . . . . • . . . • . . . • . . .. 4-39 Location of Flow Devices . . . _ . . . . . . . . . . . .. 4-40 TEMPERATURE MEASURING INSTRUMENTS. . . . . .• . . . •. . . • . . . 4-42 Glass Tube Thermometers . . . • . . ... ... 4-42 Dial Thermometers . . . . . . . . . . . . . . . . . . . • . . . . . . • . . .. 4-44 Pyrometers . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . • . . . • . . .. 4-45 HUMIDITY MEASURING DEVICES . . . . . . . . . . . • . . . . . . . . . . . .. 4-46

Psychometric Measurement Devices . . . . . . . 4-46

Dry Bulb Thermometer. . . . . . . . . . • . . . . • . . . • . . . • . . . .. 4-46 Wet Bulb Thermometer .. . . • . . . • . . . 4-46 Psychrometer . . . . . . . . . . . . . . . . . . . . . . . . • . . . .. 4-47

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HEATING, VENTILATION, AND AIR CONDmONING TABLE OF CONTENTS

Dew Point .. . . • . . . • . . . . . • . . . • . .. 4·50 Wick·Type Dewcells . . . .. ••. .• . . . •. . . • . .. . 4·50 Capacitance Probe Dewcell . . . • . . . . .. •. .•.. . • . . . 4·52 Chilled Mirror Dewcell . . . •.. . •.. .. . . •. . . .•. . .. 4·53

ELECTRICAL MEASURING DEVICES .. .. . . . .. . • ..•• . . . .. 4·54

Volt-Anuneter . . . . . . . . . . . . . . . . • . . . . . • . . . . . . . . . .. 4-54 Insulation Resistance Monitoring . . . .. . . • . . . 4-57 Two Fundamental Properties of Insulation . . . • .. • . . . 4-57 Factors Affecting Insulation Resistance . . . • . . . •. .. . . 4-58 Measuring Insulation Resistance . . . . . . . . . . . . • . . . . . . .. 4-58 Conditions for Measuring Insulation Resistance . . . 4~59 Instruments . . . .. . . • . . . 4·61 Testing Guidelines .. . . . . . . . . . . . . . . . . . . . . . .. 4·61 MINIMUM VALUES AND FREQUENCY OF INSULATION

RESISTANCE TEST . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . .. 4·64 Minimum Insulation Resistance Value . . . . . . . . . . . . . . . • . . . • .. 4-64 Frequency of Inspection . . . . . . . . . . . . . • . . . • . . • . . . • . . .. 4·65 Interpretation of Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . 4-65

ROTATION MEASURING INSTRUMENTS . . . • •. . •.. . • . . . • .. 4·67

Revolution Counter (Odometer) . . . . . . . • . . . . . . . . . . • . . . .. 4-68 Tachometers. Centrifugal . . . . . . . . . . . . . . . • . . . . . . • . . . .. 4-68 Tachometer, Chronometric . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-69 Tachometer, Electronic . . . . . . . . . . . . . . . . . . . . . .. 4-70 Tachometer, Photo. . . . . . . • • . . . • . . . • . . . . • . . . .. 4-71 VIBRATION MEASUREMENT. . . . . • . . . • . . . . . . . . . . • . . . 4-72 Vibration Probe. . . . . . . . . . . . . . • . . . . • . . . . . . • . . . . . . . . . 4·72 Measuring Vibration . . . . . . . . . . . . . • . . . . • . . • . . . . . . . • . . . .. 4-75 Measuring Displacement . . . . . . . . . . . . . . . . . . . . . .. 4-77 Measuring Velocity . . . • . . . • . . . • . . . • .. 4-77 GAUGE MANIFOLD . . . .. . . • . . . .. . . 4-77 Using the Gauge Manifold . . . . • . . . • . . • . . . . .. . . .. 4-78 Special Attaching Devices . . . .. . . • . . . • . . . • .. 4·82 SUMMARY . . . .. . . • . . . . .. . . • . . 4·83

HEATING, VENTILATION, AND AIR CONDmONING TABLE OF CONTENTS

CHAPTER F1VE: SYSTEM TEST AND BALANCE PROCEDURES

INTRODUCTION 5-1

AIR FLOW MEASUREMENT IN DUCTS. . . . . . . . . . . . . • . . . • . .. 5-1 Air Flow Measurement of Diffusers . . . .. . . . 5-2 Air Flow Measurements of Supply Grilles and Registers. . . . . . . . . . 5-2 Use of Hoods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5-3 Air Flow Measurement of Return Grilles and Registers . . . .. . . 5-4 Testing of Motor Amperage . . . .. .. . .. . .. . . . • . . . 5-5 Measuring Static Pressures .. . . . .. . . , 5-5 Testing of Hot and Cold Mixing Dampers . . . • . . . •. .. . . . .. . . .. 5-5 Testing and Setting Static Pressure Dampers . . . • . . . • • . . . . .. 5-5 Testing of Face Velocities Across Coils . . . . . . . . . . . . • . . .. 5-6 Conditions of System During Tests . . . . . . . . . . . . . • . . . • • . . . • . . .. 5-6 Setting of Outside Air and Return Air Volumes . . . . . • . . . . . . .. 5-7 Testing of Ceiling Plenum Systems . . . . . . . . . . . . . • . . . • . . . • . . . .. 5-8 Testing of Air Shafts. . . . . . . . . . .. . . . . . . . . . . . • . . . • .. 5-11 Procedure for Testing Air Shafts . . . •• . . . . • . . . •.. ..•... 5-12 Procedure for Testing Shaft Wall. . . . . . . . . . . . . . . . • . . . • . .. 5-13 Fume Hood Testing .. . . •. . . 5-14 Air Distribution Duct Leakage Test ...•. .. . • . . . • . . . •. . .• . . . . 5-18 Methods and Standards . . . . . . . . . . . . . . . . . . • . . • . . .. 5-18 Test Equipment . . . •. . . •... •. . . • . . . •.. . 5-19 Field Test Procedure . .. . . ... •. . .. . . • .. . • . .. 5-19 Test Verification . . . . . . . . . . . . • . . . . . . . • . . . . • . . . • . .. 5-22 HYDRONIC SYSTEM TESTING . . . .. . ; . . . . . . • . . . • . . . .. 5-22 Balance Procedure - General . . . . . . . . . . . . . • . . • • . . . • . . . . . . .. 5-23 Chilled and/or Hot Water Systems . . . ..• .. . . 5-26 Condenser Water/Cooling Tower Systems . . . . • . . • • . . . •. . . •.. .. 5-27 Steam and Hot Water Boilers ... ... . . • . . . • • . . . •... • . . . . 5-29 Heat Exchangers/Converters . . . . . . . . . . . . . . . . . • . . .. 5-30 Balancing Data Required. . . . . . . . . . . . . . . . . . • . . .. 5-31 Water Balance with Coil, Control Valve and Measuring Station . . . • . . 5-33 GPM ESTABLISHED THRU COIL . . . .. . .. . .. . . 5-35 Cabinet Unit Heaters . . . • . . . • . .. . •. ... 5-36 Fan Coil Unit and Unit Ventilator .. •. . . .. . .. 5-37 Unit Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . • . . .. 5-38 Pumps . . . .. . . • . .. . . • . . . 5-39

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-HEATING, VENTILATION, AND AIR CONDmONING TABLE OF CONTENTS

HEPA FILTERS . . . • • . . . .. .• • . . . • . . 5-42 Problems in HEPA Filter Use . . . . . . . . . . . . . . . . . . . . . • . . . 5-42 HEPA Filter Testing . . . . . . . . . . . . . . . . . . • . . . • . . . .. 5-45 HEPA Filter Testing Problems . . . .. . . • . . . • . . . • . . . • .. . 5-45 HEPA Filter Test Procedures . .. . . . • . . . • . . . 5-46 Summary of Method . . . . . . . . • . . . . . . . • . . . • • . .. 5-46 Prerequisites for Test . . . . . . . . . . . . . . . . . .. 5-47 Apparatus . . . . . . . . . . . . . . . . • . . . . . .. 5-47 CHARCOAL ADSORBER REQUIREMENTS AND TESTING . . . • . . . 5-48 Adsorberl Adsorbent Requirements . .. . . • . . . 5-48 Charcoal Adsorber Test Procedures .. . . 5-51 Purpose . . . • . . . • . . • . . . • . . . • . . . . 5-52 Summary of Method .. . . • . . . • . . •. .. . . . .• . .. . 5-52 Prerequisites for Test . . . . . . . . . . . . . . . . . . . . . .. 5-52 Apparatus . . . . . . . . . • . . • . . • . . . • . . . .. 5-53 SUMMARY

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CHAPTER SIX: SOUND AND VIBRATION TESTING

INTRODUCTION

Sound .. . . • . . . • •... .. Sound Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sound Pressure Level .. . . ... . . .. . . . ... .. . . Loudness and Frequency .. . ... . . ... . . . NC Curves . . . .. .. . . • . . . .. . .. . • .. .•. . . Architectural Acoustics . . . • . . . • . . . • . . . • .. . . Reverberation Time . .. . . ... .. .. . ... . . ... . . Sound Trap Selection . .. . . • . . . • . . . • . . . . .. Sound Testing . . . • • .. • . . . • . . .

Sound Testing Specification . . . .. . . VIDRATION . . . .• .. . •• .. . . • . ..• .. . . .

Vibration Testing ... .. . . •.. . ... Vibration Testing Procedure . ... . . . .. . . .. . ... . Vibration and Noi~ Identification . . . . . . . .. .. . Analysis Procedure . . . .. . .. . . .. . Vibration and Noise Source Identification . . . . ... .. .. . Noise Analysis .. .. .. . . .. . . .. . Relative Probability Ratings . . . • . .. ... ..

Application of the Chart . . . • . . . ...• . . . .. . . • . ..

5-53 6-1 6-3 6-3 6-3 6-5 6-6 6-7 6-8 6-10 6-10 6-11 6-12 6-13 6-15 6-16 6-24 6-26 6-28 6-28 6-29

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HEATING, VENTILATION, AND AIR CONDmONING TABLE OF CONTENTS

CHAYfER SEVEN: MAINTENANCE AND TROUBLESHOOTING

INTRODUCTION . . . .. . . 7-1 SYSTEMATIC TROUBLESHOOTING TECHNIQUES . . . __ . . . _ . . .. 7-2 TROUBLESHOOTING FANS . . . .. . . . _ . . . _ . .. _ • .. 7-3 Noise . . . .. . . . . 7-4 Performance Reduction . . . .. . .. . . .. . . .. ... . . ... 7-4 Checking for Spin . . . . . . . . • • . . . . . . • . . . • . . . . . . . • • . .. 7-5

Rotation . . . 7-18 TROUBLESHOOTING ABNORMAL AIR CONDITIONING OPERATIONS . . . 7-20 High Head Pressure . . . .. . . .. . . .. . . 7-22 Dirty or Partially Blocked Condenser . . . . . . . . . . . . . 7-22 Air or Noncondensable Gases in System . . . • .. . • • .. • . . . . 7-22 Overcharge of Refrigerant . . . . . . . . . . . . . . • . . . • • . . . • . .. 7-24 Insufficient Condensing Medium . . . . . . . . • . . . • . . . . . . .. 7-25 High Temperature Condensing Medium .. .. • . . . .. . .. 7-25 Restricted Discharge Line . . . . . . . . • . . . . • . . . • . .. 7-25 Low Suction Pressure . . . . . . . . . . . . . . . • . . . • . . . • • .. 7-26 Insufficient Air on Evaporator Coil . . . • . . . • .. . .. .. . 7-26 Poor Distribution of Air on Evaporator Coil. . . • . . . . . . . • . . .. 7-27 Restricted Refrigerant Flow . . . . . . . . . • . . . • . . . • . . .. 7-27 Undercharge of Refrigerant . . . • _ ..• _ . . . . _ .. 7-28 Faulty Metering Device . . . • . . . •.. . . • . . .. • . . . 7-28 Low Discharge Pressure . . . • . . . _ . . . . • . . . _ • . .. 7-29 High Suction Pressure. . . . . . . . . . . . . • . . . . . . . • . . . • . . . . • . .. 7-29 Heavy Load Conditions . . . • • . . . • . . . 7-29 Low Superheat Adjustment . . . .. . . • . . . • . . . . _ . . .. 7-30 Improper Expansion Valve Adjustment . . . _ • . . . . . . . .. 7-30 PoorInstallation of Feeler Bulb . . . 7-30 Inefficient Compressor .. . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-31 High Discharge Pressure on Capillary Tube Systems . . . • . . . • .. 7-31

," .. (

CHAPTER ONE

I .

CHAPTER ONE

BASIC LAWS

AND APPLICATIONS

OBJECTIVES

At the completion of this chapter, the student will be able to: 1. State the primary objective of HV AC.

2. Explain the fan laws as they relate to fan performance.

3. Define static and velocity pressure.

4. Calculate duct capacity.

5. Explain how the fan laws are used to determine the affect of various fan speeds.

6. Use the fan laws to determine fan speed.

7. Given a change in rpm, be able to determine the charge in amperage for a motor.

8. Given a change in cfm, be able to determine a new pulley

setting.

9. Explain Pascal's Principle. 10. Explain Charles's Law.

11. Explain Gay-Lussac's Law. 12. Explain Boyle'S Law.

15. State the three ways heat can flow.

16. Explain the relationship of thermal resistance and thermal conductance.

17. Given two points of information on a psychrometric chart, be

CHAPTER ONE

BASIC LAWS AND APPLICATIONS

INTRODUCTION

The primary objective of Heating, Ventilation and Air Conditioning (HV AC) is to control the characteristics of the air in a controlled environment. This chapter introduces the characteristics and properties of air that affect HVAC system design and construction. These basics must be comprehended in order to proceed effectively with the course.

The glossary in the back of the text provides an extensive listing of HV AC terms and should be referred to as needed throughout this course. Standard Heating, Ventilating and Air Conditioning symbols are located in Appendix A.

BASIC AIR LAWS

The performance of air handling, transmission and distribution systems will follow certa·in established laws which make it possible to calculate the expected performance of an air moving system after adjustment or changes within the system have been made.

Tllese laws are the most commonly used 10 system design and balancing and are listed in Figure 1-1.

FAN PERFORMANCE:

a. CFM varies in direct proportion to RPM.

or

CFM

=

b. SP varies as the square of the RPM.

SP,

RPM2

,

RPM,

or

SP,

=

SP

_j

x

=

SP

_j

RPM;

RPM

_j

SP

or RPM

,

=

RPM

x

- '

SP

_j

c. Hp varies as the cube of the RPM. =

RPMf3

RPM

_,

or

RPM,

=

RPM

x

d. BHP varies as the cube of the CFM.

BHp

_-,-I

=

CFMf3

BHp;

Figure 1-1 Air Laws

1-2

CFMf3

CFM

_j

e. CFM & RPM varies as the square root of the pressure ratio.

or

RPM -

_-

RPM

_/

x

f. HP varies as the square root of the pressure ratio cubed.

HP

_/

-

-g. CFM varies as the square of the fan size ratio (at given SP &

rating).

h. RPM varies as the square of the fan size ratio (at given SP &

rating).

RPM

=

RPM x

I. HP varies inversely as the fan size ratio (at given SP & rating).

J. CFM varies as the size ratio cubed times the RPM ratio.

x

k. SP varies as the size ratio squared times the RPM ratio

squared.

x

RPM

f

2

RPM

_I

1. Hp varies as the size ratio raised to the 5th power times the

RPM ratio cubed.

x

Figure 1-1 Air Laws (cant'd.)

DVCfED AIR FLOW: ), ~1v\.J

'Y

'

"

.-/ J~c. iii '1':1 "-a. Total Pressure TP. \\\iJ ~~~,\

TP

=

SP (Static Pressure)

~\~ .,) ,,(~ ~\i

fI'

\

+

VP (Velocity Pressure)

b. Duct Capacity CFM.

CFM

=

Duct Velocity FPM x Duct Area A

c. CFM varies in direct proportion to duct FPM.

=

FPM

_----'-f

or CFM

=

FPM

f

I

d. SP varies as the square of the duct CFM and FPM.

SP

_f

=

CFM

f

2

=

FPM

f

2

SP

_i

CFM

_i

FPM

_i

CFM

_f

2

FPM

2

SP

_f

=

SP

_i

x

=

SP

_i X f

CFM

_i

FPM

_I

e. CFM varies as the square root of the static pressure ratio.

CFM

_f

-

-f. CFM varies in direct proportion to duct area A (at given velocity).

CFM

_f

-

-g. Duct FPM varies as the square root of the static pressure ratio.

FPM

_f

-

-Figure 1-1 Air Laws (coned.)

: .j " .

,

1 I I . .

.

h. The velocity indicated is for dry air at 70"F, 29.9" Barometric Pressure and a resulting density of .075#/cu. ft. Air Velocity = where: Air Density where: Pv =

D

= T = 11096.2 Pv D

velocity pressure in inches of water

Air density in -leu. ft.

= 1.325 x PB

T

Barometric Pressure in inches of

mercury

Absolute Temperature (indicated

temperature plus 460)

The three main fan laws are identical to the three "pump laws".

PumQ Fan

V a

N

CFM a RPM

Hp a

N'

SP a RPM'

P a

N'

BHP a RPM'

where: V = volumetric flow rate

CFM = air flow rate

Hp = pump head

SP = static pressure

P = power

BHP

-

horsepower

The above relationships are extremely useful in determining the affect that varying fan speed has on overall fan performance.

Static pressure is the pressure exerted by reason of weight or existence of fluid or gas confined within a space. Velocity pressure, commonly referred to as impact pressure is pressure exerted by air moving through a

confined space and impinging on a stationary object. The sum of static and

velocity pressures is total pressure which is defined as total air "pressure"

energy or energy of the air within the duct relating to atmospheric pressure.

This pressure energy results in air flow within a duct, and if duct size is

known (area), then total duct air flow may be determined. Static and velocity

pressures are measured in inches of water gauge or inches of mercury.

Mathematically, these pressure relationships are:

TP =

CFM =

SP + VP

Duct Velocity (FPM) x Duct Area

Duct velocity in feet per minute is determined using velocity pressure and a conversion factor to convert inches of water gauge to feet per minute and will be discussed in detail under air flow measurement.

PULLEY LAWS

Drive sets for fans and blowers consist of a driver pulley on the motor

shaft, a driven pulley on the blower shaft, and a belt or set of matched belts

to transmit the power. Pulley formulas are usually given in pulley diameters;

for accuracy, they should be considered in actual pitch diameters. Figure 1-2 shows an example of a fully closed

shows the same sheave in the fully open position.

SHEAVE FULLY CLOSED

Figure 1-2 Fully Closed Sheave

sheave. Figure 1-3

BREAK CORNERS

Figure 1-3 Fully Open Sheave

Figure 1-4 is an example of a multiple groove pulley with fully open

sheaves.

F Open ~ SeN 9 - 1) + 2S_e

1-

- - - -

-

(N_g ~ No. of Grooves)

-S

e-

"~

I

~

-Figure 1-4 Multiple Groove Pulley with Fully Open Sheave

c,"" Section 14JOV 1930V 2S30V 32JOV 4430V

Table I-I gives dimensions for standard variable sheaves.

Table 1-1 Variable Sheave Groove Dimensions

b, b, h, 20

Closed 0"," Minimum

(Inches) (Inches) (Inches) (Inches)

0.875 !. 0.005 1.582 !. 0.005 1.758 0.20

LlSB ! 0,005 2.142! Q,()()5 2.]41 0.25

1.563 :!: 0.007 2.823 :!: 0.007 3.0:'18 0.30

2.000 ! 0.007 3.665 ! 0.007 3.855 0.35

2.750 ! 0.007 5.132 ! 0.007 5.258 0.40

The four basic pulley laws are:

rpm P, rpm Pm dia P, dia Pm where: diaPmxrpm dia P_r dia P, X rpm dia Pm dia Pm X rpm dia P, X rpm

P, fan pulley or driven sheave 2a. (Inches) 2.64 3.56 4.74 6.21 8.89

Pm = motor pulley or driver sheave

S. Open Minimum (Inches) 0.882 1.163 1.501 [.954 2.687 S Minimum (Inches) 1.765 2.325 l003 3.908 5.375

Pulley Speed-O-Graph for rapid calculations of the pulley laws is shown in Figure 1-5. Using this nomograph, the speed or size of either

pulley can be determined when the other three factors are known.

1. Enter the chart from any given factor and follow the straight grid line to the point where it intersects, on the diagonal, the other given factor.

2. Follow the diagonal line to the point where it meets the third given factor.

3. From this point of intersection, move along the straight grid line to the fourth side of the margin for the solution.

EXAMPLES:

Example 1: Given:

Diameter of Drive = 3 in.

DIAMETER OF DRIVEN = 12 in. rpm of Driver = 5000

FIND: RPM OF DRIVEN

RI'~1 or: TIll; driull" ( dl Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q -. N ~ ~ ~ ~ , ~ 0 - . N ~ ~ ~ ~ , ~. 9000 8000 7000 '0 6000 SO SOOO <0 4000 • • ~ _JO JOOO u c

.

-

25 -. ₂₀ 2000 ·u

-

_<

•

_"

,

...

< -. ·u Q

-~ ,0 1000

,

~ e-

,

. . 0 • ~ ~ _{8 800} ~ 0

,

0 ~ 7 700 ~ ~ e- O 600 ~ ~

e-'"

~ ~ ₅ SOD ₀ c

'"

,

<00 ~ ~ J JOO 2 ZOO 100

/lIMtETER or TIlE DRIVeN (0) -inches

Example 2: Given:

Diameter of Drive = 30 in.

DIAMETER OF DRIVEN = 4 in.

RPM OF DRIVER = 3750

FIND: rpm of Driver

Example 3: Given:

rpm of Driver = 1000

RPM OF DRIVER = 5000

Diameter of Driver = 10 in.

FIND: DIAMETER OF DRIVEN

I .

I

Example 4: Given:

rpm of Driver = 2500

RPM OF DRIVEN = 5000

FIND: Diameter of Driver

FINDING RPM INCREASE OR DECREASE BY AMPERAGE

To determine the percent of rpm increase or decrease by reading the

ammeter, the following formula applies:

rpm23

(

)

Example:

A fan is turning 600 rpm and reading 20 amps. To deliver the proper

cfm it is necessary to increase the fan speed to 700 rpm. Find the new

amperage.

20

x (

700)3

=

200

x

1.588

=

31.75

amps2

Table 1-2 gives calculated data for this equation. Using the above

example, the increase in speed is 100 rpm; this is an increase of 100/600 or

16.66%. By interpolation the table shows that the original amps would have to be multiplied by 1.588, or 20 x 1588 = 31.75 amps.

Table 1-2 Rpm Increase/Decrease

(To determine the required change in fang speed multiply the measured amps by the given faclor)

% RPM Multiply Amps % RPM Multiply Amps

Increase By: Decrease By: 1 I 2 1.06 2 0.94 3 1.09 , J 0.92 4 1.13 4 0.88 5 1.16 5 0.86 6 1.19 6 0.83 7 1.23 7 0.80 8 1.26 8 0.78 9 1.33 9 0.75 to 1.33 to 0.73 11 l.37 II 0.70 12 1.40 12 0.68 13 1.44 13 0.66 14 1.48 14 0.64 15 1.52 15 0.61 16 1.56 16 0.59 17 1.60 17 0.57 18 1.64 18 0.55 19 1.69 19 0.53 20 1.73 20 0.51 21 1.77 21 0.49 22 1.82 22 0.47

"

_J _{1.86 23 0.45} 24 1.90 24 0.44

,-

-) 1.95 25 0.42 30 2.20 30 0.34 35 2.46 35 0.28 40 2.75 40 0.22 45 3.05 45 O.t 7 50 3.38 50 0.12 1-16

FORMULAS

FOR A

DJUSTI

NG

SHEA YES

1. Given a change in cfm, find the new pulley setting.

=

Example:

(

Cfm2)

_X

_P

d

₁

cfml

Determine the new pitch diameter for 4000 frm when a fan output is

3500 cfm at a 10 in. pitch diameter.

10

=

11.43

In.

2. Given an increase in cfm, will the new brake horsepower

Example:

Determine the new brake horsepower required to increase the cfm from 5000 to 5500 when the bhp is 0.8 and the motor is rated at 1 hp.

=

1.06

Therefore, the motor needs to be changed to 1-1/2 hp.

3. Given a maximum brake horsepower, find the new pitch diameter

required to change from an existing pitch diameter.

3[jrnax

bhP

₂

y(

bh

)

PI

Example:

Determine the new pitch diameter to bring a 1 hp motor up to maximum when the present pd is 10 in. and bhp is 0.8.

10

3jI.25

X

10

=

1.077

X

10 10.77pd

where:

pd = pitch diameter

bhp = brake horsepower cfm = air quantity at the fan

PERFECT GAS LAWS

Pascal's Principle.

Pascal's Principle is defined as follows: "Fluid pressure is due to the

weight of the fluid pushing on an area. [t is normally measured in pounds

per square inch. If it is referenced to atmospheric pressure, it is psig.

Pressures below atmospheric are measured in inches of mercury (in Hg). If

the reference point for pressures is below atmospheric it is a vacuum, then

the units are in psia or in Hg abs.

The expansion and contraction of solids and liquids with change in

temperature are enough that they must be considered in many situations. But

they are comparatively small, for the molecules of the solids and liquids are

held rather closely and not allowed to fly off by themselves. [n gases, the

expansion and contraction with change of temperature are very large

compared to those of solids and liquids. [n addition, the volume of the gas

varies with the container. A gas automatically fills any container that it is

put into, regardless of whether the container is small or large.

Neither a liquid nor a solid does this. A container for solids or liquids

can be partly filled, but a gas container is always full.

Charles's Law

Pressure Varies Directly with Absolute Temperature if Volume Stays the

Same

Because a gas adapts itself to its container, regardless of container size

or the amount of gas in the container, another factor is introduced -- pressure.

If the container is already filled, then a rise in temperature cannot cause an

increase in volume, but it does result in an increase in the pressure of the gas

This change in pressure with a change in temperature can be easily

calculated as long as the volume stays the same. The calculation is very

simple: Gas pressure goes up at the same rate as the temperature. If the

temperature rises 25% or 1/4, the pressure goes up 1/4. [f the gas cools down to 2/3 its temperature, the pressure does down to 2/3 of what it was.

Years ago, a scientist named Charles discovered this principle, so it is called

"Charles' Lawrl •

Charles' Law says that if the volume remains the same,

pressure of a gas varies as the absolute temperature varies.

illustrates Charles' Law.

1 CU.FT. 70'F 700.0 PSIG 714.7PSIA 1 CU.FT. gOT 727.0PSIG 741.7PSIA

Figure 1-6 Illustration of Charles' Law

1-20

the absolute

Gay-Lussac's Law

Volume Varies Directly with Absolute Temperature if Pressure Stays the

.'

_,

Same

Now suppose that instead of having the gas in a steel cylinder that

keeps the gas from expanding, the gas is in a cylinder that has a loose bottom

that can slide up and down just like a piston in a compressor. If the gas in

the cylinder is warmed, it can expand and push the piston downward, but the

pressure inside the cylinder remains the same, because the piston would

merely slide downward if the pressure inside the cylinder tended to become

greater than that outside the cylinder and below the piston.

Now we have a condition of the volume changing with change of

temperature, but the pressure remaining constant. This principle, known as

Gay-Lussac's Law, states that the volume changes with the change in

temperature. Figure 1-7 illustrates this principle.

700 PSIG 700 PSIG

707.dF gOT

1 CU.FT. 1.04 CU.FT.

©

©

Boyle's Law

Pressure Varies Inversely with Volume if the Temperature Stays the Same

There is a third condition: What happens to the pressure if the volume changes but the temperature stays the same?

In the two previous conditions, the proportion was direct; that is, the pressure and volume went up as the temperature went up and down as the temperature went down.

In this third condition, the temperature remains constant, and the pressure goes down as the volume goes up, or vice versa, the pressure goes

up as the volume goes down. This relationship is called an inverse proportion, known as Boyle's Law.

For example, Figure 1-8 shows a cylinder with a loose piston. In the piston shown in Figure 1-8(a), the volume of the cylinder above the piston

is one cubic foot and the pressure is 20 psig or 34.7 psia. In Figure 1-8(b), the piston has been slowly lowered twice as far, so now the volume is two cubic feet. What happens to the pressure? It goes down in the same proportion as the volume went up.

70T 1 cu. FT.

34.7 PSIA 70T

2 cU.n.

17.35 P$lA

Figure 1-8 Illustration of Boyle's Law

Effects of Changing Temperature, Pressure, and Volume at Same Time

[n Charles' Law, the volume remains constant and the pressure varies

with a change of temperature.

[n Gay-Lussac's Law, the pressure remains constant and volume varies

with change of temperature. [n Boyle's Law, the temperature remains

constant and the pressure varies with change in volume. All three of these

laws help us understand how pressures, volumes, and temperatures change in

containers of gas. In each of these three laws, one of the variables remains

constant: The pressure, the volume, or the temperature.

However, gases are not always so considerate: Pressure, volume and

temperature may change at the same time with none of them remaining

constant. Therefore, combination of these laws must be used, namely the

general law of perfect gases. Figure 1-9 shows an actual case where

pressure, temperature, and volume have changed.

70·F 1 CU.FT. , .014.7 PSIA

@

40·F 2 CU.FT. 478.6 PSIA

@

Figure 1-9 lllustration of Temperature, Volume,

HEAT TRANSFER

Conservation Of Energy

Chemical energy in coal can be changed into heat energy in steam.

Heat energy in steam is changed into mechanical energy in a turbine and then

into electrical energy in a generator. Electrical energy can then be changed

back into heat energy in a toaster, to mechanical energy in a motor, or to chemical energy in a storage battery. In this example, all of the electrical

energy in the motor did not go into mechanical energy. Some of it was

"lost" as heat. We say "lost", because we did not get any use out of the heat

of the motor. Actually it was not "lost", for the heat was energy, transformed from electrical energy.

Energy cannot be destroyed nor created. It can merely be transformed to or from some other kind of energy. This principle is known as the Law of Conservation of Energy, also known as the first Law of Thermodynamics.

It is well to remember it, for it explains many things.

For example, the Law explains efficiency. In changing the electrical

energy in the motor, some went into mechanical energy (or power) and some

into heat. The efficiency is the percentage of the electricity that becomes

power. The mechanical energy (the output energy) determines the efficiency.

If three-fourths of the electrical energy became mechanical energy, then the efficiency is 75%.

In a good boiler, over one-third of the heat energy in the coal burned

goes up the chimney or is radiated from the boiler; about two-thirds goes into

heat energy in the steam, so the efficiency is about 60% to 65%. Efficiency

is the useful output energy from a machine, divided by the input energy to the machine, and expressed as a percentage.

The speed with which heat transfers by means of conduction varies

with different substances or materials if the substances or materials are of the

same dimensions. The rate of heat transfer varies according to the ability of

the materials or substances to conduct heat. Solids, on the whole, are much

better conductors than liquids; liquids conduct heat better than gases or

vapors.

Most metals, such as silver, copper, steel, and iron, conduct heat fairly

rapidly, whereas other solids such as glass, wood, or other building materials

transfer heat at a much slower rate and, therefore, are used as insulators.

Copper is an excellent conductor of heat, as is aluminum. These

substances are ordinarily used in the evaporators, condensers, and refrigerant

pipes connecting the various components of a refrigerant system although

iron is occasionally

used with some refrigerants.

Heat transfer by conduction depends upon (1) the driving force, which

is caused by a temperature difference Il.T, and (2) the resistance to heat

transfer, which depends on the nature and dimensions of the heat transfer

medium. There are several ways to relate these parameters. One of the most

useful relates the rate of heat transfer

Q

to the cross-sectional area A, the

temperature difference Il. T and a quantity called the heat transfer coefficient

U.

Q

=UMT

where:

Q

= rate of heat transfer (Btu/hr)

U = heat transfer coefficient (Btu/hr-ft2 _OF)

A = cross-sectional area for heat transfer (ft2)

The rate of heat transfer Q divided by the cross-sectional area A is

commonly referred to as the heat flux. The heat transfer coefficient U is

equivalent to the reciprocal of resistance to heat transfer. The temperature

difference l>T is the driving force.

where Heat Flux =

Q

= l>T

A

1

U Driving Force Resistance

Q

= heat flux (Btu/hr-ft') A

l>T - temperature difference ("F)

U = heat transfer coefficient (Btu/hr-ft'-'F)

The heat transfer coefficient U is a measure of the resistance of the medium to heat transfer. It depends on both the heat transfer characteristics

and the dimensions of the heat transfer medium. The heat transfer characteristics of a material are measured by a property called the thermal

conductivity k. The thermal conductivity of liquids and solids depends on

temperature. For vapors, it depends also on pressure. Table 1-3 gives the thermal conductivity for zirconium, aluminum and water at several temperatures.

Table 1-3 Thermal Conductivity of Common Materials

Material Thermal Conductivity Temperature

(BtulHr-Ft-OF) (OF) Zirconium

12.1

120

11.

8

200

11.5

300

11.0

500

11.6

750

Aluminum

132

68

131

390

131

750

Water

0.343

3

2

0.393

200

0.4

30

0

0.356

60

0

The heat transfer coefficient U depends also on the dimensions of the

heat transfer medium. For the simplest case of steady-state heat transfer by

conduction through a slab, the temperature profile is linear and the heat

transfer coefficient U equals the thermal conductivity k divided by the

thickness of the slab x. Thus, the basic relationship for heat transfer by conduction through a slab can be written as follows:

Q

= kA t.T

x

where: Q ₌ rate of heat transfer (Btu/hr)

k = thermal conductivity (Btu/hr-ft-OF)

A = cross-sectional area for heat transfer (ft')

x = thickness of slab (ft)

Metals with a high conductivity are used in the refrigeration system

itself because it is desirable that rapid transfer of heat occur in both

evaporator and condenser. The evaporator is where heat is removed from the

conditioned space or substance or from air that has been in direct contact

with the substance; the condenser dissipates this heat to another medium or

space.

Convection

In convection, heat is transferred by motion of the heated material itself and is limited to liquid or gas. When a material is heated, convection

currents are set up within it. The warmer portions rise, since heat brings about the decrease of a fluid's density and an increase in its specific volume.

Figure 1-10 shows a generalized diagram of heat transfer by

convection. It involves the transfer of heat between a surface at temperature

T, and a fluid at temperature T" referred to as the bulk temperature of the

fluid. The exact definition of the temperature of the fluid T, is the

temperature far from the surface. For boiling or condensation, T, is the

saturation temperature.

'!-<;..-- SURFACE

t

FLOW

Figure 1-10 Heal Transfer by Convection

The basic relationship for heat transfer by convection has the same

form as that for heat transfer by conduction.

where Q h A t.T = = = = Q = hN.T

rate of heat transfer (Btu/hr)

heat transfer coefficient (Btu/hr-ft2_0F)

cross-sectional area for heat transfer (ft')

temperature difference ("F)

The heat transfer coefficient h, more precisely referred to as the

convective heat transfer coefficient, has been measured and tabulated for the

commonly encountered situations for heat transfer by convection. Table 1-4

shows representative values of the convection heat transfer coefficient h.

Table 1-4 Representative Values of the convective

Heat Transfer Coefficient

Operation Heat Transfer Coefficient

(Btu/hr-ft2-OF)

Drop-wise condensation of Steam

5000

-

20,000

Film condensation .

1000

-

₃₀₀₀

Boiling of water

300

-

9000

Heating of water

50

-

3000

Superheating of steam

5

-

20

The temperature difference t. T in heat transfer by convection is the

difference between the temperature of the surface T, and the bulk temperature

Air in a refrigerator and water being heated in a pan are examples of

the results of convection currents (see Figure 1-11). The air in contact with

the cooling coil of a refrigerator becomes cool and more dense and begins to

fall to the bottom of the refrigerator. [n doing so, it absorbs heat from the

food and the walls of the refrigerator, which through conduction, has picked

up heat from the room .

._...... _... ..._.... .._... ... -... . ...

...

_{, .. ....}

..

...

-

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After heat has been absorbed by the air, it expands, becoming lighter, and rises until it again reaches the cooling coil where heat is removed from

it. The convection cycle repeats as long as there is a temperature difference

between the air and the coil. [n commercial-type units, baffles may be constructed within the box so that the convection currents will be directed to

take the desired patterns of air flow around the coil.

[n the case of the evaporator, the product or air is at a higher

temperature than the refrigerant in the tubing and there is a transfer of heat

downhill. In the condenser, the refrigerant vapor is at a higher temperature

than the cooling medium traveling through or around the condenser, and here

again there is a downhill transfer of heat.

Plain tubing, whether copper, aluminum, or another metal, transfers heat

according to its conductivity or "k" factor, but this heat transfer can be

increased through the addition of fins on the tubing. They increase the area

of heat transfer surface, thereby increasing the overall efficiency of the

system. [f the addition of fins doubles the surface area, it can be shown that

the overall heat transfer should itself be doubled, when compared to that of

plain tubing.

Water heated in a pan is affected by the convection currents set up in

it through the application of heat. The water nearest the heat source becomes

warmer and expands. As it becomes lighter, it rises and is replaced by the other water which is cooler and more dense. This process continues until all of the water is at the same temperature.

Convection currents are natural, and, as in the case of the refrigerator,

a natural flow is a slow flow. [n some cases, convection must be increased

through the use of fans or blowers. [n the case of liquids, pumps are used for forced circulation to transfer heat from one place to another.

Radiation

A third means of heat transfer is through radiation by waves similar to

light or sound waves. The sun's rays heat the earth by means of radiant heat

waves which travel in a straight path without heating the intervening matter

of air. The heat from a light bulb or from a hot stove is radiant in nature and

is felt by those near them, although the air between the source and the object,

which the rays pass through, is not heated.

If you have been relaxing in the shade of a building or a tree on a hot

sunny day and move into direct sunlight, the direct impact of the heat waves

will hit like a sledge hammer even though the air temperature in the shade

is approximately the same as the sunlight.

At low temperatures, there is only a small amount of radiation and only

minor temperature differences are noticed, so radiation has very little effect

in the actual process of refrigeration itself. But the results of radiation from

direct solar rays can cause an increased refrigeration load in a building in the

path of these rays.

Radiant heat is readily absorbed by dark or dull materials or substances,

while light colored surfaces or materials reflect radiant heat waves, just as

they do light rays. Wearing apparel designers and manufacturers make use

of this by supplying light-colored

materials for summer clothes.

This principle is also carried over into the summer air-conditioning field

where, with light colored roofs and walls, less of the solar heat will penetrate

into the conditioned space, reducing the size of the overall cooling equipment

required. Radiant heat also readily penetrates clear glass in windows, but

will be absorbed by translucent or opaque glass.

When radiant heat or energy is absorbed by a material or substance, it

is converted into sensible heat - that which can be felt or measured. Every body or substance absorbs radiant energy to some extent, depending upon the temperature difference between the specific body or substance and other

substances. Every substance will radiate energy as long as its temperature

is above absolute zero and another substance within its proximity is at a

lower temperature.

If an automobile has been left out in the hot sun with the windows

closed for a long period of time, the temperature inside the car will be much

greater than the ambient air temperature surrounding it. This demonstrates that radiant energy absorbed by the materials of which the car is constructed

is converted to measurable sensible heat. Insulation

In the section on heat transfer by conduction, it was pointed out that certain substances are excellent conductors of heat, while others are poor conductors. The poor conductors are classified as insulators. Any material

that deters or helps to prevent the transfer of heat by any means is called and

may be used as insulation. Of course, no material will completely stop the flow of heat. If there were such a substance, it would be very easy to cool a given space down to a desired temperature and keep it there.

Such substances as cork, glass fibers, wool, and polyurethane foams are good examples of insulating materials; but numerous other substances are

used in insulating refrigerated spaces or buildings. The compressible

materials, such as fibrous substances, offer better insulation if installed loosely packed or in blanket or batt form than if they are compressed .or tightly packed.

The thermal conductivity of materials, the temperature to be maintained in the refrigerated space, the ambient temperature surrounding the enclosed space, permissible wall thicknesses of insulating materials, and the cost of the various types of insulation are all points to consider in selecting the proper materials for a given project. Most service personnel are not involved in the selection or the installation of insulating material in a refrigeration

application, but they may come in contact with different types of insulation, and under various conditions.

Insulation should be fire and moisture resistant, and also vermin proof. Large refrigeration boxes or walk-in types of coolers are usually insulated with a rigid-type of insulation such as corkboard, fiber glass, foam blocks, and the like, while smaller boxes or receptacles might be filled or insulated with a foam that flows like a liquid and expands to fill up the available cavity with foam.

Low temperature boxes require an insulation that is vapor-resistant, such as unicellular foam, if the walls of the refrigerated enclosure are not made of metal on the outside. This foam ensures that water vapor will not readily penetrate through into the insulation and condense there, reducing the insulating efficiency. The most common unit for evaluating insulation materials is thermal resistance (R) or resistance to heat flow. Basically thermal resistance "R" is the inverse of thermal conductance "k". R = 11k.

The units for "R" are (hr)x(ft") x ("F).

BTU x in

Psychrometric PROPERTIES OF AIR

Psychrometry is the science and practice of dealing with air mixtures

and their control. The science deals mainly with dry air and water vapor mixtures.

Psychrometry deals with the specific heat of dry air and its volume.

It also deals with the heat of water, heat of vaporization or condensation, al)d the specific heat of steam in reference to moisture mixed with dry air.

Tables and graphs have been developed to show the pressure, temperature, heat content (enthalpy), and volume of air and its steam content. The tables and charts are based on one pound of dry air, plus the water vapor

to produce the air conditions being studied.

A standard pressure of 29.92 m. Hg. abs. IS used as the standard

atmospheric pressure. Psychrometric Chart

The psychrometric chart in Figure 1-12 is probably the best way of showing what happens to air and water vapor as these properties are changed. The chart is published by ASHRAE and is one most commonly used in the industry. Some manufacturers have developed their own charts which vary

only in style and construction but the relationship of the air properties are all the same.

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