Tài liệu Engineering manual of automatic control for commercial building honeywell

HONEYWELL ENGINEERING MANUAL of AUTOMATIC CONTROL for COMMERCIAL BUILDINGS ENGINEERING MANUAL OF AUTOMATIC CONTROL i Copyright 1934, 1940, 1953, 1988, 1991 and 1997 by Honeywell Inc. All rights reserved. This manual or portions thereof may not be reporduced in any form without permission of Honeywell Inc. Library of Congress Catalog Card Number: 97-72971 Home and Building Control Honeywell Inc. Honeywell Plaza P.O. Box 524 Minneapolis MN 55408-0524 Home and Building Control Honeywell Limited-Honeywell Limitée 155 Gordon Baker Road North York, Ontario M2H 3N7 Honeywell Latin American Region 480 Sawgrass Corporate Parkway Suite 200 Sunrise FL 33325 Honeywell Europe S.A. 3 Avenue du Bourget 1140 Brussels Belgium Honeywell Asia Pacific Inc. Room 3213-3225 Sun Hung Kai Centre No. 30 Harbour Road Wanchai Hong Kong Printed in USA ii ENGINEERING MANUAL OF AUTOMATIC CONTROL FOREWORD The Minneapolis Honeywell Regulator Company published the first edition of the Engineering Manual of Automatic Control in l934. The manual quickly became the standard textbook for the commercial building controls industry. Subsequent editions have enjoyed even greater success in colleges, universities, and contractor and consulting engineering offices throughout the world. Since the original 1934 edition, the building control industry has experienced dramatic change and made tremendous advances in equipment, system design, and application. In this edition, microprocessor controls are shown in most of the control applications rather than pneumatic, electric, or electronic to reflect the trends in industry today. Consideration of configuration, functionality, and integration plays a significant role in the design of building control systems. Through the years Honeywell has been dedicated to assisting consulting engineers and architects in the application of automatic controls to heating, ventilating, and air conditioning systems. This manual is an outgrowth of that dedication. Our end user customers, the building owners and operators, will ultimately benefit from the efficiently designed systems resulting from the contents of this manual. All of this manual’s original sections have been updated and enhanced to include the latest developments in control technology. A new section has been added on indoor air quality and information on district heating has been added to the Chiller, Boiler, and Distribution System Control Applications Section. This twenty-first edition of the Engineering Manual of Automatic Control is our contribution to ensure that we continue to satisfy our customer’s requirements. The contributions and encouragement received from previous users are gratefully acknowledged. Further suggestions will be most welcome. Minneapolis, Minnesota October, 1997 KEVIN GILLIGAN President, H&BC Solutions and Services ENGINEERING MANUAL OF AUTOMATIC CONTROL iii iv ENGINEERING MANUAL OF AUTOMATIC CONTROL PREFACE The purpose of this manual is to provide the reader with a fundamental understanding of controls and how they are applied to the many parts of heating, ventilating, and air conditioning systems in commercial buildings. Many aspects of control are presented including air handling units, terminal units, chillers, boilers, building airflow, water and steam distribution systems, smoke management, and indoor air quality. Control fundamentals, theory, and types of controls provide background for application of controls to heating, ventilating, and air conditioning systems. Discussions of pneumatic, electric, electronic, and digital controls illustrate that applications may use one or more of several different control methods. Engineering data such as equipment sizing, use of psychrometric charts, and conversion formulas supplement and support the control information. To enhance understanding, definitions of terms are provided within individual sections. For maximum usability, each section of this manual is available as a separate, self-contained document. Building management systems have evolved into a major consideration for the control engineer when evaluating a total heating, ventilating, and air conditioning system design. In response to this consideration, the basics of building management systems configuration are presented. The control recommendations in this manual are general in nature and are not the basis for any specific job or installation. Control systems are furnished according to the plans and specifications prepared by the control engineer. In many instances there is more than one control solution. Professional expertise and judgment are required for the design of a control system. This manual is not a substitute for such expertise and judgment. Always consult a licensed engineer for advice on designing control systems. It is hoped that the scope of information in this manual will provide the readers with the tools to expand their knowledge base and help develop sound approaches to automatic control. ENGINEERING MANUAL OF AUTOMATIC CONTROL v vi ENGINEERING MANUAL OF AUTOMATIC CONTROL ENGINEERING MANUAL of AUTOMATIC CONTROL CONTENTS Foreward ............................................................................................................................................................. iii Preface ................................................................................................................................................................ v Control System Fundamentals ............................................................................................ 1 Control Fundamentals ....................................................................................................................................... Introduction .......................................................................................... Definitions ............................................................................................ HVAC System Characteristics ............................................................. Control System Characteristics ........................................................... Control System Components .............................................................. Characteristics And Attributes Of Control Methods ............................. 3 5 5 8 15 30 35 Psychrometric Chart Fundamentals ................................................................................................................ Introduction .......................................................................................... Definitions ............................................................................................ Description of the Psychrometric Chart ............................................... The Abridged Psychrometric Chart ..................................................... Examples of Air Mixing Process .......................................................... Air Conditioning Processes ................................................................. Humidifying Process ............................................................................ ASHRAE Psychrometric Chart ............................................................ 37 38 38 39 40 42 43 44 53 Pneumatic Control Fundamentals .................................................................................................................... Introduction .......................................................................................... Definitions ............................................................................................ Abbreviations ....................................................................................... Symbols ............................................................................................... Basic Pneumatic Control System ........................................................ Air Supply Equipment .......................................................................... Thermostats ........................................................................................ Controllers ........................................................................................... Sensor-Controller Systems ................................................................. Actuators and Final Control Elements ................................................. Relays and Switches ........................................................................... Pneumatic Control Combinations ........................................................ Pneumatic Centeralization .................................................................. Pneumatic Control System Example ................................................... 57 59 59 60 61 61 65 69 70 72 74 77 84 89 90 Electric Control Fundamentals ......................................................................................................................... Introduction .......................................................................................... Definitions ............................................................................................ How Electric Control Circuits Classified .............................................. Series 40 Control Circuits.................................................................... Series 80 Control Circuits.................................................................... Series 60 Two-Position Control Circuits ............................................... Series 60 Floating Control Circuits ...................................................... Series 90 Control Circuits.................................................................... Motor Control Circuits .......................................................................... 95 97 97 99 100 102 103 106 107 114 ENGINEERING MANUAL OF AUTOMATIC CONTROL vii Electronic Control Fundamentals ..................................................................................................................... Introduction .......................................................................................... Definitions ............................................................................................ Typical System .................................................................................... Components ........................................................................................ Electtonic Controller Fundamentals .................................................... Typical System Application .................................................................. 119 120 120 122 122 129 130 Microprocessor-Based/DDC Fundamentals .................................................................................................... Introduction .......................................................................................... Definitions ............................................................................................ Background ......................................................................................... Advantages ......................................................................................... Controller Configuration ...................................................................... Types of Controllers ............................................................................. Controller Software .............................................................................. Controller Programming ...................................................................... Typical Applications ............................................................................. 131 133 133 134 134 135 136 137 142 145 Indoor Air Quality Fundamentals ..................................................................................................................... Introduction .......................................................................................... Definitions ............................................................................................ Abbreviations ....................................................................................... Indoor Air Quality Concerns ................................................................ Indoor Air Quality Control Applications ................................................ Bibliography ......................................................................................... 149 151 151 153 154 164 170 Smoke Management Fundamentals ................................................................................................................. Introduction .......................................................................................... Definitions ............................................................................................ Objectives ............................................................................................ Design Considerations ........................................................................ Design Principles ................................................................................ Control Applications ............................................................................ Acceptance Testing ............................................................................. Leakage Rated Dampers .................................................................... Bibliography ......................................................................................... 171 172 172 173 173 175 178 181 181 182 Building Management System Fundamentals ................................................................................................. Introduction .......................................................................................... Definitions ............................................................................................ Background ......................................................................................... System Configurations ........................................................................ System Functions ................................................................................ Integration of Other Systems ............................................................... 183 184 184 185 186 189 197 viii ENGINEERING MANUAL OF AUTOMATIC CONTROL Control System Applications ............................................................................................... 199 Air Handling System Control Applications ...................................................................................................... Introduction .......................................................................................... Abbreviations ....................................................................................... Requirements For Effective Control .................................................... Applications-General ........................................................................... Valve and Damper Selection ............................................................... Symbols ............................................................................................... Ventilation Control Processes ............................................................. Fixed Quantity of Outdoor Air Control ................................................. Heating Control Processes .................................................................. Preheat Control Processes ................................................................. Humidification Control Process ........................................................... Cooling Control Processes .................................................................. Dehumidification Control Processes ................................................... Heating System Control process ......................................................... Year-Round System Control processes .............................................. ASHRAE Psychrometric Charts .......................................................... 201 203 203 204 206 207 208 209 211 223 228 235 236 243 246 248 261 Building Airflow System Control Applications ............................................................................................... Introduction .......................................................................................... Definitions ............................................................................................ Airflow Control Fundamentals ............................................................. Airflow Control Applications ................................................................. References .......................................................................................... 263 265 265 267 281 292 Chiller, Boiler, and Distribution System Control Applications ....................................................................... Introduction .......................................................................................... Abbreviations ....................................................................................... Definitions ............................................................................................ Symbols ............................................................................................... Chiller System Control ......................................................................... Boiler System Control .......................................................................... Hot And Chilled Water Distribution Systems Control ........................... High Temperature Water Heating System Control .............................. District Heating Applications ................................................................ 293 297 297 297 298 299 329 337 376 382 Individual Room Control Applications ............................................................................................................ Introduction .......................................................................................... Unitary Equipment Control .................................................................. Hot Water Plant Considerations .......................................................... 399 401 412 428 ENGINEERING MANUAL OF AUTOMATIC CONTROL ix Engineering Information ....................................................................................................... 429 Valve Selection and Sizing ................................................................................................................................ Introduction .......................................................................................... Definitions ............................................................................................ Valve Selection .................................................................................... Valve Sizing ......................................................................................... 431 432 432 436 441 Damper Selection and Sizing ............................................................................................................................ Introduction .......................................................................................... Definitions ............................................................................................ Damper Selection ................................................................................ Damper Sizing ..................................................................................... Damper Pressure Drop ....................................................................... Damper Applications ........................................................................... 451 453 453 454 463 468 469 General Engineering Data ................................................................................................................................. Introduction .......................................................................................... Weather Data ...................................................................................... Conversion Formulas And Tables ........................................................ Electrical Data ..................................................................................... Properties Of Saturated Steam Data ................................................... Airflow Data ......................................................................................... Moisture Content Of Air Data .............................................................. 471 472 472 475 482 488 489 491 Index ....................................................................................................................................... 494 x ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS CONTROL SYSTEMS FUNDMENTALS 1 ENGINEERING MANUAL OF AUTOMATIC CONTROL Control Fundamentals Contents Introduction ............................................................................................................ 5 Definitions ............................................................................................................ 5 HVAC System Characteristics ............................................................................................................ General ................................................................................................ Heating ................................................................................................ General ................................................................................................ Heating Equipment .............................................................................. Cooling ................................................................................................ General ................................................................................................ Cooling Equipment .............................................................................. Dehumidification .................................................................................. Humidification ...................................................................................... Ventilation ............................................................................................ Filtration ............................................................................................... 8 8 9 9 10 11 11 12 12 13 13 14 Control System Characteristics ............................................................................................................ Controlled Variables ............................................................................ Control Loop ........................................................................................ Control Methods .................................................................................. General ........................................................................................... Analog And Digital Control .............................................................. Control Modes ..................................................................................... Two-Position Control ....................................................................... General ....................................................................................... Basic Two-Position Control ......................................................... Timed Two-Position Control ........................................................ Step Control .................................................................................... Floating Control ............................................................................... Proportional Control ........................................................................ General ....................................................................................... Compensation Control ................................................................ Proportional-Integral (Pi) Control .................................................... Proportional-Integral-Derivative (Pid) Control ................................. Enhanced Proportional-Integral-Derivative (epid) Control .............. Adaptive Control ............................................................................. Process Characteristics ....................................................................... 15 15 15 16 16 16 17 17 17 17 18 19 20 21 21 22 23 25 25 26 26 CONTROL FUNDAMENTALS Load ................................................................................................ Lag .................................................................................................. General ........................................................................................... Measurement Lag ........................................................................... Capacitance .................................................................................... Resistance ...................................................................................... Dead Time ....................................................................................... Control Application Guidelines ............................................................ 26 27 27 27 28 29 29 29 ............................................................................................................ Sensing Elements ............................................................................... Temperature Sensing Elements ...................................................... Pressure Sensing Elements ............................................................ Moisture Sensing Elements ............................................................ Flow Sensors .................................................................................. Proof-Of-Operation Sensors ........................................................... Transducers ........................................................................................ Controllers ........................................................................................... Actuators ............................................................................................. Auxiliary Equipment ............................................................................. 30 30 30 31 32 32 33 33 33 33 34 Characteristics And Attributes Of Control Methods .............................................................................................. 35 Control System Components ENGINEERING MANUAL OF AUTOMATIC CONTROL 4 CONTROL FUNDAMENTALS INTRODUCTION Automatic controls can optimize HVAC system operation. They can adjust temperatures and pressures automatically to reduce demand when spaces are unoccupied and regulate heating and cooling to provide comfort conditions while limiting energy usage. Limit controls ensure safe operation of HVAC system equipment and prevent injury to personnel and damage to the system. Examples of limit controls are lowlimit temperature controllers which help prevent water coils or heat exchangers from freezing and flow sensors for safe operation of some equipment (e.g., chillers). In the event of a fire, controlled air distribution can provide smoke-free evacuation passages, and smoke detection in ducts can close dampers to prevent the spread of smoke and toxic gases. This section describes heating, ventilating, and air conditioning (HVAC) systems and discusses characteristics and components of automatic control systems. Cross-references are made to sections that provide more detailed information. A correctly designed HVAC control system can provide a comfortable environment for occupants, optimize energy cost and consumption, improve employee productivity, facilitate efficient manufacturing, control smoke in the event of a fire, and support the operation of computer and telecommunications equipment. Controls are essential to the proper operation of the system and should be considered as early in the design process as possible. HVAC control systems can also be integrated with security access control systems, fire alarm systems, lighting control systems, and building and facility management systems to further optimize building comfort, safety, and efficiency. Properly applied automatic controls ensure that a correctly designed HVAC system will maintain a comfortable environment and perform economically under a wide range of operating conditions. Automatic controls regulate HVAC system output in response to varying indoor and outdoor conditions to maintain general comfort conditions in office areas and provide narrow temperature and humidity limits where required in production areas for product quality. DEFINITIONS The following terms are used in this manual. Figure 1 at the end of this list illustrates a typical control loop with the components identified using terms from this list. Controlled medium: The medium in which the controlled variable exists. In a space temperature control system, the controlled variable is the space temperature and the controlled medium is the air within the space. Analog: Continuously variable (e.g., a faucet controlling water from off to full flow). Controlled Variable: The quantity or condition that is measured and controlled. Automatic control system: A system that reacts to a change or imbalance in the variable it controls by adjusting other variables to restore the system to the desired balance. Controller: A device that senses changes in the controlled variable (or receives input from a remote sensor) and derives the proper correction output. Algorithm: A calculation method that produces a control output by operating on an error signal or a time series of error signals. Corrective action: Control action that results in a change of the manipulated variable. Initiated when the controlled variable deviates from setpoint. Compensation control: A process of automatically adjusting the setpoint of a given controller to compensate for changes in a second measured variable (e.g., outdoor air temperature). For example, the hot deck setpoint is normally reset upward as the outdoor air temperature decreases. Also called “reset control”. Cycle: One complete execution of a repeatable process. In basic heating operation, a cycle comprises one on period and one off period in a two-position control system. Cycling: A periodic change in the controlled variable from one value to another. Out-of-control analog cycling is called “hunting”. Too frequent on-off cycling is called “short cycling”. Short cycling can harm electric motors, fans, and compressors. Control agent: The medium in which the manipulated variable exists. In a steam heating system, the control agent is the steam and the manipulated variable is the flow of the steam. Cycling rate: The number of cycles completed per time unit, typically cycles per hour for a heating or cooling system. The inverse of the length of the period of the cycle. Control point: The actual value of the controlled variable (setpoint plus or minus offset). 5 ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS Deadband: A range of the controlled variable in which no corrective action is taken by the controlled system and no energy is used. See also “zero energy band”. related. Also, the delay in response of the sensing element of a control due to the time required for the sensing element to sense a change in the sensed variable. Deviation: The difference between the setpoint and the value of the controlled variable at any moment. Also called “offset”. Load: In a heating or cooling system, the heat transfer that the system will be called upon to provide. Also, the work that the system must perform. DDC: Direct Digital Control. See also Digital and Digital control. Manipulated variable: The quantity or condition regulated by the automatic control system to cause the desired change in the controlled variable. Digital: A series of on and off pulses arranged to convey information. Morse code is an early example. Processors (computers) operate using digital language. Measured variable: A variable that is measured and may be controlled (e.g., discharge air is measured and controlled, outdoor air is only measured). Microprocessor-based control: A control circuit that operates on low voltage and uses a microprocessor to perform logic and control functions, such as operating a relay or providing an output signal to position an actuator. Electronic devices are primarily used as sensors. The controller often furnishes flexible DDC and energy management control routines. Digital control: A control loop in which a microprocessorbased controller directly controls equipment based on sensor inputs and setpoint parameters. The programmed control sequence determines the output to the equipment. Droop: A sustained deviation between the control point and the setpoint in a two-position control system caused by a change in the heating or cooling load. Modulating: An action that adjusts by minute increments and decrements. Enhanced proportional-integral-derivative (EPID) control: A control algorithm that enhances the standard PID algorithm by allowing the designer to enter a startup output value and error ramp duration in addition to the gains and setpoints. These additional parameters are configured so that at startup the PID output varies smoothly to the control point with negligible overshoot or undershoot. Offset: A sustained deviation between the control point and the setpoint of a proportional control system under stable operating conditions. On/off control: A simple two-position control system in which the device being controlled is either full on or full off with no intermediate operating positions available. Also called “two-position control”. Electric control: A control circuit that operates on line or low voltage and uses a mechanical means, such as a temperature-sensitive bimetal or bellows, to perform control functions, such as actuating a switch or positioning a potentiometer. The controller signal usually operates or positions an electric actuator or may switch an electrical load directly or through a relay. Pneumatic control: A control circuit that operates on air pressure and uses a mechanical means, such as a temperature-sensitive bimetal or bellows, to perform control functions, such as actuating a nozzle and flapper or a switching relay. The controller output usually operates or positions a pneumatic actuator, although relays and switches are often in the circuit. Process: A general term that describes a change in a measurable variable (e.g., the mixing of return and outdoor air streams in a mixed-air control loop and heat transfer between cold water and hot air in a cooling coil). Usually considered separately from the sensing element, control element, and controller. Electronic control: A control circuit that operates on low voltage and uses solid-state components to amplify input signals and perform control functions, such as operating a relay or providing an output signal to position an actuator. The controller usually furnishes fixed control routines based on the logic of the solidstate components. Proportional band: In a proportional controller, the control point range through which the controlled variable must pass to move the final control element through its full operating range. Expressed in percent of primary sensor span. Commonly used equivalents are “throttling range” and “modulating range”, usually expressed in a quantity of engineering units (degrees of temperature). Final control element: A device such as a valve or damper that acts to change the value of the manipulated variable. Positioned by an actuator. Hunting: See Cycling. Lag: A delay in the effect of a changed condition at one point in the system, or some other condition to which it is ENGINEERING MANUAL OF AUTOMATIC CONTROL 6 CONTROL FUNDAMENTALS Step control: Control method in which a multiple-switch assembly sequentially switches equipment (e.g., electric heat, multiple chillers) as the controller input varies through the proportional band. Step controllers may be actuator driven, electronic, or directly activated by the sensed medium (e.g., pressure, temperature). Proportional control: A control algorithm or method in which the final control element moves to a position proportional to the deviation of the value of the controlled variable from the setpoint. Proportional-Integral (PI) control: A control algorithm that combines the proportional (proportional response) and integral (reset response) control algorithms. Reset response tends to correct the offset resulting from proportional control. Also called “proportional-plusreset” or “two-mode” control. Throttling range: In a proportional controller, the control point range through which the controlled variable must pass to move the final control element through its full operating range. Expressed in values of the controlled variable (e.g., degrees Fahrenheit, percent relative humidity, pounds per square inch). Also called “proportional band”. In a proportional room thermostat, the temperature change required to drive the manipulated variable from full off to full on. Proportional-Integral-Derivative (PID) control: A control algorithm that enhances the PI control algorithm by adding a component that is proportional to the rate of change (derivative) of the deviation of the controlled variable. Compensates for system dynamics and allows faster control response. Also called “threemode” or “rate-reset” control. Time constant: The time required for a dynamic component, such as a sensor, or a control system to reach 63.2 percent of the total response to an instantaneous (or “step”) change to its input. Typically used to judge the responsiveness of the component or system. Reset Control: See Compensation control. Sensing element: A device or component that measures the value of a variable. Two-position control: See on/off control. Zero energy band: An energy conservation technique that allows temperatures to float between selected settings, thereby preventing the consumption of heating or cooling energy while the temperature is in this range. Setpoint: The value at which the controller is set (e.g., the desired room temperature set on a thermostat). The desired control point. Short cycling: See Cycling. Zoning: The practice of dividing a building into sections for heating and cooling control so that one controller is sufficient to determine the heating and cooling MEASURED VARIABLE ALGORITHM IN CONTROLLER RESET SCHEDULE OUTDOOR AIR 60 130 SETPOINT 30 160 0 OUTDOOR AIR 190 OA TEMPERATURE CONTROLLED VARIABLE MEASURED VARIABLE INPUT OUTPUT PERCENT OPEN CONTROL POINT 41 VALVE HOT WATER SUPPLY TEMPERATURE CONTROLLED MEDIUM SETPOINT HW SETPOINT 159 FINAL CONTROL ELEMENT STEAM CONTROL AGENT FLOW HOT WATER SUPPLY MANIPULATED VARIABLE 148 HOT WATER RETURN AUTO M10510 requirements for the section. Fig. 1. Typical Control Loop. 7 ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS HVAC SYSTEM CHARACTERISTICS Figure 2 shows how an HVAC system may be distributed in a small commercial building. The system control panel, boilers, motors, pumps, and chillers are often located on the lower level. The cooling tower is typically located on the roof. Throughout the building are ductwork, fans, dampers, coils, air filters, heating units, and variable air volume (VAV) units and diffusers. Larger buildings often have separate systems for groups of floors or areas of the building. GENERAL An HVAC system is designed according to capacity requirements, an acceptable combination of first cost and operating costs, system reliability, and available equipment space. DUCTWORK COOLING TOWER DAMPER AIR FILTER COOLING COIL HEATING UNIT VAV BOX DIFFUSER FAN CHILLER BOILER PUMP CONTROL PANEL M10506 Fig. 2. Typical HVAC System in a Small Building. The control system for a commercial building comprises many control loops and can be divided into central system and local- or zone-control loops. For maximum comfort and efficiency, all control loops should be tied together to share information and system commands using a building management system. Refer to the Building Management System Fundamentals section of this manual. ENGINEERING MANUAL OF AUTOMATIC CONTROL The basic control loops in a central air handling system can be classified as shown in Table 1. Depending on the system, other controls may be required for optimum performance. Local or zone controls depend on the type of terminal units used. 8 CONTROL FUNDAMENTALS Table 1. Functions of Central HVAC Control Loops. Control Loop Classification Ventilation Description Basic Better Controls cooling tower fans to provide the coolest water practical under existing wet bulb temperature conditions. Water coil control Adjusts chilled water flow to maintain temperature. Direct expansion (DX) system control Cycles compressor or DX coil solenoid valves to maintain temperature. If compressor is unloading type, cylinders are unloaded as required to maintain temperature. Basic Turns on supply and return fans during occupied periods and cycles them as required during unoccupied periods. Better Adjusts fan volumes to maintain proper duct and space pressures. Reduces system operating cost and improves performance (essential for variable air volume systems). Coil control Adjusts water or steam flow or electric heat to maintain temperature. Boiler control Heating Maintains chiller discharge water at preset temperature or resets temperature according to demand. Cooling tower control Fan Measures and controls the volume of outdoor air to provide the proper mix of outdoor and return air under varying indoor conditions (essential in variable air volume systems). Low-temperature protection may be required. Chiller control Cooling Coordinates operation of the outdoor, return, and exhaust air dampers to maintain the proper amount of ventilation air. Low-temperature protection is often required. Operates burner to maintain proper discharge steam pressure or water temperature. For maximum efficiency in a hot water system, water temperature should be reset as a function of demand or outdoor temperature. HEATING GENERAL Building heat loss occurs mainly through transmission, infiltration/exfiltration, and ventilation (Fig. 3). TRANSMISSION VENTILATION ROOF Transmission is the process by which energy enters or leaves a space through exterior surfaces. The rate of energy transmission is calculated by subtracting the outdoor temperature from the indoor temperature and multiplying the result by the heat transfer coefficient of the surface materials. The rate of transmission varies with the thickness and construction of the exterior surfaces but is calculated the same way for all exterior surfaces: 20°F PREVAILING WINDS DUCT 70°F EXFILTRATION DOOR WINDOW Energy Transmission per Unit Area and Unit Time = INFILTRATION C2701 (T - T ) x HTC IN OUT Where: Fig. 3. Heat Loss from a Building. T = indoor temperature T = outdoor temperature HTC = heat transfer coefficient IN The heating capacity required for a building depends on the design temperature, the quantity of outdoor air used, and the physical activity of the occupants. Prevailing winds affect the rate of heat loss and the degree of infiltration. The heating system must be sized to heat the building at the coldest outdoor temperature the building is likely to experience (outdoor design temperature). OUT HTC 9 = Btu Unit Time x Unit Area x Unit Temperatur ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS Infiltration is the process by which outdoor air enters a building through walls, cracks around doors and windows, and open doors due to the difference between indoor and outdoor air pressures. The pressure differential is the result of temperature difference and air intake or exhaust caused by fan operation. Heat loss due to infiltration is a function of temperature difference and volume of air moved. Exfiltration is the process by which air leaves a building (e.g., through walls and cracks around doors and windows) and carries heat with it. Infiltration and exfiltration can occur at the same time. STEAM OR HOT WATER SUPPLY FAN COIL CONDENSATE OR HOT WATER RETURN UNIT HEATER STEAM TRAP (IF STEAM SUPPLY) C2703 Ventilation brings in fresh outdoor air that may require heating. As with heat loss from infiltration and exfiltration, heat loss from ventilation is a function of the temperature difference and the volume of air brought into the building or exhausted. Fig. 5. Typical Unit Heater. HOT WATER SUPPLY HEATING EQUIPMENT HOT WATER RETURN Selecting the proper heating equipment depends on many factors, including cost and availability of fuels, building size and use, climate, and initial and operating cost trade-offs. Primary sources of heat include gas, oil, wood, coal, electrical, and solar energy. Sometimes a combination of sources is most economical. Boilers are typically fueled by gas and may have the option of switching to oil during periods of high demand. Solar heat can be used as an alternate or supplementary source with any type of fuel. GRID PANEL HOT WATER SUPPLY HOT WATER RETURN SERPENTINE PANEL C2704 Figure 4 shows an air handling system with a hot water coil. A similar control scheme would apply to a steam coil. If steam or hot water is chosen to distribute the heat energy, highefficiency boilers may be used to reduce life-cycle cost. Water generally is used more often than steam to transmit heat energy from the boiler to the coils or terminal units, because water requires fewer safety measures and is typically more efficient, especially in mild climates. Fig. 6. Panel Heaters. Unit ventilators (Fig. 7) are used in classrooms and may include both a heating and a cooling coil. Convection heaters (Fig. 8) are used for perimeter heating and in entries and corridors. Infrared heaters (Fig. 9) are typically used for spot heating in large areas (e.g., aircraft hangers, stadiums). DISCHARGE AIR WALL THERMOSTAT VALVE HOT WATER SUPPLY FAN HEATING COIL DISCHARGE AIR FAN HOT WATER RETURN COOLING COIL C2702 Fig. 4. System Using Heating Coil. DRAIN PAN An air handling system provides heat by moving an air stream across a coil containing a heating medium, across an electric heating coil, or through a furnace. Unit heaters (Fig. 5) are typically used in shops, storage areas, stairwells, and docks. Panel heaters (Fig. 6) are typically used for heating floors and are usually installed in a slab or floor structure, but may be installed in a wall or ceiling. MIXING DAMPERS RETURN AIR OUTDOOR AIR C3035 Fig. 7. Unit Ventilator. ENGINEERING MANUAL OF AUTOMATIC CONTROL 10