77-1131 - Building Airflow System Control .BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS 2 Airflow

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  • 77-1131-1

    Building Airflow SystemControl Applications

    Contents

    Introduction ............................................................................................................ 3

    Definitions ............................................................................................................ 3

    Airflow Control Fundamentals ............................................................................................................ 5Need For Airflow Control ..................................................................... 5What Is Airflow Control ........................................................................ 5Types Of Airflow Systems.................................................................... 5

    Variable Air Volume ......................................................................... 5Constant Air Volume ....................................................................... 6Variable Versus Constant Air Volume .............................................. 7

    Ventilation ............................................................................................ 7Pressurization ..................................................................................... 8

    Building Pressure Balance .............................................................. 8Containment Pressurization ............................................................ 9Wind Pressure Effects ..................................................................... 9Stack Effect ..................................................................................... 9

    Characteristics Of Fans And Fan Laws ............................................... 10General ........................................................................................... 10Fan Types ........................................................................................ 10Fan Performance Terms .................................................................. 11Fan Laws ......................................................................................... 11Fan Horsepower .............................................................................. 11Duct System Curves ....................................................................... 12Fan Curve And System Curve Comparison .................................... 12

    Characteristics Of Airflow In Ducts ...................................................... 12General ........................................................................................... 12Pressure Changes Within A Duct.................................................... 12Effects of Fittings............................................................................. 14Effects Of Dampers ......................................................................... 15Effects Of Air Terminal Units ........................................................... 15

    Measurement Of Airflow In Ducts ........................................................ 15General ........................................................................................... 15Pressure Sensors ........................................................................... 15Pitot Tube Sensors .......................................................................... 16Total And Static Pressure Sensors ................................................. 16Airflow Measuring Devices .............................................................. 18

    SECTION OF ENGINEERING MANUAL OF AUTOMATIC CONTROL 77-1100

    U.S. Registered TrademarkCopyright 1998 Honeywell Inc. All Rights Reserved

  • 77-11311

    BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS

    2

    Airflow Control Applications ............................................................................................................ 19Central Fan System Control ................................................................ 19

    Supply Fan Control For VAV Systems ............................................. 19General ....................................................................................... 19Duct Static High-Limit Control..................................................... 20

    Return Fan Control For VAV Systems ............................................. 21Open Loop Control ..................................................................... 21Direct Building Static Control ...................................................... 21Airflow Tracking Control .............................................................. 21Duct Static Control ...................................................................... 22

    Relief Fan Control For VAV Systems .............................................. 22Return Damper Control For VAV Systems ...................................... 23Sequencing Fan Control ................................................................. 24Other Control Modes....................................................................... 24

    Warm-Up Control ........................................................................ 24Smoke Control ............................................................................ 24Night Purge Control .................................................................... 24

    Zone Airflow Control ............................................................................ 24Airflow Tracking/Space Static Pressure ........................................... 24Multiple Fan Systems ...................................................................... 26

    Exhaust System Control ...................................................................... 26Types ............................................................................................... 26Fume Hoods ................................................................................... 26Laboratory Pressurization ............................................................... 28

    References ............................................................................................................ 30

  • 77-11311

    BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS

    3

    INTRODUCTION

    This section explains the need for airflow control in a centralair handling system, describes the various means of airflowmeasurement, provides fan and duct characteristics, anddiscusses suggested means of airflow control. The final controlsystem design depends, of course, on specific job requirements.

    There are several types of airflow control that relate directlyto the control of airflow in a central air handling system. Thesetypes of airflow control include space pressurization, zonepressurization, and exhaust air control. Space or zonepressurization is used when an enclosed area within a building(e.g., a clean room, hospital space, laboratory, fire and smokecontrol area) must be kept at a positive or negative pressure so

    contaminated air does not migrate to unwanted areas. Basictypes of space pressure control are static pressure, airflowtracking, and constant airflow. Exhaust air control regulatesthe amount of air exhausted to keep it at the minimum safelevel. Space pressure control is generally required with exhaustair control, and control of airflow in a central air handling systemis generally required with space pressure control and/or exhaustair control.

    For information on air terminal units used in building airflowcontrol system applications, refer to the Individual RoomControl Applications section.

    DEFINITIONS

    Airflow: The rate at which a volume of air moves through aduct. In this section, airflow is denoted Q and ismeasured in cubic feet per minute (cfm). Airflow isderived:

    Q = A x VAVG

    Where:Q = Airflow in cfmA = Cross-sectional area of duct in

    square feet (ft2)V

    AVG= Average velocity

    Axial fan: A propeller type fan where airflow within the wheelis substantially parallel to the shaft and in-line withthe duct. Axial fan airflow can be controlled by speed,variable inlet vanes, or variable pitch blades dependingon the fan type.

    Centrifugal fan: A fan where airflow within the wheel issubstantially radial to the shaft and the air must maketwo turns before being expelled from the fan housing.Centrifugal fan airflow can be controlled by speed,variable inlet vanes, or less commonly by dampers.

    Constant Air Volume (CAV) system: A central fan system inwhich airflow in the duct is maintained at a constantvolume.

    Differential: The difference between supply and return airflowsnecessary to maintain a positive or a negative pressurein an area. For example, if supply airflow is 1800 cfmand return airflow is 1500 cfm, the differential(positive) is 300 cfm. The 300 cfm surplus leaves thebuilding through exhaust fans or vents and exfiltration.

    Duct: A circular or rectangular tube for conveying air.

    Duct cross-sectional area: For round ducts, the duct cross-sectional area is r2, where r is the radius. Forrectangular ducts, the duct area is X times Y, where Xand Y are the height and width dimensions. In thissection, a duct cross-sectional area is measured insquare feet (ft2).

    NOTE: If duct dimensions are in inches (in.) andthe result of the duct area is in squareinches (in2), divide the result by 144 in2/ft2

    to obtain square feet (ft2).

    Duct diameter: For round ducts, the diameter is twice the radius(2r). For rectangular ducts, an equivalent diameter isderived: 2XY (X + Y), where X and Y are the heightand width.

    Fan surge: A condition that occurs when air passing over thefan blades causes a stall. A fan surge causes afluctuation in duct static pressure and an increase noiselevel.

    Flow Measuring Station (FMS): A device containing multiplestatic pressure sensors and multiple total pressuresensors manifolded separately f