Fundamentals of HVAC Control Systems
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ASHRAE Hong Kong Chapter Technical Workshop Fundamentals of HVAC Control Systems 18, 19, 25, 26 April 2007 © 2007 ASHRAE Hong Kong Chapter Slide 1 Chapter 3 Control Valves and Dampers © 2007 ASHRAE Hong Kong Chapter Slide 2 Control Valves Important component of fluid distribution systems Common types: Regulate the flow of fluid to the process under control Globe valves (for modulating) Ball valves (less expensive) Butterfly valves (for isolation) Valve material Bronze, cast iron, steel © 2007 ASHRAE Hong Kong Chapter Slide 3 Control valve components [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 4 2-Way and 3-Way Valve Circuits 2-way control valve Mixing 3-way control valve Diverting 3-way control valve © 2007 ASHRAE Hong Kong Chapter Slide 5 2-Way and 3-Way Valves 2-way: for variable flow More sensitive to high differential pressure Harder to close off against line pressure 3-way: for constant flow Actuator does not need to be as powerful © 2007 ASHRAE Hong Kong Chapter Slide 6 Advantages of 2-Way Valves Less expensive to buy and install Result in variable flow which reduces pumping energy Reduced piping heat losses and pump energy Potentially lower costs for pumping and distribution systems System balancing is reduced or eliminated © 2007 ASHRAE Hong Kong Chapter Slide 7 Disadvantages of 2-Way Valves Most chillers and some boilers cannot handle widely varying flow rates Differential pressures will increase across control valves, reducing system controllability © 2007 ASHRAE Hong Kong Chapter Slide 8 Normally Open 2-Way Valve © 2007 ASHRAE Hong Kong Chapter Slide 9 Normally Closed 2-Way Valve © 2007 ASHRAE Hong Kong Chapter Slide 10 Two-way globe valve application [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 11 Double-Seated 2-Way Valve © 2007 ASHRAE Hong Kong Chapter Slide 12 Butterfly Valve © 2007 ASHRAE Hong Kong Chapter Slide 13 Ball Valve © 2007 ASHRAE Hong Kong Chapter Slide 14 Ball valve Butterfly valve [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 15 Linear Valve © 2007 ASHRAE Hong Kong Chapter Slide 16 Equal Percentage Valve © 2007 ASHRAE Hong Kong Chapter Slide 17 Quick-Opening Valve © 2007 ASHRAE Hong Kong Chapter Slide 18 Characteristics of Control Valves Valve flow characteristics Relationship between the stem travel of a valve, expressed in percent of travel, and the fluid flow through the valve, expressed in percent of full flow Typical flow characteristics Linear Equal percentage Quick opening © 2007 ASHRAE Hong Kong Chapter Slide 19 Typical Flow Characteristics © 2007 ASHRAE Hong Kong Chapter Slide 20 [Source: Shadpour, F., 2001. The Fundamentals of HVAC Direct Digital Control] © 2007 ASHRAE Hong Kong Chapter Slide 21 Flow coefficient: AV = q ρ ΔP q = volume flow (m3/s) ρ= fluid density (kg/m3) ΔP = static pressure loss across the valve (Pa) For different units and locations, CV and KV are used. For valve used in water application: KV = Q ρ ΔP ⋅10 Q = volume flow (m3/h) [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 22 Control Valve Ratings Flow coefficient Close-off rating: Pressure drop: The maximum pressure drop that a valve can withstand without leakage while in the full closed position The difference in upstream and downstream pressures of the fluid flowing through the valve Maximum pressure and temperature: The maximum pressure and temperature limitations of fluid flow that a valve can withstand © 2007 ASHRAE Hong Kong Chapter Slide 23 Location of Control Valves At the outlet on the top of cooling/heating coils Avoid coil starvation from water flow (lower pressure) Flow of water from the bottom to the top (avoid air bubble) Flow measuring & balancing device should be placed after the control valve Provide a means of shut-off to allow a proper means for servicing © 2007 ASHRAE Hong Kong Chapter Slide 24 [Source: Shadpour, F., 2001. The Fundamentals of HVAC Direct Digital Control] © 2007 ASHRAE Hong Kong Chapter Slide 25 3-Way Mixing Valve © 2007 ASHRAE Hong Kong Chapter Slide 26 3-Way Diverting Valve © 2007 ASHRAE Hong Kong Chapter Slide 27 Three-way valves [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 28 3-Way Mixing Valves © 2007 ASHRAE Hong Kong Chapter Slide 29 Flow Through 3-Way Valve © 2007 ASHRAE Hong Kong Chapter Slide 30 Selecting & Sizing Valves Control valve selection depends on: The fluid being controlled Valve style: 2-way or 3-way Control mode: modulating or 2-position Maximum fluid temperature Maximum inlet pressure Desired flow characteristic Maximum fluid flow rate Desired pressure drop when valve is full open Turn-down ratio Close-off pressure © 2007 ASHRAE Hong Kong Chapter Slide 31 Coil Subsystem © 2007 ASHRAE Hong Kong Chapter Slide 32 Representative Values of Cv © 2007 ASHRAE Hong Kong Chapter Slide 33 Flow Characteristic Selection The desired flow characteristic is a function of: The heat transfer device being controlled and its flow versus capacity characteristic The control of fluid supply temperature The control of the differential pressure across the valve © 2007 ASHRAE Hong Kong Chapter Slide 34 Capacity Versus Flow Rate © 2007 ASHRAE Hong Kong Chapter Slide 35 Authority Distortion of Linear Flow Characteristic © 2007 ASHRAE Hong Kong Chapter Slide 36 Authority Distortion of Equal Percentage Flow Characteristic © 2007 ASHRAE Hong Kong Chapter Slide 37 Pump & System Curves With Valve Control © 2007 ASHRAE Hong Kong Chapter Slide 38 Control Dampers For controlling air distribution, such as Fire damper: A thermally actuated damper arranged to automatically restrict the passage of fire and/or heat at a point where an opening violates the integrity of a fire partition or floor Smoke damper: A damper arranged to control passage of smoke through an opening or a duct Volume control damper (VCD): A device used to regulate the flow of air in an HVAC system © 2007 ASHRAE Hong Kong Chapter Slide 39 Control Dampers Common types: Opposed blade dampers (e.g. in AHU) Parallel blade dampers Butterfly dampers (e.g. in VAV box) Linear air valves (e.g. in fume hood) Specialty dampers © 2007 ASHRAE Hong Kong Chapter Slide 40 [Source: Shadpour, F., 2001. The Fundamentals of HVAC Direct Digital Control] © 2007 ASHRAE Hong Kong Chapter Slide 41 Typical Multiblade Dampers © 2007 ASHRAE Hong Kong Chapter Slide 42 Parallel blade damper Opposed blade damper [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 43 Round damper Volume control damper (opposed blade) [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 44 Typical (opposed blade) damper construction [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 45 Externally mounted pneumatic actuator Internally mounted electric actuator [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 46 Damper Sizing Typically chosen based on duct size and convenience of location Proper selection and sizing provides the following benefits: Lower installation cost (damper sizes are smaller) Smaller actuators or a fewer number of them are required Reduced energy costs (smaller damper, less overall leakage) Improved control characteristics (rangeability) because the ratio of total damper flow to minimum controllable flow is increased Improved operating characteristics (linearity) © 2007 ASHRAE Hong Kong Chapter Slide 47 Performance Data Leakage ratings Torque requirements Closing torque Dynamic torque Velocity ratings Temperature ratings Pressure ratings UL classification (fire/smoke) © 2007 ASHRAE Hong Kong Chapter Slide 48 [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 49 Control loop for a damper system Resistance to airflow in actual system [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 50 [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 51 Mixed air control system (parallel blade dampers) [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 52 Mixed air system with louvers [Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings] © 2007 ASHRAE Hong Kong Chapter Slide 53 Selecting and Sizing Dampers The three basic damper applications are: Two-position duty Capacity control duty Mixing duty © 2007 ASHRAE Hong Kong Chapter Slide 54 Fans in Parallel © 2007 ASHRAE Hong Kong Chapter Slide 55 Centrifugal Fan With Inlet Vane Damper © 2007 ASHRAE Hong Kong Chapter Slide 56 Characteristic Curves of Parallel Blade Dampers © 2007 ASHRAE Hong Kong Chapter Slide 57 Characteristic Curves of Opposed Blade Dampers © 2007 ASHRAE Hong Kong Chapter Slide 58 Flow Pattern Through Dampers © 2007 ASHRAE Hong Kong Chapter Slide 59 Mixing Box Arrangements © 2007 ASHRAE Hong Kong Chapter Slide 60 Pressure Drop Across Mixing Box © 2007 ASHRAE Hong Kong Chapter Slide 61 Economizer With Supply and Return Fans © 2007 ASHRAE Hong Kong Chapter Slide 62 VAV System at 100% Outdoor Air and Minimum Outdoor Air © 2007 ASHRAE Hong Kong Chapter Slide 63 Economizer With Relief Fan © 2007 ASHRAE Hong Kong Chapter Slide 64
