HVAC2_InstructorNotesRev1.1.doc

此文档收集于网络，如有侵权，请联系网站删除HVAC APPLICATION IISIZING AND SELECTING VALVESFOR HVAC APPLICATIONSBEFORE STARTING :-1. OBTAIN FEEDBACK FROM STUDENTS RE BACKGROUND ETC.2. GO THROUGH AND EXPLAIN MANUAL.3. ASK WHAT DETAIL THEY THINK IS REQUIRED TO SIZE A CONTROL VALVE.Table Of Contents1VALVE SELECTION FOR SUCCESSFUL CONTROL41.1Course Overview41.2Aims And Objectives Of The Course51.3Course Agenda51.4Appendix To Course Notes52BASICS OF FLUID FLOW62.1Characteristics Of Pumps82.1.1Typical Centrifugal Pump Curve Combined With A System Curve83PUMPING SYSTEMS123.1Single Chiller Set With 3 Way Valves133.2Single Chiller Set With 2 Way Valves143.3Multiple Chiller Sets With 2 Way Valves153.4Primary / Secondary System With Multiple Chiller Sets And 2 Way Valves On The Field Units163.5Reverse Return Piping Systems173.5.1Single Chiller System With Two Way Valves173.5.2Multiple Chiller System With Two Way Valves183.6Normal Return Piping Systems193.7Primary / Secondary Piping Systems213.8Cooling Tower Piping System For Reciprocating Or Screw Chiller Sets224VALVE SELECTION AND SIZING244.1Valve Flow Terms244.1.1Rangeability244.1.2Turndown244.1.3Tight Shut Off / Close Off244.2Valve Ratings254.2.1Flow Coefficient Or Capacity Index254.2.2Valve Body Pressure Rating (Nominal)254.2.3Valve / Actuator Close Off Rating For Two Way Valves264.2.4Valve / Actuator Close Off Rating For Three Way Valves264.2.5Critical Pressure Drop264.3Valve Types274.3.1General Overview274.3.2Three Way Mixing Valves284.3.3Three Way Diverting Valves284.3.4Two Way Single Seated Valves294.3.5Two Way Double Seated Valves304.3.6Butterfly Valves304.4Seat And Plug Arrangements For Honeywell Valves314.4.1General Overview314.4.2Quick Opening Valves324.4.3Linear Valves334.4.4Equal Percentage Valves345CONSIDERATIONS IN SELECTING VALVES.365.1Overall System Operating Pressure365.2Close Off Rating Of Actuator / Valve Assembly376FORMULAE FOR SIZING CONTROL VALVES.386.1Imperial Formulae For CV Rated Valves386.2Metric Formulae For KVs Rated Valves387STANDARD RATINGS FOR HONEYWELL VALVES.397.1Standard Honeywell CV Rated Control Valves397.2Standard Honeywell KVs Rated Control Valves398SIZING OF TWO WAY CONTROL VALVES.408.1General Overview408.2Selection Of Control Valves418.3Typical Control Valve Schedule428.4Alternate Method Of Determining Valve Pressure Drop439SIZING OF THREE WAY CONTROL VALVES449.1General Overview449.2Selection Of Control Valves449.3Alternate Method Of Determining Valve Pressure Drop4410SIZING OF STEAM VALVES.4510.1General Overview4510.2Required Input Data4510.3Formulae For Determining Size Of Steam Valve4510.3.1To Determine Pressure Drop Across The Valve4610.3.2To Determine The Critical Pressure Drop4610.3.3To Determine “Pout”4610.3.4Steam Traps4610.4Control Valves For Use In Steam Services4711GENERAL DATA AND INFORMATION4811.1Centrifugal Pumps And Fans4811.2Formulae For Determining Kilo Watts501 VALVE SELECTION FOR SUCCESSFUL CONTROL1.1 Course OverviewThis course provides a review of the following areas of a standard air conditioning system as applied to a typical commercial building or campus: - Fluid flow basics as applicable to the HVAC industry. Pump characteristics. Fluid thermodynamics. Valve types. Pumping system types.These characteristics TOGETHER determine the appropriate valve selection criteria.A correctly sized control valve and actuator in a typical HVAC system results in: - The lowest initial capital cost device for the required application. A unit which will control the process variable within the specified limits subject to the performance of the overall mechanical plant. A unit which will result in the minimum amount of energy use for that particular control loop while still maintaining the required specified limits.An automatic control system has three major GAIN components: - The controller gain, which is adjustable. The gain of the process itself, which is usually not known and essentially non-adjustable. The gain of the final control device, in this case the valve, which depends on the selection of the unit.The total loop gain of the overall control system is the product of the above three individual gain components.For a control system to be stable it must have a TOTAL loop gain of LESS THAN ONE.Hence if the control valve is incorrectly oversized then the total loop gain can only be made less than one by a corresponding decrease in controller gain. This is achieved by means of increasing the proportional band or throttling range of the controller which results in a corresponding increase in offset of the process variable from the required set point.The ideal valve selection produces a VALVE gain of ONE.Should the input into the valve actuator change by 20% and the output from the coil or process change by 30% then the gain is 30 / 20 = 1.5.For a valve to have a gain of one it is sized and selected so that when it is driven to 100% open it will pass the required design flow rate value. For all actuator inputs below 100% the valve will pass a flow rate value less than design.1. EXPLAIN AND DRAW SINE WAVE FOR LOOP GAIN 1,GAIN = 1 AND GAIN 1PTOGAIN = 100 / PBOR SENSOR RANGE / TR1.2 Aims And Objectives Of The CourseUpon completion of this course the attendee will be able to: - Plot the system curve on the appropriate pump curve given details on the index run pressure difference. Identify the characteristics of a constant volume / variable temperature chilled water system. Identify the characteristics of a variable volume / constant temperaturechilled water system. Select the appropriate valve type for a specific application in the HVAC system. Select the appropriate actuator for the required valve close off pressure. Select valve sizes for an application to minimise the valve gain. Select a Honeywell preferred valve and actuator assembly for the application.1.3 Course AgendaTIMETOPIC8-30 to 8-45Opening Exercise For Students8-45 to 9-30Basics Of Fluid Flow9-30 to 10-00Characteristics Of Pumps10-00 to 10-15Coffee Break10-15 to 12-00Pumping Systems12-00 to 12-45Lunch12-45 to 1-15Valve Types1-15 to 1-45Characteristics Of Valves And Actuators1-45 to 2-15Application Of Different Valve Types2-15 to 2-30Coffee Break2-30 to 4-00Valve Selection For Normal HVAC Application4-00 to 4-15Valve Selection For Steam Application4-15 to 5-00Practical Workshop On Sizing Control Valves1.4 Appendix To Course Notes Details On Preferred Honeywell Valve And Actuator Assemblies.2 BASICS OF FLUID FLOWA fluid is defined as any LIQUID or GAS.All fluids, at ordinary temperatures and pressures as employed in HVAC systems, behave in a similar manner in regards to: - FRICTION LOSS. FLOW PROFILE. FLOW CHARACTERISTICS.FRICTION is defined as the energy dissipated as heat in overcoming the viscous forces generated in a moving fluid. Friction is normally identified as a PRESSURE or HEAD loss in a system.The relationship follows a square law, where PRESSURE LOSS is proportional to the VELOCITY SQUARED.This is shown in the pressure versus velocity diagram below for an increasing volume in a given pipe size: -The flow profile of a fluid in a pipe is largely dependent on the geometry of the pipe preceding the point under observation. If the pipe is straight for many pipe diameters, then the velocity profile ranges from ZERO at the pipe wall, to a MAXIMUM value at the pipe centre.If the preceding section of pipe has a hand valve, multiple bends, and or tee pieces, then the velocity profile may be rotating within the pipe, or some distorted flow pattern not concentric with the pipe.In general the flow profile does not change the valve selection, BUT it does change the noise generated within the valve.Noise in valves is not normally a problem at system pressure differences less than about 300 kPa but increased turbulence before the valve can increase the generation of noise.Differential pressures in excess of 300 kPa across the control valve will cause excessive fluid velocity to pass through the valve plug and seat assembly. This flow rate increases until the critical pressure drop is reached after which any further increase is dissipated as noise and cavitation rather than an increase in flow rate.Over time the noise and cavitation can destroy the valve and adjacent piping components.Fluid flow in a pipe has two distinct characteristics: - LAMINAR flow where the individual particles essentially travel in straight lines. TURBULENT flow where the individual particles are moving in random directions while moving collectively along the pipe.The Reynolds number for flow systems is a dimensionless number that is indicative of the flow profile in the pipe. At Reynolds number less than 2000 the flow is considered to be LAMINAR while at Reynolds number greater than 3000 the flow is TURBULENT.The range 2000 to 3000 is referred to as the transition range.Pipe sizes in HVAC systems are normally selected in the turbulent range at 100% flow but often fall in the laminar range at light load flow conditions.Although the actual flow characteristic has negligible effect on the valve selection for HVAC applications the general observation is that the valve size will be significantly smaller than the pipe size, and to a greater extent where the pipe flow is laminar.The exceptions occur in such applications as chiller discharge high limit flow control valves or cooling tower bypass valve which are both normally line size.2.1 Characteristics Of Pumps Pumps used in the HVAC industry are almost always Centrifugal Pumps.There are other types of pumps, particularly positive displacement types, but these are not used in HVAC.A typical centrifugal pump has a head (pressure difference), versus flow curve that has two limiting points. Maximum head / Zero flow, and Zero head / Maximum flow.2.1.1 Typical Centrifugal Pump Curve Combined With A System CurveThis curve shows the crossover point of the pump and system curves which is the overall system operating point for this combination.A similar graph can have the individual sections of the overall system, i.e. pipe work and coil combined to depict the pressure difference across the coil / valve at various flow rates.This depicts that the pressure difference between the coil loss and system pressure, representing the sum of pipe friction plus pressure across the valve, varies from a high value at 100% flow through the coil, to 100% of design operating pressure at zero flow through the coil.THUS the valve is required to have a close-off capability equal to the MAXIMUM system pressure difference. (i.e. NO pipe friction and NO coil loss )The various pressure differences are further clarified and detailed in the following graph and for purely clarification purposes the “System Curve” has been removed.1 = Pressure drop across coils and piping at the design flow rate.2 = Pressure drop across control valves at the design flow rate.1a = Pressure drop across coils and piping at minimum flow rate.2a = Pressure drop across the control valves at minimum flow rate.There are two distinct centrifugal pump types. Prior to 1970 most pumps used in HVAC systems were of the STEEP curve type. This means that the high efficiency part of the curve occurred where the flow change is small for large changes in pressure.Since then the FLAT curve pump has predominated. This type has the high efficiency part of the curve occurring where the flow changes dramatically for small changes in pressure.The “High Efficiency” part of the pump curve is defined as the point on the curve where maximum flow occurs at maximum head.There are potential problems when the pump has to be replaced in a system installed prior to 1970. i.e. replacing a steep curve pump with a flat curve pump.In general these can be summarised as:- It is unlikely that the operating point can be matched if a mixture of steep and flat curve pumps operate in parallel. The pressure or flow control system may not work with the new pump.This will always be true where the existing system has a proportional only differential pressure control system.Another problem that has developed with the advent of flat curve pumps is that the motor may not be selected as Non Overloading, whereas the steep curve pump automatically fall into the Non Overloading condition.This means that the control system must be chosen to avoid any possibility of overloading the motor and this can be achieved by means of a high limit flow control function.1. HAND OUT A COPY OF THE CURVE AND DISCUSS.2. ALSO DISCUSS PUMP AFFINITY LAWS AND PUMPING EFFICIENCIES. PUMP ELECT W = flow x pressure / efficiency3 PUMPING SYSTEMSThe main relationship that is relevant to all HVAC water systems is that of heat, flow, and temperature and these are expressed in the formulae: -K.Watts =Litres / SecondXTemp. DifferenceX4.187Where: - The actual flow rate is in Litres / Second. The temperature difference is the difference between the flow and return water temperatures. The constant 4.187 is the Specific Heat Capacity of Water at 14 C.This actual flow rate and temperature difference identifies the two basic variables of Volume and Temperature.All chilled and heating water pumping systems used in HVAC applications fall into one of the following two categories:- Constant volume and variable temperature. Variable volume and constant temperature.Any chilled water system can be divided into the two segments: - The chiller segment which is always constant volume and variable temperature. The load segment which is selected to suit the application.The options available for the load segment are identified by the valve type installed which can be either 2 way throttling valves or 3 way mixing valves.The 2 way throttling valve system will result in a variable volume constant temperature load segment while the 3 way mixing valve system will result in a constant volume variable temperature system.In BOTH cases the flow rate through the air handling unit chilled water coil is variable volume.Heating water systems are normally constant volume, variable temperature at the boiler, but variable volume systems are not unknown. Again the load system may be either 2 way throttling or 3 way mixing valve types.The different types of pumping systems are detailed in the following diagrams: -3.1 Single Chiller Set With 3 Way ValvesThis arrangement provides a constant volume variable temperature system for both the chiller and load segments of the installation.1. EXPLAIN CONSTRAINTS OF CONSTANT VOLUME SYSTEM.CANNOT ADD ANY ADDITIONAL UNITS WITHOUT CHANGING PUMP AND CHILLER.2. IF PARALLEL CHILLERS ARE USED THEN BOTH SETS MUST RUN IF ANY ONE VALVE OPENS MORE THAN 50%.3.2 Single Chiller Set With 2 Way ValvesThis arrangement provides a constant volume variable temperature system for the chiller segment and a variable volume constant temperature system for the load segment of the installation.1. EXPLAIN PURPOSE OF BYPASS VALVE.2. EXPLAIN OPERATION OF TYPICAL COIL AND WHY LOAD SEGMENT IS CONSTANT TEMPERATURE.3. TALK ABOUT AND EXPLAIN “INDEX” UNIT.3.3 Multiple Chiller Sets With 2 Way ValvesThis arrangement provides a constant volume variable temperature system for the chiller segment and a variable volume constant temperature system for the load segment of the installation. It should be noted that the flow rate or volume within the chiller segment of the system will be changed in steps as the chiller sets are staged On and Off1. FLOW RATE IN CHILLER SEGMENT CHANGED IN STEPS.2. CHILLER SETS USUALLY SELECTED AS A “BASE” LOAD RECIPROCATING OR SCREW SET AND THEN 2 EQUAL SIZED CENTRIFUGAL SETS.3.4 Primary / Secondary System With Multiple Chiller Sets And 2 Way Valves On The Field UnitsThis arrangement provides a constant volume variable temperature system for the chiller segment (Primary) and a variable volume constant temperature system for the load segment (Secondary) of the installation. It should be noted that the flow rate or volume within the chiller segment (Primary) of the system will be changed in steps as the chiller sets are staged On and Off.1. EXPLAIN PRIMARY PUMPING SYSTEM (ONLY SMALL HEAD REQUIRED).2. EXPLAIN SECONDARY PUMPING SYSTEM AND CONTROL OF VARIABLE SPEED SECONDARY PUMP.3. EXPLAIN IMPORTANCE OF HEADER ARRANGEMENT FOR CORRECT TEMPERATURE DISTRIBUTION.3.5 Reverse Return Piping Systems3.5.1 Single Chiller System With Two Way Valves3.5.2 Multiple Chiller System With Two Way ValvesThe reverse return piping system is usually designed to have the SAME pressure difference across each coil / valve combination.This is achieved by making the pipe route length from the primary plant (Boiler or Chiller) to EACH coil unit and then back to the plant equal in length and frictional loss.These systems are not common due to the higher installation costs and are normally only used where the site is arranged in such a way that the pipe work is installed in a square, rectangle or circle arrangement and the individual coil flow rates and pressure drops are similar.Reverse return piping systems cannot be used