楼宇第07次课-CHAPTER-2-Pneumatic-Control-Fundamentals-电课件.ppt

1、CHAPTER 2Pneumatic Control FundamentalsINTRODUCTION This section provides basic information on pneumatic control systems and components commonly used to control equipment in commercial heating and air conditioning applications.The information in this section is of a general nature in order to explai

2、n the fundamentals of pneumatic control.Some terms and references may vary between manufacturers(e.g.,switch port numbers).Pneumatic control systems use compressed air to operate actuators,sensors,relays,and other control equipment.Pneumatic controls differ from other control systems in several ways

3、 with some distinct advantages:Pneumatic equipment is inherently proportional but can provide two-position control when required.Many control sequences and combinations are possible with relatively simple equipment.Pneumatic equipment is suitable where explosion hazards exist.The installed cost of p

4、neumatic controls and materials may be lower,especially where codes require that low voltage electrical wiring for similar electric controls be run in conduit.Quality,properly installed pneumatic equipment is reliable.However,if a pneumatic control system requires troubleshooting or service,most bui

5、lding-maintenance people have the necessary mechanical knowledge.DEFINITIONS Actuator:A mechanical device that operates a final control element(e.g.,valve,damper).Branch line:The air line from a controller to the controlled device.Branch line pressure(BLP):A varying air pressure signal from a contro

6、ller to an actuator carried by the branch line.Can go from zero to full main line pressure.Discharge air:Conditioned air that has passed through a coil.Also,air discharged from a supply duct outlet into a space.Direct acting(DA):A direct-acting thermostat or controller increases the branch line pres

7、sure on an increase in the measured variable and decreases the branch line pressure on a decrease in the variable.A direct-acting actuator extends the shaft on an increase in branch line pressure and retracts the shaft on a decrease in pressure.Main line:The air line from the air supply system to co

8、ntrollers and other devices.Usually plastic or copper tubing.Mixed air:Typically a mixture of outdoor air and return air from the space.Reset changeover:The point at which the compensation effect is reversed in action and changes from summer to winter or vice versa.The percent of compensation effect

9、(authority)may also be changed at the same time.Restrictor:A device in an air line that limits the flow of air.Return air:Air entering an air handling system from the occupied space.Reverse acting(RA):A reverse-acting thermostat or controller decreases the branch line pressure on an increase in the

10、measured variable and increases the branch line pressure on a decrease in the variable.A reverse-acting valve actuator retracts the shaft on an increase in branch line pressure and extends the shaft on a decrease in pressure.Sensor Span:The variation in the sensed media that causes the sensor output

11、 to vary between 3 and 15 psi.ABBREVIATIONS The following port abbreviations are used in drawings of relays and controllers:B Branch C Common E Exhaust M Main O Normally connected*X Normally disconnected*P Pilot(P1 and P2 for dual-pilot relays)S Sensor(S1 and S2 for dual-input controllers)N.C.Normal

12、ly closed N.O.Normally open SYMBOLS BASIC PNEUMATIC CONTROL SYSTEM GENERAL A pneumatic control system is made up of the following elements:Compressed air supply system Main line distribution system Branch lines Sensors Controllers Actuators Final control elements(e.g.,valves,dampers)A basic pneumati

13、c control system consists of an air supply,a controller such as a thermostat,and an actuator positioning a valve or damper(Fig.1).The controller receives air from the main line and regulates its output pressure(branch line pressure)as a function of the temperature,pressure,humidity,or other variable

14、.The branch line pressure from the controller can vary from zero to full mainline pressure.The regulated branch line pressure energizes the actuator,which then assumes a position proportional to the branch line pressure applied.The actuator usually goes through its full stroke as the branch line pre

15、ssure changes from 3 psi to 13 psi.Other pressure ranges are available.In a typical control system,the final control element(a valve or a damper)is selected first because it must produce the desired control results.For example,a system designed to control the flow of water through a coil requires a

16、control valve.The type of valve,however,depends on whether the water is intended for heating or cooling,the water pressure,and the control and flow characteristics required.An actuator is then selected to operate the final control element.A controller and relays complete the system.When all control

17、systemsfor a building are designed,the air supply system can be sized and designed.AIR SUPPLY AND OPERATION The main line air supply is provided by an electrically driven compressor pumping air into a storage tank at high pressure(Fig.2).A pressure switch turns the compressor on and off to maintain

18、the storage tank pressure between fixed limits.The tank stores the air until it is needed by control equipment.The air dryer removes moisture from the air,and the filter removes oil and other impurities.The pressure reducing valve(PRV)typically reduces the pressure to 18 to 22 psi.For two-pressure(d

19、ay/night)systems and for systems designed to change from direct to reverse acting(heating/cooling),the PRV switchesbetween two pressures,such as 13 and 18 psi.The maximum safe air pressure for most pneumatic controls is 25 psi.From the PRV,the air flows through the main line to the controller(in Fig

20、.1,a thermostat)and to other controllers or relays in other parts of the system.The controller positions the actuator.The controller receives air from the main line at a constant pressure and modulates that pressure to provide branch line air at a pressure that varies according to changes in the con

21、trolled variable,as measured by the sensing element.The controller signal(branch line pressure)is transmitted viathe branch line to the controlled device(in Fig.1,a valve actuator).The actuator drives the final control element(valve)to a position proportional to the pressure supplied by the controll

22、er.When the proportional controller changes the air pressure to the actuator,the actuator moves in a direction and distance proportional to the direction and magnitude of the change at the sensing element.RESTRICTOR The restrictor is a basic component of a pneumatic control system and is used in all

23、 controllers.A restrictor is usually a disc with a small hole inserted into an air line to restrict theamount of airflow.The size of the restrictor varies with the application,but can have a hole as small as 0.003 inches.NOZZLE-FLAPPER ASSEMBLY The nozzle-flapper assembly(Fig.3)is the basic mechanis

24、m for controlling air pressure to the branch line.Air supplied to the nozzle escapes between the nozzle opening and the flapper.At a given air supply pressure,the amount of air escaping is determined by how tightly the flapper is held against the nozzle by a sensing element,such as a bimetal.Thus,co

25、ntrolling the tension on the spring also controls the amount of air escaping.Very little air can escape when the flapper is held tightly against the nozzle.PILOT BLEED SYSTEM The pilot bleed system is a means of increasing air capacity as well as reducing system air consumption.The restrictor and no

26、zzle are smaller in a pilot bleed system than in a nozzle flapper system because in a pilot bleed system they supply air only to a capacity amplifier that produces the branch line pressure(Fig.4).The capacity amplifier is a pilot bleed component that maintains the branch line pressure in proportiont

27、o the pilot pressure but provides greater airflow capacity.The pilot pressure from the nozzle enters the pilot chamber of the capacity amplifier.In the state shown in Figure 4,no air enters or leaves the branch chamber.If the pilot pressure from the nozzle is greater than the spring force,the pilot

28、chamber diaphragm is forced down,which opens the feed valve and allows main air into the branch chamber.When the pilot pressure decreases,the pilot chamber diaphragm rises,closing the feed valve.If the pilot chamber diaphragm rises enough,it lifts the bleed valve off the feed valve disc,allowing air

29、 to escape from the branch chamber through the vent,thus decreasing the branch line pressure.Main air is used only when branch line pressure must be increased and to supply the very small amount exhausted through the nozzle.SENSING ELEMENTSBIMETAL A bimetal sensing element is often used in a tempera

30、ture controller to move the flapper.A bimetal consists of two strips of different metals welded together as shown in Figure 6A.Asthe bimetal is heated,the metal with the higher coefficient of expansion expands more than the other metal,and the bimetal warps toward the lower-coefficient metal(Fig.6B)

31、.As the temperature falls,the bimetal warps in the other direction(Fig.6C).A temperature controller consists of a bimetal element linked to a flapper so that a change in temperature changes the position of the flapper.Figure 7 shows a direct-acting thermostat(branch line pressure increases as temper

32、ature increases)in which the branch line pressure change is proportional to the temperature change.An adjustment screw on the spring adjusts the temperature at which the controller operates.If the tension is increased,the temperature must be higher for the bimetal to develop the force necessary to o

33、ppose the spring,lift the flapper,and reduce the branch pressure.RELAYS AND SWITCHES Relays are used in control circuits between controllers and controlled devices to perform a function beyond the capacity of the controllers.Relays typically have diaphragm logic construction(Fig.9)and are used to am

34、plify,reverse,average,select,and switch controller outputs before being sent to valve and damper actuators.The controlling pressure is connected at the pilot port(P),and pressures to be switched are connected at the normally connected port(O)or the normally disconnected port(X).The operating point o

35、f the relay is set by adjusting the spring pressure at the top of the relay.When the pressure at the pilot port reaches the relay operating point,it pushes up on the diaphragm in the control chamber and connects pressure on the normally disconnected port(X)to the common port as shown.If the pilot pr

36、essure falls below the relay set point,the diaphragm moves down,blocks the normally disconnected(X)port,and connects the normally connected port(O)to the common port.AIR SUPPLY EQUIPMENTGENERAL A pneumatic control system requires a supply of clean,dry,compressed air.The air source must be continuous

37、 because many pneumatic sensors,controllers,relays,and other devices bleed air.A typical air supply system includes a compressor,an air dryer,an air filter,a pressure reducing valve,and air tubing to the control system(Fig.10).The following paragraphs describe the compressor,filter,pressure reducing

38、 valves,and air drying techniques.For information on determining the moisture content of compressed air,refer to the General Engineering Data section.AIR COMPRESSOR The air compressor provides the power needed to operate all control devices in the system.The compressor maintains pressure in the stor

39、age tank well above the maximum required in the control system.When the tank pressure goes below a minimum setting(usually 70 to 90 psi),a pressure switch starts the compressor motor.When the tank pressure reaches a high limit setting,the pressure switch stops the motor.A standard tank is typically

40、large enough so that the motor and compressor operate no more than 50 percent of the time,with up to twelve motor starts per hour.Some applications require two compressors or a dual compressor.In a dual compressor,two compressors operate alternately,so wear is spread over both machines,each capable

41、of supplying the average requirements of the system withoutoperating more than half the time.In the event of failure of one compressor,the other assumes the full load.Contamination in the atmosphere requires a compressor intake filter to remove particles that would damage the compressor pump.The fil

42、ter is essential on oil-less compressors because a contaminated inlet air can cause excessive wear on piston rings.The intake filter is usually located in the equipment room with the compressor,but it may be located outdoors if clean outdoor air is available.After the air is compressed,cooling and s

43、ettling actions in the tank condense some of the excess moisture and allow fallout of the larger oil droplets generated by the compressor pump.A high pressure safety relief valve which opens on excessively high tank pressures is also required.A hand valve or automatic trap periodically blows off any

44、 accumulated moisture,oil residue,or other impurities that collect in the bottom of the tank.AIR DRYING TECHNIQUESGENERAL Air should be dry enough to prevent conden-sation.Condensation causes corrosion that can block orifices and valve mechanisms.In addition,dry air improves the ability of filters t

45、o remove oil and dirt.Moisture in compressed air is removed by increasing pressure,decreasing temperature,or both.When air is compressed and cooled below its saturation point,moisture condenses.draining the condensate from the storage tank causes some drying of the air supply,but an air dryer is oft

46、en required.An air dryer is selected according to the amount of moisture in the air and the lowest temperature to which an air line will be exposed.CONDENSING DRYING The two methods of condensing drying are high-pressure drying and refrigerant drying.High-Pressure Drying High-pressure drying may be

47、used when main air piping is kept away from outside walls and chilling equipment.During compression and cooling to ambient temperatures,air gives up moisture which then collects in the bottom of the storage tank.The higher the tank pressure,the greater the amount of moisture that condenses.Maintaini

48、ng a high pressure removes the maximum amount of moisture.The compressor should have a higher operating pressure than is required for air supply purposes only.However,higher air pressure requires more energy to run the compressor.The tank must include a manual drain valve or an automatic trap to con

49、tinually drain off accumulated moisture.With tank pressures of 70 to 90 psi,a dew point of approximately 70F at 20 psi can be obtained.Refrigerant Drying(作业）作业）DESICCANT DRYING A desiccant is a chemical that removes moisture from air.A desiccant dryer is installed between the compressor and the PRV.

50、Dew points below 100F are possible with a desiccant dryer.The desiccant requires about one-third of the process air to regenerate itself,or it may be heated.To regenerate,desiccant dryers may require a larger compressor to produce the needed airflow to supply the control system and the dryer.It may