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

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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.Direct acting(DA):A direct-acting thermostat or controller increases the branch line pressure on an increase in the measured variable and decreases the branch line pressure on a decrease in the variable.A direct-acting actuator extends

6、 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 controllers and other devices.Usually plastic or copper tubing.Mixed air:Typically a mixture of outdoor air and return air from the space.Reset chan

7、geover: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(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 hand

8、ling system from the occupied space.Reverse acting(RA):A reverse-acting thermostat or controller decreases the branch line pressure on an increase in the measured variable and increases the branch line pressure on a decrease in the variable.A reverse-acting valve actuator retracts the shaft on an in

9、crease 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 to vary between 3 and 15 psi.SYMBOLS BASIC PNEUMATIC CONTROL SYSTEM GENERAL A pneumatic control system is made up of the following elements:Compr

10、essed air supply system Main line distribution system Branch lines Sensors Controllers Actuators 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.The branch line pressure from th

11、e 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 pressure changes from 3 psi to 13 ps

12、i.Other pressure ranges are available.for 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 com

13、pressor on and off to maintain 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

14、18 to 22 psi.For two-pressure(day/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.The actuator drives the final control elem

15、ent(valve)to a position proportional to the pressure supplied by the controller.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 res

16、trictor is a basic component of a pneumatic control system and is used in all 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.NO

17、ZZLE-FLAPPER ASSEMBLY The nozzle-flapper assembly(Fig.3)is the basic mechanism 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 flap

18、per is held against the nozzle by a sensing element,such as a bimetal.Thus,controlling 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

19、air capacity as well as reducing system air consumption.The restrictor and nozzle 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

20、 pilot bleed component that maintains the branch line pressure in proportionto 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 t

21、he pilot pressure from the nozzle is greater than the spring force,the pilot 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 diaphra

22、gm rises enough,it lifts the bleed valve off the feed valve disc,allowing air 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

23、.SENSING ELEMENTSBIMETAL A bimetal sensing element is often used in a temperature 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

24、 other metal,and the bimetal warps toward the lower-coefficient metal(Fig.6B).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.Figur

25、e 7 shows a direct-acting thermostat(branch line pressure increases as temperature 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 t

26、emperature must be higher for the bimetal to develop the force necessary to oppose the spring,lift the flapper,and reduce the branch pressure.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 disconn

27、ected port(X).The operating point of 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 c

28、ommon port as shown.If the pilot pressure 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 ai

29、r.The air source must be continuous 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

30、compressor,filter,pressure reducing 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 compre

31、ssor maintains pressure in the storage 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

32、motor.A standard tank is typically 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 spre

33、ad over both machines,each capable 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

34、 damage the compressor pump.The filter 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

35、the air is compressed,cooling and settling 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 automa

36、tic trap periodically blows off any 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 ai

37、r improves the ability of filters to 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

38、 air supply,but an air dryer is often 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

39、Drying High-pressure drying may be 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 o

40、f moisture that condenses.Maintaining 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 drai

41、n valve or an automatic trap to continually 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

42、between the compressor and the PRV.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 c

43、ontrol system and the dryer.It may be necessary to install a desiccant dryer after the refrigerant dryer in applications where the 12F dew point at 20 psi mainline pressure does not prevent condensation in air lines(e.g.,a roof-top unit exposed to severe winters).The desiccant dryer most applicable

44、to control systems uses the adsorbent principle of operation in which porous materials attract water vapor.The water vapor is condensed and held as a liquid in the pores of the material.The drying action continues until the desiccant is saturated.The desiccant is regenerated by removing the moisture

45、 from thepores of the desiccant material.The most common adsorbent desiccant material is silica gel,which adsorbs over 40 percent of its own weight in water and is totally inert.Another type of adsorbent desiccant is the molecular sieve.A desiccant is regenerated either by heating the desiccant mate

46、rial and removing the resulting water vapor from the desiccant chamber or by flushing the desiccant chamber with air at a lower vapor pressure for heatless regeneration.To provide a continuous supply of dry air,a desiccant dryer has two desiccant chambers(Fig.14).While one chamber is being regenerat

47、ed,the other supplies dry air to the system.The cycling is accomplished by two solenoid valves and an electric timer.During one cycle,air passes from the compressor into the left desiccant chamber(A).The air is dried,passes through the check valve(B),and flows out to the PRV in the control system.Si

48、multaneously,some of the dried air passes through the orifice(G)to the right desiccant chamber(E).The air is dry and the desiccant chamber is open to the atmosphere,which reduces the chamber pressure to near atmospheric pressure.Reducing the air pressure lowers the vapor pressure of the air below th

49、at of the desiccant,which allows the moisture to transfer from the desiccant to the air.The timer controls the cycle,which lasts approximately 30 minutes.During the cycle,the desiccant in the left chamber(A)becomes saturated,and the desiccant in the right chamber(E)becomes dry.The timer then reverses the flow by switching both of the solenoid valves(D and H).The desiccant in the right chamber(E)then becomes the drying agent connected to the compressor while the desiccant in the left chamber(A)is dried.