BASIC AUTOMATIC CONTROLS UNIT 16 ADVANCED AUTOMATIC CONTROLS SECTION 3 BASIC AUTOMATIC CONTROLS UNIT 16 ADVANCED AUTOMATIC CONTROLS

UNIT OBJECTIVES Recognize advanced control terminology After studying this unit, the reader should be able to Recognize advanced control terminology Demonstrate control applications Describe electronic control circuits Describe pneumatic control circuits Discuss a control loop Discuss sensitivity or gain in controls

CONTROL APPLICATIONS Older controls were manually activated and operated Thermostats and other simple controls were added to reduce human contact with the equipment Automatic control systems require little human attention Large buildings can be maintained by a small staff of technicians Technicians need to be educated Should be able to calibrate controls Need to be able to interpret blueprints

TYPES OF CONTROL SYSTEMS First control systems started and stopped main system components Newer controls can modulate (start, stop, or adjust) the flow or operation of system components Control systems are divided into control loops Three basic components of a control system The sensor measures changes in conditions The controller sends an output signal The controlled device reacts to the controller

Controller Air gage Diaphragm Main air Branch air Chilled water coil Return air Supply air Main air Hot water coil T-stat

PNEUMATIC CONTROL SYSTEMS Mechanical systems Use air to transmit signals between the sensor and the controlled device Require a clean and dry permanent air supply Air pressure (100 to 125 psig) generated by an air compressor Regulator reduces pressure to working pressure (20 to 30 psig)

Pressure reducing valve Compressor Air drier Service valve Pressure reducing valve Intake filter Belt Motor Tank To system Submicron filter To drain Normally closed test cocks Auto separator filter Auto trap

Air tank Float Water level To drain

Air tank Float Water level To drain

CLEANING AND DRYING CONTROL AIR Prefilter to the compressor Moisture is removed from the air Chilled water coil Desiccant Refrigerated drier (most common) High-density filters Final filtering stage Removes oil vapor and remaining liquid

CONTROL COMPONENTS The pressure on control valves is equal to the air pressure multiplied by the cross sectional area of the diaphragm Pilot positioners use a small volume of air to control a larger volume of air Fluid flow can be modulated Pressure gages used to measure pressure at the controls Can be used to control electric switches or circuits Electric devices can be used to control pneumatic controls

Diaphragm in actuator assembly 20 psig Diaphragm in actuator assembly 20 psig exerted on a 4” diaphragm can result in a pressure of over 250 pounds which can be used to open valves, etc. Area of a 4” round diaphragm = πr2= 3.14 x 22= 3.14 x 4 = 12.56 in2 Total pressure = 20 x 12.56 = 251.2 pounds

0 psig Air cylinder Shaft position when there is 0 psig of air

Shaft position when there is 7.5 psig of air Air cylinder Shaft position when there is 7.5 psig of air

Shaft position when there is 20 psig of air Air cylinder Shaft position when there is 20 psig of air

Air line from thermostat Pilot positioner Main line

Valve is positioned for full flow 20 psig 15 psig from thermostat (calling for cooling) Valve is positioned for full flow 20 psig (main line)

CONTROL COMPONENTS Fluid flow can be modulated Sensitivity or gain Pressure and temperature at fully open and fully closed positions Sensitivity = Δ pressure ÷ Δ temperature High sensitivity will cause the control to “hunt” Low sensitivity will result in noticeable change in space temperature

Valve positioned for full flow through the cooling coil 55 degree return water from cooling coil Bypass line 45 degree supply water to cooling coil

Valve positioned for partial flow through the cooling coil 55 degree return water from cooling coil Bypass line 45 degree supply water to cooling coil

Valve positioned for no flow through the cooling coil 55 degree return water from cooling coil Bypass line 45 degree supply water to cooling coil (no water actually flowing to coil)

DIRECT DIGITAL CONTROLS Referred to as DDC Operate at very low voltages Can be used to open, close, or modulate valves, dampers, motor speeds, etc. Can be controlled from remote locations via computers Corrective actions can be taken from remote locations via computers System parameters can be observed

RESIDENTIAL ELECTRONIC CONTROLS Entire board is evaluated, not the individual board components Use manufacturer’s diagnostic systems Boards are often treated as switches Boards often have diagnostic codes for troubleshooting Thermistors are often used Often equipped with DIP switches Allows control to be configured Allows user to customize system operation Status lights indicate mode of operation

UNIT SUMMARY - 1 Older controls manually activated and operated Automatic control systems require little attention Newer controls modulate fluid flow Control systems are divided into control loops Control systems are made up of the sensor, controller and the controlled device Pneumatic control systems use air to transmit signals between the sensor and the controlled device

UNIT SUMMARY - 2 Pneumatic systems require clean and dry air supply Control air used to position controlled devices Sensitivity = Δ pressure ÷ Δ temperature DDC can be used to open, close, or modulate valves, dampers, motor speeds, and other devices Corrective actions can be taken from remote locations via computers Entire board is evaluated, not the individual board components