HVAC Controls & Control Loops |
Categories of HVAC & Refrigeration Control Systems
1.Electric Control
-Extra low voltage
-Low voltage
-High voltage
2.Pneumatic Control
3.Electronic Control
All Control Systems Require The Following Components |
Energy source |
Air
Electrical potential (AC or DC) |
Controller |
Press/ Temp/ Humidity/ Fluid Flow |
Transmission Lines |
Conductors/ Tubing |
Operator (Actuator) |
Two Position (on or off)
Proportional (Variable Positions) |
Controlled Device |
Normally open
Normally closed |
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| HVAC Control Terminology |
Controller: Is a instrument that measures variations in the controlled variable and transmits energy to produce an appropriate corrective action.
Controlled Variable: Is the variable such as temp/ humidity/ or pressure which is controlled or measured (by the operator).
Controlled Medium: Is the material in which a controlled variable such as temp is controlled (air or water).
Control Point: Is the actual value of the controlled variable which the controlled is causing to be maintained.
Set Point: Is the point at which the controller is set to maintain the correct controlled variable value.
Measuring Elements: Are those elements which sense and communicate the value of the controlled variable (temp/ humidity/ or pressure sensors).
Operator(Actuator): Is a device which responses to the output of the controller and positions the controlled device.
Controlled Device: Is the final element which is activated by the operator in response to the output from the controller.
Normally Closed: Applies to the condition of the controlled device which closes when all operating force is removed.
Normally Open: Applies to the condition of the controlled device which opens when all operating force is removed.
Control Agent: Is the source of energy such as steam/ Hot water/ air… which is being regulated by the controlled device
Two Position: Controlled Device can be positioned only to a maximum or minimum position with no intermediate steps.
Proportional: Applies to the controller which changes its output in proportion to the amount of change in the controlled variable
Fail Safe: Is the term applied to “dead man” controls and to automatic controls and controlled devise that will move to a required position when the source of energy is removed
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| HVAC Control Terminology And Systems |
All system control loops have a pattern which recognizes the relationship between cause and affect
Cause and Affect: Is simply the intemperance of one thing upon another (this is communally called “feedback”). Feedback is what makes automatic control or self regulation possible.
2 Types of Control Loops
Open loop: No feedback
Closed loop: Feedback
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| Energy Sources for HVAC/ System Controls |
Pneumatic Source |
Air Compressor |
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-Tank press. 100 PSIG(main)
-Air to controller: 15 to 20 PSIG(supply) |
Electrical Potential Source |
A/C or D/C Voltage |
AC Voltage |
-Extra low: 30 and below
-Low: 30 plus to 750
-High: 750 volts plus |
Transformers (AC) |
Step up
Step down
Delta star line control |
Electronic Potential Source |
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DC Voltage |
Pure rectified |
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| Controllers |
Controller: |
Is an instrument that measures variations in controlled variable and transmits a signal to produce a corrective action |
Type: |
Electric/ Electronic |
Action: |
-Two position (simple electrical pole switch)
-Floating (SPDT electrical contact with neutral zone)
-Proportional (Output varies with change in variable using a variable resistor) |
Type: |
Pneumatic |
Action: |
Proportional DA or RA (output varies with change in variable) |
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| Controller Configurations |
T-stat
Humidistat
Press-stat
*Note: The above can be self contained or receiver controller arranged
-Self contained sensor is built in a bulb or remote bulb
-Receiver Controller transmitter is remote from main body
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| Sensing Elements for Controllers |
Pressure Sensing Elements
-Bourdon Tube and spring (press. gauge)
-Bellows (press. switch)
Temp. Sensing Elements
-1: Bi-metal
-2: Bellows
-3: Electrical resistance
-Bi-Metal: Is 2 dissimilar elements such as brass (high) and nickel (low expansion or invar) are placed together to obtain the desired effect. Types: Strip, u-shaped, helix, cylindrical, sinuous, ridge stem.
-Low expansion = normally silver in color
-High expansion = normally black strips
-Bellows: Used with coiled capillary or bulb and capillary filled with a volatile fluid
-3 Types of capillary sensors: Averaging, immersion and insertion.
-Electrical Resistance: Positive/ negative temp co-efficient sensor
Humidity Sensing Elements
Definition: This type of sensor uses a material which will expand or contract in reaction to changes in humidity of the controlled medium. Any material that responds to changes in humidity is called a hygroscopic material.
Types Of Sensors: |
Wood (Bi-wood) |
Cedar(low) and Yew(high expansion rate) |
Hair |
Human hair (contracts on decrease) |
Gold Leaf |
Current across the metal changes in response to humidity |
Membrane |
(Lab synthetic material) Cellulose acetate butyrate |
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| Operators and Controlled Devices |
Operator: Is a device that receives the signal from the controller and reforms the corrective action on the controlled device.
Controlled Device: Is the component that responds to the change made from the operator
Types of Operators or Actuators
-Damper Motor
-Solenoid Coil
-Valve Motor
Types of Controlled Devices
-Solenoid Valve
-Damper
-Water or Fluid Valve
Basic Components Of All Control Loops |
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| Pneumatic Additional Terminology |
Direct Acting: Applies to a pneumatic controller when an increase in controlled variable results in an increase in control air (output)
Reverse Acting: Applies to a pneumatic controller when an increase in controlled variable results in an decrease in control air (output)
Control Pressure: Is the output air pressure of a controller which will vary as the controlled variable changes
Supply Pressure: Is the air pressure supplied to the controller normally between 15 to 25 PSIG.
Storage Tank Pressure: Is the pressure maintained with in the air tank. Normally at 80 PSIG
Sensitivity: In a pneumatic controller the relationship between changes in the controlled variable and the controlled pressure.
-Example: PSI/PSI or PSI/ ºF or PSI/ RH %
-Example: The temp of the controlled variable increases by 1ºF. The output pressure from the controller would increase by 3 PSIG. Therefore the sensitivity would be 3PSI / ºF.
Electromagnetic Relay: A device used to control pneumatic pressure systems by an electrical means (EP switch). For example, an electric powered solenoid.
Pneumatic Electric Relay: A device used to control electric systems by an pneumatic means (PE switch). For example, an pneumatic powered solenoid.
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| Basic Series 90 Low Voltage Control Loop |
Major components of a series 90 control loop
-Controller
-Transmission lines
-Operator or actuator
-Controlled Device
A series 90 control circuit provides for modulating control and can be applied to:
-Motorized valves
-Motorized Dampers
-Sequence of Controlled Devices
Controller Types
-Extra low voltage 30 volts and under (is 24 volts)
-Single or dual potentiometers( 135 ohm control)
-With or with out remote sensors
-Press/ temp/ or humidity
Actuator Types
Master motor (2 potentiometer)(135 ohm(Feedback)
Slave motor (1 potentiometer)(135 ohm(Feedback)
*Either of the above actuators are operated on extra low voltage and are equipment with a reversible drive motor which may have a max shaft rotation of 90º or 160º depending on the model
*Shaft rotation is limited by the use of a limit switch built into the motor assembly
Major Components Of a series 90 Actuator
-Feedback potentiometer
-Balance relay (activates motor rotation CW or CCW)
-Limit switches (Limits Travel to 160º)
-Reversible motor
-Wiring connections |
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