1. Air-conditioning and Refrigeration Control -1 Instructor: Eng. Raad Alsaleh Grading system: Exam 1 - 15 points Exam 2 - 15 points Att. - 10 points Hw - 10 points Lab. - 20 points Final - 30 points Total - 100 points

2. Course Contents: I. Introduction II. Control Fundamentals A.Feedback control systems. B.System representation. C.Modes of automatic control. D.Performance requirements of control systems. E.Classification of control systems. III. Components of control circuits A.Controlled Devices. B.Sensors. C.Controllers. D.Auxiliary control devices.

3. I. Introduction: A refrigeration system can be built with only 4 essential components: Compressor Condenser Evaporator Expansion Valve But for ease, economy and safety of operation, and to assist the maintenance function, system control must be fitted

4. Purpose of Control System 1.Provide automatic operation; avoid the cost of an attendant labor force. 2.Maintain the controlled conditions closer than could be achieved by manual operation. 3.Provide maximum efficiency and economy of operation. 4.Ensure safe operation at all times.

5. II. Control Fundamentals II.1 Feed Back Control System The position of temperature dial sets the desired Temperature (input signal) Set Point The actual Temperature of the system is the Controlled Variable (The quantity which being controlled).

6. Sensor measures the controlled variable and convey values to the Controller. Controller compares the actual temperature in order to measure the Error This Error signal is the actuating signal which is then sent back to the unit in order to correct the temperature.

7. Examples of Controller Thermostat Humidistat Pressure Controller

8. The Controlled Device reacts to signals received from the controller to vary the flow of the control agent. Exampled of Controlled Device Valve Damper Relay

9. Control Agent is the medium manipulated by the controlled device. It may be Air flowing through damper. Gas, Steam, Water flowing through a valve. Current flowing through a relay.

10. The Control Planet is the air-conditioning apparatus being controlled, it reacts to the output of the control agent and affects the change in the controlled variable. It may include a Coil Fan Duct

12. Examples of controlled Variable Temperature Humidity Pressure

13. II.2 System Representation The mathematical relationship of control systems are usually represented by Block Diagrams These diagrams have the advantage of indicating more realistically the actual processes which are taking place. In addition it is easy to form the overall block diagrams for one entire system by merely combining the block diagrams for each component or part of the system.

14. A controller subtracts the feedback signal from the set- point (r). For the case in which the controlled variable (c) is fed back directly, the signal coming from the controller is (r-c), which is equal to the actuating signal (e). The mathematical relationship for this operation is e = r-c

15. Circle is the symbol which is used to indicate a summing operation.

16. The relationship between the actuating signal (e), which enters the control element, and the controlled variable (c) which is the output of the control, is expressed by the equation: C = Ge Where (G) represents the operation of control element ( Transfer Function)

17. Box or Square is the symbol which is used to indicate a multiplication operation.

18. The complete block diagram for the feedback control system

19. For more general representation of feedback control system, the signal which is fed back is b = Hc Where (H) represents the operation of feedback control.

20. The control loop of discharge air temperature can be represented in the form of block diagram as follows

21. II.3 MODES OF AUTOMATIC CONTROL Feedback control systems are most frequently classified by the types of corrective action the controller is programmed to take after it senses a deviation between controlled variable and the desired set point which are: 1.Two-position action control. 2.Timed two-position action control. 3.Floating action control. 4.Proportional control.

22. 1.Two-position action control It is also referred to as ON-OFF Control. This type of control provides for only two positions of the controlled device. There are no intermediate positions, or degrees of motion, between the two extremes of full ON and full OFF. Two-position control is the simplest form of automatic regulation, but it has definite disadvantage that it is applicable only to small systems.

23. The Fig. below illustrates a simple application of two-position control. The difference between the Full ON and Full OFF is called a "Differential”.

24. 2.Timed two-position action control It is common variation of two-position action often employed in room thermostats to reduce operating differential. In heating thermostats, a heater element is energized during the ON periods; prematurely shortening the ON time as the heater falsely warms the thermostat (heat anticipation). The same anticipating action can be obtained in cooling thermostats by energizing a heater during thermostat OFF periods.

26. 3.Floating action control Lite modulating control, floating control differs from the two methods above in that the actuator, such as a damper motor or control valve, may assume any position between its maximum and minimum points. It is called floating because the actuator comes to rest when the controller is floating between its high and low operating points.

28. 3.Floating action control That is, as the controlled conditions fluctuates, the damper motor or valve motor is put into motion in the direction which counteracts the change on more till the controller indicates that corrections has been accomplished. Thus the controlled device stops only when the controlled variable stabilizers between comparatively narrow limits.

29. 3.Floating action control Normally, floating control is used only in those applications where there is a little lag between a change in the actuating control and the sensing of the result of that change by the controller. Lacking this means of control, there would be marked overshooting and an obvious control limit.

30. 4.Proportional control It is also called modulating control. Like floating control, proportional control provides for many positions of the controlled device between its maximum and minimum. But unlike the floating control, the proportional controller stops the controlled device as soon as it reaches a position corresponding to the new demand measured by the controller. That is, for each movement of the controller there is a proportional amount of movement on the controlled device.

32. 4.Proportional control Throttling Range: is the amount of change in the controlled variable to run the controlled device from one end to the other. Control Point: is the actual value of the controlled variable. If the control point lies within the throttling range the system in control. When it exceeds the throttling range the system out of control. Off set(error): is the difference between the set point and the control point..

33. 4.Proportional control Proportional control has three modes: a.Proportional: The mathematical expression is O= A + Kp e Where: O - Controller output A - Controller output with no error (constant) e - The error, difference between the set point and the control point. Kp - Proportional gain constant..

34. a.Proportional

36. If the Kp then controller response system stability If the Kp then controller response system stability

37. a.Proportional

38. b.Proportional Plus Integral The mathematical expression is O = A + Kp e + Ki edt where Ki = integral gain constant. This means that the output of the controller is now affected by the error signal integrated over time and multiplied by (Ki). (Ki) is a function of time and it equal Ki = x/t Where x = number of times variable sampled per unit time.

39. b.Proportional Plus Integral The effect of this term is that the controller output will continue to change until the offset will be eliminated.

40. c.Derivative For derivative control mode, another term is added O= A + Kp e + Ki edt + Kd de/dt Where kd - derivative gain constant Adding the derivative term gives faster response and greater stability. Most HVAC control loops perform satisfactorily with (PI) without the need for adding the derivative term. Because most HVAC systems have a relatively slow response to changes in controller output, the use of derivative mode may tend to Over Control.

42. II.4 Performance requirements of control systems A.Stability of control system. B.Accurate measurements. C.Rapid system response. D.Proper space and HVAC design.

43. II. 5 Classification of Control Systems: Control systems can be classified into categories according to the primary source of energy: A.Electric Systems. A.Pneumatic Systems. A.Self-Contained Systems.

44. A.Electric Systems Electric Systems: Electric systems provide control by starting and stopping the flow of electricity or varying the voltage and current. Electronic Systems: The systems use very low voltages (24 V or less) and currents for sensing and transmission, with amplification by electronic circuits for operation of controlled devices.

45. B.Pneumatic Systems Pneumatic Systems: These systems use low-pressure compressed air. Changes in output pressure from the controller will cause a corresponding position change at the controlled device. Hydraulic systems: These are similar in principle to pneumatic systems but use a liquid or gas rather than air. Fluidic Systems: These uses air or gas and are similar in operating principles to electronic as well as pneumatic systems.

46. C.Self-Contained Systems This type of system incorporates sensor, controller and controlled device in a single package. No external power is required. Energy needed by the controlled device is provided by the reaction of sensor with the controlled variable.

47. III. Control Components While control components may be classified in several ways, one is by their function within a complete control system. They are: 1.Controlled device, or final control element. 2.Sensing element, that measures changes in controlled variable. 3.Controllers, that do a control action to maintain the desired condition (set point). 4.Auxiliary control components, they are neither sensing elements nor controlled devices or controllers, including Transducers, Relays, Switches.

48. III.1 controlled Devices The controlled devices are most frequently used to regulate or vary the flow of steam, water, or air within the HVAC system. They are of two types: A.Valves: to regulate water and steam flow. B.Dampers: to control air flow.

49. A.Valves An automatic valve is considered as a variable orifice positioned by an electric or pneumatic Operators in response to Signals from the Controller.

50. A.Valves

51. Types of automatic valves a. Single-Seated Valve: Is designed for tight shutoff.

52. Types of automatic valves b. Double-seated Valve: Is designed so that the media pressure acting against the valve disc is essentially balanced reducing the operator force required.

53. Types of automatic valves c.Three Way Mixing: Has two inlet and one outlet, and used to mix two fluids entering through the inlet and leaving through the outlet according to the position of the valve stem.

54. Types of automatic valves include d.Three Way Diverting: Has one inlet and two outlets, and used to divert the flow to either of the outlets.

55. Valve Characteristics

56. The change in: Pressure drop. Flow in relation to stroke. Travel of valve stem. Is a function of valve plug design?

57. Types of valve plugs a.Quick Opening: These are two position valves, where maximum flow is approached rapidly as the valve begins to open.

58. Types of valve plugs b. Linear or V-Port: Opening and flow are related in direct proportion.

59. Types of valve plugs c. Equaled Percentage: Each equal increment of opening increases the flow by an equal percentage over the previous valve.

60. Flow characteristics

61. Valve Operators a.Solenoid operator : Consists of a magnetic coil operating movable plunger.

62. Valve Operators b. Electric Motor operator :

63. Valve Operators c. Pneumatic Operator :

64. B.Dampers Automatic dampers are used in air- conditioning and ventilation systems to control airflow. They may be used for modulating control or a two- position controller.

65. B.Dampers Two damper arrangements are used for air handling system flow control. Parallel-blade dampers - for two position control. Opposed-blade dampers - for modulating control.

67. Damper Operators: Like valve operators, damper operators are available using either electricity or compressed air as a power source.

68. Damper Operators: Dampers operators are mounted in several different ways, depending on : Damper size Power required to move the dampers

69. Damper Operators: Dampers are mounted : Mounted on the damper frame. Mounted outside the duct, and connected to one of the blades by a crank arm.

70. III.2 – Sensors A sensor is the component in the control system that measures the value of the controlled variable. A change in the controlled variable produces a change (physical or electrical) of the primary sensing element, which is then available for translation or amplification by mechanical or electrical signal.

71. III.2 – Sensors When the sensor uses conversion from one form of the energy (Mechanical or thermal) to another (electrical), the device is known as a transducer, such as a thermistor.

72. III.2 – Sensors In selecting sensors the following elements should be considered: 1.Operating Range of the Controlled Variable. 2.Compatibility of the Controller Input. 3.Set Point Accuracy and Consistency. 4.Response Time. 5.Control Agent Properties. 6.Ambient Environment Characteristics.

73. III.2 – Sensors A. Temperature Sensors: Temperature - sensing element are of 3 categories: 1. Those that use a change in relative dimension due to differences in thermal expansion. (Thermal to Mechanical). 2.Those that use a change in state of a vapor or liquid- filled bellows. (Thermal to Pneumatic). 3. Those that use a change in some electrical property. (Thermal to Electrical).

74. III.2 – Sensors A. Temperature Sensors: 1.Bimetal element: is composed of two thin strips of dissimilar metals fused together.

75. III.2 – Sensors A. Temperature Sensors: 2. A Sealed Bellows : element is vapor, gas, or liquid- filled Bellows after being evacuated of air.

76. III.2 – Sensors A. Temperature Sensors: 3. Remote bulb: element is a sealed diaphragm to which a bulb or capsule is attached by means of a capillary tube.

77. III.2 – Sensors A. Temperature Sensors: 4. A Thermistor : (resistance temperature detector RTD) makes use of the change of electrical resistance of a semiconductor material for a representative change in temperature.

78. III.2 – Sensors A. Temperature Sensors: 5. A Thermocouple: is formed by the junction of two wires of dissimilar metals. The constant temperature junction is called the Cold junction.

79. III.2 – Sensors B. Humidity Sensors: Hygrometers 1.Mechanical Hygrometers: operates on the principle that a hygroscopic material, when exposed to water vapor, retains moisture and expands. Hygroscopic materials are: 1.Human hair 2.Wood fibers 3.Cotton 4.Nylon

80. III.2 – Sensors B. Humidity Sensors: Hygrometers 2. Electronic Hygrometers: can be of the resistance or capacitance type. It uses a conductive grid coated with hygroscopic substance.

81. III.2 – Sensors C. Pressure Sensor: 1.Bourdon tube mechanism: A pneumatic pressure transmitter converts a change in absolute gage, or differential pressure to a mechanical motion.

82. III.3 – CONTROLLER: Controllers take the sensor effect (Controlled Variable), compare it with the desired control condition (set point), and regulate an output signal (Error) to cause a control action on the controlled device.

83. III.3 – CONTROLLER: A. Electrical / Electronic Controllers: a. For two-position control: The controller output may be a simple: Electric Contact: Start pump, and valve or damper operator. Single Pole Single Throw (SPST): Start heating or cooling. Single Pole Double Throw (SPDT): For heating-cooling applications. b. For timed two-position control: A heat anticipator is added to SPDT.

85. III.3 – CONTROLLER: A. Electrical / Electronic Controllers: c. For floating control the controller output is an SPDT switching circuit with a neutral zone where neither contact is made. d. For Proportional controller output: Gives continuous or incremental changes in output signal.

86. III.3 – CONTROLLER: B. Indicating or Recording Controllers: a. Indicating Controller: Has a pointer added to the sensing element.

87. III.3 – CONTROLLER: B. Indicating or Recording Controllers: b. Recording Controller: A recording pen added to the sensing element that record on a chart paper.

88. III.3 – CONTROLLER: C. Pneumatic Controllers: Pneumatic Controllers are normally combined with sensing elements with a force or position output to obtain a variable air pressure output. The control action is usually proportional.

89. III.3 – CONTROLLER: C. Pneumatic Controllers: a.Bleed-type (None Relay) : Pneumatic controller uses a restrictor in its air supply and a bleed nozzle.

90. III.3 – CONTROLLER: C. Pneumatic Controllers: b. None bleed (Relay Type ): controller which uses positive movement from the sensor to close or open supply air valve.

91. III.3 – CONTROLLER: C. Pneumatic Controllers: c. Pilot bleed (Relay Type ): Controller which utilizes a reduced - airflow bleed- type pilot circuit combined with amplifying non- bleed relay.

92. III.3 – CONTROLLER: C. Pneumatic Controllers: c. Pilot bleed (Relay Type ):