Instructor Eng Raad Alsaleh Grading system Exam 1 15 points Exam 2 15 points Att 10 points ID: 569229
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Slide1
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 pointsSlide2
Course Contents:
I . Introduction
II. Control
Fundamentals
Feedback control
systems.
System
representation.
Modes of automatic
control.
Performance requirements of control
systems.
Classification of control
systems.
III.
Components of control circuits
Controlled
Devices.
Sensors.
Controllers.
Auxiliary control
devices.Slide3
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
Slide4
Purpose of Control System
Provide automatic operation; avoid the cost of an attendant labor force
.
Maintain the controlled conditions closer than could be achieved by manual operation
.
Provide maximum efficiency and economy of operation.
Ensure safe operation at all times.Slide5
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
).
Slide6
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. Slide7
Examples of Controller
Thermostat
Humidistat
Pressure ControllerSlide8
The
Controlled Device
reacts
to signals received
from the
controller to vary the flow of the control agent.
Exampled of Controlled Device
Valve
Damper
RelaySlide9
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.Slide10
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
DuctSlide11
Set point
Controller
Error
Plant
Feedback
Sensor
Controlled
Variable
Control
device
plantSlide12
Examples of controlled Variable
Temperature
Humidity
PressureSlide13
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.Slide14
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-cSlide15
Circle
is the symbol which is used to indicate a summing operation.
r
e
cSlide16
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
) Slide17
Box or Square
is the symbol which is used to indicate a
multiplication operation.
c
e
GSlide18
The complete block diagram for the feedback control system
r
e
c
G
cSlide19
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.
r
e
b
G
c
HSlide20
The control loop of discharge air temperature can be represented in the form of block diagram as follows
Input Signal
Controlled Device
(Valve)
Set point
G1
c
H4
G3
G2
Controlled Variable
(Temp.)
Process Plant
(Fan)
Sensing Element
(Remote bulb)
Feedback Signal
Controller
(Thermostat)Slide21
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
:
T
wo-position action control.
Timed
two-position action
control.
Floating
action control
.
Proportional control
.Slide22
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
.Slide23
The Fig.
below illustrates
a simple application of two-position control.
The
difference between the Full ON and Full OFF is called a "Differential”.Slide24
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. Slide25Slide26
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. Slide27Slide28
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
.Slide29
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
.Slide30
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
.Slide31Slide32
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.
.Slide33
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.
.Slide34
a. Proportional
The proportional gain is related inversely to the throttling range. For example, in a pneumatic temperature controller, the output ranges from 3 to 13 psi (10 psi range).
If the throttling is 10 degrees,
then
.
=
1
(
1 PSI per degree)
Slide35
a. Proportional
If the throttling is 4 degrees,
then:
=
2.5
(2.5 PSI per degree)
Slide36
a. Proportional
If the
Kp
then
controller
response
system stability
If the
Kp
then controller
response
system stabilitySlide37
a. ProportionalSlide38
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.
Slide39
b. Proportional Plus Integral
The
effect of this term is that the controller output will continue to change until the offset will be eliminated.Slide40
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
. Slide41Slide42
II.4 Performance requirements of control systems
A. Stability of control system.
B. Accurate measurements.
C. Rapid system response.
Proper
space and
HVAC
design
.Slide43
II. 5 Classification of Control Systems:
Control systems can be classified into categories according to the primary source of energy:
Electric Systems
.
Pneumatic Systems.
Self-Contained Systems.Slide44
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.Slide45
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.
Hyd
r
aul
ic 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.Slide46
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. Slide47
III. Control Components
While control components may be classified in several ways, one is by their function within a complete control system.
They are:
Controlled device,
or final control element
.
Sensing element,
that measures changes in controlled variable.
Controllers
, that
do a control action to maintain the desired condition (set point).
Auxiliary
control components
, they are neither sensing elements nor controlled devices or controllers, including Transducers, Relays, Switches .Slide48
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
:
Valves
: to regulate water and steam flow.
Dampers
: to control air flow.Slide49
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
.Slide50
A. ValvesSlide51
Types
of automatic
valves
a. Single-Seated
Valve:
Is designed for tight shutoff.Slide52
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.Slide53
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.Slide54
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.Slide55
Valve Characteristics
The performance of a valve is expressed in terms of its flow characteristics.
The flow rate through a valve is a function of the pressure drop across the valve according to the formula
:
Where:
V
- Fluid velocity
K
- Constant (function of valve design)
g
- Acceleration due to gravity
h
- Pressure
drop across the valve.
Slide56
Valve Characteristics
The change in:
• Pressure drop.
• Flow in relation to stroke.
• Travel of valve stem.
Is a function of
valve plug
design?Slide57
Types of valve plugs
Quick
Opening
:
These are two position valves, where maximum flow is approached
rapidly as
the valve begins to open.Slide58
Types of valve plugs
b.
Linear
or V-Port
:
Opening and flow are related in direct proportion.Slide59
Types of valve plugs
c.
Equaled
Percentage:
Each equal increment of opening increases the flow by an equal
percentage over
the previous valve.Slide60
Flow characteristicsSlide61
Valve Operators
Solenoid
operator
:
Consists of a magnetic
coil operating
movable plunger
.Slide62
Valve Operators
b. Electric
Motor operator
:
Slide63
Valve Operators
c. Pneumatic
Operator : Slide64
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.Slide65
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.Slide66Slide67
Damper Operators:
Like
valve operators, damper operators are available using either electricity or compressed air as a power source
.Slide68
Damper Operators:
Dampers
operators are mounted in several different ways, depending
on :
D
amper size
P
ower
required to move the dampersSlide69
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. Slide70
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. Slide71
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
.Slide72
III.2 – Sensors
In selecting sensors the following elements should be considered:
Operating
Range of the Controlled Variable.
Compatibility
of the Controller
Input.
Set
Point Accuracy and Consistency.
Response
Time.
Control
Agent
Properties
.
Ambient
Environment Characteristics. Slide73
III.2 – Sensors
A
. Temperature Sensors
:
Temperature - sensing element are of 3 categories
:
Those
that use a change in relative dimension
due to
differences in thermal expansion. (
Thermal to Mechanical
).
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). Slide74
III.2 – Sensors
A
. Temperature Sensors
:
Bimetal element:
is composed of two thin strips of dissimilar metals fused together. Slide75
III.2 – Sensors
A
. Temperature Sensors
:
2
. A Sealed Bellows
:
element
is vapor, gas, or liquid- filled
Bellows after
being evacuated of air. Slide76
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.Slide77
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. Slide78
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
.
Slide79
III.2 – Sensors
B. Humidity
Sensors:
Hygrometers
Mechanical Hygrometers:
operates on the principle that a hygroscopic material, when exposed to water vapor, retains moisture and expands
.
H
ygroscopic
materials are:
Human hair
Wood fibers
Cotton
Nylon Slide80
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. Slide81
III.2 – Sensors
C. Pressure Sensor:
B
ourdon
tube
mechanism:
A pneumatic pressure transmitter converts a change in absolute gage, or differential pressure to a mechanical motion
.Slide82
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. Slide83
III.3 – CONTROLLER:
A. Electrical / Electronic Controllers
:
a.
For
two-position
control:
T
he
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. Slide84Slide85
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:
G
ives
continuous or incremental changes in output signal. Slide86
III.3 – CONTROLLER:
B. Indicating or Recording Controllers:
a
.
Indicating Controller:
H
as
a pointer added to the sensing
element.Slide87
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. Slide88
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
. Slide89
III.3 – CONTROLLER:
C. Pneumatic Controllers:
Bleed-type
(
None Relay) :
Pneumatic controller
uses
a restrictor in its
air supply
and a bleed nozzle. Slide90
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
.Slide91
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
.Slide92
III.3 – CONTROLLER:
C. Pneumatic Controllers:
c
.
Pilot bleed (Relay Type ):