WorksheetsPneumatic Control Systems Quiz
Total questions: 97
Worksheet time: 49mins
What is an actuator in pneumatic control systems?
A device that senses a variable other than the controlled variable and sends a feedback signal.
A mechanical device that operates a final control element such as a valve or damper.
The actual value of the controlled variable.
A varying air pressure signal from a controller to an actuator.
What does a controller do in a pneumatic control system?
It operates the final control element directly.
It adjusts the control point of a given controller to compensate for changes in a measured variable.
It senses the controlled variable or receives an input signal from a remote sensing element, compares the signal with the setpoint, and outputs a control signal (branchline pressure) to an actuator.
It reverses the compensation effect when switching from summer to winter or vice versa.
What does the term "Manipulated variable" refer to in pneumatic control systems?
The air line from the air supply system to controllers
Media or energy controlled to achieve a desired controlled variable condition
A device that detects and measures the controlled variable
The variation in the sensed media that causes the sensor output to vary
What is the purpose of a "Restrictor" in an air line?
It controls the pressure of the air supply
It detects and measures the controlled variable
It limits the flow of air
It provides a setpoint on the controller scale
What is the definition of "Offset" in the context of pneumatic control systems?
The pressure required to change the controlled variable from 3 to 13 psi
The difference between the actual system control point and its controller setpoint under stable operating conditions
The air leaving an air handling system
A device that responds to changes in temperature and outputs a control signal
What does "N.C." stand for in the abbreviations section?
Normally connected
Normally closed
Normally controlled
Normally clear
What does "Sensor Span" mean?
The air line from the air supply system to controllers
The pressure required to change the controlled variable from 3 to 13 psi
The variation in the sensed media that causes the sensor output to vary between 3 and 15 psi.
The value on the controller scale at which the controller is set
What are the main elements that make up a pneumatic control system?
Compressor, filter, and pressure gauge
Main line distribution system, sensors, and actuators
Valves, dampers, and compressed air supply system
Compressed air supply system, main line distribution system, branch lines, sensors, controllers, actuators, and final control elements
What is the function of the controller in a basic pneumatic control system?
It filters the air coming from the main line.
It regulates its output pressure as a function of temperature, pressure, humidity, or other variable.
It stores the air until it is needed by control equipment.
It removes moisture from the air.
What is the typical pressure range that a controller can vary from zero to energize the actuator in a basic pneumatic control system?
0 to 5 psi
3 to 13 psi
13 to 18 psi
18 to 25 psi
What is the maximum safe air pressure for most pneumatic controls?
13 psi
18 psi
25 psi
30 psi
What is the purpose of the air dryer in the compressed air supply system?
To regulate the output pressure
To remove moisture from the air
To store the air until it is needed
To increase the air pressure
According to the text, what is the role of the final control element in a typical control system?
To filter and dry the air supply
To regulate the branchline pressure
To produce the desired control results
To store the air in a tank
What is the function of a restrictor in a pneumatic control system?
To increase the air pressure in the main line
To control the air supply to the nozzle-flapper assembly
To measure the temperature, humidity, and pressure changes
To amplify the pilot pressure in a bleed system
What happens when the flapper is near the nozzle in a nozzle-flapper assembly?
The branch line pressure decreases significantly
The controller positions the actuator at a preset pressure
Very little air can escape, and the branch line pressure is off zero
The spring tension prevents air from moving through the nozzle
How does a "bleed" control because air "bleeds" continuously from the nozzle?
By increasing the branch line pressure to the mainline pressure
By reducing the branch line pressure to zero
By maintaining the branch line pressure at a constant level
By preventing the spring tension from affecting the nozzle
What is the role of the spring in a nozzle-flapper assembly?
It controls the air supply to the branch line
It measures the changes in the sensed variable
It controls the amount of air escaping by holding the flapper against the nozzle
It restricts the air flow to the main line
In a pilot bleed system, what is the function of the capacity amplifier?
To control the direction and magnitude of the change at the sensing element
To supply air only to a pneumatic amplifier that provides a proportional response
To increase the branch line pressure by supplying air to the system
To provide greater airflow capacity to the pilot pressure but not to the main system
What is the primary function of a signal amplifier in a pneumatic system?
To decrease the level of the input signal
To convert the input signal into an electrical signal
To increase the level of the input signal
To maintain the input signal at a constant level
Why are modern nozzle-flapper systems preferred over feed and bleed systems?
Because they are more complicated and efficient
Because they consume more air
Because they consume less air and are more efficient
Because they require constant adjustment
What is the role of the bleed valve in a feed and bleed system?
To supply main air to the branch line
To exhaust air from the branch line
To increase the branch line pressure
To maintain a constant pressure in the feed valve
What happens to a bimetal strip in a sensing element when it is heated?
It contracts and bends towards the metal with a higher coefficient of expansion
It expands and bends towards the metal with a lower coefficient of expansion
It expands and bends towards the metal with a higher coefficient of expansion
It remains straight and unchanged
Which of the following is a reason why feed and bleed systems are no longer popular?
They are too sensitive to changes in temperature
They are more efficient than modern systems
They consume more air than modern systems
They provide a higher level of amplification
What does a temperature controller consist of according to the text?
A bimetal element linked to a diaphragm
A bimetal element linked to a flapper
A capillary tube linked to a flapper
A rod-and-tube sensor linked to a diaphragm
What is the purpose of the setpoint screw in a temperature controller with a bimetal sensing element?
To adjust the temperature at which the bimetal element reacts
To connect the flapper to the spring
To adjust the branchline pressure directly
To measure the temperature in the duct, tank, or pipe
How does the rod-and-tube sensor operate when the temperature rises?
The rod contracts and the tube expands
The rod and tube both expand
The rod expands and the tube contracts
The rod and tube both contract
What is the typical capillary length for remote-bulb temperature sensors used in bleed-type controllers?
Up to 1.5 meters
Up to 2.5 meters
Up to 3.5 meters
Up to 4.5 meters
What is the main difference between the averaging-element sensor and the remote-bulb sensor?
The averaging-element sensor has no bulb
The averaging-element sensor has a shorter capillary
The averaging-element sensor uses a different liquid fill
The averaging-element sensor has a wider bore
At what temperature is the setpoint usually made for the throttling range of a temperature controller in a pneumatic system?
47°F
72°F
70°F
74°F
What is the branchline pressure for the setpoint at 72°F for a direct acting controller?
3 psi
8 psi
10 psi
13 psi
What type of device is used to adjust the throttling range in a pneumatic controller?
A bimetal element
A capillary tube
A lever arm
A diaphragm pad
What is the function of relays in control circuits between controllers and controlled devices?
To measure temperature
To adjust the throttling range
To perform a function beyond the capacity of the controllers
To control a function between the controllers and controlled devices
What is the purpose of a typical switching relay in a pneumatic control system?
To reduce the air pressure in the system
To amplify, reverse, average, select, and switch controller outputs
To filter the air supply
To connect the controlling pressure to the air compressor
What happens when the pressure at the pilot port reaches the relay operating point?
The diaphragm moves up and connects the normally connected port (O) to the common port
The diaphragm moves down, blocking the normally disconnected (X) port, and connects the normally connected port (O) to the common port
The spring pressure is adjusted
The air compressor stops
What does a pneumatic control system require?
A supply of clean, dry, compressed air
A supply of hydraulic fluid
A supply of refrigerant
A supply of natural gas
What components are typically included in an air supply system?
Compressor, filter, air dryer, air filter, pressure reducing valve, and air tubing
Compressor, hydraulic pump, air dryer, air filter, pressure increasing valve, and air tubing
Filter, air dryer, pressure reducing valve, and hydraulic tubing
Compressor, filter, refrigerant, pressure reducing valve, and air tubing
What is the function of the air compressor in a pneumatic control system?
To provide the power needed to operate all control devices in the system
To filter the air before it enters the system
To reduce the air pressure to a usable level
To cool the air before it is used in the system
How often does a standard air compressor typically start per hour?
Up to six times per hour
Up to twelve times per hour
Up to twenty-four times per hour
Up to thirty times per hour
What triggers the air compressor to stop in a pneumatic control system?
When the air pressure is too low
When the air pressure exceeds a high limit setting
When the air filter is clogged
When the pressure reducing valve fails
What is the purpose of having two compressors or a dual compressor in some applications?
To increase the air pressure
To filter the air more efficiently
To provide redundancy and ensure continuous operation
To reduce the noise level
What is the purpose of a compressor intake filter in oil-less compressors?
To regulate the air pressure
To remove particles that could damage the compressor pump
To cool down the air before it enters the compressor
To collect moisture and oil residue
Where is the intake filter usually located in the equipment room with the compressor?
Outside the room
Inside the air handling unit
At the bottom of the tank
Near the pressure safety relief valve
What is the function of a high pressure safety relief valve?
To filter the air
To cool the air
To open on excessively high tank pressures
To remove moisture from the air
What is the coldest ambient temperature to which tubing is exposed, according to the DRY AIR REQUIREMENT section?
The temperature at which moisture condenses
The temperature of the equipment room
The lowest temperature to which an air line will be exposed
The temperature at which the safety relief valve opens
According to Figure 12, Winter Dew Point Requirement, what happens to the required dew point as the outdoor air temperature increases?
The required dew point decreases
The required dew point remains constant
The required dew point increases
The required dew point is not related to outdoor air temperature
What does Figure 13, Twelve-Month Composite Dew Point Requirement, illustrate?
The relationship between indoor and outdoor temperatures
The average dew point required over a year
The maximum dew point allowed for safety
The dew point requirement for summer only
Which type of drying may be used when main air piping is kept away from outside walls and chilling equipment?
Refrigerant Drying
Desiccant Drying
High-Pressure Drying
Low-Pressure Drying
What is the main advantage of using a refrigerant dryer compared to other methods?
It requires manual drain valve operation.
It has the ability to hold more water.
It provides the greatest system reliability and requires minimal maintenance.
It uses a non cycling operation with a hot gas bypass control.
At what tank pressure can a constant dew point of approximately 35°F be maintained using a refrigerant dryer?
20 psi
50 psi
70 psi
100 psi
What is the purpose of a desiccant in pneumatic control systems?
To cool the air passing through it
To remove moisture from air
To heat the air before it enters the compressor
To filter contaminants from the air
What percentage of its own weight in water is a molecular sieve desiccant capable of absorbing?
(a)
How is a desiccant regenerated?
By cooling it down to a certain temperature
By heating the desiccant material and removing the resulting water vapor
By passing it through a refrigerant dryer
By increasing the pressure in the desiccant chamber
What is the purpose of the air filter in a pressure reducing valve station?
To regulate the air pressure
To remove solid particulate matter and oil aerosols or mist from the control air
To measure the air pressure
To introduce moisture into the system
What happens to the desiccant in the right chamber (E) of the Typical Heatless Desiccant Dryer Airflow Diagram when the air passes through the orifice (G)?
It becomes saturated with moisture
It becomes the drying agent connected to the compressor
It reduces the chamber pressure to near atmospheric pressure
It coalesces oil from the air
What is the result of oil contamination in compressed air as it appears in the system?
It enhances the performance of the system
It remains in a vapor state throughout the system and does not interfere with operations
It can coalesce while flowing through the system and collect in device filters, orifices, and small passages
It solidifies and blocks the airflow completely
How long does the moisture removal cycle last in the Typical Heatless Desiccant Dryer Airflow Diagram?
15 minutes
30 minutes
45 minutes
60 minutes
What is the most important element of the pressure reducing valve station, as mentioned in the text?
The solenoid
The check valve
The air filter and the PRV
The desiccant chambers
What is the normal setting for outlet pressure after it passes through the Single-Pressure Reducing Valve?
10 psi
20 psi
25 psi
50 psi
What is the typical set relief valve pressure in PRV assemblies to protect control system devices from overpressurization?
10 psi
20 psi
24 psi
50 psi
What happens when the switch-over occurs in a Two-Pressure Reducing Valve?
The valve switches to a lower setting.
The valve exhausts the switch-over chamber.
The valve increases the pilot pressure.
The valve operates an E/P relay to switch the main pressure.
Which type of thermostat is described as a high-capacity, single-temperature thermostat with a capacity amplifier added?
A low-capacity, single-temperature thermostat
A high-capacity, single-temperature thermostat
A pilot-bleed, two-pipe, proportioning thermostat
A dual-acting (heating/cooling) thermostat
What is a dual-acting thermostat typically used for?
Providing proportional control for one specific temperature
Serving as cooling control only
Heating and cooling control with two separate sensing elements
Heating and cooling control with a single sensing element
What is the function of the bimetal in a thermostat with a nozzle-flapper-bimetal assembly?
To control the pilot pressure
To adjust the setpoint temperature
To rebalance the lever system when ambient temperature changes
To switch the main pressure for a heating/cooling system
What is the primary difference between a two-position controller and a proportional controller?
A) A two-position controller changes branchline pressure incrementally, while a proportional controller changes it rapidly.
B) A two-position controller changes branchline pressure in response to changes in the measured condition, while a proportional controller does not.
C) A two-position controller changes branchline pressure rapidly from maximum to minimum or vice versa, while a proportional controller changes it incrementally.
D) A two-position controller operates on a fixed schedule, while a proportional controller changes branchline pressure based on temperature changes.
What does a proportional-integral (PI) controller add to the proportional controller?
A) It adds a component that takes offset into account and eliminates the control point offset from the setpoint.
B) It adds a rapid switching mechanism between maximum and minimum branchline pressure.
C) It adds a fixed schedule of operation independent of the measured condition.
D) It introduces a delay in the response to changes in the measured condition.
Which type of controller system uses a single pipe for both the controller and actuator?
A) Two-pipe system
B) One-pipe system
C) Proportional system
D) Proportional-integral system
In a one-pipe controller system, what is the path of the main air?
A) The main air goes through a restrictor to the actuator only.
B) The main air goes through a restrictor to the controller and then to the actuator.
C) The main air goes directly to the actuator without passing through the controller.
D) The main air is split into two paths, one going to the controller and one to the actuator.
What is the result of decreasing the setpoint in a direct acting thermostat as described in the text?
A) It causes the branchline pressure to increase.
B) It causes the branchline pressure to decrease.
C) It has no effect on the branchline pressure.
D) It reverses the direction of the branchline pressure.
What type of controllers use a constant main air pressure that is alternately switched between two pressures, such as 13 and 18 psi for example?
Temperature controllers
Humidity controllers
Single-pressure controllers
Two-pressure controllers
What happens to the main lever in a bleed-type, low-pressure controller when there is an increase in static pressure?
It moves closer to the nozzle, restricting the airflow through the nozzle.
It moves away from the nozzle, increasing the airflow through the nozzle.
It remains stationary, as it is not affected by static pressure changes.
It triggers the feedback bellows to balance the static pressure on the diaphragm.
What is the primary difference between temperature and humidity controllers?
The type of sensor element used.
The range of pressure they control.
The type of pressure they measure.
The size of the diaphragms.
High-pressure controllers are designed to measure and control pressures and vacuums in what units?
Pounds per square inch or inches of water.
Inches of mercury or inches of water.
Pounds per square inch or inches of mercury.
Cubic feet per minute or gallons per hour.
What is the function of the feedback bellows in the bleed-type static pressure controller diagram?
It measures the static pressure in the duct.
It balances the static pressure on the diaphragm.
It adjusts the setpoint spring tension.
It controls the airflow through the nozzle.
What is the function of a sensor-controller system in HVAC systems?
To provide a 15 to 20 psi signal to the controller
To provide a linear 3 to 15 psi signal to the controller over a fixed service range
To provide a non-linear signal to the controller for temperature adjustment
To act as a general-purpose pneumatic amplifier
What is the primary difference between single-input and dual-input controllers?
Single-input controllers use diaphragm logic, while dual-input controllers use PI logic
Single-input controllers can only adjust the setpoint, while dual-input controllers can adjust both the setpoint and reset time
Single-input controllers have one sensor input, while dual-input controllers have inputs from a primary sensor and a reset sensor
Single-input controllers are used in HVAC systems, while dual-input controllers are not
What is the purpose of the proportional band in PI controllers?
To adjust the setpoint
To eliminate deviation from setpoint by controlling the controller output
To provide a fixed service range for the controller
To amplify the signal from the sensor
How can the setpoint be manually adjusted in a controller?
By using a remote restrictor
By adjusting the reset authority
By using a dial on the controller
By changing the proportional band
What is the typical adjustable range for the proportional band setting in PI controllers?
0 to 25 percent of the primary sensor span
2.5 to 50 percent of the primary sensor span
50 to 100 percent of the primary sensor span
10 to 20 percent of the primary sensor span
What is the range of reset authority that can be set according to Figure 23?
0 to 50 percent
10 to 300 percent
20 to 100 percent
0 to 100 percent
What does the velocity sensor-controller measure?
Temperature
Humidity
Air velocity
Air pressure
What is the minimum air velocity that the velocity sensor-controller can accurately sense as mentioned in the text?
100 feet per minute
500 feet per minute
3,500 feet per minute
1,000 feet per minute
According to the text, what is the function of the gap in the velocity sensor operation as shown in Figure 24?
It measures the air temperature.
It allows air to flow into the collector tube.
It collects the air pressure data.
It emits air to the environment.
What is the throttling range of pneumatic sensors typically the same as?
The sensors in the controller range
The sensors and the throttling range adjustments
The sensors in the same sensing element range
The sensors and the air pressure range
What type of pressure do low-pressure sensor measures as per the text?
High static pressure
Differential pressure
Atmospheric pressure
Absolute pressure
What happens to the branchline pressure when the pressure differential increases, according to the text?
The branchline pressure decreases.
The branchline pressure remains constant.
The branchline pressure increases.
The branchline pressure fluctuates.
What is the typical application of a thermostat and velocity controller as shown in Figure 25?
To control the electrical power supply
To regulate the Variable Air Volume (VAV) terminal unit with hot water reheat
To monitor outdoor weather conditions
To manage the lighting system in a building
What is the function of a pneumatic actuator?
To convert electrical signals into mechanical movement
To regulate the flow of a medium through a valve or damper
To measure the temperature in a room
To provide insulation to pneumatic systems
What is the action of a direct-acting damper actuator in response to an increase in branchline pressure?
It retracts
It extends
It stops functioning
It oscillates
What is the typical stroke range for damper actuators?
Between one-quarter and one-half inch
Between one and two inches
Between one-quarter and one inch
Between one-half and one and a half inches
What is the term used for pneumatic valve and damper actuator assemblies that are positioned in the valve body upon zero actuator air pressure?
Normally open (N.O.)
Normally closed (N.C.)
Direct acting
Reverse acting
What determines the start pressure and pressure change required for full movement of the actuator from open to closed, or from closed to open in valve and damper actuators?
Electrical signals
Spring ranges
Hydraulic pressure
Manual control
What type of valve body allows flow with the stem up and shuts off flow with the stem down?
Normally open valve assembly
Normally closed valve assembly
Double-seated valve
Reverse-acting valve body
What happens to a reverse-acting valve body when the diaphragm moves downward?
The valve stem moves toward the open position.
The valve stem stays in the same position.
The valve stem moves toward the closed position.
The valve stem moves in a random direction.
Which force is the only one available to close the valve in a normally open valve body when the actuator spring is compressed?
Force F1
Force F2
Force F3
The force of gravity
In Figure 28B, what causes the diaphragm to move upward and open the valve?
Decrease in branchline pressure
Increase in branchline pressure
Decrease in controlled-medium force
Increase in controlled-medium force
Why do double-seated valves generally not provide tight close-off?
Because the actuator force needed to position the plug assembly is too high.
Because the balance of forces between the two discs of the plug assembly reduces the actuator force needed.
Because one disc can seat before the other and prevent the other disc from seating tightly.
Because the valve stem does not move.
What is the function of a double-seated valve as shown in Fig. 29?
To control the flow of air in air-handling systems
To operate butterfly valves
To mix two inlets and a common outlet
To provide isolation between branch lines
