Control system fundamentals quiz Flashcards
(33 cards)
T or F
Controls for the air handling system and zones are specifically designed for a building by the Service Technician responsible for the building. The controls are usually installed at the job site.
F
T or F
The control system for a commercial building comprises many control loops and can be divided into central system and local- or zone-control loops.
T
T or F
Dehumidification is generally required only during the cooling season.
T
T or F
Ventilation air must be kept at the minimum required level except when used for free cooling
T
T or F
For satisfactory environmental conditions, the relative humidity of the air should be 40 to 70 percent.
F
T or F
An exhaust-air system may not be incorporated into main air conditioning unit
F
T or F
HVAC control loop applications generally use three types of control loops
F
T or F
An open-loop system does not take into account changing space conditions from internal heat gains, infiltration/exfiltration,solar gain, or other changing variables in the building
T
T or F
A open-loop system relies on measurement of the controlled variable to vary the controller output
F
T or F
Pneumatic, electromechanical, and electronic systems perform limited, predetermined control functions and sequences. Microprocessor-based controllers use digital control for a wide variety of control sequences.
T
T or F
Two-position control is used to energize and deenergize electric strip heaters.
T
T or F
in basic two-position control, the presence of lag causes the controller to correct a condition that is taking place or is about to take place, rather than one that has already passed.
F
T or F
Step control uses a digital signal to attempt to obtain a digital output from equipment that is typically either on or off.
F
T or F
The throttling range of a proportional control is the amount of change in the controlled variable required for the controller to move the controlled device through its full operating range.
T
T or F
Process lag is the time delay between the introduction of a disturbance and the point at which the controlled variable begins to respond.
T
T or F
The sensing element in a temperature sensor can be a bimetal strip, a rod-and-tube element, a sealed bellows, a sealed bellows attached to a capillary or bulb, a resistive wire, or a thermistor.
T
Define control point in a controls system:
a) A calculation method that produces a control output by operating on an error signal or a time series of error signals
b) Control action that results in a change of the manipulated variable. Initiated when the controlled variable deviates from setpoint.
c) The actual value of the controlled variable
d) The quantity or condition that is measured and controlled
C
Define droop in a controls system:
a) A sustained deviation between the control point and the setpoint in a two-position control system caused by a change in the heating or cooling load.
b) A simple two-position control system in which the device being controlled is either full on or full off with no intermediate operating positions available. Also called “two-position control”.
c) An action that adjusts by minute increments and decrements.
d) The quantity or condition regulated by the automatic control system to cause the desired change in the controlled variable.
A
Define proportional control in a controls system:
a) Control method in which a multiple-switch assembly sequentially switches equipment (e.g., electric heat, multiple chillers) as the controller input varies through the proportional band. Step controllers may be actuator driven, electronic, or directly activated by the sensed medium (e.g., pressure, temperature).
b) An energy conservation technique that allows temperatures to float between selected settings, thereby preventing the consumption of heating or cooling energy while the temperature is in this range
c) A control algorithm or method in which the final control element moves to a position proportional to the deviation of the value of the controlled variable from the setpoint.
d) A control circuit that operates on air pressure and uses a mechanical means, such as a temperature-sensitive bimetal or bellows, to perform control functions, such as actuating a nozzle and flapper or a switching relay. The controller output usually operates or positions a pneumatic actuator, although relays and switches are often in the circuit
C
Which statement is false?
a) While heat pumps are usually direct expansion, a large heat pump may be in the form of a chiller.
b) Chilled water control is usually proportional, whereas control of an evaporator coil is two position.
c) In direct expansion systems having more than one coil, a main valve controls a solenoid valve for each coil and the compressor is cycled by a refrigerant pressure control
d) Absorption refrigeration systems, which use heat energy directly to produce chilled water, are sometimes used for large chilled water systems
C
Which statement is false?
a) Viscous coated filters remove fine particles by using the turbulence present in the air stream to drive particles to the fibers of the filter surface.
b) Filters are selected according to the degree of cleanliness required, the amount and size of particles to be removed, and acceptable maintenance requirements.
c) Electrostatic filter plates may be coated with an adhesive
d) A ventilation system that uses return air is more common than the 100 percent outdoor air system
A
Which statement is false?
a) Automatic control systems use feedback to reduce the magnitude of the deviation and produce system stability.
b) Self-powered systems are a type of control that use the power of the measured variable to induce the necessary corrective action
c) In a pneumatic controller, the sensor sends the controller a continuous signal
d) The controller samples digital data at set time intervals, rather than reading it continually. The sampling method is called pulsive control signaling
D
Which statement is false?
a) In many of the larger commercial control systems, an electronic-pneumatic transducer converts the pneumatic output to a variable pressure output for pneumatic actuation of the final control element.
b) The most common forms of energy for automatic control systems are electricity and compressed air.
c) The result of using a basic two-position control is cyclical operation of the controlled equipment and a condition in which the controlled variable cycles back and forth between two values (the on and off points) and is influenced by the lag in the system.
d) Traditionally, analog devices have performed HVAC control.
A
Control systems use different control modes to accomplish their purposes. Control modes in commercial applications include:
a) two-position, step, and floating control; proportional, proportional-integral, and proportional-integral-derivative control; and adaptive control.
b) two-position, step, and floating control; proportional, secondary input control, proportional-integral-derivative control; and adaptive control.
c) floating control, corrective signal, proportional, proportional-integral, proportional-integral-derivative control; and adaptive control.
d) two-position, step, and floating control; proportional, mandarin orange induction control, proportional-integral-derivative control; and adaptive control.
A
