437 Flashcards
what is the main function of a control system
maintain a variable set point
what are the basic components of a control system
sensor controller; controlled device
air conditioning and refrigeration systems are commonly controlled by ___ control
proportional
which of these control loops adjusts the setpoint automatically
reset system
the power to operate a control system may come from
fluid under pressure
an RTD responds to a change in temperature by undergoing a change in its
electrical resistance
relative humidity is defined as the amount of moisture in the air
divided by the amount it can hold
in a pressure sensor that uses a bourdon tube changes in pressure are sensed by changes in the ___ of the tube
position of sealed end
an air conditioning system has a two position damper in the duct bringing outside air into the building and a second two position damper in the return air duct. when the system shuts down the outside air damper should be ___ and the return air damper should be ___
closed; open
which of the following is an advantage of variable speed drives over older methods of controlling the output of an air conditioning or refrigeration system
proportional control can be used
unlike floating control the proportional control is
continuous
the air supply for a pneumatic control system requires which of the following
drier
a PE relay is used to control a device operated by
electricity
setting the gain of a controller determines how much a change in the ___ affects the ____
controlled variable; controlled device
adding integral action to a controller improves system performance by reducing ____ adding derivative action improves performance by reducing ______
offset; overshoot and undershoot
in pumpdown control the compressor starts when the pressure ___ in the ____
increases; suction line
the high pressure control in the discharge line of an air conditioning or refrigeration system responds to high pressure by
shutting down the compressor
a differential pressure oil safety control shuts off the compressor if the difference on pressure between the ____ and the oil pump ___ does not become high enough within a given time
compressor crankcase; discharge
what kind of capacity control is commonly used on large reciprocation compressors
cylinder unloading
in modulating the flow of water to hydronic terminals the temperature control method has the advantage of
preventing system freeze up in cold weather
in setting up a preventive program your first step should be to
inspect the system in detail
to maintain oil pressure in a 10_hp reciprocating compressor a ____ oil pump is used
positive displacement
to adjust the water regulating valve on a water cooled condenser you should turn the adjusting screw until the ____ is at the design value. you should make this adjustment when the equipment is running at ___ load
head pressure; maximum
the main requirements for maintaining a pneumatic control system include
removing and disassembling valves
the main requirements for maintaining an electric or electronic control system include
checking electrical contacts for pitting
