Heating Ventalation and Air conditioning control systems Flashcards
what two devices are used to control flow of refrigerant into the evaporator core
thermal expansion valve and a fixed orifice tube
what is a flow restrictor located in the engine coolant water valve used for
to slow down the flow of hot coolant to allow the heat to be dissipated into the cab
an adjustable thermostatic switch will always open the switch contacts at
32F but can be set to close as high as 50 F
what are passive controls
regulating the flow of refrigerant or heater coolant without driver input
how are switches that sense low pressure connected into the system
in series with the electromagnetic clutch
the compressor will not engage if the thermostatic switch detects an
evaporator freeze up
when will the system resume operation after the low-pressure switch disengages the compressor
after the low pressure switch disengages and the 15 second delay has passed
when the air conditioning system contains a fixed expansion tube and an accumulator, the thermostatic switch may be replaced with a
clutch cycling switch ( CCOT cycling switch )
how can you check an electromagnetic clutch if it is not labeled or color coded
check the resistance of the coil a 24 v will have double resistance of 12 v
when the high pressure switch indicates a high -pressure and the compressor is disengaged what two conditions need to be met in order to resume normal operation
the high pressure switch needs to reset and the 15 second delay must have passed
what temperatures does a fixed thermostatic switch operate in
32 F to 38 F
if a electromagnetic clutch has a black lead wire it will be a
12 v clutch
how are most CCOT cycling switches and low pressure switches installed
on a 1/4 inch flair port equipped with a Schrader valve
what are the three inputs of an air conditioning management system
high pressure switch
low pressure switch
evaporator thermostatic switch
how can the doors on a HVAC duct system be controlled
. push or pull cables
. vacuum motors
. compressed air motors
. electric motors
what function does a binary switch provide
works as a low pressure switch preventing the system from operating without refrigerant or if ambient temperatures are to cold
it also de-energizes the compressor clutch if refrigerant pressures rise to unsafe levels
switches, levers, knobs and air louvers or diffusers are what type of control
Active controls
ambient temperature, multi - function pressure switches, radiator fan clutches and radiator shutter systems are what type of control
combination and supplemental control devices
why are high pressure cut out switches installed in the air condition system
to protect the air conditioning system compressor from high discharge pressures and temperatures
where is the thermostatic switch located
it has a heat sensing capillary tube placed between the fins of the evaporator core
what would a electrically controlled refrigerant solenoid valve be used for
to split and direct part of the refrigerant flow to the second evaporators expansion device
when the engine is cranking during start up the compressor will
have a 15 second delay
the air condition system will be turned off if any of these faults are detected
. low static refrigerant pressure
. open in air conditioning electromagnetic clutch circuit
. short in air conditioning electromagnetic clutch circuit
at what pressure would the CCTO cycling switch close and start the compressor
48 psi
the blower speed switch has three ON positions and regulate the HVAC system air velocity measured in
CFM cubic feet per minute
thermostats, pressure - activated or regulating valves and pressure temperature switches are what type of control
Passive controls
what are active controls
used by the driver to turn the system ON and adjust air temperature, air flow direction and velocity
how are high pressure cut out switches connected into the system
in series with the compressor electromagnetic clutch coil
what might be the problem if the electromagnetic clutch is slipping
not enough voltage being supplied to the clutch to hold
why would some air conditioning systems have two evaporators
one for the cab and one for the sleeper
what does a trinary pressure switch do
. prevents the system from operating with no refrigerant or if ambient temperatures are to cold
. disengage the clutch if pressures are to high
. activates fan clutch and or radiator shutter assembly as pressure reaches mid range
what is an H fitting used for
allows some coolant water to pass through the heater lines at all times and protects the truck engine from hot spots, it is connected in series with the heater core
why would a low-pressure sensing switch be installed on the high pressure side on the receiver dryer
can be use to interrupt the compressor clutch circuit when the system is low in refrigerant or the ambient temperature is too cold
what are the two types of blower wheels used in HVAC systems
single entry blowers and double entry blower
what are the three HVAC control systems
Active controls - Passive controls - Combination and supplemental control devices
what does the temperature control lever control
controls the amount of engine coolant allowed to pass through the heater core
what are combination and supplemental control devices
can affect system operation. they may prevent or support system function
what can be used to heat sleepers when the truck is not running
fuel-fired air heaters
if an electromagnetic clutch has a green lead wire it will be a
24 v clutch
where is the CCOT cycling switch located in the system
on the accumulator and senses the low side of the air conditioning system refrigerant pressure
at what pressure would the CCOT cycling switch open and disengage the compressor
24 psi
what is the air conditioning cycle limit
once every 15 seconds
how is the speed in a blower motor controlled in the electrical system
with a resistor
