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HET4 > Heating Ventalation and Air conditioning control systems > Flashcards

Heating Ventalation and Air conditioning control systems Flashcards

1
Q

what two devices are used to control flow of refrigerant into the evaporator core

A

thermal expansion valve and a fixed orifice tube

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2
Q

what is a flow restrictor located in the engine coolant water valve used for

A

to slow down the flow of hot coolant to allow the heat to be dissipated into the cab

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3
Q

an adjustable thermostatic switch will always open the switch contacts at

A

32F but can be set to close as high as 50 F

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4
Q

what are passive controls

A

regulating the flow of refrigerant or heater coolant without driver input

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5
Q

how are switches that sense low pressure connected into the system

A

in series with the electromagnetic clutch

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6
Q

the compressor will not engage if the thermostatic switch detects an

A

evaporator freeze up

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7
Q

when will the system resume operation after the low-pressure switch disengages the compressor

A

after the low pressure switch disengages and the 15 second delay has passed

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8
Q

when the air conditioning system contains a fixed expansion tube and an accumulator, the thermostatic switch may be replaced with a

A

clutch cycling switch ( CCOT cycling switch )

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9
Q

how can you check an electromagnetic clutch if it is not labeled or color coded

A

check the resistance of the coil a 24 v will have double resistance of 12 v

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10
Q

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

A

the high pressure switch needs to reset and the 15 second delay must have passed

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11
Q

what temperatures does a fixed thermostatic switch operate in

A

32 F to 38 F

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12
Q

if a electromagnetic clutch has a black lead wire it will be a

A

12 v clutch

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13
Q

how are most CCOT cycling switches and low pressure switches installed

A

on a 1/4 inch flair port equipped with a Schrader valve

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14
Q

what are the three inputs of an air conditioning management system

A

high pressure switch
low pressure switch
evaporator thermostatic switch

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15
Q

how can the doors on a HVAC duct system be controlled

A

. push or pull cables
. vacuum motors
. compressed air motors
. electric motors

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16
Q

what function does a binary switch provide

A

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

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17
Q

switches, levers, knobs and air louvers or diffusers are what type of control

A

Active controls

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18
Q

ambient temperature, multi - function pressure switches, radiator fan clutches and radiator shutter systems are what type of control

A

combination and supplemental control devices

19
Q

why are high pressure cut out switches installed in the air condition system

A

to protect the air conditioning system compressor from high discharge pressures and temperatures

20
Q

where is the thermostatic switch located

A

it has a heat sensing capillary tube placed between the fins of the evaporator core

21
Q

what would a electrically controlled refrigerant solenoid valve be used for

A

to split and direct part of the refrigerant flow to the second evaporators expansion device

22
Q

when the engine is cranking during start up the compressor will

A

have a 15 second delay

23
Q

the air condition system will be turned off if any of these faults are detected

A

. low static refrigerant pressure
. open in air conditioning electromagnetic clutch circuit
. short in air conditioning electromagnetic clutch circuit

24
Q

at what pressure would the CCTO cycling switch close and start the compressor

A

48 psi

25
Q

the blower speed switch has three ON positions and regulate the HVAC system air velocity measured in

A

CFM cubic feet per minute

26
Q

thermostats, pressure - activated or regulating valves and pressure temperature switches are what type of control

A

Passive controls

27
Q

what are active controls

A

used by the driver to turn the system ON and adjust air temperature, air flow direction and velocity

28
Q

how are high pressure cut out switches connected into the system

A

in series with the compressor electromagnetic clutch coil

29
Q

what might be the problem if the electromagnetic clutch is slipping

A

not enough voltage being supplied to the clutch to hold

30
Q

why would some air conditioning systems have two evaporators

A

one for the cab and one for the sleeper

31
Q

what does a trinary pressure switch do

A

. 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

32
Q

what is an H fitting used for

A

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

33
Q

why would a low-pressure sensing switch be installed on the high pressure side on the receiver dryer

A

can be use to interrupt the compressor clutch circuit when the system is low in refrigerant or the ambient temperature is too cold

34
Q

what are the two types of blower wheels used in HVAC systems

A

single entry blowers and double entry blower

35
Q

what are the three HVAC control systems

A

Active controls - Passive controls - Combination and supplemental control devices

36
Q

what does the temperature control lever control

A

controls the amount of engine coolant allowed to pass through the heater core

37
Q

what are combination and supplemental control devices

A

can affect system operation. they may prevent or support system function

38
Q

what can be used to heat sleepers when the truck is not running

A

fuel-fired air heaters

39
Q

if an electromagnetic clutch has a green lead wire it will be a

A

24 v clutch

40
Q

where is the CCOT cycling switch located in the system

A

on the accumulator and senses the low side of the air conditioning system refrigerant pressure

41
Q

at what pressure would the CCOT cycling switch open and disengage the compressor

A

24 psi

42
Q

what is the air conditioning cycle limit

A

once every 15 seconds

43
Q

how is the speed in a blower motor controlled in the electrical system

A

with a resistor

HET4 (43 decks)

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