Air conditioning fundamentals Flashcards
most people feel most comfortable when surrounded by air temperatures of
21 - 27 C
why are cabs difficult to heat
because of the larger surface area of windows and poor insulation
what is the most desirable cab humidity level
40 to 45 %
the purpose of the heater/ air conditioning unit is to
provide an air exchange , control humidity , heat and cool the cab and allow fresh filtered air to enter the cab
heat energy always moves from
a hotter area to a colder area or from a high heat energy to a lower one
how dose the air conditioning system remove hot air from a cabs interior
by moving the heat from inside the cab to the evaporator core ( the cooling element ) and transferring unwanted heat outside the cab
what are the three heat movement methods
conduction , convection , radiation
what is conduction
heat dissipation by transferring heat energy from one particle to another within and object
what is convection
is heat transfer by movement of warm gas or liquids from one place to another
what is radiation
is heat movement by electromagnetic rays or moving particles
what are the two methods to measure heat
heat intensity and heat quality
how is heat intensity measured
by measuring the intensity of molecular motion of a substance
how is heat quality measured
by measuring how much heat energy a substance contains
what are the three states of matter
solid liquid vapour
what are the two important changes of state that happen in a air conditioning system
vaporization and condensation
the temperature at which a substance changes state depends on
the pressure surrounding the substance
the higher the pressure surrounding the substance
the higher the temperature at which the substance changes state
the lower the pressure that surrounds a substance
the lower the temperature at which a substance will change state
what is the normal refrigerant used in truck air conditioning systems
R-134a refrigerant
what is superheat
when all the liquid refrigerant has gone through a state of change into a vapour and continues to absorb heat energy and will continue to absorb heat with no change in pressure
what are the two applications for calculating superheat in an air conditioning system
to determine how well the expansion devise is working and to determine how efficiently the evaporator is working
where will most manufacturers run superheat temperatures
between 8 - 12 F
what is subcooling
after the refrigerant is completely in liquid form as more heat is removed the refrigerant will drop in temperature without a change in pressure
what are the two practical applications for determining subcooling
to be able to fine tune the air conditioning system and to be able to determine the efficiency of the system
most manufacturers will run subcooling between
12 - 21 F
why is it dangerous when refrigerant is exposed into the atmosphere
because the refrigerant boil away and become vapour and the temperature will drop INSTANTLY causing frostbite or eye damage
what is the purpose of oil in the air conditioning system
provides lubrication , acts as a sealant and lubricates the compressor
what are the four events in an airconditioning system
expansion - evaporation - compression - condensation
what is the first event in an airconditioning system
expansion
what is the second event in an airconditioning system
evaporation
what is the third event in a airconditioning system
compression
what is the fourth event in a airconditioning system
condensation
how is the compressor driven
by the engine through a clutch pulley
what is the compressors function
it functions as a pump to move the refrigerant and oil through the air conditioning system
how is the clutch pulley activated
electromagnetically
what is the purpose of the unique R-134a service valve fittings
to avoid mixing R-134a with other refrigerant
what is the purpose of the condenser
to transfer cab heat energy to the air outside the truck cab
what are the condenser types
tube and fin - serpentine - parallel flow
what is the advantage of tube and fin condenser
it is the strongest type of condenser
what is the advantage of the serpentine condenser
high heat transfer in a compact size
what is an advantage of parallel flow condenser
highly efficient and used when mounting space is limited
where are most condensers mounted
in front of the radiator
what are the functions of the receiver dryer
to act as a dryer, filter and temporary storage tank
how is moisture trapped in the receiver dryer
with use of a desiccant material
why is it important to remove water from the air conditioning system
because water can form ice blocking the system , it can cause rust and mix with other chemicals causing acid erosion
receiver dryers may also be equipped with
a sight glass to determine moisture contamination
where dose the expansion event take place
in the refrigerant metering valve
how is the refrigerant metering valve actuated
by temperature or pressure
what is the refrigerant metering valve designed to do
to control the flow of refrigerant into the evaporator and create a refrigerant pressure drop at the evaporator inlet port
how is the pressure drop in a refrigerant valve controlled
by controlling the size of the orifice in the thermal expansion valve
when the discharge port of the evaporator valve is warm
the metering orifice in the thermal expansion valve opens
when the discharge port of the evaporator valve is cold
the metering orifice in the thermal expansion valve closes
when is an accumulator used in a air conditioning system
when an expansion tube is used in place of a thermostatic expansion valve liquid refrigerant may be able to enter the compressor therefore and accumulator is added to the system in the suction line to prevent this from happening
why should hydraulic hoses not be used in airconditioning systems
because their porous construction allows the refrigerant to leak to the outside environment
what are the three major differences from fixed orifice tube system compared to the thermal expansion valve system
1 the thermal expansion valve is replaced by a fixed orifice tube
2 the receiver / dryer is replaced by an accumulator
3 the thermostatic switch is replaced by a clutch cycling pressure switch
