A lesson about Thermal Comfort - Notes 2.pdf

a lesson about thermal comfort

1. FOCUS ON THERMAL COMFORT

4. Moving heat
An air conditioning system transfers heat from inside the cabin to the
condenser where it is transferred to the outside air.
Heat from the cabin air is absorbed in the evaporator and pumped to the
condenser where it is transferred to the air flowing through the condenser.
Condenser
Evaporator

5. Refrigeration Cycle
4 1
23
Enthalpy kJ/kg
1-2 : Compression
2-3 : Condensation
3-4 : Expansion
4-1 : Evaporation
1
2
3
4
Into cabin
space

6. Refrigerant cycle
Compressor
Evaporator
Accumulator
[reservoir]Condenser
Restriction
[orifice]
4 1
23
Enthalpy
kJ/kg
1
2
3
4
Into cabin
space
43
2
1
1-2 : Compression
2-3 : Condensation
3-4 : Expansion
4-1 : Evaporation

7. Refrigerants
The chemicals used as refrigerants in modern cars normally boil at about -26
deg. F if the pressure in the refrigeration system was released.
In an enclosed system filled with refrigerant the pressure the pressure [in psi]
will be roughly the same as the temperature.
At 70 deg F the pressure in the A/C system is about 70 PSI [when the A/C
compressor is not running]
The ratio of temp to pressure is not exactly 1 to 1
At 100 deg F the pressure will rise to 124 psi.

8. R-134a
The refrigerant currently used in the United States is R-134a.
R-134a has been installed on all cars sold in the USA since model year 1993
The chemical name for R-134a is tetrafluoroethane (CF
3CH
2F).
R-134a does not contain chlorine and does not harm the earth's ozone layer.
There are plans to replace R-134a with a more environmentally friendly alternative.

9. Reusing the refrigerant
•Heat energy is absorbed by liquid refrigerant when the refrigerant changes
from a liquid to a vapor [gas].
•A pound of refrigerant can absorb only a small amount of heat.
•The A/C system uses the refrigerant over and over again in a continuous cycle
of boiling and condensing.
•To reuse the refrigerant it must be pressurized and cooled so that it turns
back into a liquid.

10. Recovering the refrigerant
•The process of turning a vapor
into a liquid is called
condensation.
•When the refrigerant is
pressurized to 200 psi it will
condense back into a liquid
when it is cooled to 130 deg. F.
or lower.
Compressor
Condenser
Low
pressure
vapor
High
pressure
vapor
•Air passing over the condenser
coils removes the heat that was
absorbed by the refrigerant.
High
pressure
Liquid

11. Refrigerant cycle
Compressor
Evaporator
Accumulator
[reservoir]
Condenser
Restriction
[orifice]

12. Pressure drop
•The A/C system runs about 2 lbs. of refrigerant through a continuous cycle of
pressurization and pressure release.
•When high pressure refrigerant passes through a restriction [orifice] the
pressure drops.
•There are two ways the cause this drop in pressure:
–Fixed orifice [orifice tube]
–Expansion valve

13. Compressor
•The compressor takes
refrigerant vapor at low
pressure [ typically 20 –30 psi]
and compresses it to 180 psi or
higher.
Inlet and
outlet ports
Compressor
clutch
Compressor clutch
electrical connector
Mounting
boss
Image courtesy of General Motors corp.

14. Compressor
•The compressor is driven by engine power via a serpentine belt.
•An electrically actuated clutch allows the compressor to be turned off when
the A/C system is not needed.
•In some systems the compressor clutch is cycled on and off to control the
temperature of the evaporator.

15. Compressor
•A/C compressors use power from the engine
crankshaft to compress the vapor leaving
the evaporator.
•There are three types of compressors used
in modern A/C systems:
–Piston
–Vane
–Scroll
Vapor inlet
[suction]
Outlet
[discharge]
Intake reed
valve
Outlet reed
valve
Piston type
compressor

16. Condenser
•The condenser changes refrigerant
vapor back into liquid.
•Refrigerant vapor enter the condenser
at close to 200 psi.
•At this pressure refrigerant will change
into a liquid when cooled below 130
deg. F.
•Air passing through the condenser fins
absorbs the heat from the refrigerant.
Outlet
[liquid]
Inlet [vapor]

17. Condenser
•The condenser is normally mounted ahead of the radiator.
•Normally the same electric fan that draws air through the radiator pulls air
through the condenser as well.
•In some vehicles the condenser has a dedicated fan
Outlet
[liquid]
Inlet [vapor]

18. Fixed orifice
•The tube is enclosed in a nylon sleeve that has mesh filters of prevent small
particles of dirt clogging the orifice.
•The orifice tube is located at the end of the liquid line where it connects to
the evaporator inlet.
•The orifice tube is inexpensive and is normally replaced anytime an A/C
component is replaced.
•An orifice tube is a small
brass tube with an inner
diameter of about .060”.

19. Expansion valve
•An expansion valve opens and closes to
control the flow of refrigerant into the
evaporator.
•The valve is operated by a diaphragm that is
connected to a thermal bulb via a capillary
tube.
•The diaphragm responds to changes in the outlet
temperature of the evaporator.
•The valve is often called a TXV[Thermal Expansion
Valve].

20. Expansion valve
•The expansion valve
connected to the evaporator
inlet.
•The thermal bulb is clipped to
the evaporator outlet .
•When the temperature of the
bulb drops below 32 deg. F
the diaphragm closes the
expansion valve, stopping the
flow of refrigerant into the
evaporator.
Liquid
line
Capillary
tube
Thermal
bulb
Equalizer
line
Outlet
Inlet

21. Evaporator
•Liquid refrigerant under low
pressure enters the evaporator at
the bottom.
•The superheated refrigerant liquid
pulls heat from the cabin air passing
over the evaporator fins.
•As the refrigerant absorbs heat from
the cabin air vapor bubbles float to
the top.
•By the time the refrigerant gets to
the top all of the refrigerant should
have changed from liquid to vapor
Inlet [liquid]
Coldest
point in the
A/C system
Outlet
[vapor]
Orifice

22. Evaporator core
•Since space iaat a premium under the
dash the evaporator core is much
thicker than the condenser.
•Note: the evaporator shown here is
upside down. It would not work if it
was installed in the car this way.
Evaporator
outlet
Evaporator
inlet

23. Evaporator case
•The evaporator is located inside the heater box between the blower fan and
the heater core.
•All of the air from the blower fan passes through the evaporator.
•If the temperature is desired to be warmer than maximum cool the blend
door diverts some of the cooled air through the heater core.
•This has the added benefit of dehumidifying the air as well as cooling it.
Heater core
Evaporator
Blend door

24. Refrigerant reserve -Accumulator
•In normal operation the system needs a little over one pound of refrigerant
for normal operation plus a few additional ounces of reserve for expected
leakage over time.
•Cars that use a fixed orifice have an accumulator [reserve] located between
the evaporator and compressor.
Compressor
Evaporator
Accumulator
Condenser
Orifice tube

25. Accumulator
•This accumulator has been cut open
to reveal the desiccant bag.
•Desiccant is a chemical that absorbs
water.
•Beside being a reservoir of liquid
refrigerant the accumulator functions
to prevents liquid refrigerant from
entering the compressor.
Service
fitting
Outlet
Inlet
Desiccant

26. Refrigerant reservoir –Receiver Drier
•Systems that use an expansion valve store their refrigerant reserve in a
receiver drier between the condenser output and the thermal expansion
valve.
Expansion
valve
Receiver
drier
Thermal bulb
Capillary
tube
Desiccant
bag

27. Light-Duty Vehicles Heavy-Duty Vehicles
Other Methods and Technologies to Improve Thermal Comfort
Source: NREL