1. Condensers and Evaporators

2. Condensers
Condensers change high pressure refrigerant vapor into high pressure liquid by transferring the heat within the refrigerant to the outside air.
The pressure inside the condenser is tied to the temperature of the outside air and the rate of air flow through the condenser.

3. Condenser pressure
In normal summer driving with the outside air temperature in the 90 degree range the condenser pressure will range between 150 psi and 280 psi.
If there is insufficient air flowing through the condenser the pressure can rise above 450 psi.
Most systems will automatically shut down the compressor if the pressure exceeds 350 psi.
On older A/C systems the pressure relief valve in the compressor will open to prevent the condenser or hoses from exploding.

4. Condenser construction
Because of the high pressure levels condensers are normally made of steel and copper tubing.
Most condensers are serpentine tube design where vapor enters at the top and liquid collects at the bottom.

5. Condenser construction
Steel fins are soldered or welded to the tubes to provide sufficient surface area to transfer heat to the air passing over the fins.
Steel plates are often attached to the sides to prevent air circumventing the fins.

6. Condenser location
Normally the condenser is mounted ahead of the radiator.
The condenser can be located anywhere there is a source of cooling air.
Condensers have been mounted in the fender wells, under the rear seat or on the engine lid in rear engine cars.

7. Condenser air flow
The A/C system works hardest directly after engine start when the vehicle is parked in the sun.
Cabin temperatures are well over 100 degrees and the vehicle is normally driving less than 40 mph for the first few minutes.
Most vehicles use an electric motor that provides maximum air flow through condenser when the vehicle is stationary.

8. Electric fan[s]
FWD vehicles with transverse engines require an electric fan for both A/C and the radiator.
In some cases 2 fans are used.
Most vehicles have one electric fan that provides air flow for both condenser and radiator.
The radiator requires air flow only when the vehicle is stopped or traveling at speeds less than 25 mph.
The radiator fan is normally not needed if the vehicle is only momentarily stopped.

9. Condenser fan
The condenser fan is needed whenever the vehicle is stopped of moving at speeds less that 25 mph.
Older cars wired the condenser fan in parallel with the compressor clutch so that whenever the compressor was operating the condenser fan was also running.

10. Fan Shroud
The plastic cover that encloses the fan and radiator/condenser is called a fan shroud.
The fan shroud ensures that all of the air moved by the fan passes through the condenser and radiator.

11. Condenser fan
Newer vehicle turn the operation of the fan over to the PCM [power train control module]
The PCM looks at coolant temp, high side pressure, vehicle speed and A/C request signals and then decides whether or not to turn on the fan.

12. Two speed fan The fan produces a lot of noise when it is running.
To minimize noise may vehicles have a 2 speed fan.
A resistor is placed in series when the PCM decides that only a small amount of air flow is needed.
The resistor is bypassed so that it receives full current when the PCM determines that maximum air flow is necessary.

13. Mechanical fans
Rear drive cars and trucks normally use mechanically driven fans.
The fan is driven by the water pump pulley via a fan clutch.
The fan clutch is filed with a viscous fluid that transfers power from the water pump shaft to the fan blade.

14. Fan clutch
To reduce noise and parasitic drag the fan clutch is designed to limit fan speed to about 2000 rpm.
At speeds above 2000 rpm the viscous fluid is pushed away from the center of the clutch reducing it’s ability to couple the fan to the water pump shaft.

15. Thermostatic fan clutch
To further reduce noise and parasitic drag some fan clutches have a thermostatic valve.
The valve is closed when a coil spring on the front of the clutch unwinds as the temperature rises.
When the coil spring is cold the valve inside the clutch is open – allowing centrifugal force to push the fluid away from center and disengaging the clutch from the pump shaft.

16. Thermostatic fan clutch
As the coil warms the valve closes allowing the fluid to transfer power from the water pump shaft to the fan blades.

17. Threaded and bolted fan clutches
There are two designs of fan clutch mounts in use today.
Bolted …is attached to the pump flange with 4 studs and nuts
Threaded… has an internal thread and hex that screws onto the water pump shaft

18. Fan shroud for RWD vehicles
The fan shroud on vehicles with mechanically driven fans is normally a split case design.
The sprit case shroud allows the fan, fan clutch and water pump to be serviced without having to remove the radiator.

19. Auxiliary electric fans
Many RWD cars have an electrically driven fan mounted ahead of the condenser and a mechanically driven fan behind the radiator.
The electric fan provides cooling air for the condenser when the mechanically driven fans are running at relatively low speeds when the engine is idling.
The principle advantage of an auxiliary fan is that the water pump pulley ratio can be lowered to reduce fan noise when idling.

20. Auxiliary fan
The important thing to remember about auxiliary fans is that the pushers – not pullers.
If if an auxiliary fan that was designed to be mounted ahead if the condenser was to be installed behind the radiator it will push air out of the grill.
The engine will overheat but only when it is traveling at 10 to 15 mph when the speed of the vehicle cancels the air flow of the fan.

21. Evaporator Evaporators absorb heat from the cabin air.
Low pressure liquid enter at the bottom and low pressure vapor exits at the top.

22. Evaporator pressure
The range of pressure in the evaporator when the compressor is cycling on/off is 20 to 40 psi.
The normal pressure inside the evaporator is in a VDOT system is about 28 psi.
The pressure in the evaporator right after engine shut down will be about 130 psi.
After the engine cools down the pressure throughout the A/C system will be about tha same as the ambient temperature [71 psi at 70 deg. F].

23. Evaporator design
Since the evaporator runs at relatively low pressure it is often made of aluminum.
Since space is limited under the dash the evaporator is has small height and width but is relatively thick.
Evaporators are broken down into two major groups.
Single pass
Multi pass

24. Single pass evaporator
Single pass evaporators use a single tube folded into a serpentine [S] curve with fins welded to the tube to increase surface area.
Single pass evaporators are normally found on smaller cars where under dash space is limited.

25. Plate and fin design.
Plate and fin design is often used on single pass evaporators.
Flattened aluminum tubes about 3’ wide with a ribbon fin attached are stacked like a battery – with the outlet of one plate connected to the inlet of the plate above.
The problem with plate and fin type evaporators is that refrigerant oil tends to collect at the bottom.
To correct this an oil return line [oil bleed line] connects the base of the evaporator to the outlet.

26. Multi-pass evaporators
Multi-pass evaporators operate more like radiators.
Liquid refrigerant enters a header tank at the top where it passes downward through several tubes into a collector.

27. Multi-pass evaporators
The refrigerant then passes upward through another series of tubes into outlet header.
Refrigerant flow is very rapid in the multi-pass system so oil does not get a chance to collect at the bottom.

28. Evaporator drain
Whenever the A/C system is running moisture will condense and collect on the surface of the evaporator fins.
In summer weather the moisture that condenses on the evaporator will drip down into the bottom of the evaporator case.
This condensate needs to be removed from the cabin.
A drain tube carries the condensate through the firewall where it drips harmlessly onto the pavement.

29. Condensation drain tube
Evaporator drain
Condensation drain tube

30. Evaporator location
The evaporator is located inside the heater box [heater case] which is normally bolted to the inside surface of the firewall.
The evaporator is enclosed inside the case so servicing the evaporator normally requires removing the heater box assembly with the heater core, mode doors and blower motor from the vehicle.
With the heater/evaporator box on the bench the spring clips that hold the plastic case halves together can be remove and the case split to reveal the evaporator and heater cores.

31. Evaporator/heater case removal
In some cases the entire dashboard may need to be removed to gain access to the heater/evaporator case.