1. Evaporators For Air Conditioning

2. Evaporators
Discussed in much more detail in HVACR312 the refrigeration term.
In air conditioning there are two primary types of evaporators used:
Natural Draft
Forced Convection

3. Natural Draft

4. Forced Draft

5. Operating Design Direct Expansion
Refrigerant directly cools the air. The evaporator coil is full of refrigerant and air is blowing across the coil.

6. Operating Design Indirect expansion
Refrigerant cools secondary medium, such as water or glycol.
The secondary medium flows through a coil in the air stream and that cools the space.

7. Indirect Expansion

8. Operating Design Two types of Direct Expansion coils exist: Dry Type
Flooded Type

9. Dry Types Use 25% less refrigerant than the flooded type.
Have more vapor in the evaporator
Have less chance of floodback to the compressor.

10. Dry Type The disadvantages of the dry type coil are:
Slower pull-down with heavy loads
System runs with higher head pressures.

11. Evaporator Purpose There are two purposes of evaporators: Cooling
Dehumidification

12. Cooling Changes the sensible heat content in the air.
This you can actually measure.

13. Dehumidification
Dehumidification changes the latent heat and the moisture in the air.
This is the process described in the psychometric chart.
Must keep indoor humidity under 50%.

14. Evaporator Design Most often done by mechanical engineers.
You will have a catalogue to choose evaporator and condenser combinations based on cooling requirements and size.

15. Design Factors
There are several factors looked at for evaporator design:
Pressure Drop
Evaporator Capacity

16. Design Factors Causes of pressure drop: Long evaporators
Not actual size, but the length of a run.
Solved by multiple evaporator circuits.
Tubing too small

17. Design Factors

18. Poor Evaporator Design
Low Gas Velocity
Poor oil return
No “scrubbing” effect, refrigerant debris build up in evaporator tubes.
Oil clogged evaporator

19. Evaporator Capacity Factors that effect evaporator capacity:
Surface Area
Temperature Difference
Refrigerant Velocity
Conductibility (How fast heat moves through metal)
Metal thickness
Air Volume

20. Superheat

21. Superheat
A sensible heat added to the vapor refrigerant after the change of state has taken place.
The difference between the boiling refrigerant and the suction line temperature.

22. Superheat
Is used to check if the evaporator has proper level of refrigerant.
Superheat is gained in the evaporator – refrigerant picks up additional sensible heat after the change in state takes place.

23. Superheat

24. Superheat Normal superheat is between 8-12 degrees for a TXV system.
Depending on the application this can be much lower or higher.
If the superheat is high
Starved coil
Low refrigerant

25. Superheat If the superheat is low
Flooded coil
To much refrigerant
DO NOT ADJUST REFRIGERANT WITH JUST SUPERHEAT UNLESS YOU ARE SURE THAT YOU KNOW HOW THE SYSTEM SHOULD WORK!

26. Superheat
Complete vaporization of refrigerant should occur around the last bend of the evaporator.
Any additional heat absorbed is now referred to as superheat.
The TXV as a metering device is designed to maintain proper superheat.

27. Measuring superheat
Take the temperature of the suction line with a thermometer.
Best to do within 6 inches of the evaporator.
Take the suction pressure and convert to the temperature of saturation.

28. Measuring Superheat
Subtract the saturation temperature from the suction line temperature.
Example:
R22 system
Suction Pressure is 68.5psi (40 degrees)
Suction line temp is 50 degrees
50 – 40 = superheat of 10 degrees

29. Measuring Superheat Add 2 psi to your suction line if:
Condenser is in remote location.
Suction line is well over 8 feet.
You are working on a split system.

30. Trouble shooting with superheat
Domestic and commercial units:
8 to 12 degrees of superheat is the rule of thumb.
Whatever must be done to superheat the opposite must be done to the refrigerant.

31. Troubleshooting with superheat
If you have a superheat of 20 degrees
Superheat must be lowered
Increase refrigerant charge (or flow).
If you have a superheat of 2 degrees
Superheat must be raised
Decrease refrigerant charge (or flow).

32. Troubleshooting with superheat
Anytime you make a superheat adjustment you must wait 10 to 15 minutes prior to making next adjustment.
This wait is so the system will stabalize.

33. Superheat With a fixed orifice metering device or a cap tube:
Adding charge lowers superheat
Removing charge raises superheat

34. Additional Notes
The difference between the temperature of the refrigerant boiling in the evaporator and the temperature at the evaporator outlet is known as the evaporator superheat.

35. Additional Notes
When measuring evaporator superheat on a commercial system with a long suction line the pressure reading should be taken at the evaporator outlet, not the compressor inlet.

36. Additional Notes
Superheat measurements are best taken with the system operating at design conditions.

37. Additional notes
Evaporators can by multi-pass. This means the coil has been folded over on itself or is actually 2 or three coils clamped together and fed by a distributor.

38. Additional Notes
When an evaporator coil is multi pass and has a superheat that is higher than others this can be caused by un-even air distribution, a blocked distributor, or even a dirty coil section.

39. Additional Notes
Evaporators that are used to chill liquids, like the ones found in slurpey machines and soda dispensers can have a normal superheat measurement but not be cooling properly. This is caused by deposits built up on the liquid side of the evaporator or poor circulation of the liquid.