Air Conditioning

1. Air Conditioning 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.