HVACR317 – Refrigeration

1. HVACR317 – Refrigeration Evaporators and Superheat

2. Evaporators • Two principle types • Natural convection: Does not use any mechanical means to move cold air away from evaporator. • Forced convection: Uses fans or blowers to move air around coil.

3. Evaporator Operating Design • Direct expansion • The refrigerant directly cools the air. • Most HVAC systems and refrigeration systems. • Indirect expansion • The refrigerant cools a medium such as water and this medium cools the air. • Chilled water systems.

4. Evaporator Types • Two types of evaporators • Dry type • Flooded type

5. Dry Type Evaporator • 25% less refrigerant than the flooded type, which means more vapor in the evaporator. • The advantages are: • Less refrigerant • Less chance of flood back

6. Dry Type Evaporator • Disadvantages: • Slower pull-down under heavy loads • System runs with higher head pressures

7. Flooded Type Evaporator • Almost all liquid in the evaporator. • Advantages: • 50% more effective than dry expansion. • Liquid refrigerant is in direct contact with most coil surfaces. • Used in chillers where the water coil is submerged in refrigerant.

8. Flooded Type Evaporator • Disadvantages • Larger charges of refrigerant are required. • There is a greater chance of flood back to the compressor.

9. Evaporator Types • There are several types of evaporators: • Plate • Shelf • Wall • Fin and Tube • Bare Coil • Gravity • Forced Air

10. Plate Type Evaporator • The plate type (when found at the back of the refrigerator) is the last part of the evaporator. • As the suction gas comes out of the plate, it returns directly to the compressor. • If plate used in shelves, it is used for contact freezing (conduction). • Has no fans.

11. Plate Type Evaporator, Cont’d • Used in conduction and convection to absorb heat. • Used in domestic refrigerators and freezers.

12. Shelf Type Evaporator • Has no fans. • Used in domestic refrigerators and freezers. • Use conduction and convection to absorb heat. • Built directly into the shelf of domestic refrigerators and freezers. • Can be damaged by using knives and hammers to remove ice from shelves.

13. Wall Type Evaporator • Used in chest freezers and coolers like the white reach-in freezers found in stores. • No coil visible; it is built into the wall of the freezer.

14. Fin and Tube Type Evaporator • Fin and Tube with forced circulation is a commercial type or high-end residential type evaporator. • Requires a fan • More efficient operation.

15. Bare Coil Type Evaporator • Bare Coil type is not as efficient; has less surface area. • Used in older systems. • Used in immersed systems where liquid is in contact with entire coil.

16. Gravity Type Evaporator • Gravity type evaporator is used where high relative humidity is desired. • Lower coil temperature difference between supply and return. • Used in deli cases. • No fan; cold air falls on its own, hot air rises.

17. Forced Air (Blower) Type Evaporator • Forced Air (Blower) type is used in frost- free domestic refrigerators (i.e., those without a defrost cycle). • Normal air flow draws through the coil. • The most widely used type in commercial refrigeration.

18. Evaporator Purposes • Cooling • Remove the sensible heat • Dehumidification • Remove the latent heat and cause a change of state from vapor to water.

19. Evaporator Design • Things that affect evaporator efficiency and capacity: • Surface Area • Temperature Difference • Refrigerant velocity (speed) • Conductibility • Metal Thickness • Air Volume

20. Evaporators, General Info • A low temperature evaporator must be defrosted periodically to prevent ice buildup. • This defrost is required any time the evaporator operates under 32° F. • Ice will cause superheat problems, a loss of efficiency, and compressor problems.

21. Evaporators, General Info. • Defrosting a low temperature evaporator coil can be accomplished by the use of an electric heater or a hot gas bypass from the compressor discharge line.

22. Additional Notes • A dirty evaporator and subsequent low evaporator pressures will cause low head pressure. • The defrost cycle is initiated by a time clock. • The defrost cycle is terminated by time, temperature, or pressure.

23. Additional Notes • With a direct expansion evaporator coil, the refrigerant must boil away as close to the end of the coil as possible in order to a) ensure that frost does not accumulate; and b) to operate at high efficiency.

24. Superheat • Is a sensible heat added to the vapor refrigerant after the change of state has taken place. • Is the difference between the boiling refrigerant and the suction line temperature.

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

26. Superheat • Normal superheat is between 8-12° F for a TXV system. • Depending on the application, this can be much lower or higher. • If the superheat is high, causes can be: • Starved coil • Low refrigerant

27. Superheat • If the superheat is low, causes can be: • Flooded coil • To much refrigerant • Caution: DO NOT adjust refrigerant with superheat alone, unless you are sure that you know how the system should work! • Complete vaporization of refrigerant should occur around the last bend of the evaporator.

28. Superheat • Any additional heat absorbed is now referred to as superheat. • The TXV as a metering device is designed to maintain proper superheat. • With a fixed orifice metering device or a cap tube: • Adding charge lowers superheat • Removing charge raises superheat

29. 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. • Subtract the saturation temperature from the suction line temperature.

30. Measuring Superheat • Example: • R22 system • Suction Pressure is 68.5psi (40°F) • Suction line temp is 50°F • 50 – 40 = superheat of 10°F

31. 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.

32. Troubleshooting 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.

33. 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).

34. Troubleshooting with superheat • Any time you make a superheat adjustment you must wait 10 to 15 minutes prior to making the next adjustment. • This wait allows the system to stabilize.

35. 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.

36. 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.

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

38. 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.

39. 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.

40. Additional Notes • Evaporators that are used to chill liquids, like the ones found in ‘slushy’ 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.