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.