evaporators_and_superheat

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

Home appliances

16 cards

Clocks

33 cards

Auto parts

16 cards

Create Flashcards