Transcript evaporators_and_superheat

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HVACR317 – Refrigeration
Evaporators and Superheat
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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.
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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.
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Evaporator Types
• Two types of evaporators
– Dry type
– Flooded type
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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
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Dry Type Evaporator
• Disadvantages:
– Slower pull-down under heavy loads
– System runs with higher head pressures
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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.
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Flooded Type Evaporator
• Disadvantages
– Larger charges of refrigerant are required.
– There is a greater chance of flood back to the
compressor.
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Evaporator Types
• There are several types of evaporators:
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Plate
Shelf
Wall
Fin and Tube
Bare Coil
Gravity
Forced Air
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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.
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Shelf Type Evaporator
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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.
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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.
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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.
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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.
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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.
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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.
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Evaporator Purposes
• Cooling
– Remove the sensible heat
• Dehumidification
– Remove the latent heat and cause a change of state
from vapor to water.
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Evaporator Design
• Things that affect evaporator efficiency and
capacity:
– Surface Area
– Temperature Difference
– Refrigerant velocity (speed)
– Conductibility
– Metal Thickness
– Air Volume
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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
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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.
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Measuring Superheat
• Example:
– R22 system
– Suction Pressure is 68.5psi (40°F)
– Suction line temp is 50°F
– 50 – 40 = superheat of 10°F
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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.
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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.
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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).
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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.
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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.
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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.
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Additional Notes
• Superheat measurements are best taken with the
system operating at design conditions.
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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.
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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.
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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.