1 air conditioning evaporators for air conditioning
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Air ConditioningAir Conditioning
Evaporators For Air Conditioning
Evaporators For Air Conditioning
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EvaporatorsEvaporators
• 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
• 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
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Natural DraftNatural Draft
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Forced DraftForced Draft
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Operating DesignOperating Design
• Direct Expansion– Refrigerant directly cools the air. The evaporator
coil is full of refrigerant and air is blowing across the coil.
• Direct Expansion– Refrigerant directly cools the air. The evaporator
coil is full of refrigerant and air is blowing across the coil.
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Operating DesignOperating 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.
• 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.
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Indirect ExpansionIndirect Expansion
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Operating DesignOperating Design
• Two types of Direct Expansion coils exist:– Dry Type– Flooded Type
• Two types of Direct Expansion coils exist:– Dry Type– Flooded Type
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Dry TypesDry Types
• Use 25% less refrigerant than the flooded type.
• Have more vapor in the evaporator• Have less chance of floodback to the
compressor.
• Use 25% less refrigerant than the flooded type.
• Have more vapor in the evaporator• Have less chance of floodback to the
compressor.
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Dry TypeDry Type
• The disadvantages of the dry type coil are:– Slower pull-down with heavy loads– System runs with higher head pressures.
• The disadvantages of the dry type coil are:– Slower pull-down with heavy loads– System runs with higher head pressures.
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Evaporator PurposeEvaporator Purpose
• There are two purposes of evaporators:– Cooling– Dehumidification
• There are two purposes of evaporators:– Cooling– Dehumidification
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CoolingCooling
• Changes the sensible heat content in the air.
• This you can actually measure.
• Changes the sensible heat content in the air.
• This you can actually measure.
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DehumidificationDehumidification
• 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%.
• 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%.
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Evaporator DesignEvaporator Design
• Most often done by mechanical engineers.• You will have a catalogue to choose
evaporator and condenser combinations based on cooling requirements and size.
• Most often done by mechanical engineers.• You will have a catalogue to choose
evaporator and condenser combinations based on cooling requirements and size.
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Design FactorsDesign Factors
• There are several factors looked at for evaporator design:– Pressure Drop– Evaporator Capacity
• There are several factors looked at for evaporator design:– Pressure Drop– Evaporator Capacity
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Design FactorsDesign Factors
• Causes of pressure drop:– Long evaporators
• Not actual size, but the length of a run.• Solved by multiple evaporator circuits.
– Tubing too small
• Causes of pressure drop:– Long evaporators
• Not actual size, but the length of a run.• Solved by multiple evaporator circuits.
– Tubing too small
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Design FactorsDesign Factors
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Poor Evaporator DesignPoor Evaporator Design
• Low Gas Velocity– Poor oil return– No “scrubbing” effect, refrigerant debris build up
in evaporator tubes.– Oil clogged evaporator
• Low Gas Velocity– Poor oil return– No “scrubbing” effect, refrigerant debris build up
in evaporator tubes.– Oil clogged evaporator
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Evaporator CapacityEvaporator Capacity
• Factors that effect evaporator capacity:– Surface Area– Temperature Difference– Refrigerant Velocity– Conductibility (How fast heat moves through
metal)– Metal thickness– Air Volume
• Factors that effect evaporator capacity:– Surface Area– Temperature Difference– Refrigerant Velocity– Conductibility (How fast heat moves through
metal)– Metal thickness– Air Volume
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SuperheatSuperheat
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SuperheatSuperheat
• 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.
• 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.
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SuperheatSuperheat
• 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.
• 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.
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SuperheatSuperheat
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SuperheatSuperheat
• 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
• 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
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SuperheatSuperheat
• 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!
• 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!
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SuperheatSuperheat
• 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.
• 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.
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Measuring superheatMeasuring 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.
• 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.
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Measuring SuperheatMeasuring 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
• 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
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Measuring SuperheatMeasuring 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.
• 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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Trouble shooting with superheatTrouble 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.
• 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 superheatTroubleshooting 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).
• 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 superheatTroubleshooting 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.
• 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.
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Superheat Superheat
• With a fixed orifice metering device or a cap tube:– Adding charge lowers superheat– Removing charge raises superheat
• With a fixed orifice metering device or a cap tube:– Adding charge lowers superheat– Removing charge raises superheat
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Additional NotesAdditional 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.
• 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 NotesAdditional 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.
• 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 NotesAdditional Notes
• Superheat measurements are best taken with the system operating at design conditions.
• Superheat measurements are best taken with the system operating at design conditions.
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Additional notesAdditional 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.
• 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 NotesAdditional 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.
• 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 NotesAdditional 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.
• 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.