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HVACR317 - RefrigerationRefrigerant Cycle

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Refrigerants• Compressors are not meant to run

in a vacuum. Running a compressor in a vacuum will damage the windings.

• Refrigerants provide the cooling for compressors.

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Refrigerants

• At atmospheric pressure, water boils at 212°F.

• If the pressure drops, the boiling point will drop as well - for example, in higher altitudes.

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Refrigerants

• All refrigerants have a specified boiling point temperature at a specified atmospheric pressure.− Example 1: R134A at 0 psig will boil

at -14°F.− Example 2: R12 at 0 psig has a

boiling point of -20°F.− So, knowing the boiling point is at 0

psig, a chart can be created to show the boiling point at other pressures.

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Using Temp / Pressure Chart

• The chart on the next slide is the temperature / pressure chart.– Find the column for R134A, and locate

.1 psig on the column. Look to the left and find the temperature. R134A boils at -15°F at atmospheric pressure.

– Most systems do not need to run at temperatures this cold so the pressures are increased (by the use of a compressor), which makes the boiling point increase.

6Temperature Pressure Chart

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CFCs and HCFCs

• CFCs– Chloro Fluoro Carbons– Old refrigerants being phased out.– Contain chlorine

• HCFCs– HydroChloroFluoroCarbons– Less harmful– Contains some hydrogen

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HFCs and Oils

• HFCs– HydroFluoroCarbon – Newer refrigerants– No damage to Ozone layer

• Refrigeration Oils– Lubrication for compressors– Polyol Ester & Alkyl Benzene are

synthetic oils.

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Refrigerant Containers• Color Coded Containers

– Refer to your handout from National Refrigerants. You must know the colors for R22, R134A, R410A, R409A, R502 and R12.

– The oils used are specific for the type of refrigerants. Take care not to use the wrong oil with the refrigerant.

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System Components

• System Components Consist of the

Following: – Compressor– Condenser– Metering Device– Evaporator Coil– Liquid Line– Suction Line– Filter Drier– Sight Glasses

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Compressor

• Compressor– Pumps or circulates the refrigerant

through the system. The compressor converts low pressure vapor refrigerant to high pressure vapor refrigerant.

– The compressor does not pump liquid.

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Condenser

• Condenser: – Rejects heat from the system.– Condenses high pressure vapor into

high pressure liquid refrigerant approximately 30F above ambient temperature.

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Metering Device

• Metering Device:– Controls the flow of refrigerant

entering the evaporator.– Restricts the flow of refrigerant which

starts the boiling process. (Almost like a resistor in electrical circuits).

• TXV is a type of metering device. TXV stands for Thermostatic Expansion Valve.

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Evaporator Coil & Liquid Line

• Evaporator Coil:– Absorbs heat from the medium being

cooled.– Evaporates the refrigerant into a

100% low pressure vapor.• Liquid line:

– Carries the high pressure liquid refrigerant from the condenser to the metering device.

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Suction Line

• Suction Line:– Carries the low pressure vapor from

the evaporator to the compressor inlet.

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Filter Dryer

• Filter Dryer:– Located on the liquid line and

removes the moisture and/or debris from the system.

– The drier material is made out of Silica gel or a molecular sieve which both filter and absorb moisture.

– The liquid line filter should be replaced any time the system is opened.

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Filter Dryer

• Filter Dryer, cont’d– Special cleanup driers and suction

line “core type” driers are usually installed on systems that are badly contaminated.

– Make sure you are aware of the direction of the arrow on filter driers. Most are not bi-directional.

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Sight Glasses

• Sight Glass– Located in the liquid line. Allows a

view of the refrigerant flowing in the system.

– Some sight glasses have a moisture indicator on them to let you know if moisture is in the system.

– Do NOT rely on the sight glass to charge a system. A clear sight glass can mean the system is either empty or full.

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System Components

• Sight Glass, cont’d– DO NOT charge or diagnose a system

by the sight glass. Always use the gauges.

– Customers will call and complain about “seeing bubbles in the sight glass” and assume it is a low charge. You cannot rely on that.

– The only correct way to diagnose a refrigeration system is with Superheat and subcooling.

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Refrigerant Cycle Sequence

• Refrigerant leaves the compressor as a high pressure vapor in the discharge line.

• Heat from the compressor and the refrigerant is released in the condenser coil.

• At this pressure, the boiling point is MUCH higher than the outdoor temperature (ambient air).

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Refrigerant Cycle Sequence

• As the heat is removed from the refrigerant, it starts to condense to a liquid state. This is the heat of condensation.

• Once the refrigerant is 100% liquid, a sensible heat change occurs as the liquid subcools.

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Refrigerant Cycle Sequence

• The liquid line now carries the refrigerant to the metering device.

• The refrigerant is forced through the pinhole in the metering device.

• Flash gas occurs (80% liquid, 20% vapor) as the refrigerant exits the metering device into the evaporator coil.

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Refrigerant Cycle Sequence

• Latent heat of evaporation now occurs in the evaporator coil as the refrigerant boils off.

• When the refrigerant is 100% vapor, a sensible heat increase will further heat the vapor (this is superheat).

• The refrigerant returns to the compressor through the suction line.

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Superheat Defined

• Superheat:– A sensible heat change that occurs in

approximately the last pass of the tubing in the evaporator coil.

– It is a sensible heat gain once the latent heat change has been completed.

– The latent heat change is the liquid changing to gas.

– When superheat is normal the heat transfer in the evaporator coil is working properly.

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Subcooling Defined

• Subcooling:– Sensible heat change in the condenser

coil following the change of state.– After the condenser coil completely

condenses the high pressure - high temperature vapor to high pressure liquid, the temperature should drop an additional 10 - 20°F as it enters the liquid line. This additional drop is called "subcooling."

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Additional Notes

• Flash Gas:– Right after the metering device.– 80% liquid and 20% vapor.

• Saturation Point:– The point at which the refrigerant is

100% liquid or 100% vapor. The liquid saturation point is in the condenser. The vapor saturation point is in the evaporator.

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Additional Notes

• It is important to remember that refrigerant is NEVER used up. If it is not in the system, there must be a leak.

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Refrigerant Cycle Diagram

CondenserEvaporator

Compressor

Metering Device

FilterDrier

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Refrigerant State Diagram

FilterDrier

VAPOR

LIQUID