Refrigerant Pressure-Temperature Charts

• Pressure-Temperature charts correlate pressures and temperatures of certain refrigerants, which is used to convert pressure to a saturation temperature in condensers and evaporators to calculate superheat and subcooling.
• These charts are only applicable when the refrigerant is saturated, where both liquid and vapor are present.
• The refrigerant is saturated in the condenser, evaporator, and receiver.
• The charts correlate refrigerant temperature to pressure readings, and determine refrigerant type in a recovery cylinder.

Dew Point - PT Chart

• Dew point is the temperature at which refrigerant starts to condense
• PT charts provide refrigerants' dew point temperature at a specific pressure to find superheat
• 'Dewperheat' is a mnemonic to remember that dew point is used for superheat and refrigerants with glides.

Bubble Point - PT Chart

• Bubble point is the temperature, at a given pressure, when refrigerant starts to boil.
• Bubble Point is used in calculating subcooling
• 'Bubcooling' is a mnemonic to remember that bubble point is used when calculating subcooling on refrigerants with glides.

Triple Point - PT Chart

• Triple point is a unique combination of pressure and temperature where a substance exists as a solid, liquid, and vapor simultaneously

Subcooling

• Subcooling is typically measured in degrees Fahrenheit, and occurs when a liquid's temperature is below its boiling point.
• In the refrigeration cycle, subcooling takes place in the condenser when a gas is cooled below its saturation/boiling point.
• For example, R410a at 450 PSIG has a saturation temperature of 125°F.
• When the refrigerant is cooled to 113°F, the subcooling is 12°F, which requires subtracting liquid line temperature from the saturated condensing temperature.

Superheat

• Superheat is the condition where a vapor is heated above its boiling point, typically measured in degrees Fahrenheit.
• In the refrigeration cycle, superheat occurs in the evaporator when the gas is heated above its boiling temperature.
• For example, R410a at 119 PSIG has a saturation temperature of 40°F.
• If the gas is heated to 52°F, the superheat becomes 12°F when subtracting the suction line temperature from the saturated suction temperature.

Pressure and Boiling Point

• The pressure of a liquid directly affects its boiling point: increased pressure raises the boiling point, while decreased pressure lowers it.
• Water boils at 212°F at sea level pressure (14.7 PSIA)
• In Denver, Colorado, at 5600 miles above sea level, the pressure is around 11 PSIA, causing water to boil at 203°F due to lower pressure.

Pressure and Evaporation

• Lower pressure leads to an increased evaporation rate because lowering the pressure also lowers the boiling point, enabling faster heat transfer due to a larger temperature difference.
• Low pressure occurs in the evaporator, where the refrigerant's boiling point needs to be lower than the load temperature to facilitate heat transfer from hot to cold.

Pressure and Condensing

• Increased pressure results in increased condensing temperature, leading to more heat being rejected.

Vapor Compression Cycle

• The vapor compression cycle transfers heat from a lower to a higher temperature region, using a working fluid (refrigerant) that changes phase between liquid and vapor states.
• The main components are the compressor, condenser, metering device (expansion), and evaporator.
• It works by creating pressure differences, causing the boiling point of the refrigerant to also change.
• Energy flows from high to low, following the path of least resistance.

Compressor

• The compressor moves a fixed volume of refrigerant internally, regardless of its density, with a variable mass flow rate.
• Refrigerant enters the compressor as a low temperature, low pressure, superheated vapor.
• Refrigerant exits the compressor as a high pressure, high temperature, superheated vapor.

Discharge Line

• The discharge line contains high pressure, high temperature, superheated vapor, and connects the compressor to the condenser.
• The refrigerant gets compressed and travels into the discharge line.
• Heat gain comes from compressing the refrigerant, and it is also picked up by the compressor motor itself.

Condenser

• The condenser rejects heat absorbed by the evaporator plus heat of compression, and is where the condenser de-superheats, condenses, and subcools.
• Heat of compression is typically an additional 20%.
• Heat is rejected to the condenser's ambient air or supplied water, and the temperature falls below the refrigerant's bubble point or boiling point.
• This condenses the vapor into a saturated liquid. Continued heat rejection further lowers the temperature, subcooling the refrigerant.

Liquid Line

• The liquid line is located on the outlet of the condenser/receiver and feeds the metering/expansion device.
• The liquid line needs to maintain a solid column of liquid for the metering device to operate properly, and can be susceptible to flashing of the refrigerant if it picks up too much additional heat.
• Flashing before the expansion valve is undesirable.

Expansion Device

• The expansion device separates the high/low side of the system and must have a certain pressure drop to operate properly.
• It receives subcooled liquid (no flashing can occur prior), which drops in pressure, flashing off into a saturated liquid.
• It lowers the boiling point of the refrigerant, and the refrigerant expands when the liquid flashes off into a saturated liquid.

Evaporator

• The evaporator can be thought of as a sponge, with the purpose of absorbing heat it must be colder than the product/air.
• A 10 row evaporator operates best if the refrigerant starts to boil off around the 9th row.
• The most efficient way is to feed the coil with enough liquid refrigerant, so it starts to boil off around the last 10% of the coil because because liquid is denser and can absorb more heat.

Condensate Line

• When the evaporator coil becomes colder than the air's dew point temperature, moisture will condense on the coil.
• The moisture falls into the drain pan, and drains through the condensate drain line.
• Condensate lines located in an area below the freezing temperature of water need a heater.

Refrigeration Cycle Summary

• The compressor creates a pressure difference, allowing flow.
• The condenser rejects the heat absorbed by the evaporator plus the heat of compression, subcooling the refrigerant.
• Subcooled liquid feeds into the expansion device, where it becomes a superheat vapor, and drops in pressure entering the evaporator.
• The evaporator absorbs the heat, boiling off refrigerant near the end of the coil.
• Additional heat on the refrigerant gains after it's boiled off to become a superheat vapor.