When the liquid refrigerant reaches the evaporator its pressure has been reduced, dissipating its heat content and making it much cooler than the fan air flowing around it. This causes the refrigerant to absorb heat from the warm air and reach its low boiling point rapidly.
When the refrigerant absorbs heat inside the evaporator, there is a rapid change from the liquid state to the vapor state, which is called boiling. This boiling isn't actually hot; refrigerants have very low boiling points compared to water (which is what many of us have observed boiling).
The expansion valve reduces the pressure of the refrigerant before it enters the evaporator. By lowering the pressure, it allows the refrigerant to undergo vaporization (boiling) at a lower temperature, enabling effective heat absorption.
For example the refrigerant commonly used in refrigerators boils between 40° and 50°F as compared to water's boiling point of 212°F. Let's look at the process to see how boiling and condensing a refrigerant can move heat. The process is the same whether it is operating a refrigerator, an air conditioner or a heat pump.
As refrigerant travels through the evaporator, it absorbs heat from the air. As it absorbs heat, it vaporizes. If the system operates according to design, the refrigerant will be 100% vapor as it nears the exit of the evaporator. Before leaving the evaporator, the vapor continues absorbing heat, becoming superheated.
It's boiling temperature is -51.7°C. It's freezing point temperature is -136°C.
The evaporator functions by allowing the refrigerant to evaporate and expand in a controlled environment. As the liquid refrigerant enters the evaporator, it encounters low pressure, which causes it to vaporize and absorb heat from the surrounding air or medium that needs cooling.
The boiling points of refrigerants differ with the pressure; the higher the pressure, the higher boiling points. At atmospheric pressure, their boiling points can be lower than –40°C and therefore they can be used even with low temperature heat sources or to provide low temperature for refrigerated storage.
The evaporator is simply a bank, or coil, of copper tubing. It is filled with Freon 12 at low pressure and temperature. Heat flowing from the air spaces or articles to be cooled into the coil causes the liquid Freon 12 to boil.
The refrigerant vapour enters the evaporator where it absorbs heat from the space being cooled, causing the refrigerant to boil. As it continues through the evaporator coil the vapour is superheated turning the refrigerant to gas before it enters the compressor and starts the cycle over again.
The liquid refrigerant inside the system called R410A boils at only 55.3° Fahrenheit. As the boiling point of water is affected by changing environmental pressure, so are the boiling points of refrigerant in regard to the variance of applied pressure throughout an air conditioning system.
The relationship between the vapour pressure and boiling point is that both are inversely proportional. The more volatile liquid evaporates fast as compared to the less volatile liquid at a low temperature because the volume increases with respect to temperature so it has a low boiling point.
It has the formula CF3CH2F and a boiling point of −26.3 °C (−15.34 °F) at atmospheric pressure. R-134a cylinders are colored light blue.
Typical evaporator temperature range is from –40 °C (–40 °F) to –65 °C (–85 °F).
Refrigerant can shift easily between liquid and gas states, which makes it ideal for ACs since it doesn't take significant amounts of energy to cause the phase shift. Refrigerant starts inside the compressor, where the reduction of volume turns it into a high pressure gas about 150°F.
Adding energy (heating) increases the rate of evaporation
This makes sense because at higher temperatures, more molecules are moving faster; therefore, it is more likely for a molecule to have enough energy to break away from the liquid to become a gas.
The mixture of liquid and gas from the expansion valve enters the evaporator and starts to boil, because heat is transferred from the warmer secondary fluid (b-c). The evaporating refrigerant absorbs energy from the secondary fluid, whose temperature is reduced.
Low refrigerant can also lead to frozen evaporator coils. Without refrigerant passing through the coils, there will be no way for that absorbed heat to be transferred out of the air handler. The lack of pressure within the coil continues to drop, the moisture freezes onto the coils, and ice forms as a result.
The evaporator works the opposite of the condenser, here refrigerant liquid is converted to gas, absorbing heat from the air in the compartment. When the liquid refrigerant reaches the evaporator its pressure has been reduced, dissipating its heat content and making it much cooler than the fan air flowing around it.
That simply means the compressor raises the boiling point of our refrigerant so that when it gets into the condenser, the heat is given off to the cooling water instead of absorbing heat from the cooling water. And it also circulates the refrigerant.
It depends on the refrigerant and evaporator pressure. In a freon 22 system if evaporator pressure is 68.5 psig the temperature of freon would be 40 deg F.
R-134a is not going to disappear in the same way as CFCs (chlorofluorocarbon) or R-12 did. R-12 was phased out in accordance with the Montreal Protocol in the 1990s, as it was found that the chlorine contained in the refrigerant was creating a hole in the ozone layer when it was vented into the atmosphere.
In an ideal vapor-compression refrigeration cycle, the refrigerant enters the compressor as a saturated vapor and is cooled to the saturated liquid state in the condenser. It is then throttled to the evaporator pressure and vaporizes as it absorbs heat from the refrigerated space.
Superheat is sensible heat transfer that provides very little useful cooling. This occurs because there is no change of state; only change of temperature.
Low or High Superheat
Low superheat means that there is too much in the evaporator. High superheat means that there is not enough in the evaporator. High superheat can be caused by restrictions in the line, significant airflow, or a faulty metering device.