The process that allows liquid to turn back into vapor during a heat pump’s heating cycle is a manipulation of basic physics, specifically the relationship between pressure and a fluid’s boiling point. This cycle, which is the refrigeration process running in reverse, enables a heat pump to extract heat energy from the cold outside air and transfer it indoors. By controlling the pressure inside a closed system, engineers can force a special fluid, called refrigerant, to boil and evaporate at extremely low temperatures. This low-temperature boiling allows the refrigerant to capture heat from the ambient environment to warm a home.
Preparing the Refrigerant for Vaporization
The first step in vaporization is preparing the high-pressure liquid refrigerant to absorb heat. The refrigerant, which is at a relatively high temperature and pressure after leaving the indoor coil, must have its pressure reduced. This task is handled by a metering device, such as a thermal expansion valve or a capillary tube, which acts as a precise restriction in the flow path.
As the liquid refrigerant is forced through this narrow opening, its pressure drops rapidly and significantly. This sudden drop lowers the refrigerant’s saturation temperature, which is its boiling point. This is analogous to water boiling at a lower temperature on a high mountain where atmospheric pressure is lower.
A fraction of the liquid instantly flashes into vapor as it crosses this pressure barrier, a process known as flash-gas. This initial vaporization draws heat energy from the remaining liquid refrigerant, causing its temperature to plummet further. The result is a cold, low-pressure mixture of liquid and vapor ready to enter the outdoor coil, ensuring the refrigerant’s temperature is well below that of the outdoor environment.
The Physics of Phase Change and Heat Absorption
The cold, low-pressure refrigerant mixture then enters the outdoor coil, which acts as the system’s evaporator in heating mode. Even when the outside air temperature is, for example, 35°F, the refrigerant circulating inside the coil is much colder, perhaps around 20°F. Heat energy naturally flows from a warmer substance to a colder one, so heat moves from the outdoor air into the colder refrigerant circulating in the coil.
This absorbed heat energy causes the remaining liquid refrigerant to boil and change its state into a vapor. The heat required for this phase change is known as latent heat of vaporization, which is energy absorbed by the fluid without a corresponding increase in its temperature until all the liquid has evaporated. The refrigerant maintains a near-constant, low temperature throughout the outdoor coil as it transitions entirely from a liquid to a gas.
This mechanism is analogous to the cooling sensation felt when sweat evaporates from skin. The evaporating liquid takes a large amount of heat energy from its surroundings to complete the phase change, effectively cooling the source of the heat. This process allows the system to continuously “harvest” thermal energy from the cold ambient air, and the refrigerant leaves the outdoor coil as a low-pressure, low-temperature vapor.
Completing the Heating Cycle
Once the refrigerant has completely turned into a low-pressure vapor, it must be prepared to release that heat indoors. The vapor is drawn directly into the system’s compressor. The primary function of the compressor is to increase the pressure of the vapor dramatically.
As the compressor squeezes the refrigerant gas into a smaller volume, the molecules collide more frequently, converting the mechanical energy of compression into thermal energy. This action simultaneously raises both the pressure and the temperature of the refrigerant. The low-pressure, low-temperature vapor is transformed into a high-pressure, high-temperature gas, often reaching temperatures between 130°F and 180°F.
This superheated gas then travels to the indoor coil, which now functions as the condenser. Because the refrigerant’s temperature is significantly higher than the air inside the home, heat flows out of the gas and into the circulating indoor air. As the refrigerant releases its stored heat energy, it cools down and condenses back into a high-pressure liquid, completing the cycle. This hot liquid is then ready to pass through the expansion device again, where the pressure will drop, and the cycle of vaporization and heat absorption will repeat.