Latent Heat vs. Sensible Heat
Sensible heat is heat that causes a measurable temperature change -- you can feel it and measure it with a thermometer. Latent heat is heat absorbed or released during a phase change (liquid to gas, or gas to liquid) without any change in temperature. In a refrigeration system, the real cooling work happens through latent heat: the refrigerant absorbs large amounts of heat as it boils in the evaporator, all without changing temperature during the phase change itself.
Latent heat drives refrigeration: heat is absorbed during phase change without temperature change. Sensible heat causes a measurable temperature change.
Refrigerant State at Each Point
Leaving the compressor, the refrigerant is a superheated high-pressure vapor (hotter than its saturation temperature at that pressure). As it passes through the condenser, it first desuperheats (sensible heat loss), then condenses from gas to liquid (latent heat rejection), and finally subcools below its saturation temperature. After the metering device, the refrigerant is a cold, low-pressure two-phase mixture of liquid and vapor. In the evaporator, it absorbs heat and fully boils into a saturated or slightly superheated vapor before returning to the compressor.
The refrigerant transitions through four states: superheated vapor after the compressor, subcooled liquid leaving the condenser, two-phase mixture after the metering device, and saturated vapor leaving the evaporator.
Superheat & Subcooling Defined
Superheat is the temperature of a vapor above its saturation (boiling) temperature at a given pressure. It is measured at the evaporator outlet and confirms that all liquid has boiled off, protecting the compressor from liquid slugging. Subcooling is the temperature of a liquid below its saturation (condensing) temperature at a given pressure. It is measured at the condenser outlet and confirms that the refrigerant is fully liquid before reaching the metering device, ensuring efficient system operation.
Superheat (measured at evaporator outlet) protects the compressor from liquid. Subcooling (measured at condenser outlet) ensures solid liquid reaches the metering device.
Phase Changes in the Cycle
The Filter-Drier in the Cycle
The filter-drier is installed in the liquid line between the condenser and the metering device. Its job is to remove moisture, acid, and particulate contaminants from the refrigerant before it reaches the metering device. Moisture in the system can freeze at the metering device orifice, causing a blockage. The desiccant inside the filter-drier (such as molecular sieve) traps water molecules and prevents acid formation that would corrode system components.
The filter-drier sits in the liquid line and removes moisture, acid, and particles. Moisture can freeze at the metering device and block refrigerant flow.
Vapor-Compression Cycle Review
- Latent heat is absorbed or released during phase change without temperature change; sensible heat causes a measurable temperature change.
- After the compressor: superheated high-pressure vapor. After the condenser: subcooled high-pressure liquid.
- After the metering device: cold, low-pressure two-phase mixture. After the evaporator: saturated or superheated low-pressure vapor.
- Superheat at the evaporator outlet protects the compressor from liquid slugging.
- Subcooling at the condenser outlet ensures solid liquid reaches the metering device for efficient operation.
- The filter-drier is in the liquid line and removes moisture, acid, and contaminants before the metering device.