We all sometimes tend to forget, vapor compression refrigeration revolves around temperatures, rather than pressures. The only reason we measure pressures, is to determine the saturated temperature corresponding to the measured pressure, for the particular refrigerant in use. We're not as interested in knowing what the pressure is, so much as knowing the temperature of the refrigerant inside the coil, because the coil was designed to run at a certain temperature, rather than some pressure. But, it's a lot simpler to measure pressures, than refrigerant temperatures inside evaporator or condenser coils.
But how are we able to determine a temperature from a pressure measurement? The short answer is, through use of a "P-T" (pressure-temperature) chart for a given refrigerant.
But, where does the "P-T" chart come from? It goes back to the boiling point concept. Boiling point is a temperature and a pressure. Most refrigerants have very low atmospheric pressure boiling points. R-22 will boil at around -40˚ F in an open container (atmospheric pressure: 0 psig / 14.7 psia), R-410A at around -60˚ F.
But we're always working with refrigerants in closed containers...either sealed refrigerant systems or storage containers.
Once the liquid refrigerant is hemmed up in a closed container, the vapor has no where to go, so any phase change from liquid to vapor state that might take place, results in more vapor, which has to compress, raising the pressure, and subsequently, raising the boiling point temperature.
If the refrigerant inside the container is at a constant temperature, and the container is surrounded by a similar, constant temperature environment, no heat transfer can take place, so no phase change can take place. And, the refrigerant is at it's boiling point temperature, for the particular pressure. In that particular state, the liquid is said to be saturated...it contains all the heat energy it can, without boiling. Said another way, the liquid is "saturated" with heat energy.
Conversely, the vapor phase contains as little heat as it can, without condensing.
If any heat is added to the drum, it will be consumed by the liquid as "latent heat of vaporization", and result in some liquid changing phase to vapor. If the drum were to lose heat, some of the vapor phase would condense back to the liquid state. It's important to remember, saturated conditions can only exist within a mixture of liquid and vapor phases...
With operating refrigerant systems, some portion of the evaporator and condenser coil always contains a mixture of liquid and vapor. That part of the evaporator coil begins at the discharge point of the metering device and ends at whatever location all the liquid has vaporized. The portion in the condenser coil begins wherever the vapor begins to condense, and ends wherever the refrigerant phase becomes 100% liquid.
If the systems are running at "dynamic equilibrium" or steady-state conditions, the pressures are (relatively) constant. If the pressures are constant, the liquid phase temperatures of the refrigerant have to be constant. And those temperatures are what we're looking for...the "saturated" (boiling point) temperature of the evaporator liquid phase, and the saturated (condensing point) temperature of the condenser vapor phase.
Each particular refrigerant has it's own, particular "pressure-temperature" relationship, provided to us by the manufacturers. And with most refrigeration gauges, the temperature "scales" are included with the pressure scales.
The pressures we measure are not necessarily the actual pressure at the point of saturated conditions in the coils, due simply to the mechanical "pressure losses" that take place as the refrigerant passes through the sealed system. They are, however, close enough and generally accepted as the saturated pressures, and we use the corresponding saturated temperatures in making our system evaluations.
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