Two components in a refrigeration system are specifically designed to change refrigerant pressure: the compressor raises it, and the expansion device lowers it. These two parts create the pressure difference that makes the entire cooling cycle work. But other components, including the evaporator and condenser coils, also affect pressure in ways that matter for system performance. Here’s how each one works.
The Compressor: Raising Pressure
The compressor is the primary component responsible for increasing refrigerant pressure. It takes in low-pressure vapor from the evaporator and compresses it into high-pressure, high-temperature vapor that flows to the condenser. In a common reciprocating design, a motor-driven piston moves down inside a cylinder, drawing refrigerant in through an intake valve. When the piston moves back up, it compresses the gas and pushes it out through an exhaust valve. The principle is similar to how a car engine works, just applied to refrigerant gas instead of fuel.
The pressure increase is substantial. In a typical residential system using R-410A refrigerant during cooling mode, suction pressure (the low side entering the compressor) sits between 115 and 140 psi. Discharge pressure (the high side leaving the compressor) reaches 400 to 450 psi. In heating mode, these numbers climb even higher, with suction pressure between 200 and 280 psi and discharge pressure between 500 and 600 psi.
The ratio of discharge pressure to suction pressure, called the compression ratio, directly affects system efficiency. When this ratio gets too high, single-stage compressors struggle. Reciprocating and rotary vane compressors both have physical compression ratio limits. For very cold applications where the suction temperature drops below about -25°F, systems often use two compressors in stages to split the work and maintain efficiency.
The Expansion Device: Dropping Pressure
If the compressor is the engine of the pressure cycle, the expansion device is its opposite. Installed between the condenser and the evaporator, this component forces a rapid, controlled drop in refrigerant pressure. That pressure drop causes the refrigerant’s boiling point to plunge, which is what allows it to absorb heat and produce cooling on the other side.
The physics behind this pressure drop are surprisingly violent at a microscopic level. As high-pressure liquid refrigerant passes through the narrow opening of an expansion valve, the sudden depressurization causes some of the liquid to “flash” into vapor almost explosively. This flashing process happens in a narrow, observable region and transforms the refrigerant from a warm, high-pressure liquid into a cold, low-pressure mixture of liquid and vapor.
Two main types of expansion devices handle this job:
- Thermal expansion valves (TXVs) and electronic expansion valves (EEVs) actively regulate how much refrigerant passes through based on system conditions. They adjust the flow rate to match changing cooling loads, which makes them common in central air conditioning and commercial refrigeration.
- Capillary tubes are simpler, fixed-restriction devices used in smaller systems like window units and household refrigerators. These narrow tubes, typically 1 to 3 mm in diameter and 1 to 4 meters long, create a pressure drop through friction as the refrigerant squeezes through. The tube’s length and diameter determine how much the pressure drops. Several combinations of length and diameter can produce the same pressure reduction, giving designers flexibility.
Evaporator and Condenser Coils: Friction Losses
The evaporator and condenser aren’t designed to change pressure, but they do. As refrigerant flows through the long, winding paths inside these heat exchanger coils, friction between the fluid and the tube walls causes a gradual pressure loss. This is an unavoidable side effect of the refrigerant traveling through narrow passages while exchanging heat with the surrounding air.
The size of these friction losses varies depending on the refrigerant type. Newer HFC refrigerants like R-134a and R-32 produce noticeably higher pressure drops through coils compared to the older refrigerants they replaced. Refrigerant blends such as R-407A, B, and C also show higher pressure losses. These differences matter for system design because excessive pressure drop in the coils reduces efficiency, forcing the compressor to work harder to maintain the desired temperature difference.
The pressure drop also behaves differently depending on whether the refrigerant is flowing as a single-phase fluid (all liquid or all vapor) or as a two-phase mixture (liquid and vapor together). Two-phase flow, which is common inside the evaporator as the refrigerant boils, is harder to predict and generally creates more complex pressure behavior.
How Pressure and Temperature Connect
Understanding why pressure changes matter requires one key fact: for any given refrigerant, pressure and boiling point are locked together. A single-component refrigerant boils and condenses at one specific temperature for each pressure level. Raise the pressure and the boiling point rises. Lower the pressure and the boiling point drops.
This relationship is what makes the entire refrigeration cycle possible. The compressor raises the pressure so the refrigerant’s boiling point climbs above outdoor air temperature, allowing heat to flow out through the condenser. The expansion device then drops the pressure so the boiling point falls below indoor air temperature, allowing the refrigerant to absorb heat through the evaporator. Every pressure change in the system is ultimately a temperature change in disguise.
Refrigerant blends add a wrinkle. Unlike single-component refrigerants, blends experience “temperature glide,” where the boiling temperature shifts depending on how much of the refrigerant is liquid versus vapor at a given pressure. This means the same pressure can correspond to slightly different temperatures at different points in the evaporator or condenser.
Pressure Switches: Safety Controls
Pressure switches don’t change the refrigerant pressure themselves, but they monitor it and shut the system down when pressure moves outside safe limits. These are critical safety components that protect the compressor from damage.
A low-pressure switch is installed near the evaporator outlet, on the suction side of the system. If suction pressure drops too low, perhaps from a refrigerant leak or a blocked filter, the switch cuts power to the compressor. Without this protection, the compressor could overheat, the motor could burn out, or ice could form on the evaporator.
A high-pressure switch sits on the discharge side, typically between the compressor outlet and the expansion device. If discharge pressure climbs too high, often from a dirty condenser or a failed fan, the switch shuts the compressor off before the excess pressure causes mechanical failure, overheating, or leaks at stressed joints. Many systems combine both functions into a single dual pressure switch that guards against both extremes. Larger commercial compressors may also use an oil differential pressure switch, which monitors lubrication pressure and shuts things down if the compressor starts running without adequate oil flow.