The Refrigeration Cycle

An air conditioner, a heat pump, a grocery store freezer and a household refrigerator all run on the same loop. A fluid called refrigerant picks up heat in one place, carries it, and gives it up somewhere else. The fluid is never used up. It just keeps circling. Learn this loop once and you will recognize it in every system you meet.

What You Will Be Able to Do

• Name the four main components in order and say what each one does.
• State whether refrigerant is a liquid, a vapor or a mix at each point in the loop.
• Explain why lowering pressure makes refrigerant boil at a low temperature.
• Point out the high side and the low side on a real system.
• Choose the name of a metering device from how it works.

The Idea Behind the Cycle

Water boils at 212°F at sea level. Lower the pressure, and it boils at a much lower temperature. Raise the pressure, and it needs more heat to boil. Every fluid behaves this way, and for each pressure there is one exact boiling temperature. This matched pair is called the saturation point, and a pressure-temperature chart lists these pairs for each refrigerant.

Refrigerants are chosen because they boil at very low temperatures. By controlling pressure, a system can make refrigerant boil at 40°F inside an indoor coil and condense at 105°F inside an outdoor coil. The first coil is colder than room air, so it absorbs heat. The second is hotter than outdoor air, so it gives heat away. That is why a system can dump heat outdoors even on a 95°F day.

The Four Components in Order

The usual order to learn is compressor, condenser, metering device, evaporator, and back to the compressor.

Compressor

The compressor pulls in low-pressure vapor from the evaporator and squeezes it into high-pressure, high-temperature vapor. It is the pump that keeps refrigerant moving and creates the pressure difference that makes the system work.

Compressors are built to compress vapor only. Liquid does not compress, so liquid reaching the compressor can break valves and other internal parts. This damage is called slugging or floodback. Common types include reciprocating (piston), scroll and rotary in smaller systems, with screw and centrifugal compressors in large ones.

Oil travels with the refrigerant to lubricate the compressor, so the piping has to bring that oil back.

Condenser

The condenser is where the system gets rid of heat. Hot vapor flows through the coil while a fan pushes outdoor air across it, or water carries the heat away. As it gives up heat, the vapor condenses into a high-pressure liquid. By the outlet it should be fully liquid and a little cooler than its condensing temperature. That extra cooling is called subcooling.

Metering Device

The metering device, also called the expansion device, controls how much liquid enters the evaporator and creates a sharp pressure drop. When pressure drops, the boiling point drops, so a small part of the liquid flashes to vapor and the rest gets very cold.

A TXV or EEV adjusts to the load. A fixed device cannot, so the charge has to be set more exactly.

Evaporator

The evaporator is where the system absorbs heat. A cold mix of liquid and vapor enters the coil. Warm air blows across it, and the liquid boils into vapor as it soaks up heat. Boiling is what does the work, because latent heat moves a great deal of energy without much change in temperature.

By the outlet the refrigerant should be all vapor and a few degrees warmer than its boiling point. That extra warming is called superheat, and it proves no liquid is on its way to the compressor. Water in the air also condenses on the cold coil and runs to a drain, which is how an air conditioner dehumidifies.

What the Refrigerant Is Doing at Each Point

This example uses R-410A in a home air conditioner on a warm day. Real numbers vary with load and weather.

Look at the two pressure groups. The compressor and metering device divide the loop into two halves:

• High side: compressor outlet, condenser and liquid line.
• Low side: metering device outlet, evaporator, suction line and compressor inlet.

Reading the Lines on a Split System

When a split air conditioner is running normally, you can tell the lines apart:

• The suction line is the larger line. It is cool and usually insulated, and it carries low-pressure vapor back to the compressor.
• The liquid line is the smaller line. It is warmer than the suction line, and it carries high-pressure liquid to the metering device.
• The discharge line is the short, very hot line between the compressor and the condenser inside the outdoor unit.

Warning: The discharge line can burn you, and refrigerant lines hold high pressure. Do not touch or open them until someone qualified shows you how.

Other Parts You Will See

• Filter-drier: removes moisture and dirt from the refrigerant, usually in the liquid line.
• Sight glass: a small window in the liquid line on some systems.
• Accumulator: catches liquid in the suction line before it reaches the compressor. It is standard on heat pumps.
• Receiver: a tank that stores extra liquid on the high side, common in commercial refrigeration.

A Low Charge Means a Leak

Refrigerant is not consumed like gasoline. It stays in the loop unless there is a leak. A system that is low on refrigerant has a leak somewhere, or it was charged wrong to begin with. With too little refrigerant, the evaporator runs colder than it should and can freeze. Less cool vapor returns to the compressor, so the compressor can overheat.

Why the Circuit Must Stay Clean and Dry

The cycle only works with refrigerant and oil in the circuit, and nothing else. Moisture reacts with them to form acids that destroy a compressor. That is why a system that has been opened gets evacuated to 500 microns or lower, and must hold that vacuum for at least 10 minutes before it is charged. Dirt matters too. Copper heated in air forms a black oxide scale inside the tube, so dry nitrogen flows through the lines during brazing. Without it, flakes of scale travel through the system and plug the metering device.

Check Your Understanding

A coil at 40°F is inside a duct with 75°F air blowing across it. Which way does heat flow, and what happens to the refrigerant? Heat flows from the warm air into the coil, and the liquid refrigerant boils into vapor. This coil is the evaporator.

A technician says a line "goes from hot to warm as it passes through a coil outside." Which component is that? The condenser. Vapor is giving up heat to outdoor air and turning into liquid.

A home unit has a device with a small bulb strapped to the suction line. What is it, and what is it trying to hold steady? A thermostatic expansion valve, which adjusts flow to hold a steady superheat.

Why does the compressor need vapor, not liquid, at its inlet? Liquid does not compress, so it can break internal parts.

Key Takeaways

• Refrigerant moves heat by boiling in the evaporator and condensing in the condenser.
• The order is compressor, condenser, metering device, evaporator.
• Pressure sets the boiling point, so changing pressure lets one fluid both absorb and reject heat.
• The compressor and metering device split the loop into a high side and a low side.
• Superheat protects the compressor from liquid, and subcooling proves the liquid line holds solid liquid.
• Refrigerant does not wear out. A low charge means a leak.