HVAC · Lesson 4 of 13Intermediate9 min read

The Refrigeration Cycle

Air conditioners, heat pumps, refrigerators and dehumidifiers all run the same loop. A fluid boils where it’s cold, soaking up heat; a compressor squeezes it; then it condenses where it’s warm and gives the heat away. This is the heart of the course.

The big idea: boil it cold, condense it hot

Two facts from Lesson 2 make the whole cycle work. First, a liquid soaks up a lot of heat when it evaporates, and gives the same heat back when it condenses. Second, the temperature at which a liquid boils depends on the pressure. Water boils at 100°C at normal air pressure, but at higher pressure it stays liquid at higher temperatures, which is how a pressure cooker cooks faster.

A refrigerant is a fluid picked to boil at low temperatures. Keep it at low pressure and it boils colder than your living room, so it can soak up heat from indoor air. Squeeze it to high pressure and it condenses at a temperature hotter than the outdoor air, so it gives that heat away outside. The machine’s whole job is to keep one side of the loop at low pressure and the other at high pressure.

A fluid’s boiling (and condensing) temperature rises with pressure. The cycle runs the evaporator on the low side, where the refrigerant boils colder than the room, and the condenser on the high side, where it condenses hotter than the outdoors. Schematic, not to scale. Source: OpenStax College Physics 2e, 13.5.

Press play to watch the refrigerant go around, or step through it one stage at a time. Colors show the refrigerant’s state, from cold liquid (blue) to hot gas (red). The COP readout is the Carnot limit from OpenStax: the best any machine could do between the temperatures you set. The coil temperatures use the gap you choose, an assumption of the model.

Four parts, four changes

Compressor. Takes in cool, low-pressure refrigerant vapor and squeezes it into hot, high-pressure vapor. This is where the electricity goes in.
Condenser. A coil of tubing and thin fins, with a fan blowing air across it. The hot vapor gives its heat to the air and condenses into a warm, high-pressure liquid.
Expansion device (also called a metering device). A small opening, such as a thermostatic expansion valve, an electronic valve or a fixed orifice, that the liquid squeezes through. On the far side the pressure drops; some of the liquid flashes into vapor, and that chills the rest. Out comes a cold mix of liquid and vapor.
Evaporator. Another finned coil. Warm air blowing across it gives up heat, and the cold refrigerant boils into vapor as it absorbs that heat. The vapor flows back to the compressor, and the loop starts again.

In a central air conditioner, the evaporator is the indoor coil and the condenser is in the outdoor unit. In a heat pump running in heating mode, they trade jobs (Lesson 6).

The refrigeration loop, part by part
Part	What it does	Signs of trouble
Compressor	Raises the refrigerant’s pressure and temperature; the only part that needs much power.	Won’t start, hums, trips the breaker, or runs but doesn’t cool.
Condenser coil and fan	Dumps heat from the hot refrigerant into the outdoor air.	Coil clogged with dirt or cottonwood fluff; fan not spinning.
Expansion (metering) device	Drops the pressure so the refrigerant gets cold.	Diagnosed by a technician with gauges and temperature readings.
Evaporator coil	Absorbs heat from the indoor air; water condenses on it.	Ice on the coil or lines; dirty fins block airflow.
Suction line	The larger, insulated copper line; carries cool, low-pressure vapor back to the compressor.	Missing or torn insulation; ice.
Liquid line	The smaller copper line; carries warm, high-pressure liquid toward the expansion device.	Oily spots at joints can mean a leak.
Refrigerant	The working fluid that carries heat around the loop.	Low charge means a leak, which a certified technician must find and fix.

Where the energy goes

Energy is conserved, so all the heat the evaporator picks up indoors, plus all the work the compressor puts in, comes out at the condenser. That’s why the air blowing out of an outdoor unit in summer feels hotter than the outdoor air.

The efficiency of a cooling machine is its coefficient of performance (COP): the heat it removes divided by the work it uses. OpenStax notes that real air conditioners and refrigerators have COPs from about 2 to 6, so they move two to six times more heat than the electricity they use.

The second law of thermodynamics sets a ceiling, the Carnot limit, which depends only on the two temperatures in kelvins. The smaller the temperature lift, the higher the limit. Between a 75°F room (297 K) and a 95°F afternoon (308 K), no machine could beat a COP of about 27. Real machines fall far short, partly because the indoor coil must run colder than the room and the outdoor coil hotter than the outdoor air, which widens the lift. Larger coils, clean coils and good airflow keep that gap small.

COP_cooling = Q_c ÷ W Q_h = Q_c + W
Carnot limit: COP_cooling ≤ T_c ÷ (T_h − T_c) (temperatures in kelvins)
EER (BTU/h per watt) = 3.412 × COP

Refrigerants, then and now

Refrigerants have changed as their side effects became known. R-22, an HCFC, damages the ozone layer; since January 1, 2020, no new R-22 may be made in or imported into the United States, though recycled and reclaimed R-22 remains available for existing systems. Its replacement in most new systems, R-410A, doesn’t harm the ozone layer but is a potent greenhouse gas: EPA lists its global warming potential (GWP) as 2,090. GWP compares a gas’s warming effect with the same amount of carbon dioxide, which has a GWP of 1.

New home air conditioners and heat pumps are moving to refrigerants with a GWP of 700 or less, such as R-454B (GWP 470) and R-32 (GWP 675). EPA lists both in safety class A2L and accepts them “with use conditions,” so equipment, tools and procedures are built for them. Lesson 5 covers the rules and dates.

StopRefrigerant work is for certified technicians

Under EPA’s Section 608 rules, a technician must be certified to attach gauges to a system, add or remove refrigerant, or do anything else that could release it. Intentionally venting refrigerant during service, repair or disposal is illegal. The refrigerant is also under high pressure and can cause frostbite-like burns. If your system is low on refrigerant, it has a leak, and EPA advises asking the technician to find and fix the leak rather than just adding more.

Check yourself

Tap an answer to check it, or open “Show the answer.”

In an air conditioner, where does the refrigerant absorb heat from the house?
Show the answer
In the evaporator (the indoor coil). The cold refrigerant boils in the evaporator, soaking up heat from the indoor air blowing across it.
What does the expansion device do?
Show the answer
Drops the pressure, so the refrigerant gets cold. Squeezing through a small opening drops the pressure; some liquid flashes to vapor and chills the rest.
Why must the compressor raise the refrigerant’s pressure?
Show the answer
So it condenses at a temperature hotter than the outdoor air and can give its heat away there. Higher pressure means a higher condensing temperature. Heat only flows out to the air if the refrigerant is hotter than the air.
Compared with the heat removed from the house, the heat released outdoors is:
Show the answer
Larger, by the compressor’s work. Q_h = Q_c + W: the outdoor coil dumps the indoor heat plus the energy the compressor added.
What happens to the Carnot limit on COP as the indoor and outdoor temperatures get farther apart?
Show the answer
It falls. A bigger temperature lift means a lower limit, which is why air conditioners work hardest on the hottest days.

Go deeper

Science Lab: Engines, Refrigerators and Heat Pumps Engines, refrigerators and heat pumps in the Science Lab, with the Carnot limit.
OpenStax College Physics 2e, 15.5 Applications of Thermodynamics: Heat Pumps and Refrigerators OpenStax on heat pumps, refrigerators and COP, with worked examples.
OpenStax College Physics 2e, 13.5 Phase Changes Why boiling temperature depends on pressure.
U.S. EPA: Stationary Refrigeration and Air Conditioning (Section 608) EPA’s refrigerant management program.

Sources

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A free course from Teddy Rook Book. It explains how HVAC systems work in general and follows U.S. sources; your equipment’s manuals and the local code are the final word, along with your gas utility for anything involving gas.