If you’ve ever wondered how your AC or refrigerator actually makes things cool, here’s the simple truth: it doesn’t “make cold”—it moves heat. The tool that makes that possible is called the refrigeration cycle, and it’s running quietly in homes, restaurants, supermarkets, office buildings, and even your car.
Below is a friendly walk-through of the parts, what each does, the “hot gas to liquid to vapor” dance, and a few basics on sizing and measurements. By the end, you’ll understand more about your system than most people do—and why good maintenance and proper setup matter.
Big idea: Heat transfer, not “cold making”
- Heat always flows from warm to cool.
- Refrigerants are special fluids that absorb heat indoors and dump it outdoors by changing state (boiling and condensing) at convenient temperatures.
Think of the refrigerant as a heat shuttle running a loop: pick up heat inside → drop it off outside → repeat.
Meet the four main components
Every modern AC, heat pump, and refrigeration system uses the same four “stops” in the loop:
- Compressor (the pump)
- Squeezes cool low-pressure vapor into hot, high-pressure vapor (“discharge” or hot gas).
- This adds energy (heat) to the refrigerant and keeps it moving around the loop.
- Condenser (the outdoor coil on ACs)
- The hot vapor travels through the coil and rejects heat to outdoor air.
- As it cools, it condenses into a high-pressure liquid.
- Leaving the condenser, it’s often a bit subcooled (more on that below).
- Metering device (TXV/TEV, EEV, or fixed orifice)
- A tiny “pinch point” that drops the pressure of the liquid refrigerant.
- Low pressure = low boiling point, so the refrigerant is now ready to boil at indoor temperatures.
- Evaporator (the indoor coil on ACs)
- Warm indoor air passes over the coil.
- The refrigerant boils (evaporates) inside the coil, absorbing heat from that air.
- The house air leaves cooler and drier; the refrigerant leaves as a low-pressure vapor headed back to the compressor.
That’s the loop. Over and over, quietly, all day.
The three refrigerant lines you’ll hear about
- Discharge (hot gas) line: Compressor → condenser. It’s hot because the vapor is under high pressure.
- Liquid line: Condenser → metering device. It carries high-pressure liquid.
- Suction line: Evaporator → compressor. It returns cool, low-pressure vapor to be compressed again.
The states of refrigerant (and two useful terms)
- Vapor: gas form.
- Liquid: liquid form.
- Saturated: right at the point of changing state (boiling/condensing).
- Superheat: vapor that’s hotter than its boiling point (safeguards the compressor from ingesting liquid).
- Subcooling: liquid that’s cooler than its condensing temperature (ensures a solid column of liquid reaches the metering device for steady cooling).
Why techs measure these:
- Correct superheat helps protect the compressor and confirms the evaporator is doing its job.
- Correct subcooling confirms the condenser and charge are right so the metering device gets pure liquid, not foam.
A 30-second history tour
- Early systems used ammonia (R717) and CO₂ (R744) in industrial plants.
- Mid-20th century homes saw CFCs/HCFCs (e.g., R12, R22). These worked well but harmed the ozone layer and were phased out.
- Most 2000s–2020s home systems used HFCs like R410A (no ozone harm but higher global warming potential).
- Today’s new residential systems are moving to lower-GWP refrigerants like R32 or R454B (called A2L, “mildly flammable,” handled safely under modern codes).
Different refrigerants, same four-part cycle.
How much cooling or heating do you need?
We size systems by BTU per hour (British Thermal Units).
- 1 ton of cooling = 12,000 BTU/hr.
- Caledonian uses Manual J load calculations to determine how many BTUs your home actually needs based on square footage, insulation, windows, sun exposure, and more. Right-sized equipment = better comfort and lower bills.
Where you’ll find the cycle in everyday life
- Residential & light commercial AC/heat pumps (your home comfort)
- Refrigeration (grocery cases, restaurant walk-ins, ice machines)
- Industrial (cold storage, process cooling)
Same physics, different sizes and pressures.
Why each piece matters (and common issues we look for)
- Compressor: Needs cool, superheated vapor back from the evaporator. Liquid in the compressor = bad news.
- Condenser: Must move outdoor air freely; dirty coils raise pressure, cut efficiency, and strain parts.
- Metering device: Must meter liquid, not bubbly mix; that’s why subcooling matters.
- Evaporator: Needs the right airflow; clogged filters or undersized/ leaky ducts reduce capacity and can cause freezing.
The cycle, step by step (simple version)
- Compress: Cool vapor → hot, high-pressure vapor (discharge line).
- Condense: Hot vapor → liquid in the condenser as it dumps heat outdoors (liquid line).
- Meter: Liquid’s pressure drops at the expansion valve/orifice.
- Evaporate: Low-pressure liquid boils in the indoor coil, absorbing heat from your air (suction line).
Back to #1. That’s it.
Subcooling & Superheat—how we measure
- Subcooling (condenser outlet):
- Read the saturation temperature for the condenser pressure.
- Measure the actual liquid line temperature.
- Subcooling = saturation temp − actual temp.
If it’s too low, you may be short on charge or have other issues. Too high could mean a restriction or overcharge.
- Superheat (evaporator outlet/suction line):
- Read the evaporator saturation temperature for the suction pressure.
- Measure the actual suction line temperature.
- Superheat = actual temp − saturation temp.
If it’s too low, you risk liquid at the compressor; too high may mean low charge or airflow problems.
Why airflow and ducts matter to the cycle
Refrigerant can only move as much heat as the airflow lets it. Undersized or leaky ducts, clogged filters, or wrong blower speed = poor heat transfer, low capacity, higher bills, and comfort complaints. (This is why we measure static pressure and check returns/supplies during maintenance.)
Different systems, same physics
- Split systems: Indoor coil + outdoor unit connected by lines.
- Package units: Everything in one cabinet (common on rooftops).
- Heat pumps: Run the same cycle in reverse to provide heating; they still rely on proper charge and airflow.
How Caledonian Mechanical puts this into practice
- Right-size first: We calculate the actual BTUs your home needs (Manual J).
- Design the air path: We check ducts, returns, filter sizing, and static pressure so the coils can move heat efficiently.
- Set the charge precisely: We use subcooling and superheat (and manufacturer tables) instead of guessing.
- Commission & document: We record temperatures, pressures, electrical readings, and photos so you know the system is performing as promised.
- Maintain it: Clean coils, correct airflow, tight electrical connections, and verified charge keep the cycle healthy for the long haul.
Bottom line
The refrigeration cycle is heat in, heat out, using a refrigerant that changes state at the right moments. Get the basics right—proper sizing, clean coils, correct charge, and good airflow—and any brand of equipment will perform better, last longer, and cost less to run.
If you’d like us to explain your system using your actual numbers (subcooling, superheat, static pressure, delta-T), we’re happy to walk you through it in plain English and show where small tune-ups can make a big difference.