Refrigeration Cycle Fundamentals: The Compressor’s Role and What Changes Across It

The compressor as the work-input “pump” for vapor

In a vapor-compression system, the compressor is the component that adds work to the refrigerant. Its job is not to “make cold,” but to raise the refrigerant vapor pressure so that the refrigerant can reject heat in the condenser. When pressure goes up, the vapor’s temperature also rises, which is exactly what we need: the refrigerant must be hotter than the outdoor/ambient air (or cooling water) so heat can flow out of the condenser.

Think of the compressor as a device that takes in low-pressure vapor from the evaporator and delivers high-pressure vapor to the condenser. That pressure increase is what separates the system into a low side and a high side.

Suction vs discharge: what the compressor “sees”

Suction (inlet) conditions

• Location in the system: compressor inlet, coming from the evaporator outlet and suction line.
• Pressure level: low-side pressure (often called suction pressure).
• Desired refrigerant state: vapor (typically slightly superheated vapor).
• Why it matters: suction pressure strongly influences evaporator temperature and capacity, and suction vapor temperature affects how hot the discharge will become.

Discharge (outlet) conditions

• Location in the system: compressor outlet, feeding the condenser inlet.
• Pressure level: high-side pressure (often called discharge or condensing pressure).
• Refrigerant state: high-pressure vapor (usually superheated vapor).
• Why it matters: discharge temperature affects oil life, compressor reliability, and how easily the condenser can reject heat.

Simplified diagram: low side vs high side around the compressor

          LOW SIDE (low pressure)                     HIGH SIDE (high pressure)  
  Evaporator outlet / suction line                 Discharge line / condenser inlet
                |                                              |
                v                                              v
        +----------------+     Work input (motor)      +----------------+
        |   COMPRESSOR   |  ------------------------>  |  Hot vapor out  |
        |  Cool vapor in |                             |  High pressure  |
        +----------------+                             +----------------+
                ^                                              
                |                                              
         Suction (Ps, Ts)                               Discharge (Pd, Td)

Beginner translation: the compressor takes in cooler, low-pressure vapor and pushes it out as hotter, high-pressure vapor.

Why compressors are designed for vapor (not liquid)

Compressors are built to compress vapor. Vapor is compressible; liquid is not (for practical purposes in HVAC/R). If liquid enters the compressor, several problems can occur.

Key reasons liquid is dangerous

• Liquid is (nearly) incompressible: the compressor tries to reduce volume, but the liquid won’t “squeeze.” This can cause mechanical damage (often called liquid slugging).
• Valve and scroll damage risk: sudden forces can break reeds/valves or damage scroll sets.
• Oil dilution and poor lubrication: liquid refrigerant can wash oil off surfaces or dilute oil, increasing wear.
• Unstable operation: liquid floodback can cause noisy operation, high current draw, and overheating.

Practical “what to watch” (step-by-step mindset)

1. Confirm the evaporator outlet is vapor: the refrigerant leaving the evaporator should be vapor, not a liquid-vapor mix.
2. Ensure the suction line stays vapor: suction line routing/insulation and proper metering help prevent liquid reaching the compressor.
3. Understand that “a little superheat” is intentional: slight superheat at the compressor inlet is commonly used as a safety margin to keep liquid out.

This is why many systems aim for a controlled amount of superheat at the evaporator outlet or compressor inlet: it’s a protection strategy as much as it is a performance setting.

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What changes across the compressor: pressure, temperature, and superheat

Across the compressor, three beginner-friendly ideas are most important:

• Pressure increases: suction pressure (low side) is raised to discharge pressure (high side).
• Temperature increases: compressing vapor raises its temperature, so discharge vapor is hotter than suction vapor.
• Superheat typically increases: the refrigerant is vapor at the inlet and remains vapor at the outlet, but it is usually more superheated after compression.

Why temperature rises when you compress vapor (simple explanation)

The compressor does work on the vapor. That work shows up largely as an increase in the vapor’s internal energy, which you observe as a higher temperature. Even without diving into thermodynamics, you can remember this rule of thumb: higher compression ratio (bigger pressure lift) usually means higher discharge temperature.

Superheat and discharge temperature: connecting the dots

Superheat at the compressor inlet matters because it affects discharge temperature. If the suction vapor starts warmer (more superheated), the discharge tends to end up warmer too.

Two practical cause-and-effect statements beginners can use:

• Higher suction superheat → higher discharge temperature (often).
• Higher discharge pressure (harder to “push into” the condenser) → higher discharge temperature (often).

That’s why technicians pay attention to conditions that raise discharge temperature: restricted condenser airflow, dirty condenser, overcharge, non-condensables, or very high ambient conditions can all increase the pressure the compressor must pump against.

Before/after table across the compressor

A practical walk-through: “what the compressor is doing” in 5 steps

1. Vapor enters at suction: low-pressure vapor arrives from the evaporator side.
2. Compressor traps and reduces volume: the vapor is squeezed into a smaller space.
3. Pressure rises: the vapor’s pressure is lifted from low-side to high-side.
4. Temperature rises: the vapor becomes significantly hotter due to the work input.
5. Hot vapor exits to the condenser: now it is hot enough (and at high enough pressure) to reject heat in the condenser.

Beginner checkpoints: quick “sanity checks” when thinking about compressor conditions

• If the compressor is operating normally, you should expect Pd > Ps and Td > Ts.
• The compressor outlet should be vapor, not liquid.
• Very high discharge temperature usually means the compressor is working too hard (high pressure lift) and/or the suction vapor is too warm (high superheat), and it deserves investigation.