Hermetic vs Scroll vs Semi-Hermetic Condensing Units

Choose a hermetic reciprocating condensing unit for small walk-ins up to about 5 HP, a scroll unit for medium-size chillers and freezers in roughly the 2–15 HP range where efficiency and low noise matter, and a semi-hermetic piston unit for larger rooms, deep-freeze and blast-freezing duty, or anywhere you need a compressor that can be repaired on site. The final selection should always be based on capacity in kW at your design evaporating and ambient temperatures.

What is the difference between hermetic, scroll and semi-hermetic compressors?

Hermetic reciprocating compressors have the motor and piston mechanism sealed in a welded steel shell. They are compact and low cost, but they cannot be opened for repair; a failed compressor is replaced.

Scroll compressors compress refrigerant between a fixed and an orbiting spiral. With fewer moving parts and no valves, they run smoothly and quietly and are efficient at medium temperatures. They are also hermetically sealed.

Semi-hermetic piston compressors use a bolted cast-iron body. Valve plates, bearings, pistons and even the motor can be serviced in the field. They tolerate high compression ratios and wet operating conditions better, and they support capacity control through cylinder unloading or variable-speed drives.

How do the three types compare?

Why does evaporating temperature matter more than horsepower?

Horsepower describes the motor, not the refrigeration capacity. The same compressor delivers much less cooling as the evaporating temperature drops, because the suction gas is thinner and the pressure ratio is higher. As a general pattern, a compressor's capacity at −35 °C evaporating is often only 35–45% of its capacity at −10 °C.

A practical way to set design conditions:

• Evaporating temperature: room temperature minus 6–10 K. For example −8 °C for a +2 °C chiller, −25 to −28 °C for a −18 °C freezer, and −40 to −45 °C for a −35 °C blast freezer.
• Condensing temperature: design ambient plus 10–15 K for air-cooled units. At 35 °C ambient, use about 45–50 °C; at 45 °C ambient, use 55–60 °C and confirm the compressor envelope allows it.

Ask every supplier for the capacity at the same evaporating and condensing temperatures, refrigerant and voltage. That is the only fair comparison.

As an illustration, a +2 °C vegetable chiller needing 7.1 kW at −8 °C evaporating and 35 °C ambient will often fall in the 3–4 HP class with a scroll or hermetic compressor on R404A, depending on the manufacturer. A −18 °C freezer needing the same 7.1 kW at −28 °C evaporating would typically need a much larger compressor, often a semi-hermetic in the 6–8 HP class. The horsepower label alone would never reveal that difference.

When should you choose a semi-hermetic unit?

• Required capacity is above about 15 kW at medium temperature or 8 kW at low temperature.
• The room temperature is −25 °C or lower, or it is a blast freezer with heavy pull-down loads.
• The site is remote and a technician must be able to repair the compressor rather than wait for a replacement.
• Several rooms share one condensing unit, so capacity control is needed to match changing loads.
• The compression ratio exceeds about 10:1, where single-stage hermetic designs struggle and two-stage or economized semi-hermetic compressors become appropriate.

Box-type or open-type condensing unit?

What components should a quality condensing unit include?

• High- and low-pressure switches, or a combined dual pressure switch, for compressor protection.
• Liquid receiver sized for the system charge, plus filter drier and sight glass with moisture indicator.
• Crankcase heater to prevent liquid migration during off cycles, essential for outdoor low-temperature units.
• Oil separator on low-temperature and long-piping systems, and suction accumulator where liquid floodback is possible.
• Condenser fans sized for the design ambient, with fan speed control or head-pressure control for cold climates.
• Electrical panel with contactor, overload protection and phase-failure protection for three-phase supplies.

How do voltage and frequency affect selection?

Small units up to about 2–3 HP are commonly available for 220 V single-phase. Larger units normally need three-phase supply: 380 V/50 Hz is common in China, Europe, Africa and the Middle East, while 440–460 V/60 Hz is found in parts of the Americas and on ships. A compressor running at 60 Hz turns about 20% faster than at 50 Hz, so its displacement and capacity rise by roughly that amount—provided the motor is rated for that supply. Always specify voltage, phase and frequency on the inquiry so the compressor and fan motors are correct from the factory.

Which refrigerant should the unit use?

Common choices for cold rooms are R404A and R507 for low and medium temperature, R134a for medium-to-high temperature, R22 where it is still permitted for new equipment, and lower-GWP blends such as R448A and R449A. Rules differ by country, so confirm what is allowed and serviceable locally before ordering.

How do hot climates affect condensing unit selection?

Air-cooled condensing units reject heat to the surrounding air, so their capacity falls and their power input rises as ambient temperature increases. A unit that comfortably delivers 10 kW at 32 °C ambient may deliver noticeably less at 45 °C, while drawing more current. At very high condensing temperatures, some compressors reach the limit of their operating envelope and trip on high pressure or high discharge temperature.

For sites in the Middle East, North Africa, South Asia and similar climates:

• Select the compressor at a condensing temperature of about 55–60 °C and confirm it is inside the manufacturer's envelope for your evaporating temperature.
• Specify an oversized or tropical-rated air-cooled condenser to keep condensing pressure down.
• Place the unit in shade with free airflow; avoid enclosed yards where hot discharge air recirculates.
• Use high-temperature-rated fan motors, electrical components and cable insulation.
• Consider liquid injection or a compressor with lower discharge temperatures for low-temperature duty.

What maintenance does each compressor type need?

All condensing units need the same basic care: clean condenser coils, check refrigerant charge and leaks, inspect electrical connections and confirm that safety switches work. The differences are in what happens when something fails.

• Hermetic and scroll units need little routine compressor maintenance, but a failed compressor must be replaced. Keep the system clean and dry, because acid and moisture shorten motor life.
• Semi-hermetic units benefit from periodic oil checks and, for larger units, oil analysis. Valve plates, gaskets and bearings can be replaced on site, which reduces downtime where replacement compressors are far away.

How do you select the right unit step by step?

1. Calculate the refrigeration load and required capacity in kW.
2. Set the evaporating temperature from the room temperature and the condensing temperature from the design ambient.
3. Confirm the refrigerant allowed and available in your market.
4. Choose the compressor type using the table above, considering size, temperature and service needs.
5. Choose box-type or open-type based on location and noise.
6. Specify voltage, phase and frequency, and match the evaporator capacity at the same conditions.

Browse our box-type and open-type condensing units with hermetic, scroll and semi-hermetic compressors, and pair them with matched air cooler evaporators. For a selection at your exact conditions, send us your room data.

Frequently asked questions