How to Match Evaporating Temperature and Compression Ratio in Low-Temperature Cold Storage Condensing Units
Why Evaporating Temperature Determines Compression Ratio
Evaporating temperature is not an independent choice; it is set by the required room temperature and the temperature difference (TD) across the evaporator coil. For a −25 °C cold room, a TD of 8–12 K is common, which places the evaporating temperature at approximately −35 °C to −40 °C. The compression ratio is calculated from absolute pressures:
Compression Ratio = (Discharge Pressure + 1.013 bar) ÷ (Suction Pressure + 1.013 bar)
Because suction pressure at −40 °C is very low — often below 1 bar absolute for many refrigerants — the denominator becomes small, and the ratio climbs rapidly. A high compression ratio reduces compressor volumetric efficiency, raises discharge temperature, increases the risk of oil breakdown, and reduces the mass flow of refrigerant available for motor cooling. This is why a low-temperature condensing unit cannot be treated as a scaled-down air-conditioning unit.
Typical Parameter Matching for Low-Temperature Cold Storage
The table below shows indicative ranges for common low-temperature cold storage applications. Actual values depend on refrigerant, condensing temperature, compressor design, and system TD. Use these figures as a starting point for selection, not as a substitute for manufacturer data.
| Cold Room Temperature | Typical Evaporating Temperature | Compression Ratio (indicative) | Discharge Temperature Risk | Recommended Compressor Type | Capacity vs. A/C Rating |
|---|---|---|---|---|---|
| −18 °C | −25 °C to −30 °C | 6 – 9 | Moderate | Low-temp piston / scroll | 35% – 45% |
| −25 °C | −35 °C to −40 °C | 10 – 15 | High | Low-temp piston / screw | 25% – 35% |
| −30 °C | −40 °C to −45 °C | 12 – 18 | Very high | Screw / two-stage / economized | 20% – 30% |
| −40 °C and below | −45 °C to −55 °C | 15 – 25+ | Critical | Two-stage / cascade / screw with liquid injection | 15% – 25% |
Compressor Selection and Operating Checks
When the evaporating temperature is in the −35 °C to −40 °C range, a standard air-conditioning compressor will almost certainly fail. The correct choice is a low-temperature piston or screw compressor designed for high compression ratios. For larger capacities, screw compressors with economizers or two-stage configurations are often preferred because they reduce the compression ratio per stage and improve efficiency.
1. Discharge Temperature
High compression ratio naturally produces high discharge temperature. For semi-hermetic reciprocating compressors, discharge temperature should generally be kept below 110–120 °C; for scroll compressors, below 115 °C; and for screw compressors, below 100–110 °C. If the calculated or measured discharge temperature exceeds these limits, the system may require liquid injection, an oil cooler, or a two-stage arrangement. Never allow the compressor to run continuously at excessive discharge temperature, as it accelerates oil carbonization and valve damage.
2. Oil Return
At low evaporating temperatures, refrigerant mass flow is low and oil viscosity can increase. Oil return becomes difficult, especially in long suction lines or vertical risers. The system must be designed with adequate gas velocity, properly sized suction lines, oil separators, and suction accumulators where necessary. Poor oil return leads to compressor starvation, bearing wear, and eventual seizure.
3. Motor Cooling
In semi-hermetic and hermetic compressors, the motor is often cooled by suction gas. At low evaporating temperature, the mass flow of suction gas is reduced, so motor cooling becomes less effective. This is another reason why low-temperature compressors are specifically designed with larger motor cooling passages, liquid injection ports, or external cooling. The selection must verify that the motor winding temperature remains within the manufacturer’s limit under the worst-case condensing condition.
Why Air-Conditioning Capacity Estimates Fail
Air-conditioning compressors are rated at evaporating temperatures around 5 °C to 7 °C. A compressor rated at 10 kW under A/C conditions may deliver only 2.5–3.5 kW at −40 °C evaporating temperature. This dramatic capacity decay is caused by reduced suction gas density, lower volumetric efficiency, and higher compression ratio. If a designer selects a condensing unit based on A/C catalog data, the installed system will be severely undersized, run continuously, and fail to pull down the cold room. Always use the compressor manufacturer’s low-temperature performance tables or selection software with the correct evaporating and condensing temperatures.
Parameterised Matching Checklist
Use the following checklist to verify that the condensing unit is correctly matched to the low-temperature cold storage duty.
| Parameter | Target / Check | Why It Matters |
|---|---|---|
| Evaporating temperature | −35 °C to −40 °C for −25 °C room | Sets suction pressure and compression ratio |
| Compression ratio | Typically 10–15; verify with absolute pressures | Affects volumetric efficiency and discharge temperature |
| Discharge temperature | < 110–120 °C depending on compressor type | Protects oil, valves, and motor windings |
| Oil return | Minimum gas velocity in suction risers; oil separator | Prevents compressor oil starvation |
| Motor cooling | Suction gas flow or liquid injection within limits | Prevents motor overheating and burnout |
| Capacity correction | Use low-temp performance data, not A/C ratings | Ensures sufficient cooling capacity at low evaporating temperature |
| Expansion device | EEV or TXV with low-temp charge and MOP | Maintains stable superheat and prevents flood-back |
| Condensing temperature | As low as practical to reduce compression ratio | Improves efficiency and lowers discharge temperature |
Common Mistakes in Low-Temperature Condensing Unit Selection
- Using A/C capacity tables: This is the single biggest error. Low-temperature capacity is only 20–35% of A/C capacity for the same compressor displacement.
- Ignoring compression ratio limits: A compressor may have a maximum compression ratio limit. Exceeding it causes valve damage and excessive discharge temperature.
- Underestimating oil return problems: Low mass flow and high oil viscosity require careful piping design and oil management.
- Neglecting motor cooling: Reduced suction gas flow can overheat hermetic and semi-hermetic motors. Liquid injection or external cooling may be required.
- Choosing a single-stage compressor for very low temperatures: Below −40 °C evaporating temperature, two-stage or cascade systems are often more reliable and efficient.
- Setting condensing temperature too high: Every 1 K increase in condensing temperature raises the compression ratio and discharge temperature, reducing compressor life.
Conclusion: Match the System, Not the Catalog
Matching evaporating temperature and compression ratio in a low-temperature cold storage condensing unit is a system-level design task. For a −25 °C cold room, expect evaporating temperatures of −35 °C to −40 °C and compression ratios above 10. Select a genuine low-temperature piston or screw compressor, verify discharge temperature, oil return, and motor cooling, and always use low-temperature performance data rather than air-conditioning estimates. When these parameters are correctly matched, the condensing unit will deliver reliable pull-down, stable operation, and long service life — even in the most demanding cold storage environments.
Disclaimer: The parameters and ranges in this article are indicative. Always consult the compressor manufacturer’s selection software and technical documentation for your specific refrigerant, condensing temperature, and operating conditions. Refrigeration systems must be installed and serviced by qualified technicians.
