Have you ever experienced a situation where a refrigeration system could no longer reach its previous temperature after changing the refrigerant, even though the compressor remained the same?
Or have you noticed that two compressors with the same horsepower or displacement can have different cooling capacities?
Why does this happen?
The answer is closely related to cooling capacity, refrigerant properties and compressor operating conditions. To understand these differences, we first need to distinguish three important compressor specifications: horsepower, displacement and cooling capacity.
These three specifications describe different characteristics of a refrigeration compressor.
| Parameter | Meaning |
|---|---|
| Horsepower (HP) | Indicates the compressor motor's power rating |
| Displacement (m³/h) | The theoretical volume of refrigerant vapor displaced by the compressor per hour |
| Cooling Capacity (kW) | The amount of heat removed by the refrigeration system per unit of time under specified conditions |
Among these three parameters, cooling capacity is the most important indicator when evaluating whether a compressor can meet a refrigeration system's cooling requirements.
Cooling capacity depends on the amount of refrigerant circulating through the system and the amount of heat each kilogram of refrigerant absorbs in the evaporator.
This means that even when horsepower and displacement remain unchanged, cooling capacity can vary depending on the refrigerant and operating conditions.
Several factors directly influence the cooling capacity of a refrigeration compressor.
Evaporating Temperature
When evaporating temperature decreases, suction pressure and refrigerant vapor density generally decrease. This reduces the refrigerant mass flow rate and, consequently, the compressor's cooling capacity.
Condensing Temperature
A higher condensing temperature increases the compression pressure ratio. This generally reduces cooling capacity while increasing the power required for compression.
Refrigerant Type
Different refrigerants have different vapor densities and thermodynamic properties. As a result, the same compressor can deliver different cooling capacities when operating with different approved refrigerants.
Compressor Design and Efficiency
Internal design, volumetric efficiency, valve losses and internal leakage also affect actual refrigerant mass flow. Therefore, compressors with similar displacement values may still have different cooling capacities.
The following examples illustrate how operating conditions and refrigerant selection affect compressor performance.
Take the Danfoss SZ090-4VI scroll compressor as an example.
According to Danfoss' published performance data, this compressor delivers the following cooling capacities with R407C at different operating temperatures.
| Evaporating Temperature | Condensing Temperature | Cooling Capacity |
|---|---|---|
| 0°C | 35°C | 19.30 kW |
| 0°C | 55°C | 15.00 kW |
| -5°C | 35°C | 15.60 kW |
Conditions: R407C, 50 Hz, 11.1 K suction gas superheat and 8.3 K liquid subcooling.
When the condensing temperature increases from 35°C to 55°C, cooling capacity drops from 19.30 kW to 15.00 kW, a reduction of approximately 22%.
Similarly, when the evaporating temperature decreases from 0°C to -5°C at a constant condensing temperature of 35°C, cooling capacity decreases to 15.60 kW.
The compressor model, motor and refrigerant have not changed. Only the operating temperatures are different.
This explains why the same compressor may perform differently in systems operating under different temperature conditions.
Changing the refrigerant can also change the cooling capacity, even when the compressor and operating temperatures remain the same.
Take the Copeland ZR72KCE-TFD-522 scroll compressor as an example.
Copeland's published performance data provides the following cooling capacities for this compressor model with different refrigerants:
| Compressor Model | Refrigerant | Cooling Capacity |
|---|---|---|
| ZR72KCE-TFD-522 | R22 | 17.60 kW |
| ZR72KCE-TFD-522 | R407C | 14.75 kW |
| ZR72KCE-TFD-522 | R134a | 11.80 kW |
Conditions: 5°C evaporating temperature, 50°C condensing temperature, 10 K suction gas superheat, 0 K liquid subcooling and 50 Hz.
Under these identical rating conditions:
R407C provides approximately 16% less cooling capacity than R22.
R134a provides approximately 33% less cooling capacity than R22.
The difference comes from the refrigerants' thermodynamic properties, which affect refrigerant mass flow and the amount of heat absorbed during evaporation.
These comparisons help explain why a refrigeration system may no longer achieve its previous cooling performance after a refrigerant change.
However, a refrigerant change must also account for the compressor's approved refrigerants, lubricant compatibility and system configuration. Published performance comparisons do not mean refrigerants can be directly exchanged in an existing system.
Whether replacing a compressor or changing the refrigerant in an existing system, cooling capacity should be evaluated under the intended operating conditions.
The following factors are particularly important:
Refrigerant compatibility: Confirm that the compressor and its lubricating oil are suitable for the intended refrigerant.
Required cooling capacity: Compare compressor performance at the system's actual evaporating and condensing temperatures, rather than relying only on nominal horsepower.
System components: Check whether the expansion device, evaporator and condenser are suitable for the refrigerant and required capacity.
Operating conditions: Verify refrigerant charge, superheat, subcooling, operating frequency and other relevant system parameters.
A refrigerant with lower cooling capacity under the same conditions may require changes to the system design or compressor selection to maintain the original performance.
If a refrigeration system cannot reach its previous temperature after a refrigerant change, the first step is to check whether the new refrigerant provides sufficient cooling capacity under the system's actual operating conditions.
If the cooling capacity is insufficient, a compressor with a suitable rated capacity or appropriate system modifications may be necessary.
If the expected cooling capacity is adequate, check the refrigerant charge, expansion device, heat exchangers and operating pressures to identify potential system problems.
The key is to match the refrigerant, compressor cooling capacity and system operating conditions rather than relying on horsepower or displacement alone.
