Cooling Capacity of the Condensing Unit – Why HP Power Is Not Enough for Selection?

One of the most common mistakes when selecting a condensing unit is relying solely on the nominal power of the compressor expressed in HP.In practice, a unit labelled as 2 HP, 3 HP, or 5 HP can achieve very different cooling capacities depending on the evaporation temperature, condensation temperature, refrigerant, and operating conditions of the installation.

The power of the compressor expressed in HP is not equivalent to the cooling capacity of the unit in kW.

To correctly select a unit for a cold room, freezer, or other installation, the actual cooling power requirement should be compared with the capacity of the device specified for a particular operating point.

In this guide, we explain the differences between HP, kW of electrical power, and kW of cooling capacity, how to read manufacturer tables, and what to pay attention to in order to avoid incorrect unit selection.

1. What does the power of the compressor expressed in HP mean?

HP (horsepower) is a unit of power, specifically a mechanical horsepower in the Anglo-Saxon unit system.

In conversion:

1 HP ≈ 0.746 kW of mechanical power

It is important to remember that in the refrigeration industry, the designations of compressors and units expressed in HP often have a nominal or commercial classification.

Therefore, a unit labelled by the manufacturer as 3 HP does not necessarily have exactly 2.24 kW of mechanical power on the shaft nor draw exactly the same electrical power from the network.

In particular, it is not possible to unequivocally determine its cooling capacity based on this.

Important: the designation 3 HP does not mean that the unit has a cooling capacity of 2.24 kW.

Converting HP directly to kW of cooling capacity is a technical error.

2. What is the cooling capacity of a unit?

Cooling capacity determines the amount of heat that a refrigeration system can remove from the cooled environment per unit of time.

It is most often expressed in watts (W) or kilowatts (kW).

For example:

• 1.5 kW means removing heat with an energy flow of 1500 J/s.
• 3.0 kW means removing heat with an energy flow of 3000 J/s.
• 5.0 kW means removing heat with an energy flow of 5000 J/s.

In a refrigeration installation, capacity depends on the properties of the refrigerant, its mass flow rate, and the enthalpy difference during the evaporation process.

In simplification:

Q₀ = ṁ × (h₁ − h₄)

Where:

• Q₀ – cooling capacity [kW];
• ṁ – mass flow rate of the refrigerant [kg/s];
• h₁ – enthalpy of the refrigerant at the evaporator outlet [kJ/kg];
• h₄ – enthalpy of the refrigerant at the evaporator inlet [kJ/kg].

In design practice, capacity is most often determined based on the manufacturer's catalogue tables or selection software.

It is important to distinguish between the capacity of the compressor or unit under catalogue conditions and the useful capacity of the entire installation under actual operating conditions.

3. Why is the unit's capacity not a constant value?

The cooling capacity of the compressor changes with operating parameters.

The most important are the evaporation temperature, condensation temperature, refrigerant, and — in the case of variable speed compressors — the operating frequency.

Evaporation Temperature

The evaporation temperature is related to the suction side pressure of the installation.

For given other conditions, lowering the evaporation temperature usually results in a decrease in the cooling capacity of the compressor.

For example, a unit operating at an evaporation temperature of –5°C may provide greater capacity than the same unit operating at –25°C.

This means that a device suitable for a cold room maintaining a positive temperature may not have sufficient capacity to service a freezer.

Condensation Temperature

The condensation temperature depends, among other things, on the ambient temperature of the condenser, its size, fan performance, and airflow conditions.

An increase in condensation temperature usually causes an increase in compression, a decrease in cooling capacity, and an increase in compressor power demand.

Therefore, a unit operating in summer at high ambient temperatures may achieve lower performance than in cooler conditions.

Type of Refrigerant

Different refrigerants have distinct thermodynamic properties.

The same unit, if the manufacturer allows its operation with multiple refrigerants, may provide different performances depending on the refrigerant used.

One should not assume that the performance for R404A will be identical to that for R448A or R449A.

Compressor Speed

In inverter units, cooling capacity can be regulated by changing the compressor speed.

Operating at a lower frequency may limit performance, while increasing speed can enhance it within the permissible range.

This relationship does not have to be exactly linear, as the efficiency of the compressor and other system parameters also change.

4. Performance of the Same Unit in Different Conditions

To illustrate the impact of evaporation and condensation temperatures, consider a hypothetical 3 HP refrigeration unit.

The following table presents only sample values to illustrate the selection principle. These are not catalogue data for a specific model.

The example shows that the performance of the same unit can vary significantly with operating conditions.

In the described example, the unit provides 5.0 kW at an evaporation temperature of –5°C, but only 1.7 kW at –30°C.

This means that the nominal designation of 3 HP is not sufficient to assess the suitability of the unit for a specific installation.

Additionally, each considered point must be within the permissible operating range of the compressor. Not every compressor can operate across the entire presented temperature range.

5. How Does Cooling Capacity Differ from Electrical Power Consumption?

These are two completely different parameters.

Cooling capacity determines how much heat the device can remove from the cooled space.

Electrical power consumption, on the other hand, indicates how much electrical energy the device consumes per unit of time.

The COP coefficient is used, among other things, to compare energy efficiency.

COP = Q₀ / P

Where:

• Q₀ – cooling capacity [kW];
• P – electrical power consumption [kW].

For example, if a system provides a cooling capacity of 4.5 kW and consumes 1.5 kW of electrical energy, its COP is:

COP = 4.5 / 1.5 = 3.0

This means that for every unit of electrical energy supplied, three units of heat are removed.

When comparing the COP of different devices, ensure they include the same range of components and are determined under comparable operating conditions.

The COP of a compressor will not always equal the COP of the entire refrigeration system, which also includes fans and other energy consumers.

6. Why Can Two Units with the Same HP Rating Have Different Performance?

Two condensing units marked as 3 HP can differ in compressor design, displacement, volumetric efficiency, motor type, and the conditions for which the manufacturer specified the nominal power.

The design of the entire unit, including the condenser and control system, is also of significant importance.

For example:

The data is hypothetical. We assume the same refrigerant, evaporation and condensation temperatures, and consistent performance and power consumption determination principles.

Despite having the same nominal HP class, unit B provides greater performance and a higher COP in the example.

Therefore, when comparing devices from different manufacturers, one should use actual performance data rather than just the HP rating.

7. How to Correctly Read Cooling Capacity Tables?

Manufacturers of condensing units and compressors publish performance tables specifying the devices' performance at various operating points.

During table analysis, the following steps should be taken.

Step 1. Select the appropriate refrigerant.

Performance data should correspond to the refrigerant intended for the installation and approved for the specific device.

Step 2. Determine the evaporation temperature.

The evaporation temperature should result from the selection of the cooler and the required temperature of the cooled space.

Step 3. Determine the condensation temperature.

Consider the ambient temperature, condenser design, and its design operating conditions.

Step 4. Check the table's reference conditions.

Particular attention should be paid to superheat, subcooling, the method of determining performance, and any additional assumptions by the manufacturer.

Step 5. Read the cooling capacity in kW.

This is the value that should be compared with the calculated installation demand.

Step 6. Check the electric power consumption and COP.

This allows for assessing power supply requirements and comparing the energy efficiency of devices.

Step 7. Verify the permissible operating range.

Confirm that the required operating point is within the permissible range of temperatures, pressures, and compressor speeds.

8. Example of Correct Condensing Unit Selection

Assume the designed cold room requires the following parameters:

Consider two hypothetical condensing units with the same nominal power class of 3 HP.

In this example, condensing unit A does not provide the required cooling capacity.

Condensing unit B has a surplus capacity of 0.4 kW, which is approximately 11.4% above the required demand.

However, this does not mean automatic acceptance of the device. Before the final selection, it is necessary to check the capacity control method, start-up conditions, ambient temperatures, condenser characteristics, and other installation requirements.

Conclusion: two condensing units marked as 3 HP can offer completely different application possibilities.

9. What Is the Importance of Cooling Capacity in Inverter Condensing Units?

In the case of inverter condensing units, not only the maximum cooling capacity is important, but also the entire permissible range of regulation.

For example, a hypothetical unit may provide at a specific operating point:

Values are for illustration purposes only.

When selecting an inverter condensing unit, it is necessary to check whether its maximum capacity covers the design demand, and the minimum is suitable for partial load conditions.

Too high a minimum capacity may still force cyclic compressor stops.

It is also necessary to consider frequency limitations resulting from lubrication conditions, motor cooling, and oil return.

10. Common Mistakes When Comparing Condensing Units

Mistake 1. Converting HP directly to cooling kW.

The mechanical power unit conversion does not determine the cooling capacity of the device.

Mistake 2. Comparing performance for different evaporation temperatures.

A unit described as 5 kW at –5°C does not necessarily provide that capacity at –20°C.

Mistake 3. Ignoring the condensing temperature.

A device selected for overly favourable conditions may be insufficient during high ambient temperatures.

Mistake 4. Comparing different refrigerants without data correction.

Performance for R404A should not be directly assumed as performance for R448A or another refrigerant.

Mistake 5. Ignoring the minimum capacity of an inverter unit.

The device must be properly matched for operation under partial load as well.

Mistake 6. Failing to verify the permissible operating range of the compressor.

Required capacity is not the only criterion. The compressor must operate within the range allowed by the manufacturer.

11. How to Select a BOYARD Condensing Unit?

The blueref.pl offers BOYARD condensing units with various nominal capacities and performance regulation methods.

When selecting the appropriate model, consider not only the compressor power in HP but, most importantly, the actual cooling capacity at the required operating point.

Before ordering, it is advisable to prepare:

• required cooling capacity in kW;
• evaporation temperature;
• condensation temperature;
• planned refrigerant;
• ambient temperature of the unit;
• required performance regulation method;
• available electrical supply.

Based on this data, you can compare available models and select a device that meets the specific installation requirements.

Check the offer of BOYARD condensing units at blueref.pl:

BOYARD Condensing Units – Offer and Technical Specifications

12. Summary – Select a Unit Based on kW, Not Just HP

The nominal compressor power in HP is useful for general identification of the device class but does not determine its actual cooling capacity.

To correctly select a condensing unit, use the manufacturer's data specified for the particular refrigerant, evaporation temperature, condensation temperature, and other reference conditions.

Two units with the same nominal HP power may differ in capacity, energy consumption, and permissible operating range.

Therefore, the most important parameter when selecting a unit is its actual cooling capacity in kW, determined for the installation's design conditions.

Need help selecting a unit?

The technical department at blueref.pl will help compare the parameters of available BOYARD units and select a device for the required operating conditions.

Submit the demand for cooling capacity, evaporation temperature, condensation temperature, and selected refrigerant.

Check Available BOYARD Units