Nominal capacity is one of the most frequently used concepts in the design of cooling systems, equipment selection and the evaluation of technical quotations. However, capacity definitions used by different manufacturers and equipment categories are not always based on the same principles.
In particular, the unit TR (Ton of Refrigeration), commonly used in the cooling tower industry, can sometimes be confused with the capacity ratings of chillers and other cooling equipment.
For accurate equipment selection, technical comparison and performance evaluation, nominal capacity must be considered together with its reference conditions and its relationship to actual operating capacity.
- What Is Nominal Capacity?
Nominal capacity is the capacity value assigned to equipment under specified reference conditions. It serves as an important indicator for equipment classification and comparison.
However, nominal capacity does not mean that the equipment will deliver the same performance under all operating conditions. Actual capacity may vary depending on the equipment type, design, operating conditions and the capacity definition being used.
In the heating and cooling industry, capacity may be expressed in different units, including kW, kcal/h, BTU/h and TR. Although these units can be converted into one another, the values being compared must represent the same physical quantity and the same type of capacity.
For example, the cooling capacity of a chiller and the heat rejection capacity of a cooling tower are not the same physical quantity. Even when both are expressed in kW or TR, they should not be considered directly equivalent.
- The TR Unit and Its Use in the Cooling Tower Industry
TR stands for Ton of Refrigeration and is a unit used to express cooling capacity.
According to the conventional definition used for refrigeration machines and chiller systems:
1 TR = 12,000 BTU/h ≈ 3.517 kW of cooling capacity
In the cooling tower industry, however, one of the traditional nominal capacity definitions considers one nominal cooling tower TR to represent 15,000 BTU/h, equivalent to approximately 4.396 kW of heat rejection.
This distinction must be taken into account when evaluating capacity.
Capacity definition | Equivalent BTU/h | Approximate kW equivalent |
Chiller cooling capacity, 1 TR | 12,000 | 3.517 |
Nominal cooling tower capacity, 1 TR | 15,000 | 4.396 |
The value of 15,000 BTU/h refers to traditional nominal capacity definitions used for cooling towers. It should not be assumed that every manufacturer or industrial application follows the same definition. Therefore, the applicable capacity basis must be verified in the technical specification and the manufacturer's documentation.
- Relationship Between Chiller Capacity and Cooling Tower Capacity
A chiller transfers the heat extracted from the cooled fluid to the outdoor environment through its condenser circuit. During this process, part of the electrical energy consumed by the compressor is also converted into heat and rejected through the condenser.
Consequently, the heat load that a cooling tower must handle is not limited to the chiller's cooling capacity alone.
In general:
Condenser heat rejection = Heat absorbed at the evaporator + Energy transferred to the system by the compressor
This relationship represents the basic energy balance. Actual calculations must take into account the chiller's technical data, the effects of auxiliary equipment and the system's operating conditions.
Therefore, selecting a cooling tower with a nominal capacity of 500 TR automatically for a chiller with a cooling capacity of 500 TR is not a sound engineering approach. The tower must be selected according to the actual heat rejection requirement of the condenser circuit and the specified design conditions.
- Reference Conditions Used to Define Nominal Cooling Tower Capacity
Nominal cooling tower capacity definitions may be based on specified water temperatures, water flow rate and the wet-bulb temperature of the ambient air.
The conventional reference conditions used for nominal capacity ratings of comfort-cooling towers are generally as follows:
Parameter | Reference value |
Hot water inlet temperature | 35 °C |
Cold water outlet temperature | Approximately 29.4 °C |
Water temperature range | Approximately 5.6 °C |
Inlet air wet-bulb temperature | Approximately 25.6 °C |
Water flow per nominal tower TR | 3 US gpm |
Heat rejection per nominal tower TR | Approximately 4.396 kW |
These values are provided to explain the traditional nominal cooling tower capacity approach. They do not guarantee that a specific product will achieve its rated capacity under these conditions. The manufacturer's capacity definition and technical documentation must be consulted.
Wet-bulb temperature is one of the principal environmental parameters affecting evaporative cooling performance. Water flow rate, inlet and outlet water temperatures, and approach temperature also influence tower performance.
- Calculation of Actual Heat Rejection Capacity
The heat transfer associated with circulating water in a cooling tower can be approximately calculated using the following equation:
Q = V̇ × ΔT × 1.163
Where:
- Q: Heat transfer or heat rejection load (kW)
- V̇: Water flow rate (m³/h)
- ΔT: Temperature difference between the hot water entering and the cold water leaving the tower (°C)
- 1.163: Approximate conversion factor based on the specific heat and density of water
For example, the heat rejection required to cool a water flow of 100 m³/h from 35 °C to 30 °C is:
Q = 100 × (35 − 30) × 1.163
Q ≈ 581.5 kW
This calculation indicates the amount of heat that must be removed from the water at the specified flow rate and temperature difference.
However, the calculated heat load and the tower's ability to meet that load under specific ambient conditions are two different matters. Actual tower performance must be verified through thermal selection and the manufacturer's performance data.
- Range and Approach
Two important parameters in cooling tower selection are the water temperature range and the approach temperature.
Range — Water Temperature Difference
The difference between the hot water entering the tower and the cold water leaving it.
For example:
35 °C − 30 °C = 5 °C
The range directly affects the amount of heat carried by the water. At a constant flow rate, a larger temperature difference increases the amount of heat that must be removed.
Approach — Approach Temperature
The difference between the cold water outlet temperature and the wet-bulb temperature of the entering air.
For example, if the cold water outlet temperature is 30 °C and the wet-bulb temperature is 25.6 °C:
30 °C − 25.6 °C = 4.4 °C
In general, achieving a lower approach temperature requires higher thermal performance. This can affect tower dimensions, heat transfer surface area, airflow and fan power consumption. It may also increase the initial investment cost and ongoing operating expenses.
Therefore, tower dimensions and energy consumption cannot be determined solely from the nominal TR rating.
- Nominal Capacity, Design Capacity and Actual Operating Capacity
Three fundamental concepts must be distinguished in technical evaluations.
Nominal capacity: The capacity stated under specified reference or classification conditions.
Design capacity: The heat load that must be handled under the project's specified fluid flow rate, inlet and outlet temperatures, and ambient conditions.
Actual operating capacity: The capacity delivered by the equipment under its actual operating conditions. Ambient conditions, fluid flow rate, load variations, control settings and equipment condition can all affect this value.
Knowing the nominal capacity provides a useful starting point for equipment evaluation. However, whether the required performance can be achieved in a specific application can only be established by evaluating the specified design conditions.
- Nominal Capacity in Other Heating and Cooling Equipment
Although the concept of nominal capacity is used across different equipment categories, it does not necessarily represent the same physical quantity in every case.
- Chillers: The heat removed from the cooled fluid.
- Air-conditioning and VRF systems: Cooling or heating capacity under specified test conditions.
- Heat pumps: Heating or cooling capacity depending on operating conditions.
- Boilers: Useful heat output or, in some documentation, fuel input power.
- Heat exchangers: Heat transfer capacity under specified fluid and temperature conditions.
- Condensers: Evaluated according to the total heat load that must be rejected from the refrigerant.
- Cooling towers: Heat rejection to the atmosphere under specified water flow, temperature and ambient conditions.
To compare the capacity ratings of these equipment types, the applicable definition, unit and reference conditions must be examined together.
- Correct Capacity Definitions in Technical Quotations
At CTP Engineering, we consider it important that nominal capacity should not be evaluated separately from the design conditions when selecting cooling towers and assessing technical quotations.
The following information should be evaluated together when reviewing a cooling tower requirement:
- The requested nominal capacity and the definition used for that rating.
- The design heat load (kW).
- The water or process fluid flow rate.
- The fluid inlet and outlet temperatures.
- The design wet-bulb temperature.
- The operating principle: open-circuit or closed-circuit.
- Fluid properties and, where applicable, glycol concentration.
- Dimensional restrictions, noise limits, energy consumption and other project constraints.
For closed-circuit cooling towers in particular, the flow rate and properties of the process fluid inside the coil must be distinguished from the flow rate of the external spray water. Coil design, heat transfer performance and fan selection must be evaluated accordingly.
From a technical standpoint, it is preferable for quotations to state not only the nominal capacity but also, wherever possible, the design heat load and the relevant operating conditions.
Conclusion
Nominal capacity is an important concept for classifying and comparing heating and cooling equipment. However, its correct interpretation depends on the equipment type and the capacity definition being used.
In the cooling tower industry, it is essential to recognize that nominal cooling tower TR and chiller TR may be based on different definitions. Equipment selection must therefore be based not only on nominal capacity, but also on the actual heat rejection requirement and the specified design conditions.
Correct capacity definitions, accurate thermal selection and clearly stated design conditions are fundamental to reliable performance, meaningful technical comparisons and long-term system efficiency.
