Closed circuit cooling towers are used in industrial facilities to cool process fluids without bringing them into direct contact with the outside environment. In these towers, heat is transferred from the fluid flowing inside the coil to the coil surface, and then to the spray water and airflow.
When a system must operate at low ambient temperatures or requires protection against freezing, a glycol-water mixture may be used instead of water in the coil circuit.
However, glycol does more than provide freeze protection. It also changes the fluid's specific heat capacity, viscosity, thermal conductivity and flow characteristics inside the coil. These changes can affect the thermal capacity of a closed-circuit cooling tower, the required coil surface area and the pumping requirements.
For this reason, the design of glycol-based systems must consider not only freeze protection but also heat transfer performance and energy consumption.
- What Is Glycol, and Why Is It Used in Closed-Circuit Cooling Towers?
Glycol is a fluid used to lower the freezing point of water. Two types are commonly used in industrial heat-transfer applications:
- Ethylene glycol (MEG): Generally selected for its favourable thermal properties and low-temperature performance. However, because of its toxicity, special consideration is required in applications where contact with people or products may occur.
- Propylene glycol (PG): May be preferred in applications where a lower-toxicity profile is desirable, particularly where contact with people or products is possible. At the same concentration, its thermal and flow properties are generally less favourable than those of ethylene glycol.
For both types, commercial products formulated for heat-transfer systems and containing corrosion inhibitors should be used. Automotive antifreeze products should not automatically be considered suitable for industrial cooling circuits, as their additive packages and material compatibility may differ.
- Why Does the Glycol Concentration Vary?
The glycol concentration is determined by the minimum temperature the system may encounter and the required level of protection. As temperatures fall, the risk of water freezing increases, and a higher glycol concentration may be necessary. However, the following factors must be considered when determining the correct concentration.
3.1. Ambient temperature
The minimum design temperature at the tower's installation location is one of the main inputs for glycol selection. Freeze risk must be assessed separately for systems that continue operating during winter and for systems that may remain shut down for extended periods.
3.2. Minimum temperature of the circulating fluid
The ambient temperature and the minimum temperature reached by the fluid inside the coil may not be the same. Operating conditions, system shutdown, exposure to cold air and the installation environment must all be considered when establishing the design temperature.
3.3. Required level of protection
Glycol concentration can be selected according to two different protection objectives:
- Freeze protection: The fluid is intended to remain pumpable and avoid freezing within the system.
- Burst protection: Protection is intended to reduce the risk of equipment damage if the fluid partially freezes while the system is shut down. This is not equivalent to the requirement for the fluid to remain fully liquid during normal operation and requires confirmation from the product manufacturer.
Where freeze protection is required, it is not sufficient to consider only whether the ambient temperature falls below 0°C. The freezing point of the glycol mixture must be sufficiently lower than the minimum expected fluid temperature.
3.4. Energy consumption and heat-transfer performance
As glycol concentration increases, freeze protection generally improves. However, viscosity also increases, while specific heat capacity and heat-transfer properties change. Consequently, achieving the same heat load may require a higher flow rate, a larger coil surface area or increased pumping power.
Therefore, the objective should be to select the appropriate concentration that provides the required freeze protection, rather than using the highest possible glycol concentration.
- Relationship Between Glycol Concentration and Freezing Temperature
The following table provides indicative freezing-onset temperatures for ethylene glycol and propylene glycol mixtures with water. The values are approximate and based on mass concentration; the actual product formulation and manufacturer data may affect the results.
Glycol concentration by mass | Ethylene glycol: approximate freezing point | Propylene glycol: approximate freezing point |
20% | −7.8°C | −7.1°C |
30% | −14°C | −12.7°C |
40% | −22°C | Around −19°C |
50% | Around −34°C | Verify according to the product |
Important: These values are for preliminary assessment and are not guaranteed design values for every glycol product. Mass percentage and volume percentage are not the same. The manufacturer's technical data for the specific product must be used when selecting the concentration. ASHRAE Handbook – Secondary Coolants
Practical approach to concentration selection
The glycol concentration should not be finalised solely on the basis of ambient temperature. First, determine the minimum temperature the fluid may reach, then select the appropriate concentration from the manufacturer's data. Where freeze protection is required, the mixture's freezing point should be sufficiently below the minimum expected fluid temperature.
- Effect of Glycol Concentration on Heat-Transfer Surface Area
In closed-circuit cooling towers, the use of glycol is an important design factor that affects the required coil surface area.
Three properties are particularly relevant to heat transfer:
Specific heat capacity: The specific heat capacity of a glycol-water mixture is generally lower than that of pure water. The flow rate or temperature difference must therefore be reassessed to transfer the same amount of heat.
Viscosity: As glycol concentration increases, fluid viscosity rises, particularly at low temperatures. This affects the flow inside the coil, pressure drop and pumping requirements.
Heat-transfer coefficient: Depending on the mixture properties and flow conditions inside the coil, the fluid-side heat-transfer coefficient may decrease. As a result, a larger heat-transfer surface may be required to achieve the same capacity.
However, a given reduction in the heat-transfer coefficient does not translate directly into the same percentage increase in coil surface area. Overall heat transfer depends on the combined effects of the fluid side, tube material, coil geometry, spray water, airflow and temperature differences.
For this reason, the selection of a glycol-based closed-circuit cooling tower should not be finalised simply by adding a fixed percentage to the coil surface area calculated for water. The coil's thermal performance must be calculated using the actual glycol concentration and design temperatures.
- Flow Rate and Pump Selection When Using Glycol
When a glycol-water mixture is used, the circulation pump must be reassessed along with the coil surface area.
The following relationship can be used for an approximate heat-load calculation:
Q (kW) = ṁ × Cp × ΔT
Where:
- Q: Heat load (kW)
- ṁ: Mass flow rate (kg/s)
- Cp: Specific heat capacity of the mixture (kJ/kg·K)
- ΔT: Difference between the fluid inlet and outlet temperatures (°C)
When the glycol concentration changes, the specific heat capacity and density also change. Therefore, a flow rate calculated for water should not be used without adjustment. The coil pressure drop must also be calculated, and the pump's operating point must be verified.
The effect of viscosity becomes particularly significant at higher concentrations and lower operating temperatures. This can increase pump energy consumption while adversely affecting heat transfer inside the coil.
- Factors to Consider When Selecting Glycol Concentration
For closed circuit cooling tower projects, the following information should be evaluated together to determine the appropriate glycol concentration:
- The minimum ambient temperature at the installation location.
- The minimum temperature the fluid inside the coil may reach.
- Whether the requirement is freeze protection or protection against equipment damage during shutdown.
- The type of glycol and whether its concentration is expressed by mass or by volume.
- Heat load, flow rate and fluid inlet/outlet temperatures.
- Coil tube material and compatibility with corrosion inhibitors.
- Pump pressure, flow rate and energy consumption.
- The operating temperature and concentration limits recommended by the glycol manufacturer.
In glycol-based systems, the concentration may change over time due to water make-up, leakage or incorrect mixture preparation. The concentration in the circuit should therefore be checked using a suitable method and maintained in accordance with the manufacturer's recommended maintenance procedures.
- Conclusion
The use of glycol in closed circuit cooling towers is an important solution for reducing the risk of freezing and improving system reliability at low temperatures. However, increasing glycol concentration can impose additional demands on heat-transfer performance and pumping.
The correct design approach should therefore include determining the minimum operating temperature, selecting the appropriate glycol type and concentration, recalculating the coil surface area and verifying pump performance.
At CTP Engineering, we emphasise the importance of thermal design based on actual operating conditions and the correct selection of heat-transfer fluids when evaluating closed-circuit cooling towers. By assessing glycol concentration, coil surface area and energy consumption together, a more reliable and efficient system can be designed.