- The Impact of Glycol on Free Cooling Performance
The use of glycol in closed circuit cooling towers can help reduce the risk of freezing at low ambient temperatures, allowing the system to operate over a wider temperature range. However, glycol does not automatically increase free cooling capacity or guarantee lower energy consumption.
The physical properties of glycol water mixtures differ from those of pure water. Changes in viscosity, specific heat capacity and thermal conductivity can affect heat transfer and flow conditions inside the coil.
Two key factors should therefore be considered together when evaluating a glycol-based closed-circuit cooling tower:
- Operational advantage: Reduced freezing risk at low temperatures and the potential to extend the free cooling operating range.
- Performance impact: Changes in heat transfer capacity, pressure drop, and pump and fan energy consumption.
In CTP Engineering applications, the objective is to balance these factors and provide the required process cooling capacity under reliable and energy-efficient operating conditions.
1.1. Operational Benefits at Low Temperatures
Glycol reduces the risk of freezing within the coil and piping. This is particularly important in facilities exposed to low ambient temperatures during winter.
In a properly designed system, glycol may help increase the number of hours during which ambient conditions can be used for free cooling. The actual benefit depends on the required process temperature, ambient conditions, glycol concentration and cooling tower capacity.
An increase in free cooling operating hours does not necessarily result in an equivalent increase in annual energy savings. The actual benefit must be calculated based on the total energy consumption of the system.
1.2. Heat Transfer and Hydraulic Performance
As glycol concentration increases, viscosity may rise, while specific heat capacity and thermal conductivity may change. These variations can affect heat transfer and pressure drop inside the coil.
Consequently, under identical coil geometry, flow rate and temperature conditions, the cooling capacity achieved with a glycol mixture may differ from that achieved with water.
The capacity of a glycol-based system should not be determined solely from water-based catalogue ratings or a fixed capacity correction factor. The actual fluid properties and design conditions must be taken into account.
- The Effect of Glycol Concentration on Capacity and Pressure Drop
Glycol concentration is an important parameter affecting the thermal and hydraulic design of closed-circuit cooling towers.
Increasing the concentration generally improves freeze protection. However, fluid viscosity may also increase, resulting in higher system pressure losses. The resulting change in heat transfer performance depends not only on glycol concentration but also on coil geometry, flow rate, fluid temperature and air-side conditions.
Glycol concentration | Design considerations |
10–20% | Changes in fluid properties and the required level of freeze protection should be evaluated. |
20–30% | Coil pressure drop and the pump operating point require closer assessment. |
30–40% | Thermal capacity, hydraulic performance and coil selection should be evaluated together. |
Above 40% | Detailed thermal and hydraulic calculations based on the fluid's technical properties are required. |
These ranges do not represent specific percentages of capacity loss or pressure-drop increase. Actual results must be calculated using the technical data of the selected glycol and the operating conditions of the system.
For CTP Engineering, the appropriate approach is to assess glycol concentration in conjunction with the required cooling capacity and pump operating conditions, rather than selecting it solely on the basis of freezing temperature.
- The Effect of Glycol on Pump Energy Consumption
Increasing glycol concentration can raise fluid viscosity, particularly at low fluid temperatures. This may increase pressure losses in the coil and piping, changing the pump operating point and power requirement.
The approximate hydraulic power required by a pump can be calculated using:
P = Q × Δp / η
Where:
- P: Hydraulic power (W)
- Q: Volumetric flow rate (m³/s)
- Δp: Total pressure difference (Pa)
- η: Pump hydraulic efficiency
Motor and, where applicable, variable-frequency drive efficiencies must also be considered when calculating electrical power consumption.
When selecting pumps for glycol applications, the following should be checked:
- Whether the required design flow rate can be maintained
- Total pressure drop across the coil and piping
- Actual pump operating point
- Power requirements of the pump, motor and drive
- Hydraulic effects of control valves and other circuit components
Selecting pumps according to the actual properties of the glycol mixture helps CTP Engineering systems maintain the required flow rate while avoiding unnecessary energy consumption.
- The Effect of Glycol on Coil Heat Transfer Surface Area
Glycol can alter heat transfer conditions inside the coil. Therefore, a coil surface area selected for a water-based system should not automatically be assumed to provide the same capacity in a glycol-based application.
The main parameters affecting the required coil surface area include:
- Required cooling capacity
- Fluid inlet and outlet temperatures
- Glycol type and concentration
- Fluid flow rate and flow conditions inside the coil
- Ambient dry-bulb and wet-bulb temperatures
- Airflow and air-side coil performance
- Permissible pressure drop
There is no single fixed percentage that can be applied to determine the change in required coil surface area for every glycol application.
Coil surface area should be calculated using the actual fluid properties and design conditions. This approach helps achieve the required capacity while avoiding unnecessarily large equipment and excessive investment costs.
- Evaluating Fan and Pump Energy Consumption Together
The total electrical consumption of a closed-circuit cooling tower is not limited to the pump. Fan energy consumption is also an important part of the system's operating cost.
The effect of glycol on coil capacity may alter the required airflow or operating conditions. However, the actual change in fan energy depends on the tower geometry, fan selection, control system and thermal performance.
Glycol-based systems should therefore be evaluated by considering:
- Hydraulic performance of the fluid inside the coil
- Fan operating conditions required to achieve the cooling duty
- Electrical consumption of pumps and fans
- Changeover between free cooling and mechanical cooling
- Seasonal variations in system operating conditions
Energy efficiency should be assessed not only by cooling tower capacity, but also by the total electrical consumption required to deliver that capacity.
- How to Calculate Annual Energy Savings from Free Cooling
The annual economic benefit of free cooling cannot be determined from glycol concentration or cooling tower capacity alone. The facility's operating conditions must be evaluated together with hourly ambient temperature data throughout the year.
The main inputs include:
- Hourly ambient temperatures
- Required process cooling capacity and temperatures
- Annual operating hours
- Glycol concentration and fluid properties
- Cooling tower fan power consumption
- Circulation pump power consumption
- Chiller part-load performance and electrical consumption
- Control strategy for switching between free cooling and mechanical cooling
Net energy savings are generally evaluated by subtracting the electricity consumed by fans and pumps during free cooling from the electricity saved by reducing mechanical chiller operation.
Annual savings should be determined using actual operating data or an hourly simulation. Providing a definitive energy-saving percentage without these data may be misleading.
- CTP Engineering's Design Approach for Closed-Circuit Cooling Towers
When evaluating glycol-based operation in CTP Engineering closed-circuit cooling towers, thermal capacity, coil design, hydraulic requirements and energy consumption should be considered as interdependent factors.
The following project data are important for proper equipment selection:
- Required cooling capacity and fluid flow rate
- Process fluid inlet and outlet temperatures
- Glycol type and concentration
- Minimum design ambient temperature
- Ambient dry-bulb and wet-bulb conditions
- Permissible coil pressure drop
- Pump and fan operating conditions
- Target free cooling operating range and annual operating hours
Coil and cooling tower capacity should be evaluated together with pump and fan selection.
For further information about CTP Engineering products:
- Closed-circuit cooling towers: https://www.ctpmuhendislik.com/en/products/closed-type-cooling-towers
- Closed-circuit cooling tower coils: https://www.ctpmuhendislik.com/en/products/hot-dip-galvanised-serpentine
The fundamental design principle is to optimize cooling capacity, hydraulic performance and total energy consumption while providing the required freeze protection.
- Conclusion
The use of glycol in closed circuit cooling towers can reduce the risk of freezing at low temperatures and extend free cooling operating hours under suitable conditions. However, increasing glycol concentration changes the thermal and hydraulic properties of the fluid, potentially affecting coil capacity, pressure drop and pump energy consumption.
Glycol concentration should therefore be selected by considering not only the freezing temperature, but also the required cooling capacity, coil surface area, pump and fan performance, and annual energy consumption.
The most suitable design provides the required freeze protection while delivering the necessary process cooling capacity reliably and efficiently.
At CTP Engineering, we consider the integrated evaluation of thermal performance, hydraulic requirements and operational efficiency fundamental to reliable long-term operation.
