How Do Dry Air Coolers and Adiabatic Cooling Systems Work?
Dry air coolers and adiabatic coolers are used across industrial process cooling systems to remove heat from water or glycol without relying entirely on mechanical refrigeration. They can be used with and without a chiller.
If used with a chiller they can reduce running hours, lower energy consumption and provide reliable heat rejection for production equipment, hydraulic systems, compressors, furnaces, moulding machines and other temperature-sensitive processes.
The principle is straightforward:
A dry air cooler does not create cooling. It transfers unwanted heat from a circulating fluid into the outside air.
An adiabatic system uses the same basic heat exchanger, but temporarily cools the incoming air through water evaporation. This allows the unit to maintain more capacity during hot weather and, in suitable conditions, cool the process fluid below the outdoor dry-bulb temperature.
Understanding the difference is important when selecting equipment, setting design temperatures and deciding whether a dry cooler can replace or support a chiller.
What Does a Dry Air Cooler Do?
A dry air cooler removes heat from water, glycol or another compatible heat-transfer fluid and rejects it to the atmosphere.
Warm fluid from the process is pumped through a finned coil. Fans draw or push outside air across the coil, and heat passes from the fluid, through the coil tubes and fins, into the moving air.
The cooled fluid then returns to the process to collect more heat, and the cycle continues.
Because the process fluid remains inside a closed circuit, it does not come into direct contact with the outside air. This helps keep the system clean and reduces water loss compared with an open cooling tower.
The First Law of Thermodynamics
The first law of thermodynamics tells us that energy cannot be created or destroyed. It can only be transferred or converted from one form to another.
In a dry cooling system, the heat produced by the process must go somewhere. The dry air cooler transfers it from the process fluid into the atmosphere.
For example, if a production process adds 100 kW of heat to a water circuit, the dry cooler must reject approximately 100 kW at the specified fluid and ambient temperatures.
The required fluid flow can be estimated using:
Heat load = mass flow × specific heat capacity × temperature difference
For water carrying 100 kW with a 5°C temperature difference:
100 kW ÷ (4.186 kJ/kg·K × 5 K) = approximately 4.8 kg/s, or roughly 4.8 litres per second.
If glycol is used, its lower specific heat capacity and higher viscosity must be included in the selection. A dry cooler chosen using water data alone may be undersized when it is installed on a glycol circuit.
How Does a Dry Air Cooler Work?
The operating sequence has five main stages.
1. The Process Heats the Fluid
Water or glycol circulates through the production process and absorbs unwanted heat from machinery, products or another heat exchanger.
2. Warm Fluid Enters the Coil
The heated fluid enters the dry cooler and passes through copper, stainless-steel or aluminium tubes, depending on the application and construction.
3. Fans Move Ambient Air Across the Coil
Axial fans move large volumes of outdoor air across the finned heat exchanger. Increasing airflow generally increases heat rejection, although fan power and noise also rise.
4. Heat Transfers Into the Air
Heat conducts through the tube wall into the fins and then passes into the moving air. The warmer discharge air is released back into the atmosphere.
5. Cooled Fluid Returns to the Process
The lower-temperature water or glycol leaves the dry cooler and returns to the system to absorb more process heat.
There is no refrigerant circuit, compressor or expansion valve within a standard dry air cooler. Its main active components are the fans, controls and, where fitted, the adiabatic equipment.
Why Ambient Temperature and Approach Matter
A standard dry cooler cannot cool fluid below the temperature of the air entering its coil.
The difference between the leaving fluid temperature and the entering air temperature is known as the approach temperature.
For example, if the outdoor air is 25°C and the dry cooler has been selected for a 5 K approach, the expected leaving fluid temperature would be around 30°C at the design condition.
A smaller approach temperature is possible, but it requires a larger coil (more surface area), more airflow or both. This affects equipment size, electrical consumption, sound level and cost.
Correct selection must therefore consider:
Maximum summer ambient temperature
Required fluid inlet and outlet temperatures
Fluid type and glycol concentration
Design flow rate and pressure drop
Required cooling capacity
Available space and airflow clearances
Noise limits
Part-load operation
Potential air recirculation
How Does an Adiabatic Cooling System Work?
An adiabatic cooler is essentially a dry air cooler with an air pre-cooling system.
During normal or cooler conditions, the unit operates dry. When the ambient temperature rises and dry cooling alone can no longer maintain the required fluid temperature, the adiabatic stage is enabled.
Water is introduced into the incoming airstream, usually through wetted media or a controlled spray system. As the water evaporates, it absorbs latent heat from the air. This reduces the air temperature before it reaches the main heat-transfer coil.
The sequence is:
The controller monitors ambient and fluid temperatures.
The fans increase speed as the cooling demand rises.
At a defined condition, the adiabatic stage is enabled.
Water evaporates into the incoming air and lowers its temperature.
The cooler air passes across the coil and increases heat rejection.
The system modulates fan speed and water use to maintain the required fluid temperature.
The water is not used to cool the process fluid directly. It cools the air that cools the closed-circuit fluid.
Dry-Bulb and Wet-Bulb Temperature
Dry-bulb temperature is the normal air temperature shown by a standard thermometer.
Wet-bulb temperature reflects the cooling effect available from water evaporation. It depends on both air temperature and humidity.
An adiabatic system can reduce the entering-air temperature towards the outdoor wet-bulb temperature, but it cannot reach it perfectly in real operation. The actual result depends on the system’s adiabatic effectiveness, airflow and ambient humidity.
As a simplified example:
Outdoor dry-bulb temperature: 32°C
Outdoor wet-bulb temperature: 20°C
Adiabatic effectiveness: 70%
The approximate air temperature entering the coil would be:
32 − [0.70 × (32 − 20)] = 23.6°C
The coil is therefore seeing air at approximately 24°C rather than 32°C. This can make a substantial difference to cooling capacity and leaving-fluid temperature.
The final fluid temperature must still remain above the pre-cooled air temperature by the cooler’s approach. Adiabatic assistance improves the operating condition; it does not abolish the approach temperature.
Adiabatic cooling is most effective in hot, dry conditions. In humid weather, the dry-bulb and wet-bulb temperatures are closer together, so less evaporative cooling is available.
Common Types of Adiabatic System
Wetted-Media Systems
Water is distributed across pads or media positioned before the coil. Incoming air passes through the wetted material, where some of the water evaporates and reduces the air temperature.
This arrangement can provide controlled air pre-cooling while helping to prevent large droplets from reaching the coil.
Spray or Mist Systems
Fine droplets are introduced into the incoming airstream. Correct droplet size, water pressure, positioning and control are essential so the water evaporates before it reaches sensitive components.
Direct Coil-Wetting Systems
Some designs apply water directly to the heat exchanger surface. These systems can increase heat transfer but require particular attention to coil materials, water quality, scaling, corrosion and hygiene.
The correct arrangement depends on the duty, site conditions, water quality and the cooler manufacturer’s design.
Dry Cooler Versus Adiabatic Cooler
Feature | Dry air cooler | Adiabatic cooler |
Primary cooling method | Sensible heat transfer to ambient air | Sensible heat transfer using evaporatively pre-cooled air |
Water use | None during normal heat rejection | Water used when the adiabatic stage operates |
Temperature reference | Outdoor dry-bulb temperature | Air temperature can move towards wet-bulb temperature |
Below outdoor dry-bulb operation | Normally not possible | Possible when sufficient evaporative cooling is available |
Maintenance requirement | Coil, fans, controls and fluid circuit | Dry-cooler maintenance plus water system, media or nozzles, drains and hygiene controls |
Typical benefit | Simple, closed-circuit heat rejection | Improved high-ambient capacity and lower design fluid temperatures |
How Dry Coolers Provide Free Cooling
Dry and adiabatic coolers are often used to reduce the load on an industrial chiller.
When outdoor conditions are suitable, the process fluid can be cooled by ambient air instead of running the refrigeration compressors.
This is known as free cooling.
There are two main operating modes:
Partial Free Cooling
The dry cooler removes part of the process heat, and the chiller provides the remaining cooling. This is useful when ambient temperature is low enough to assist the system but not low enough to achieve the required leaving-fluid temperature on its own.
Total Free Cooling
The dry cooler meets the complete process load and the chiller compressors can stop. Pumps, fans and controls still consume electricity, so the cooling is not literally free, but the power demand can be much lower than mechanical refrigeration.
A well-designed control system should automatically choose the most efficient operating mode, stage the fans and adiabatic section correctly, and prevent the chiller and dry cooler from fighting each other.
Why the Whole System Design Matters
A dry cooler cannot be selected in isolation. Its real performance depends on the complete hydraulic and control system.
Fluid Temperature
Higher fluid temperatures increase the difference between the fluid and ambient air, making heat rejection easier. Low-temperature processes may require a much larger cooler, adiabatic assistance or continued chiller operation.
Fluid Flow
Insufficient flow reduces heat transfer and can cause unstable process temperatures. Excessive flow increases pump energy and pressure drop without necessarily improving useful capacity.
Glycol Concentration
Glycol protects exposed pipework and coils from freezing, but excessive concentration reduces heat-transfer performance and increases pumping power. The concentration should be based on the lowest credible fluid or ambient exposure temperature, with a suitable safety margin.
Airflow and Recirculation
Hot discharge air must be able to escape. Poor positioning, restricted clearances, nearby walls or multiple units facing each other can allow warm air to recirculate into the coil and seriously reduce capacity.
Coil Condition
Dust, fibres, pollen, packaging debris and airborne contamination block the fin surface and restrict airflow. A cooler with a dirty coil may run every fan at full speed while delivering far less than its design capacity.
Water Quality
Adiabatic systems require an appropriate water-treatment and control strategy. Poor water quality can cause scale, blocked nozzles, damaged media, corrosion and loss of performance.
Controls
Fan speed, water use, pump operation, bypass valves and chiller enable signals must work together. Good controls allow the unit to operate dry whenever possible and use adiabatic assistance only when it provides a genuine benefit.
Common Dry Cooler and Adiabatic System Problems
Typical faults include:
Blocked or damaged heat-transfer fins
Failed, incorrectly rotating or poorly controlled fans
Hot discharge air recirculating into the cooler
Low system flow or air trapped in the fluid circuit
Incorrect glycol concentration
Fouled strainers or heat exchangers
Faulty temperature or pressure sensors
Poorly commissioned bypass valves
Scaled, blocked or badly positioned adiabatic nozzles
Damaged or unevenly wetted media
Inadequate drainage or standing water
Adiabatic operation enabled when it is not required
Many apparent capacity problems are not caused by an undersized cooler. They are caused by reduced airflow, incorrect flow, poor control or a design condition that was never clearly established.
Maintenance and Reliability
Dry air coolers are mechanically simpler than chillers, but they are not maintenance-free.
A planned inspection should include:
Coil condition and cleanliness
Fan operation, bearings, guards and electrical condition
EC or inverter fan controls
Fluid flow, temperatures and pressure drop
Glycol condition and freeze protection
Leaks, corrosion and damaged fins
Control sensors and setpoints
Adiabatic media, pumps, valves, filters and nozzles
Water quality, drainage and hygiene controls
Evidence of air recirculation or restricted airflow
Performance should be checked using measured fluid temperatures, flow rate and ambient conditions. Simply confirming that the fans are turning does not prove that the unit is delivering its rated duty.
Choosing the Right Cooling Solution
A dry air cooler may be the best choice where the required fluid temperature remains comfortably above the maximum summer ambient temperature, or where it can provide useful free cooling alongside a chiller.
An adiabatic cooler may be more suitable where:
Peak summer temperatures would make a dry-only selection too large
A lower leaving-fluid temperature is required
High-ambient performance must be maintained
Water use can be controlled and managed correctly
Reducing compressor operation is a priority
A chiller will still be required when the process needs fluid temperatures below what the available ambient and wet-bulb conditions can reliably provide.
The right answer is often a hybrid system: a dry or adiabatic cooler handles as much of the annual load as conditions allow, while the chiller supplies the balance and guarantees the process temperature.
Dry and Adiabatic Cooling Support from NovaCool Solutions
NovaCool Solutions designs, supplies, installs and maintains industrial dry air coolers, adiabatic coolers, free-cooling systems and complete chilled-water installations across the UK.
We assess the real process load, design temperatures, ambient conditions, glycol concentration, hydraulic performance, controls and future operating requirements before selecting the equipment.
Whether you are replacing an existing cooler, reducing chiller energy use or designing a new process cooling system, we can help develop a reliable solution around the application.
Speak to a NovaCool process cooling engineer: 0808 258 6894
Comments