How to Design Freeze Protection for a Dry Cooler System

A dry cooler can operate efficiently throughout the year, but winter conditions introduce one of the most serious risks in any water-based heat rejection system: freezing.

When water or an inadequately protected water-glycol solution freezes inside a dry cooler coil, pipe, valve, or heat exchanger, the resulting expansion can damage tubes, fittings, headers, and other components. In severe cases, a single freezing event can result in leaks, coil replacement, extended downtime, and expensive repairs.

For this reason, freeze protection should not be treated as an accessory added after the dry cooler has been selected.

It should be part of the system design from the beginning.

A reliable strategy must answer several questions:

• What is the lowest outdoor temperature the system may experience?

• Must the dry cooler continue operating during freezing weather?

• What happens if the pump stops?

• What happens during a power failure?

• Is glycol acceptable throughout the entire process loop?

• Can the outdoor circuit be completely drained?

• Could water remain trapped inside the coil or piping?

This guide explains how engineers can design freeze protection for a dry cooler system and compare the major strategies available.

Why Freeze Protection Matters in a Dry Cooler System

A dry cooler normally contains a finned-tube coil installed outdoors.

Water or a water-glycol solution circulates through the coil while axial fans move ambient air across the fins.

During normal operation, this arrangement is simple and reliable.

The risk appears when the fluid temperature falls toward its freezing point.

Pure water expands as it freezes. If water is trapped inside a rigid tube, header, valve, or heat exchanger passage, sufficient ice formation can create damaging internal pressure.

The exposed parts of a dry cooler system can include:

• finned coils;

• headers;

• supply and return piping;

• valves;

• pumps;

• strainers;

• sensors;

• expansion vessels;

• plate heat exchangers;

• drain lines.

Freeze protection therefore needs to cover the entire exposed hydraulic circuit, not just the dry cooler coil.

Start With the Lowest Possible Temperature

The first design input should be the lowest temperature the outdoor circuit could realistically experience.

This should not simply be the annual average winter temperature.

Engineers should consider:

• historical minimum ambient temperature;

• project design minimum temperature;

• nighttime temperature;

• wind exposure;

• equipment shutdown periods;

• prolonged power failures;

• temperatures during maintenance;

• unexpected cold-weather events.

A cooling system operating normally may contain warm fluid and appear to have little freezing risk.

But if circulation suddenly stops at 2:00 a.m. during a power outage, an outdoor coil can gradually approach ambient temperature.

That may represent the true worst-case condition.

Design Temperature vs. Normal Operating Temperature

Consider a dry cooler that normally operates with:

• fluid inlet temperature: 35°C

• fluid outlet temperature: 30°C

• normal winter ambient: 0°C

It would be incorrect to conclude that freeze protection is unnecessary because the operating fluid is far above freezing.

If the site can reach:

−20°C

and a power outage stops the pump for several hours, the stagnant fluid inside the outdoor coil can eventually cool toward that temperature.

Freeze protection should therefore consider the lowest credible system condition, not merely the fluid temperature while the system is running.

Freeze Protection and Burst Protection Are Not the Same

This distinction is important when selecting glycol.

Freeze Protection

Freeze protection means the fluid remains sufficiently liquid that ice crystals do not form above the selected design temperature.

This approach is appropriate where:

• the system must continue circulating at low temperature;

• ice crystals could obstruct small passages;

• pumps need to restart during cold weather;

• process reliability requires the fluid to remain fully pumpable.

Trane describes freeze protection as appropriate where ice formation cannot be tolerated and where the system must remain operational during cold-weather conditions.

Burst Protection

Burst protection has a different objective.

Some glycol-water mixtures may begin forming ice crystals below their freezing point without becoming a solid block immediately.

As water freezes first, the remaining liquid becomes more concentrated with glycol.

If sufficient expansion volume exists, the system may tolerate this partially frozen condition without bursting.

Burst protection therefore focuses on preventing mechanical damage, rather than keeping the solution completely free of ice.

This strategy may be applicable to certain systems that remain closed during winter, but it should not be automatically adopted in cases where pumps must be operated or where the restricted channels cannot tolerate the formation of mud.

Four Main Freeze-Protection Strategies

For most dry cooler systems, freeze protection is based on one or more of these approaches:

1. Glycol antifreeze

2. Continuous circulation

3. Drain-down or drain-back design

4. Active heating and control

The most reliable system may combine several of them.

1. Glycol-Based Freeze Protection

Using a glycol-water solution is one of the most common methods for protecting outdoor dry cooler circuits.

Two commonly used fluids are:

• ethylene glycol, or EG;

• propylene glycol, or PG.

Both lower the freezing point of water.

However, glycol selection should not be based only on freeze point.

Ethylene Glycol vs. Propylene Glycol

Compared with propylene glycol, ethylene glycol usually has better thermal and fluid properties at the same level of antifreeze protection.

Dow Corporation points out that the viscosity of ethylene glycol solutions is typically lower, which enables better heat transfer and pumping performance in many applications.

When lower toxicity is required, propylene glycol is often preferred.

The correct choice depends on:

• application;

• environmental requirements;

• local regulations;

• process contamination risk;

• fluid supplier recommendations;

• thermal performance requirements.

How Much Glycol Should Be Used?

There is no universal concentration that is correct for every dry cooler.

The required concentration depends primarily on:

• glycol type;

• lowest expected temperature;

• whether freeze or burst protection is required;

• fluid supplier data;

• required pumpability.

For example, Dow recommends selecting freeze protection below the lowest anticipated operating temperature and indicates a typical design margin of about 3°C / 5°F below the minimum anticipated condition.

The important principle is:

Do not select glycol concentration from a generic rule such as “always use 30%.”

Use the actual manufacturer's concentration-versus-temperature data.

More Glycol Is Not Always Better

Adding more glycol improves low-temperature protection, but it also changes the fluid's thermal properties.

Higher glycol concentration generally means:

• higher viscosity;

• different density;

• lower effective heat-transfer performance compared with water;

• increased hydraulic resistance;

• greater pumping requirements.

Dow specifically notes that increasing propylene glycol concentration improves freeze protection but can also increase viscosity and required pumping energy.

Therefore:

The optimum glycol concentration is usually the lowest concentration that safely satisfies the required protection level and fluid supplier requirements.

The dry cooler should also be thermally selected using the actual glycol concentration rather than water properties.

2. Continuous Fluid Circulation

Another method is to keep warm fluid continuously circulating through the outdoor circuit.

If the circulating fluid remains safely above freezing, the dry cooler coil and piping can be protected.

This strategy can work in systems where:

• heat is continuously available;

• pumps have high reliability;

• winter operation is continuous;

• power supply is highly reliable.

However, circulation alone should rarely be the only protection strategy without evaluating failure conditions.

What Happens if the Pump Stops?

This is the weakness of circulation-only protection.

During a pump trip or power outage:

flow = zero

The outdoor coil then begins losing heat to the surrounding air.

Eventually, trapped water can reach freezing temperature.

Therefore engineers should ask:

If circulation stops during the coldest night of the year, how long can the system remain safe?

If there is no acceptable answer, backup protection is required.

Redundant Pumps

Critical installations may use:

• duty/standby pumps;

• automatic pump changeover;

• emergency power;

• low-flow alarms.

These measures improve reliability but still do not eliminate every freeze scenario.

For example, a common electrical failure could disable both pumps.

Freeze protection should therefore consider common-mode failures, not only individual equipment failure.

3. Drain-Down and Drain-Back Freeze Protection

If the outdoor circuit can be completely emptied when the system stops, water cannot freeze inside the drained components.

This can reduce or eliminate the need for high glycol concentrations.

However, the key word is:

completely.

A system that is theoretically drainable but leaves water trapped in low points can still freeze.

Requirements for a Drainable Dry Cooler System

A reliable drain-down system should consider:

• coil orientation;

• pipe slopes;

• drain points;

• high-point vents;

• valve arrangement;

• header geometry;

• low-point traps.

All water should have a clear path to drain.

Why Trapped Water Is Dangerous

Possible trapping locations include:

• horizontal tubes;

• headers;

• valves;

• strainers;

• flexible hoses;

• piping dips;

• heat exchangers;

• pump casings.

Even a small isolated volume can freeze and expand.

Trane documentation specifically warns that water can remain trapped in heat exchanger passages and must be completely removed when drainage is being relied upon for winter protection.

Therefore, simply installing a drain valve at the bottom of a system does not automatically create a freeze-safe drain-down design.

4. Active Heating and Freeze Controls

Heat can also be used to prevent exposed water from reaching freezing temperature.

Possible methods include:

• electric heat tracing;

• pipe heaters;

• immersion heaters;

• enclosure heaters;

• heated pump rooms.

Trane identifies adding heat as one of the basic approaches for maintaining water-containing equipment above freezing conditions.

Heat Tracing

Heat tracing is commonly applied to vulnerable:

• pipes;

• valves;

• drains;

• instruments;

• pump casings.

It can be particularly useful where full draining is impossible.

However, heat tracing itself requires:

• electrical power;

• controls;

• insulation;

• inspection;

• maintenance.

A failed heat-tracing circuit can create a hidden freeze risk.

Controls Are Part of Freeze Protection

Mechanical design alone is not enough.

The control system should recognize when freezing conditions are approaching.

Important variables may include:

• ambient air temperature;

• entering fluid temperature;

• leaving fluid temperature;

• flow status;

• pump operation;

• glycol concentration settings.

Carrier documentation, for example, emphasizes outdoor-air and water-loop temperature sensing as well as antifreeze settings in systems associated with dry-cooler operation.

Low-Temperature Alarm

A low-temperature alarm can warn operators before freezing becomes critical.

Depending on system design, an alarm might trigger:

• fan shutdown;

• pump startup;

• bypass valve operation;

• backup heating;

• emergency notification.

Automatic Pump Start

When the temperature drops to the predetermined minimum value, the controller can automatically start the circulation.

This may transfer the warmer fluid from the indoor circuit through the exposed outdoor circuit.

However, this strategy also depends on the availability of power supply and the pumps.

Fan Shutdown

A dry cooler fan accelerates heat rejection.

During extremely cold ambient conditions, operating all fans may overcool the fluid.

Fan staging or variable-speed control can reduce cooling capacity as temperature falls.

Possible logic may include:

High load → full fan operation

Lower ambient → reduced fan speed

Very low ambient → fans stopped

while circulation continues.

This helps prevent excessive cooling and saves fan energy.

Bypass Control

A bypass arrangement can reduce or stop flow through the dry cooler when additional heat rejection is not required.

A typical concept may involve:

Process Loop → Three-Way Valve → Dry Cooler / Bypass → Return

However, bypassing the coil does not automatically protect it.

If water remains inside an isolated outdoor coil, it can still freeze.

The isolated section must therefore remain glycol-protected, heated, circulated, or fully drained.

Consider Power Failure as a Design Condition

One of the best questions during freeze-protection design is:

What happens if electrical power is lost at the minimum outdoor design temperature?

Power failure may disable:

• circulation pumps;

• heat tracing;

• control valves;

• fan controls;

• electric heaters.

A system depending entirely on active equipment may therefore lose all protection simultaneously.

For critical facilities, engineers may consider:

• emergency generators;

• UPS-supported controls;

• passive glycol protection;

• drain-back design;

• redundant heating.

Passive protection is particularly valuable because it does not depend on controls responding correctly.

Is Glycol Required in the Entire System?

Not always.

Using glycol throughout a large cooling loop can create penalties in:

• heat transfer;

• pumping energy;

• fluid cost;

• maintenance.

One alternative is to isolate the exposed outdoor circuit.

Separate Glycol Loop With Heat Exchanger

A common arrangement is:

Indoor Water Loop

↓

Plate Heat Exchanger

↓

Outdoor Glycol Loop

↓

Dry Cooler

Only the outdoor circuit contains glycol.

The indoor process loop can continue using water.

Trane describes this approach as an option when glycol is undesirable in the complete system, using an isolation heat exchanger and local pumping circuit to confine glycol to the outdoor portion.

Advantages

Potential benefits include:

• smaller glycol volume;

• reduced glycol cost;

• lower pumping penalties in the primary loop;

• easier fluid management;

• clear separation between indoor and outdoor circuits.

Trade-Offs

However, the plate heat exchanger introduces:

• additional approach temperature;

• pressure drop;

• pump requirements;

• equipment cost.

The extra temperature difference should be considered when sizing the dry cooler.

Don't Forget the Expansion Tank

Glycol concentration affects more than freezing point.

The system expansion tank must accommodate changes in fluid volume as temperature changes.

The design should consider:

• total system volume;

• minimum fluid temperature;

• maximum fluid temperature;

• glycol concentration;

• system pressure.

If burst protection allows partial ice formation, expansion capacity becomes even more important.

Protect the Pump and Other Components

Designers sometimes focus entirely on the coil.

But the coil is only one exposed component.

A complete freeze review should include:

Pumps

Check pump location and whether stagnant water can remain inside the casing.

Valves

Valve bodies can contain trapped fluid.

Strainers

Strainers often create low points where water remains.

Sensors

Small sensor pockets or impulse lines may be vulnerable.

Expansion Piping

Small-diameter lines can freeze quickly.

Plate Heat Exchangers

Narrow passages may retain water and may not drain completely.

Drain Lines

A frozen drain can prevent the system from emptying when protection is needed.

Insulation Helps — But It Is Not Freeze Protection by Itself

Insulation slows heat loss.

It does not generate heat.

If ambient temperature remains below freezing long enough, insulated stagnant water will eventually approach ambient temperature.

Therefore insulation should normally be considered a supporting measure, not the primary freeze-protection mechanism.

The same principle applies to wind shielding.

It can reduce heat loss but cannot guarantee protection indefinitely.

Designing Freeze Protection for Different Operating Modes

A robust design should consider more than normal operation.

Mode 1: Normal Winter Operation

The system is:

• powered;

• circulating;

• automatically controlled.

Freeze protection may rely primarily on glycol or controlled fluid circulation.

Mode 2: Standby

The dry cooler is not rejecting heat but remains filled.

Protection may require:

• glycol;

• minimum circulation;

• heaters;

• automatic drain-down.

Mode 3: Planned Shutdown

If the system will remain offline for an extended winter period, engineers may choose:

• full drainage;

• glycol protection;

• controlled heating.

Mode 4: Unplanned Power Failure

This condition deserves special attention.

The system may lose:

• pumps;

• control;

• heating.

Passive protection such as properly selected antifreeze or complete drainage becomes especially valuable.

Example Freeze-Protection Design

Consider a dry cooler serving an industrial process.

Project Conditions

• Cooling duty: 500 kW

• Fluid supply to dry cooler: 40°C

• Fluid return: 35°C

• Summer design ambient: 32°C

• Winter design minimum: −18°C

• Outdoor installation

• Year-round operation required

Because the system must continue operating during winter, relying only on draining would not be practical.

A possible concept is:

Process Water Loop

↓

Plate Heat Exchanger

↓

Protected Glycol Loop

↓

Pump

↓

Dry Cooler

↓

Return to Plate Heat Exchanger

The outdoor circuit could use inhibited glycol selected according to the glycol manufacturer's freeze-protection data for the required minimum temperature.

The control sequence might include:

1. Variable-speed fan control during normal operation.

2. Reduced fan speed as ambient temperature falls.

3. Fans stopping when further cooling is unnecessary.

4. Pump circulation maintained as required.

5. Low-fluid-temperature alarm.

6. Emergency operating logic.

7. Protection remaining adequate during a power interruption because the outdoor fluid itself is antifreeze-protected.

The final glycol percentage should be determined from the selected fluid manufacturer's data rather than assumed from this example.

How Glycol Affects Dry Cooler Sizing

One common mistake is:

Select dry cooler using water → Add glycol later.

This can produce an inaccurate selection.

Glycol affects:

• specific heat;

• thermal conductivity;

• viscosity;

• density;

• Reynolds number;

• pressure drop.

Therefore the manufacturer should know:

glycol type + concentration

before the final dry cooler selection.

Increasing glycol concentration may require:

• greater flow;

• more surface area;

• different pump sizing;

• revised pressure-drop calculations.

Common Freeze-Protection Design Mistakes

Mistake 1: Selecting Glycol From Average Winter Temperature

Design should be based on a credible minimum condition, not average weather.

Mistake 2: Assuming 20% or 30% Glycol Is Always Enough

Freeze points differ according to:

• glycol type;

• concentration;

• product formulation.

Always use fluid-manufacturer data.

Mistake 3: Adding Glycol After Selecting the Dry Cooler

Thermal and hydraulic calculations should use the actual mixture.

Mistake 4: Depending Only on Pump Circulation

Pump or power failure can remove protection.

Mistake 5: Assuming the Coil Can Fully Drain

Internal geometry may retain water.

Drainability must be verified.

Mistake 6: Protecting the Coil but Ignoring Piping and Valves

The system is only as freeze-resistant as its most vulnerable fluid-filled component.

Mistake 7: Using Excessive Glycol

More antifreeze does not automatically mean better system design.

Higher viscosity can increase pumping energy and reduce thermal performance.

Freeze-Protection Decision Guide

This table should be used as a design framework rather than a substitute for project-specific engineering.

What Information Should Be Sent to the Dry Cooler Manufacturer?

For reliable winter selection, engineers should provide:

Thermal Data

• required cooling capacity;

• entering fluid temperature;

• leaving fluid temperature;

• fluid flow rate.

Fluid Data

• water or glycol;

• ethylene glycol or propylene glycol;

• glycol concentration.

Summer Conditions

• maximum design ambient temperature;

• required summer performance.

Winter Conditions

• minimum design ambient temperature;

• whether winter cooling is required;

• shutdown strategy.

Electrical and Control Requirements

• voltage;

• frequency;

• variable-speed control requirements;

• control-system interface.

Installation Information

• outdoor or indoor installation;

• exposed piping length;

• installation elevation;

• equipment spacing;

• wind exposure.

The freeze-protection strategy and dry cooler selection can then be evaluated together.

Aidear Dry Cooler Design for Cold-Climate Applications

For cold-climate projects, dry cooler selection should consider more than nominal cooling capacity.

Aidear can configure dry cooler systems around project operating requirements including:

• design heat rejection;

• entering and leaving fluid temperature;

• glycol type and concentration;

• minimum winter ambient temperature;

• summer design temperature;

• required fan arrangement;

• variable-speed control;

• coil and casing material requirements;

• site restrictions.

When requesting a dry cooler for a freezing climate, provide both the maximum summer design temperature and minimum winter design temperature.

The first determines whether the system can reject enough heat during hot weather.

The second helps determine how the outdoor hydraulic circuit should be protected during winter.

Engineering Checklist for Dry Cooler Freeze Protection

Before finalizing the system, verify:

1. What is the minimum design ambient temperature?

2. Must the dry cooler operate during that condition?

3. What fluid is used?

4. What is the glycol type and concentration?

5. Is freeze protection or burst protection required?

6. Has glycol concentration been verified against supplier data?

7. Has the dry cooler been selected using actual glycol properties?

8. What happens if the circulation pump fails?

9. What happens if site power fails?

10. Can the coil truly drain completely?

11. Are valves, strainers and low points protected?

12. Are exposed pipes insulated and/or heat traced where required?

13. Are low-temperature alarms provided?

14. Can fans stage down or stop during very cold weather?

15. Is there an emergency freeze-protection strategy?

If several of these questions do not have clear answers, the freeze-protection design probably needs further review.

Final Takeaway

Reliable dry cooler freeze protection is not achieved by simply adding an arbitrary amount of glycol.

It requires engineers to evaluate:

• *Minimum Ambient Temperature

• Fluid Properties

• Operating Mode

• Pump Failure

• Power Failure

• Drainability

• Control Strategy**

For many year-round systems, a properly selected inhibited glycol solution provides one of the most robust forms of passive protection.

For systems where glycol is undesirable, alternatives such as a separate outdoor glycol circuit, reliable drain-back arrangement, continuous circulation, or active heating can be considered.

The most important principle is:

Design freeze protection around the worst credible condition—not only normal operation.

A dry cooler may operate normally for thousands of hours, but freeze damage can occur during just one unprotected winter shutdown.

Planning for that condition during the initial system design can significantly improve reliability and reduce the risk of costly equipment damage.

Need a Dry Cooler for a Cold-Climate Project?

Provide:

• required cooling capacity;

• entering and leaving fluid temperatures;

• fluid type;

• glycol concentration;

• maximum summer ambient temperature;

• minimum winter ambient temperature;

• installation location.

Aidear can use these operating conditions as the basis for selecting and configuring a dry cooler for the intended project environment.