What Should the Dew Point Be on an Air Dryer?

A dryer can be working normally and still leave water downstream if its dew point is too high for the temperature reached by the compressed-air piping.

For most indoor industrial compressed-air systems, a pressure dew point around +3°C to +7°C is normally suitable and can be achieved with a refrigerated dryer. Applications exposed to freezing conditions or requiring very dry air commonly use about -40°C PDP, while critical processes may require -70°C. The correct dew point depends on the lowest system temperature and process requirements.

I do not select the lowest dew point available automatically. I first identify how cold the compressed air and piping can become, then choose a dryer that keeps the pressure dew point safely below that temperature.

What Is a Good Dew Point for a Compressed Air Dryer?

There is no single correct dew point for every compressed-air system. General factory air and critical instrument air can require very different moisture levels.

A good pressure dew point for ordinary indoor factory air is typically around +3°C to +7°C. If compressed-air piping can fall below freezing, I normally consider a desiccant dryer with a PDP around -20°C or -40°C. For highly moisture-sensitive applications, -70°C may be required. I specify only the dryness the process actually needs.

+3°C is a common industrial target

For many manufacturing plants, a refrigerated dryer is sufficient.

Atlas Copco currently describes typical refrigerated dryer performance as approximately:

+3°C to +7°C pressure dew point

and its FD refrigerated dryers can achieve a stable PDP as low as approximately +3°C.

Kaeser similarly describes a properly sized refrigerated dryer as commonly producing a pressure dew point around 40°F, or roughly +4°C.

For ordinary indoor pneumatic applications, this is often adequate because the piping remains considerably warmer than the dew point.

Typical applications can include:

• General manufacturing
• Pneumatic tools
• Packaging machinery
• Assembly equipment
• Indoor automation
• Normal plant-air distribution

The key condition is that the air must not later cool below its pressure dew point.

-40°C is a common desiccant-dryer target

When a much lower moisture content is required, I normally move from refrigerated drying to adsorption/desiccant drying.

A typical industrial desiccant dryer can achieve approximately:

-40°C PDP

with specialized systems capable of going lower.

Atlas Copco describes -40°C as a typical desiccant dryer pressure dew point, while molecular-sieve systems can reach approximately -70°C and below.

Kaeser gives a broader adsorption-dryer range extending from around 0°F to -40°F and even approximately -100°F, depending on dryer design.

For me, -40°C is one of the most common design targets when:

outdoor piping + cold climate + instrumentation + moisture-sensitive process

makes refrigerated drying insufficient.

ISO 8573-1 provides useful moisture classes

ISO 8573-1 compressed-air purity classes are frequently used to specify moisture requirements.

Current Atlas Copco guidance summarizes the water pressure-dew-point classes as:

This table gives me a much better specification than simply saying:

“I need dry air.”

Instead, I can define the actual PDP or purity class the process requires.

How Should I Choose the Correct Pressure Dew Point?

The dryer outlet temperature alone does not determine whether downstream condensation will occur. I need to know the coldest temperature the compressed air will experience anywhere after the dryer.

I select the pressure dew point below the lowest temperature expected anywhere in the compressed-air system. A useful design guideline is to keep the PDP at least about 10°C below the lowest pipe or ambient temperature the system may experience. This provides a safety margin against downstream condensation during seasonal temperature changes.

The coldest pipe temperature matters more than the compressor room

Suppose my compressor room is maintained at:

25°C

A refrigerated dryer provides:

+3°C PDP

Inside the factory, there is no problem.

But imagine the compressed-air pipe then travels outdoors in winter where the temperature reaches:

-10°C

The air can cool below its +3°C pressure dew point.

Water vapor then begins condensing.

The dryer has technically achieved its specification, but the system design is still wrong for the environmental conditions.

Atlas Copco specifically recommends choosing a pressure dew point at least 10°C below the lowest temperature any part of the compressed-air network will experience, including outdoor and unheated sections.

I therefore check:

compressor room → production area → roof piping → outdoor piping → underground piping → point of use

I use the coldest credible temperature.

An example makes the principle clearer

Suppose my lowest expected pipe temperature is:

+15°C

A +3°C refrigerated dryer provides plenty of margin.

Now suppose my pipe temperature can fall to:

0°C

A +3°C PDP is no longer safely below the pipe temperature.

And if the pipe can fall to:

-20°C

I may need a considerably lower PDP, often around -40°C.

My practical selection might look like this:

This table is a design starting point, not a substitute for the process specification.

Pressure dew point is not the same as atmospheric dew point

This distinction is important.

A compressed-air dryer is normally specified using pressure dew point, or PDP.

PDP is the temperature at which water vapor begins condensing while the air is still at its operating pressure.

Atlas Copco emphasizes that this differs from atmospheric dew point because compressed air contains water vapor at elevated pressure.

If someone tells me:

“The dew point is -20°C”

I ask whether that means:

pressure dew point

or:

atmospheric dew point

For evaluating an industrial air dryer, I normally want the pressure dew point at the system operating pressure.

What Dew Point Should Refrigerated and Desiccant Dryers Produce?

Different dryer technologies operate economically in different dew-point ranges. I select the technology after identifying the moisture requirement.

A refrigerated dryer normally provides approximately +3°C to +7°C PDP and is suitable for most general industrial compressed air. A desiccant dryer commonly provides around -40°C PDP, with systems available around -20°C or as low as -70°C. Refrigerated drying is usually more economical when subzero dew points are unnecessary.

Refrigerated dryers are efficient for normal factory air

A refrigerated dryer cools compressed air.

As the compressed air temperature falls, water vapor condenses into liquid.

The dryer separates and drains that liquid and then normally reheats the outgoing air.

A typical result is:

+3°C to +7°C PDP

Atlas Copco's current FD range reaches approximately +3°C PDP and corresponds to ISO 8573-1 water Class 4 performance in applicable configurations.

For me, refrigerated drying is normally the first technology I consider if:

the air remains indoors + freezing is impossible + the process does not require ultra-dry air

It generally provides a good balance between:

air quality + purchase cost + energy consumption + maintenance

Desiccant dryers go below freezing

A desiccant dryer removes water vapor through adsorption rather than cooling the air near freezing.

This allows it to reach much lower dew points.

Common design targets include:

-20°C

-40°C

-70°C

Atlas Copco notes that -40°C is a typical adsorption-dryer PDP and that molecular-sieve systems are used for -70°C and lower applications.

Kaeser likewise states that desiccant dryers can provide dew points below freezing, including around -40°F and as low as -100°F depending on system design.

This makes desiccant drying appropriate for:

• Outdoor compressed-air lines
• Instrument air
• Cold-climate installations
• Electronics manufacturing
• Pharmaceutical applications
• Sensitive process air
• Certain food and beverage applications
• Critical control systems

Membrane dryers can fill another niche

Membrane dryers are also available where compact point-of-use drying is valuable.

Kaeser's current membrane dryer range can provide pressure dew points down to approximately -40°F/-40°C and is aimed particularly at smaller and medium compressed-air systems or point-of-use treatment.

However, membrane dryers continuously consume some purge air.

So I compare their simplicity with the cost of compressed-air loss.

Lower dew point is not automatically better

A -70°C dryer produces much drier air than a +3°C dryer.

That does not mean it is automatically the better dryer.

Producing extremely dry compressed air generally increases:

capital cost + energy requirement + purge loss + maintenance complexity

Kaeser notes that desiccant dryers are considerably more expensive to purchase and operate than refrigerated dryers, so very low PDP should be specified only when it is genuinely required.

My goal is therefore:

dry enough, not unnecessarily dry

What Causes an Air Dryer Dew Point to Become Too High?

If a dryer that normally produces +3°C or -40°C suddenly produces wetter air, I investigate operating conditions before assuming the entire dryer has failed.

A high dryer dew point can result from excessive inlet temperature, excessive airflow, low operating pressure, dirty heat exchangers, failed condensate drains, refrigeration problems, contaminated or saturated desiccant, switching-valve faults, poor regeneration, or inadequate prefiltration. A dryer must be sized for the worst credible operating conditions, not just nominal flow.

High inlet temperature can overload the dryer

Hot compressed air contains more water vapor.

If an aftercooler is dirty or ambient temperature becomes unusually high, the dryer may receive a larger moisture and thermal load.

This can push the outlet PDP upward.

For refrigerated dryers, I inspect:

aftercooler → inlet temperature → condenser → refrigerant circuit → drain

For desiccant dryers, I additionally check:

desiccant condition → regeneration → switching valves → purge flow → prefilters

Excessive airflow can reduce drying performance

A dryer is rated for a particular flow under defined operating conditions.

If a plant expands and airflow rises beyond the dryer's corrected capacity, compressed air may pass through too quickly for the dryer to achieve its intended PDP.

The nameplate flow alone is not enough.

Dryer capacity is affected by:

airflow + inlet pressure + inlet temperature + ambient temperature

Kaeser specifically recommends sizing compressed-air dryers for unfavorable operating conditions, including maximum air consumption, high inlet and ambient temperature, and low operating pressure.

Desiccant contamination can increase dew point

Desiccant needs clean inlet air.

Oil contamination can coat the adsorbent and reduce its ability to remove moisture.

That is why desiccant dryers normally use high-quality prefiltration.

A failure upstream can therefore appear later as a dew-point problem.

I monitor dew point instead of waiting for visible water

Visible water is a late warning.

Atlas Copco recommends PDP monitoring because a dew-point sensor can reveal dryer deterioration before liquid moisture starts appearing downstream.

For a critical system, I therefore prefer:

dryer outlet PDP sensor → alarm → trend monitoring

rather than relying only on manual drain inspection.

My insights: What Should the Dew Point Be on an Air Dryer

I do not think the correct answer is simply “+3°C” or “-40°C.” The correct pressure dew point is the one that prevents moisture problems under the coldest system conditions while satisfying the process air-quality specification.

For most indoor industrial compressed air, I target approximately +3°C PDP with a refrigerated dryer. If piping can experience freezing temperatures or the application requires very dry air, I typically evaluate -40°C PDP from a desiccant dryer. Critical processes may require -70°C. I keep the selected PDP safely below the lowest expected system temperature.

My first rule is to start with the coldest downstream temperature

I do not begin with:

Which dryer is cheapest?

I begin with:

How cold can the compressed air become after the dryer?

If the coldest pipe remains at 20°C, +3°C PDP gives substantial margin.

If pipes run outside at -15°C, +3°C drying is clearly unsuitable.

Atlas Copco's current guideline of keeping PDP approximately 10°C below the lowest system temperature gives me a practical design margin.

My second rule is to check the process requirement

The environmental temperature is only part of the answer.

Some processes require a specific moisture class regardless of ambient temperature.

For example:

ISO Class 4 water → +3°C PDP

ISO Class 3 → -20°C PDP

ISO Class 2 → -40°C PDP

ISO Class 1 → -70°C PDP

If the customer's production specification calls for Class 2 water, a +3°C refrigerated dryer is not sufficient even if all pipes remain warm.

So I check both:

environmental requirement + process requirement

and use whichever is stricter.

My third rule is to avoid unnecessary over-drying

Suppose my entire plant is indoors and never falls below:

15°C

and the pneumatic equipment requires only Class 4 moisture performance.

Installing a -70°C system gives me much drier air, but that dryness may have no production value.

Instead it can add:

higher purchase cost → regeneration energy → purge-air consumption → additional filtration → greater maintenance

Kaeser specifically notes that desiccant drying should be used where very low dew points are actually required because it costs substantially more to purchase and operate than refrigerated drying.

So I do not specify -40°C just because it sounds more professional.

My fourth rule is to specify PDP rather than saying “dry air”

A purchase specification such as:

“air dryer for 20 m³/min compressor”

is incomplete.

I want:

20 m³/min + operating pressure + maximum inlet temperature + ambient temperature + required PDP

For example:

or:

Now suppliers are quoting equipment for the same duty.

My fifth rule is to monitor actual dew point after installation

When someone asks me, “What should the dew point be on an air dryer?”, my practical answer is:

Use around +3°C PDP for most ordinary indoor industrial compressed-air systems, around -40°C when freezing conditions or very dry process air require adsorption drying, and around -70°C only for highly demanding applications. Most importantly, keep the PDP below the coldest temperature the compressed-air network will experience.

My selection sequence is:

application → required ISO class → lowest pipe temperature → target PDP → dryer technology → airflow → pressure → inlet temperature → ambient temperature → energy cost

Then I verify the result with a dew-point sensor.

That approach prevents both major dryer-selection mistakes:

dew point too high → downstream condensation

and:

dew point unnecessarily low → excessive purchase and operating cost

For most factories, +3°C is a very practical starting point.

For cold outdoor piping or moisture-sensitive industrial processes, -40°C becomes much more appropriate.

And for specialized applications requiring exceptionally dry air, -70°C may be justified.

Conclusion

Most indoor industrial systems use about +3°C PDP; cold or critical applications often require -40°C, while highly moisture-sensitive processes may need -70°C.