What Is a Heatless Type Air Dryer?

Compressed air can remain saturated with water vapor even after liquid condensate is removed, creating corrosion, freezing, and process problems when very dry air is required.

A heatless type air dryer is a regenerative desiccant compressed-air dryer that removes water vapor by adsorption. It normally uses two desiccant towers: one dries the compressed air while the other regenerates using expanded dry purge air. It is called “heatless” because no external regeneration heater is required.

I usually consider a heatless dryer when an application requires a pressure dew point far below what a refrigerated dryer can economically achieve, especially around -40°C PDP.

How Does a Heatless Type Air Dryer Work?

Heatless dryers operate continuously by alternating two desiccant vessels between adsorption and regeneration.

A heatless dryer works through pressure-swing adsorption. Wet compressed air flows through one tower, where desiccant adsorbs the water vapor. Meanwhile, part of the dry outlet air is expanded to low pressure and passed through the second tower to regenerate its desiccant. The second tower is then repressurized, and the towers switch roles.

One tower dries the compressed air

The basic process begins with:

compressed air → prefiltration → drying tower

The desiccant inside the tower attracts water molecules onto its surface.

The process is called:

adsorption

not:

absorption

The desiccant does not simply soak up liquid water like a sponge.

Instead, water vapor adheres to the large internal surface area of the adsorbent.

Atlas Copco describes industrial desiccant drying as a cyclic adsorption and regeneration process, with one bed drying compressed air while the second is restored for reuse.

Dry compressed air then exits the tower and continues toward the plant.

The second tower is depressurized

While Tower A is producing dry air:

Tower B is offline

and must be regenerated.

The first part of regeneration is pressure reduction.

The tower moves from approximately system pressure toward atmospheric pressure.

This pressure change makes it much easier to remove the moisture that was previously adsorbed onto the desiccant.

That is why heatless drying is often described as:

pressure-swing adsorption

or:

PSA

The dryer continuously alternates between:

high-pressure adsorption

and:

low-pressure regeneration.

Dry compressed air becomes purge air

The dryer takes a portion of the dry air produced by the online tower.

Instead of sending all of it to production, some is diverted for regeneration.

This is:

purge air

The purge stream expands from compressed-air pressure toward atmospheric pressure.

That expansion makes the air extremely dry relative to the moisture-loaded desiccant.

It then flows through the offline tower.

The process becomes:

dry compressed air → expansion → regeneration tower → moisture removal → exhaust

Atlas Copco currently describes heatless regeneration as using expanded dry compressed air and notes typical purge consumption around 16–19% of total flow for its general heatless technology discussion.

Kaeser notes that conventional pressure-swing dryers can have purge losses as high as roughly 15–20% of dryer capacity.

The exact value varies by:

• Dryer design
• Pressure
• Dew-point target
• Cycle timing
• Flow
• Control system

So I do not assume every heatless dryer automatically loses exactly 15%.

The regenerated tower is repressurized

Once regeneration is complete, the exhaust path closes.

The offline vessel is gradually brought back toward system pressure.

This prepares it to receive the compressed-air flow without creating an abrupt pressure disturbance.

The sequence is therefore:

adsorption → depressurization → purge regeneration → repressurization

The towers switch automatically

Once Tower B has been regenerated:

Tower B becomes the drying tower

and:

Tower A begins regeneration

The cycle repeats continuously.

Ingersoll Rand describes its current heatless dryers as using twin desiccant towers and diverting part of the dried compressed air through the offline tower for regeneration.

That alternating twin-tower arrangement allows the dryer to deliver dry air continuously even though each desiccant bed periodically needs regeneration.

Why Is It Called a Heatless Air Dryer?

The term “heatless” refers specifically to the regeneration method. It does not mean that the dryer uses no energy.

It is called a heatless dryer because it does not use an external electric, steam, or other supplemental heater to regenerate the desiccant. Instead, it relies on pressure reduction and dry compressed purge air. The main energy penalty is therefore the compressed air sacrificed for regeneration rather than electrical heater consumption.

Heatless does not mean energy-free

This is one of the most important concepts for a buyer.

The dryer may not have:

a large electric heater

but the purge air was already compressed.

Producing that purge air required:

compressor motor power → compression → cooling → filtration → drying

Then the dryer exhausts it.

So the energy cost is hidden inside the compressor's electricity consumption.

For example, imagine:

dryer inlet flow = 1,000 cfm

and:

purge requirement = 15%

A simplified purge flow would be:

150 cfm

That means only part of the dry air reaches the production system during purge operation.

Heatless dryers trade energy efficiency for simplicity

This is the central engineering tradeoff.

Heatless systems avoid:

• Regeneration heaters
• Regeneration blowers
• High-temperature control equipment

That generally gives them:

simple construction + lower initial cost + easier maintenance

Ingersoll Rand describes heatless dryers as having the lowest capital investment among its comparable desiccant options, although operating costs are higher than heated systems.

Kaeser similarly notes that pressure-swing dryers are generally inexpensive to purchase but can be expensive to operate because of compressed-air purge loss.

Heated dryers solve the purge-loss problem differently

An externally heated desiccant dryer adds thermal energy to the regeneration process.

Because hot regeneration air can carry more moisture, much less compressed purge air is needed.

Kaeser currently specifies about 7% process air for regeneration on its externally heated range.

A blower-purge design goes further.

Instead of using large quantities of compressed air, it can use:

ambient air → blower → heater → desiccant bed

Kaeser's current blower-purge range can operate with approximately 1% or no process-air loss, depending on cooling configuration.

So I summarize the difference as:

The drying principle is still adsorption.

The regeneration method changes.

What Dew Point Can a Heatless Type Air Dryer Achieve?

Heatless dryers are mainly selected when air must be significantly drier than the level produced by normal refrigerated dryers.

A heatless desiccant dryer commonly produces approximately -40°C (-40°F) pressure dew point. Higher-performance designs can reach about -70°C (-94°F) under specified operating conditions. The actual PDP depends on dryer design, inlet temperature, pressure, airflow, desiccant condition, cycle time, and purge performance.

-40°C PDP is the common industrial target

Ingersoll Rand's current HL heatless range is specified for approximately:

-40°C / -40°F PDP

for critical compressed-air applications.

That level is widely useful for:

• Instrument air
• Outdoor compressed-air piping
• Cold climates
• Sensitive pneumatic controls
• Electronics
• Process applications
• Moisture-sensitive production

It provides much greater moisture protection than a standard refrigerated dryer.

Some heatless dryers reach about -70°C

Heatless technology is not limited to -40°C.

Kaeser currently publishes heatless regenerative dryers with pressure dew points down to approximately:

-94°F / -70°C.

That can be useful for highly moisture-sensitive applications or extremely low air-quality specifications.

However, I do not automatically specify -70°C.

A lower PDP can increase:

purge demand + dryer size + operating cost

depending on the dryer design.

The correct question is:

How dry does the application actually need the compressed air to be?

Heatless versus refrigerated drying

For ordinary indoor factory air, a refrigerated dryer is usually more economical.

A refrigerated dryer removes water by:

cooling → condensation → separation

A heatless dryer removes water vapor by:

adsorption → regeneration

So the practical comparison is:

I therefore do not use heatless technology if the customer only requires ordinary positive-temperature PDP.

Dryer performance depends on actual operating conditions

A dryer advertised as:

500 cfm at -40°C PDP

does not necessarily provide the same performance at every:

pressure + temperature + flow

Higher inlet temperature usually increases the water load.

Lower operating pressure can reduce effective dryer capacity.

Higher flow reduces contact time.

That is why I request:

maximum flow

minimum/normal pressure

maximum inlet temperature

and:

required PDP

before selecting a dryer.

Desiccant must be protected

The dryer is designed primarily to remove:

water vapor

It should not be forced to handle uncontrolled amounts of:

liquid water + oil + dirt

Appropriate pretreatment normally includes some combination of:

aftercooler → moisture separator → prefilter/coalescing filter

depending on the compressor and air-quality requirement.

Contaminated desiccant can lose adsorption capacity and shorten dryer service life.

The complete drying system therefore matters more than the dryer vessel alone.

When Should You Use a Heatless Type Air Dryer?

Heatless dryers are especially useful where low pressure dew point, simple design, and reliable regeneration are more important than minimizing purge-air consumption.

I normally evaluate a heatless dryer for small-to-medium airflow, instrument air, freezing environments, remote locations, intermittent operation, or processes requiring approximately -40°C PDP. For large 24/7 systems, I also compare heated or blower-purge dryers because reducing purge-air loss can lower lifecycle energy cost.

Good application: outdoor piping

Suppose compressed air leaves the dryer at:

+3°C PDP

but outdoor pipe temperature falls to:

-15°C

The air can cool below its dew point.

Water can condense.

It can then freeze.

Kaeser specifically positions current heatless dryers for compressed-air systems exposed to freezing temperatures because their low PDP helps prevent water from condensing and freezing in piping, valves, and end uses.

In that situation, a -40°C dryer can provide a much safer margin.

Good application: instrument air

Instrumentation often requires:

dry + clean + reliable compressed air

Moisture can cause:

corrosion + sticking valves + frozen control lines + unreliable signals

Heatless dryers are therefore widely used for instrument-air systems.

Good application: remote or demanding sites

Heatless regeneration is mechanically simple.

Atlas Copco positions heatless dryers as particularly suitable for applications where simplicity and reliability are important, including hazardous or remote installations.

This can matter for:

mines

remote industrial installations

drilling sites

and other facilities where complex maintenance support may be limited.

Less attractive application: large continuous airflow

Now suppose the dryer handles:

4,000 cfm

and operates:

8,000 hours per year

Even a relatively small percentage of purge air becomes significant.

Kaeser notes that as airflow increases, heatless operating cost rises because of purge loss, while heated technologies use different regeneration methods to reduce or eliminate that loss.

At this scale, I would compare:

heatless

versus:

heated purge

versus:

blower purge

and possibly:

heat-of-compression

depending on the compressor system.

Heatless can still win despite higher purge cost

Energy is not the only purchasing factor.

A heatless dryer may still be attractive because it offers:

lower initial cost

simple controls

no regeneration heater

no regeneration blower

strong low-PDP capability

straightforward maintenance

So the correct comparison is:

capital cost + energy + purge loss + maintenance + downtime

not only:

purge percentage

My insights: What Is a Heatless Type Air Dryer

The simplest answer is that it is a desiccant dryer without a regeneration heater, but that definition misses its most important engineering tradeoff.

A heatless type air dryer is a twin-tower regenerative desiccant dryer that dries compressed air by adsorption and regenerates the saturated desiccant with expanded dry compressed air instead of external heat. Its key advantage is simplicity; its key disadvantage is purge-air consumption, which can make large continuously operating systems expensive to run.

My first insight is that heatless is a regeneration type

“Heatless” does not describe a completely different drying mechanism.

It describes:

how the desiccant is regenerated

The drying process remains:

adsorption

The regeneration process is:

pressure reduction + dry purge air

That makes the classification much easier to understand.

My second insight is to remember the five-stage cycle

The complete operating sequence is:

1. Adsorption

One tower removes water vapor.

2. Depressurization

The saturated tower reduces pressure.

3. Purge regeneration

Expanded dry air removes moisture.

4. Repressurization

The regenerated vessel returns to line pressure.

5. Tower switching

The two vessels exchange jobs.

That cycle defines the heatless dryer.

My third insight is to calculate net dry air, not just dryer capacity

Suppose a heatless dryer is listed at:

1,000 cfm

but requires purge air.

I need to know whether that 1,000 cfm represents:

inlet capacity

or:

guaranteed net usable outlet flow

This matters when sizing the compressor.

If production requires:

1,000 cfm

I may need more compressor capacity to supply:

plant demand + regeneration demand

The dryer itself is therefore a compressed-air consumer.

My fourth insight is that modern controls can reduce unnecessary purge

Traditional heatless dryers often switch on fixed time cycles.

Modern controls can adapt regeneration more closely to actual moisture loading.

Kaeser's current heatless KAD range includes optional automatic purge-saving control that matches purge consumption more closely to reduced demand.

For variable-flow systems, this can materially improve efficiency.

So I consider:

dryer controller

part of the energy specification.

My fifth insight is to choose heatless only when it makes lifecycle sense

When someone asks me, “What is a heatless type air dryer?”, my practical answer is:

It is a regenerative desiccant dryer that uses a portion of dry compressed air instead of external heat to regenerate its desiccant.

For equipment selection, I use this sequence:

application → required PDP → flow → pressure → inlet temperature → purge rate → net outlet flow → annual hours → energy cost → dryer type

My starting decision table is:

For a small application, I may prefer heatless because:

the equipment is simple

and:

the purge volume is manageable

For a large continuous application, I may prefer heated regeneration because:

saving compressed air can offset higher equipment cost

Ingersoll Rand makes this tradeoff explicit: its heatless systems have lower capital investment but higher operating cost than heated desiccant technology.

My final engineering principle is therefore:

Heatless dryers use compressed-air energy instead of heater energy.

They are excellent when the project values:

simple construction + reliable low dew point + modest flow

They become less attractive when:

high airflow + 24/7 operation + expensive electricity

make purge loss a major lifecycle expense.

The best heatless dryer is not simply the model with the lowest advertised dew point.

It is the system that provides the required net airflow and PDP reliably while keeping purge consumption, pressure drop, maintenance, and total lifecycle cost within the project's target.

Conclusion

A heatless type air dryer is a twin-tower desiccant dryer that regenerates with dry compressed purge air instead of an external heater, providing simple, reliable low-dew-point drying.