A refrigerated dryer will get your air to +3°C. If your process runs in a cold store, outdoor pipework, or a food production environment, that’s not dry enough. Moisture gets through, corrosion starts, and the contamination problem becomes a production problem.
J Ll Leach, Atlas Copco authorised distributor in the West Midlands, has been specifying and servicing desiccant dryer systems across Birmingham and the wider Midlands since 1936. This guide covers how desiccant drying works, which type suits which application, what it actually costs to run, and what UK law requires before you switch the system on.
Why Refrigeration Alone is Not Enough
Compressed air always carries moisture. The question is how much you can afford to leave in it.
Liquid water and high humidity in compressed air systems cause aggressive corrosion, microbial growth, product spoilage in food and pharmaceutical manufacturing, and equipment failure. A refrigeration dryer removes the bulk of it by cooling air to approximately +3°C. That works in a temperature-controlled indoor environment.
It doesn’t work in a freezing ambient environment, instrument air pipework, or any process where the air contacts food. Refrigeration cannot achieve outlet conditions below 0°C, so any application exposed to sub-zero temperatures will see ice formation in the lines.
When Desiccant Drying is Technically Mandatory
Desiccant drying is the only viable solution in these situations:
- Outdoor pipework in the UK winter
- Instrument and control air where moisture causes valve failure
- Food and beverage manufacturing under BCAS Best Practice Guideline 102 (bcas.org.uk)
- Pharmaceutical and electronics production requiring Class 1 air purity
- Any process specifying an outlet condition of -40°C or lower
For compressed air systems requiring an outlet moisture level below -20°C, a refrigerated dryer is not a partial solution. It is the wrong solution entirely.
How Desiccant Dryers Work
A desiccant dryer uses hygroscopic material to pull water vapour directly out of the airstream through adsorption. The industry-standard twin-tower configuration runs two vessels simultaneously: one adsorbs moisture from the live airstream while the other regenerates, purging accumulated water. Cycle time is typically 5 to 10 minutes per tower.
The desiccant bed needs to be dried out between cycles. How you power that regeneration process is where the cost difference between dryer types becomes significant.
Desiccant Materials: What Goes Inside
The choice of fill material affects performance, maintenance, and downstream filtration requirements.
Atlas Copco’s Cerades structured desiccant eliminates loose bead agitation entirely. Compressed air flows through straight ceramic channels rather than navigating loose beads. The result is zero dust generation, a pressure drop reduction of up to 70%, and the ability to operate at 100% of rated airflow where traditional bead dryers derate to 70–80%.
For silica gel installations, dust generation is a genuine health hazard for service technicians and requires expensive downstream particulate filtration.
The Real Cost of Heatless Desiccant Drying
Heatless units are the cheapest to buy and the most expensive to run.
They use Pressure Swing Adsorption to regenerate the offline tower without external heat. A portion of already-dried compressed air, the purge flow, is expanded to atmospheric pressure and passed backward through the saturated bed. That process works, but it consumes approximately 15 to 20% of the dryer’s rated capacity on every cycle.
The Purge Penalty in Real Numbers
If your compressed air is generated at 20 kW per 100 cfm, purge air waste alone equates to 3 to 4 kW per 100 cfm of rated capacity. On a system costing around £15,000 annually in electricity, that is a significant and continuous operating penalty from the day the unit is commissioned.
Comparing Dryer Types on Running Cost
Heated purge units use thermodynamics to reduce the required regeneration volume to roughly 7–8%. Vacuum-assisted regeneration, as used in the Hi-line Industries Hi-Plex system, drops that requirement to just 1 to 2%. The Hi-Plex mounts a refrigeration unit and a PSA unit on a single skid: the refrigeration stage knocks out bulk moisture first, leaving the towers handling only residual vapour with vacuum-assisted regeneration.
Read our guide to compressed air piping systems for how dryer type selection interacts with pipework sizing and pressure drop across the distribution network.
Dew Point Dependent Switching: the Central Cost Lever
Most older units regenerate on a fixed time cycle, regardless of actual moisture loading. If demand drops overnight or across a weekend, the dryer keeps regenerating at full rate. You pay for regeneration you don’t need.
Dew Point Dependent Switching (DPDS) monitors actual moisture content at the dryer outlet and triggers regeneration only when the desiccant bed needs it. Atlas Copco’s Elektronikon controllers handle this on current CD and BD series dryers.
What DPDS Delivers in Practice
Regeneration tracks real demand rather than a fixed schedule, so the dryer is not purging when the moisture load does not justify it. Desiccant media life extends because cycles are not running unnecessarily. The Energy Saving Trust (energysavingtrust.org.uk) specifically advises equipping dryers with demand-based controllers to curtail continuous waste in older unregulated units.
- Regeneration tracks real demand rather than a fixed schedule.
- Desiccant media lasts longer because the beds are not cycling unnecessarily.
- Demand-based control cuts wasted purge energy in older fixed-cycle dryers.
For facilities managers reviewing energy spend, DPDS is the single biggest lever for reducing operating costs in these systems. It does not require replacing the whole unit and is often available as a retrofit on existing installations.
PSSR Compliance: What UK Law Requires
Operating a pressurised compressed air system without a Written Scheme of Examination is a statutory offence under the Pressure Systems Safety Regulations 2000 (hse.gov.uk).
PSSR 2000 applies to any compressed air system operating above 0.5 bar above atmospheric pressure, which covers almost every industrial desiccant dryer installation. Non-compliance is prosecuted by the Health and Safety Executive.
What PSSR 2000 Requires From You
Before you run a qualifying system, you need three things in place:
- Written Scheme of Examination: Documents which parts of the system require periodic examination, at what intervals, and by whom. Must be in place before the system is operated.
- Competent Person inspections (Regulation 9): Examinations must be conducted by an impartial Competent Person, such as a TÜV SÜD engineering surveyor or equivalent chartered engineer. Your own maintenance team cannot self-certify.
- Safe Operating Limits: Maximum allowable working pressure and temperature must be defined and displayed.
Scope and Exemptions
Systems with a stored energy product below 250 bar litres are exempt from the formal examination scheme requirement, but they are not exempt from the broader safety and maintenance obligations under the regulations. Pipework associated with a pressure vessel falls under the scheme unless a Competent Person risk assessment justifies its exclusion. The Provision and Use of Work Equipment Regulations 1998 (PUWER) adds a parallel obligation: equipment must be suitable for its intended use, properly maintained, and fitted with appropriate safety controls.
Air Purity Standards for Food and Pharmaceutical Applications
Food and beverage manufacturers operating in the West Midlands can’t treat desiccant drying as optional. It is a compliance requirement.
The British Compressed Air Society’s Best Practice Guideline 102 sets the standard for food contact applications: an outlet condition of -40°C, equating to Class 2 under ISO 8573-1:2010. Because a refrigeration dryer can realistically only achieve +3°C, adsorption drying is the only route to compliance for food contact air.
ISO 8573-1:2010 Classification for Food Contact Applications
For pharmaceutical manufacturing and electronics production, Class 1 across all three pillars is the target. Breathing air applications are governed separately under BS EN 12021, with laboratory test methods validated to ISO 17025:2017 and quality management systems certified to ISO 9001:2015. See our guide on common causes of pressure drop in compressed air systems a guide for UK businesses for how filter sizing and placement affects the ability to maintain these purity classes across a distribution network.
Sizing a Desiccant Air Dryer
Flow rate alone does not size a unit correctly. Three additional inputs matter.
- Inlet temperature: Every 11°C drop in inlet air temperature roughly halves the moisture content the desiccant bed must handle. Aftercooling before the dryer reduces load and extends media life.
- Inlet pressure: Dryers are rated at a reference pressure, typically 7 bar. If your system runs at higher or lower pressure, apply a correction factor.
- Duty cycle and demand profile: Size at approximately 75% of maximum calculated demand to allow for production growth and demand peaks.
Do not size tight. The cost of undersizing is not a marginal variance. It is a contaminated product line or a failed instrument air system.
Frequently Asked Questions
What is the Purpose of a Desiccant Air Dryer?
A desiccant air dryer removes water vapour from compressed air using hygroscopic material, achieving outlet conditions as low as -70°C. It is used where refrigeration cannot achieve sufficiently low moisture levels: outdoor pipework, instrument air, food manufacturing under BCAS Guideline 102, and pharmaceutical applications requiring ISO 8573-1:2010 Class 1 or Class 2 air quality.
What Are the Disadvantages of a Desiccant Dryer?
The primary disadvantage of heatless desiccant drying is the purge air penalty: 15 to 20% of rated capacity is consumed regenerating the offline tower on every cycle. This adds 3 to 4 kW per 100 cfm to operating costs. Silica gel media also generates dust during operation, creating a health hazard for service technicians and requiring downstream particulate filtration.
How Do You Size a Desiccant Air Dryer?
Size to your maximum demand flow rate corrected for inlet pressure and temperature, then target 75% of that calculated figure to allow for growth. Inlet temperature matters: every 11°C drop halves the moisture load on the bed. Aftercooling before the dryer is the most cost-effective way to reduce load and extend media life.
What Are the Different Types of Desiccant Dryers?
Heatless PSA units use 15–20% regeneration air and carry the lowest capital cost. Heated purge models reduce that to approximately 8% using internal or external heaters. Blower purge variants draw in ambient air and heat it for regeneration.
Vacuum-assisted hybrid systems such as the Hi-line Hi-Plex drop consumption to 1–2% by combining refrigeration pre-drying with vacuum-assisted regeneration.
Does PSSR 2000 Apply to My Compressed Air System?
If your system operates above 0.5 bar above atmospheric pressure and stored energy exceeds 250 bar litres, a Written Scheme of Examination is legally required before operation. Regulation 9 requires a Competent Person to conduct the examination. Systems below the bar litre threshold still fall under the broader safety and maintenance obligations of the regulations.
If your West Midlands facility needs a desiccant dryer specified, serviced, or brought into PSSR 2000 compliance, talk to our engineers at J Ll Leach. We cover Birmingham, the Black Country, and the wider Midlands from our Cuckoo Road depot. Call us with your flow rate, your required outlet condition, and your application, we’ll tell you exactly what fits.