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Department for Business, Energy and Industrial Strategy

Desiccant Air Dryers with Energy Saving Controls

1.1           Scope

Desiccant air dryers are commonly fitted to compressed air systems to prevent moisture from condensing within pipe work and equipment. They are typically utilised where compressed air is needed at higher quality or with a lower dew point than can be achieved by a refrigerated air dryer. 

They contain a desiccant material which absorbs the moisture and is then regenerated, for example, by blowing air through the dryer. 

The direct energy usage of a desiccant air dryer typically increases the energy used in compressed air generation by between 10% and 25% depending on the product design and how it is controlled. Indirect energy usage, in the form of the pressure drop across the dryer and the compressed air used for purging can increase the overall energy usage in compressed air generation by between 20% and 50%.  

1.2           Definitions

Desiccant air dryers are products that are specifically designed to extract water vapour from industrial compressed air systems by absorbing moisture using a desiccant material which is then, for example, regenerated by blowing air through the dryer.  

The aim of the Energy Technology List (ETL) Scheme is to encourage the purchase of higher efficiency models which use energy efficient methods of desiccant regeneration, have low pressure drops across them and include energy savings controls. 

The ETL Scheme covers three categories of product: 

  • Heatless dryers: Heatless dryers take a small proportion of the dry process air, known as purge air, expands this purge air back to atmospheric pressure where it becomes even drier and then passes it over the off-line desiccant bed undergoing regeneration. The dry air strips the moisture from the desiccant material. Heatless dryers can further be vacuum assisted, wherein a vacuum pump is installed to assist purge. These are more energy efficient than the conventional heatless dryers. 
  • Heated dryers: These dryers use a heater (located inside or outside the vessel) to heat the purge air. This heated purge air regenerates the desiccant material.  
  • Hybrid dryers: These dryers use a combination of refrigeration dryer and adsorption dryer to achieve the desired dew point. The adsorption dryer size is comparatively smaller as the incoming water vapour level has already been reduced by the refrigeration dryer. As the adsorption dryer bed size is reduced, so is the volume of desiccant requiring regeneration. 

To be eligible for inclusion on the ETL, products shall meet the requirements as set out below. 

1.3           Requirements

1.3.1       Eligibility requirements 

To be eligible, products shall: 

  • Have a dew point rating of <= -40ºC i.e. Class 2 specifications for moisture removal in BS ISO 8573-1:2010, Table 2 Compressed air purity classes for humidity and liquid water.
  • Utilise a regeneration method which is either heatless or internally/externally electrically heated or utilises a blower or vacuum system. Hybrid dryers using a combination of refrigeration dryer and adsorption dryer are eligible. Desiccant dryers that utilise steam, heat of compression or are heated in any way other than electrically are not eligible.
  • Incorporate dew point sensing controls that automatically control the regeneration cycle to optimise the time between regenerations depending on the dew point of the exit air in a manner that reduces the energy consumption of the product.
  • Not exceed the limits set out in the performance criteria below for the composite specific energy consumption (SEC) at 100% load (i.e. rated air flow), corrected for the pressure drop across the dryer and any compressed air used by the dryer for regeneration, purging and/or cooling.
  • Conform with the requirements of The Pressure Equipment (Safety) Regulations 2016 in respect of their design, manufacture, and testing procedures, or have an appropriate Conformity Assessment mark.

1.3.2       Performance requirements

Eligible products in all three categories shall not exceed the values for composite specific energy consumption (SEC) at instantaneous purge, corrected for the pressure drop across the dryer and any compressed air used by the dryer, set out in the Table 1.1 below at 100% load (i.e. rated air flow). 

Table 1.1   Maximum Allowable Composite SEC in kW/m3/min


Product Category

Maximum allowable Composite SEC at Instantaneous value/purge for 100% load (kW/m3/min)

Heated Dryers

<= 0.66

Hybrid Dryers

<= 0.88

Heatless Dryers

<= 1.41

The composite SEC should be calculated as follows:

$$SEC = \frac{P + (1.67 \times \Delta p \times Q) + (5 \times C)}{Q - C}$$


P =   Total electrical power consumed by air dryer, inclusive of any

         external heaters, blowers, vacuum pumps or other associated

              equipment, kW

Δp = Pressure drop across air dryer, bar

Q =   Flow rate of air, m3/min

C =   Total compressed air loss of air dryer for regeneration, purging, cooling

         or any other purpose, m3/min

1.4           Measurement and Calculations

1.4.1       Measurement Standards and Test Requirements

All products shall be tested in accordance with the procedures and test conditions laid down in BS ISO 7183:2007, which specifies how to measure the electrical power consumed by the product at full load, the pressure drop across the dryer, compressed air loss and the flow rate of air through the product. The test results should be presented in the format laid down in Annex B of BS ISO 7183:2007.

Products shall also meet the Class 2 specifications for moisture removal in BS ISO 8573-1:2010, “Table 2 Compressed air purity classes for humidity and liquid water”.

1.4.2       Rounding 

For the avoidance of doubt composite SEC data shall be presented to 2 decimal places. As an example, a heated dryer with a composite SEC of 0.67 at 100% load would be deemed to be a fail.

1.5           Verification for ETL Listing

Any of the following testing routes may be used to demonstrate the conformity of products against the requirements:

  • In-house testing – Self-tested and verified or cross-checked by an independent body
  • Witnessed testing
  • Independent testing
  • Representative Testing (See clause 1.5.1)

Further information regarding the first three routes can be found in the ETL Testing Framework.

1.5.1       Representative testing 

Where applications are being made for two or more models that are variants of the same basic design and category, test data may be submitted for a single ‘representative model’ provided that all variants: 

  • Use the same method of regeneration
  • Fit within the same product category

It should be noted that:

  • If a manufacturer voluntarily removes the representative model from the ETL then other products linked with that representative model may or may not be permitted to remain on the ETL.
  • If any product submitted under these representative model rules is later found not to meet the performance criteria when independently tested, then all products based on the same representative model will be removed from the ETL.

1.6           Conformity testing

Products listed on the ETL may be subject to the scheme’s conformity testing programme in order to ensure listed models continue to meet the ETL requirements.

1.7           Review

1.7.1       Indicative review date

This specification is scheduled to be reviewed during the 2023/24 review cycle. 

1.7.2       Illustrative future direction of the requirements

The next technical review will consider:

  • Amending the energy efficiency thresholds in light of the performance data collected during the intervening period.
  • Introducing SEC performance thresholds at partial flows (e.g., 25%, 50%, 75%) following the test method prepared by the British Compressed Air Society.