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Department for Business, Energy and Industrial Strategy

Refrigerated Air Dryers with Energy Saving Controls

1.1           Scope

Refrigerated air dryers are commonly fitted to compressed air systems to prevent moisture from condensing within pipe work and equipment. They work by cooling the air to a desired dew point temperature, thus forcing moisture to condense out of the air. This resulting condensate is then drained from the compressed air system.

A refrigerated air dryer typically increases the energy used in compressed air generation by between 2% and 5% depending on the type of product selected and how it is controlled. The pressure drop across the refrigerated air dryer is also a key factor in the amount of additional energy consumed as a result of the use of refrigerated air dryers.

1.2           Definitions

Refrigerated air dryers are products that are specifically designed to extract water vapour from industrial compressed air systems by means of cooling with a refrigeration cycle.

The aim of the Energy Technology List (ETL) Scheme is to encourage the purchase of higher efficiency models, which have low pressure drops across them.

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 <= +3C i.e., Class 4 specifications for moisture removal in BS ISO 8573-1:2010, “Table 2 Compressed air purity classes for humidity and liquid water”.
  • Incorporate energy saving controls that automatically reduce the cooling output of the refrigerated air dryer as the average flow rate and temperature of the inlet air decreases in a manner that reduces the energy consumption of the product.
  • Automatically control their output between 20% and 100% in response to changes in the flow rate and/or temperature of the inlet air and/or outlet air.
  • Not exceed the limits set out in the performance criteria below for pressure drop corrected composite specific energy consumption (SEC) at 100% load (i.e. rated air flow).
  • 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

Products shall not exceed the values for pressure drop corrected composite specific energy consumption (SEC) set out in the Table 1.1 below at the specified percentage of full load. 

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

Percentage of full load (i.e. rated air flow) Maximum allowable Composite SEC (kW/m3/min)
100% <=0.41

The pressure drop-corrected composite SEC should be calculated as follows:

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


P = Total electrical power consumed by air dryer, kW

Δp = Pressure drop across air dryer, bar

Q = Flow rate of air, 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 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. 

1.4.2       Rounding 

For the avoidance of doubt composite SEC data shall be presented to 2 decimal places. As an example, a product with a composite SEC of 0.42 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 refrigerant as the representative model.
  • Have the same compressor type (i.e., manufacturer, method of compression and type of enclosure) as the representative model.
  • Fit within the same product category.

The representative models shall be selected by dividing the range of products into groups of models with similar design characteristics and testing a model in each group. The performance of each model in the group shall be predicted using a validated mathematical model. As a minimum, at least one model shall be tested in each range of products. 

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.
  • Amending the moisture removal class, as per BS ISO 8573-1: 2010, from the existing class 4 to class 5.