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Evaporative Air Coolers

1.1    Scope

Evaporative air coolers can be direct or indirect. With direct evaporative air cooling, outside air is blown through a water-saturated medium and cooled by evaporation. The cooled air is circulated by a fan.  With indirect evaporative air cooling, a secondary air stream is cooled by water. The cooled secondary air stream passes through a heat exchanger, where it cools the primary air stream. The cooled primary air stream is circulated by a fan. 

Evaporative air coolers are available with a wide variety of efficiencies. The Energy Technology List (ETL) Scheme aims to encourage the purchase of direct evaporative air coolers and higher efficiency indirect evaporative air coolers.

The ETL Scheme covers two categories of product:

  1. Direct evaporative air coolers 
  2.  Indirect evaporative air coolers

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

1.2    Definitions

An evaporative air cooler is a device that cools air through the evaporation of water. 

1.3    Requirements 

1.3.1    Eligibility requirements  

General functional criteria for evaporative air coolers 
Direct and Indirect evaporative air coolers shall conform to the following functional criteria:

  • Incorporate one or more electrically powered fans
  • Incorporate an electrically powered pump to circulate water to a water-saturated medium through which an air stream passes or to spray nozzles in an air stream.
  • Minimise scale build up with optimised water bleed rates
  • Maintains conditions compliant with ACoPl8 legionella code of practice and guidance.
  • Not use air to cool a water stream.

Direct and indirect evaporative air coolers shall be have an appropriate Conformity Assessment mark.

In addition to the general functional criteria above, evaporative air coolers shall also conform to specific functional criteria for either direct or indirect evaporative air coolers.

Functional criteria for direct evaporative air coolers:

In order for the product to be classified as a direct evaporative air cooler, all of the following criteria shall be met:

  • The product cools an air stream by moving air through a water-saturated medium which is cooled by evaporation. Moisture must be added to the air stream until it is close to the point of saturation (i.e., the wet bulb depression is close to zero).
  • The product dry bulb temperature should reduce while the wet bulb temperature remains constant.

Functional criteria for indirect evaporative air coolers:

The product will be classified as an indirect evaporative air cooler should any one of the following criteria be met:

  • The product incorporates a secondary air stream that is cooled by means of evaporation of water.
  • The product incorporates a secondary air stream that passes through a heat exchanger in order to provide further cooling to a primary air stream.
  • The product incorporates a primary airstream with no change in absolute moisture content. 

1.3.2    Performance requirements 

Eligible indirect evaporative air coolers shall meet the following criteria:

1.    Energy Efficiency Ratio (EER). The EER is the ratio of cooling capacity (kW) to electrical power input (kW). 

$$ EER=\frac{Cooling\ capacity\ (kW)}{electrical\ power\ input\ (kW)}$$

$$Cooling\ capacity,\ q=1.21Q_p(t_{d1}-t_{d2})$$

td1 and td2 are the primary air inlet and outlet dry-bulb temperatures, respectively. 
Qp is the primary standard airflow rate (m3/s).

The total electric power input is the sum of pump, air-moving device, and any other electric power input due to appurtenances required to produce cooling. 

Table 1.1    EER performance threshold for indirect evaporative air coolers

Product category EER
Indirect ≥7.0

'≥' means 'greater than or equal to'


2.    Cooling Effectiveness (ε), which is the primary air dry-bulb temperature reduction divided by the primary air entering dry-bulb temperature less the entering secondary air wet-bulb temperature. 

$$ Ɛ=\frac{t_{d1}-t_{d2}}{t_{d1}-t_{w3}}$$

Where, tw3 is the secondary air inlet wet bulb temperature.

Table 1.2    Cooling effectiveness performance threshold for indirect evaporative air coolers

Product category Cooling effectiveness (%)
Indirect ≥95.0

'≥' means 'greater than or equal to'


3.     Water consumption. For information purposes only, provide details on the amount of water consumed by the indirect evaporative cooling unit (m3/hr).  

1.4   Measurement and Calculations

1.4.1   Measurement standards

All indirect evaporative air coolers shall be tested in accordance with the procedures and test conditions laid down in:

ANSI/ASHRAE Standard 143-2015: Method of Test for Rating Indirect Evaporative Coolers.

1.4.2   Calculation Requirements

The EER and cooling effectiveness of the evaporative air cooler will be calculated when operated at an inlet psychometric condition of 35°C dry bulb temperature, and a 24°C wet bulb temperature. Furthermore, the following external resistance (system static pressure) will be applied:

  • Units up to 4 m3/s  = 80 Pa resistance
  • Units greater than 4 m3/s = 120 Pa resistance

1.4.3   Rounding  

For the avoidance of doubt test data should be presented to 1 decimal place. As an example, a Cooling Effectiveness of 94.9 % for an indirect unit 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:

  • Witnessed testing
  • Independent testing
  • Representative testing (see clause 1.5.1)

Further information regarding the first two routes can be found in Guidance Note 5 on the ETL product testing framework . 

1.5.1   Representative testing

Where applications are being made for a range of two or more products that are variants of the same basic design, test data may be submitted for a representative selection of models, provided that it can be demonstrated that all variants:

  • Utilise the same core technology as the tested model;
  • Utilise the same key components as the tested model. 

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