Energy Recovery Wheel Purge

  • Energy Recovery Wheel Purge: How it works and why it is needed to limit the exhaust air transfer ratio (EATR) to be in compliance with ASHRAE Standard 62.1

    Technology Training: How a purge works and why you need it.

    What is the Exhaust Air Transfer Ratio (EATR) and How is it Limited?

    Energy recovery wheels function by rotating a honeycomb transfer media alternately through an airstream being exhausted from a building and then through an outdoor airstream being delivered to a building. The outdoor airstream is delivered to the building for the sole purposes of improving indoor air quality. Effective total energy recovery allows increased quantities of outdoor air to be provided to the occupied space in a cost effective manner.

    During the recovery process, a portion of the contaminated exhaust air is trapped inside the flutes of the transfer media as the recovery wheel rotates through the contaminated exhaust airstream into the supply outdoor airstream. Once that section of the wheel media enters into the opposing supply airstream, this contaminated “carry-over” air, if not addressed, is pushed into the supply air, resulting in an unwanted compromise in indoor air quality. Unaddressed, this carry-over air can amount to as much as 10% of the supply airstream volume. 

    energy recovery wheel purge schematicA properly designed “purge section” (See Figure 1) can be easily employed to effectively remedy this carry-over for recovery wheels using fluted media. The purge section allows for strategic leakage of unconditioned outdoor air into the return airstream. This “purge airflow”, driven by system static pressures around the recovery wheel, “flushes” the contaminated air that is otherwise carried over to the supply air back into the contaminated airstream and is exhausted from the building. 

    By incorporating a properly designed purge section, the carry-over associated with the wheel geometry during rotation (the purge does not eliminate contaminant carryover transferred by the desiccant materials used by some energy wheels) can be limited to less than .04% of the exhaust air contamination level. This carry-over is reported as the exhaust air transfer ratio (EATR), quantified by sulfur hexafluoride tracer gas testing, as part of the AHRI certification ratings. 

    How to Calculate the Magnitude of EATR if a Purge Section is Not Utilized

    It is common for manufacturers of energy recovery systems to install recovery wheels without the use of a purge section. The thinking is that without the purge, fans can be sized for a somewhat lower airflow and the recovery wheel components may have a lower cost. However, this decision is often made without understanding the negative impact of the excessive carry-over (EATR) and code compliance.

    Example 1:
    An energy recovery wheel utilizing a 150 mm (6”) deep transfer media and operated at 40 revolutions per minute is sized to provide a total energy efficiency of 75%, with a supply and return airflow of 4,500 cfm. How is the carry-over percentage (EATR) estimated to determine if a purge section is appropriate?


    • The recovery wheel operates at a face velocity of 510 ft/min to achieve 75% recovery efficiency

    • The wheel area per side is therefore 8.8 square feet (17.6 square feet for the total wheel)

    • The volume of the wheel is then calculated from the face area 17.6 square feet multiplied times the depth .5 feet which equals 8.8 cubic feet 

    The carry-over volume can be easily calculated as shown below:

    Carry-over volume = 8.8 cubic feet x the 40 revolutions per minute = 352 cfm
    EATR or carry-over percentage of the supply airflow = 352 cfm/4,500 cfm or 8%

    So, if a purge section is not utilized, 8% of the supply air delivered to the building will be recirculated return air, not outdoor air. This has significant ramifications with regard to compliance with ASHRAE 62.

    Since both logic and ASHRAE 62 requirements do not allow for recirculated air to be considered outdoor air, the omission of an effective purge would require that the supply airflow delivered to the space be increased to 4,852 cfm (4,500 plus 352 cfm). This 8% increase in airflow has to be accommodated by any cooling coils, heating device, filters and ductwork downstream of the recovery wheel. The internal and external static pressures increase substantially due to the higher flow, increasing fan horsepower.

    If an effective purge is employed, the supply airflow remains at 4,500 cfm and only the exhaust fan airflow capacity (sometimes both the supply and exhaust fan depending on fan placement) is increased to provide the necessary purge airflow volume.

    Purge Operation Basics: How it Works

    energy recovery wheel without purgeThe schematic labeled Figure 2 depicts an energy recovery wheel that is operated without the use of a purge section. The recovery wheel media has a volume, and that volume contains (traps) contaminated exhaust air as the wheel rotates from the return airstream into the supply airstream. Without a purge section, the volume of exhaust air trapped within the media is pushed into the supply air and introduced to the occupied space. As presented within Example 1, the magnitude of this recirculated or “carry-over” air is a function of the recovery wheel volume and the rotational speed of the wheel.

    Figure 3 shows how a small portion of unconditioned outdoor air is used to effectively “flush” the trapped exhaust air from the recovery wheel as it rotates from the return airstream into the supply airstream. As shown the contaminated air that would otherwise be introduced into the “clean” supply airstream is recycled back into the return airstream and exhausted.

    Years of laboratory and field testing has proven that a well-designed purge section properly operated can easily achieve 99.96% efficiency and, for critical applications, can routinely be adjusted to achieve 99.99%. For documenting purge efficiency, see manufacturer’s AHRI listings for purge inefficiency (EATR ratings) at the rated pressures. Most manufacturers AHRI ratings only show EATR values with a purge. To approximate carry-over without a purge, the procedure outlined in Example 1 must be used.

    To optimize purge efficiency, the purge flow (see Figure 3) should be slightly greater than the energy recovery wheel with purgeair trapped within the wheel (carry-over volume calculated in Example 1) during normal operation. Achieving the proper purge airflow is a simple function of allocating adequate purge area and ensuring the necessary purge airflow velocity. Therefore, there are two parameters that can regulate purge volume.

    Purge pressure: The purge pressure differential at a given purge area determines the purge airflow. The purge pressure differential is the static pressure difference between the outdoor and return airstreams when the purge section is positioned within the supply airstream section of the air handling system as shown in Figure 3. For the purge to function, the outdoor air static pressure must be higher than the return air static pressure plus the pressure loss across the supply side of the wheel. This pressure differential is the “purge driving force”. 

    If a system is designed such that the outdoor static pressure is less than the return air static pressure, the purge airflow will be flowing in the wrong direction and a purge section should not be employed. In this case, the EATR will be dominated by seal leakage moving from the contaminated into the clean airstreams. This pressure scenario should almost always be avoided.

    The purge pressure is seldom an adjustable design variable since it is a result of optimizing the pressure loss through components within the air system and the ductwork. Therefore most purge airflow adjustments where purge pressure differentials greater than about 2” exist are best made by a modification of the purge area.

    Purge area: Some energy wheel products have a fixed purge area while others have an adjustable purge. A fixed purge area is effective if system pressures across the purge are always modest and moderate purge efficiency is adequate for a given application. 

    An adjustable purge is essential when system pressures across the purge section are high, unknown, variable or where minimizing carry-over is important (i.e. high purge efficiency is required). By adjusting the purge angle, the net area of the purge section is varied. This allows for the necessary purge airflow to be achieved whether the differential pressure across the purge is very high (small purge area required) or low (large purge area required). For a detailed discussion of this topic see the SEMCO Energy Recovery Wheel Technical Guide pages 14–16 (available for download here).

    Exhaust Air Transfer Ratio (EATR) Does Not Reflect Cross-Contamination Due to the Desiccant Surface

    The EATR measured in accordance with ASHRAE Standard 84 “Method of Testing Air to Air Heat Exchangers” uses sulfur hexafluoride tracer gas to quantify purge inefficiency and seal leakage. SF6 is a very large and non-polar molecule, and was strategically chosen for EATR testing since it will not be adsorbed nor transferred by the desiccant surface of a total energy recovery wheel.

    While omitting a purge angle can result in up to 10% EATR or carry-over, utilizing the wrong desiccant coating can result in more than 40% of unwanted indoor contaminants being transferred from the exhaust airstream back into the occupied space. This carry-over is not avoided by a purge section. ASHRAE Standard 84 has provisions for testing the carry-over of contaminants of concern but few manufacturers provide this important information despite the fact that it has a substantial impact on the resultant indoor air quality. For more on this important topic see the SEMCO white paper entitled “Not All 3A Recovery Wheels Limit Contaminant Transfer.”

    ASHRAE 62.1 Committee Interpretation IC_62.1-2010-8

    During January 2016 the ASHRAE 62.1 Committee approved an interpretation request (IC_62.1-2010-8) confirming that recirculated air within an energy recovery device due to lack of purge or seal leakage from the exhaust into the supply airstream, cannot be considered outdoor air, and that the supply air must be increased to account for this recirculation (i.e supporting the values shown in Example 1).

    With this official interpretation, there is no ambiguity regarding recirculated room air within all ASHRAE air classes. Depending upon the air classification (1 through 4), recirculated air may be allowed within an energy recovery device (wheel or plate) but this recirculated air cannot be considered outdoor air or used to satisfy the minimum outdoor air requirement specified by ASHRAE 62.1.