Purge: How it Works and Why You Need It

SEMCO Technology Training Session

Why a Purge Is Needed for ASHRAE 62 Compliance

SEMCO Technology Training Session 2

Example:

o 10’ diameter recovery wheel (with and without a purge)o 20,000 cfm of supply air to building

Without Purge:

o Recirculated carry‐over ‐1,300 cfmo Outdoor air required – 21,300 cfm*o Supply air required – 21,300 cfmo Air through coils, filters – 21,300

With Purge:

o Recirculated carry‐over ‐ 0 cfmo Outdoor air required – 21,300 cfm*o Supply air required – 20,000 cfmo Air through coils, filters – 20,000

* Plus seal leakage

A portion of the outside air stream is used to flush out the contaminated

return air still contained in the flutes of the wheel

Supply Air

Outside Air

Purge section: How it Works

SEMCO Technology Training Session 3SEMCO TECHNOLOGY TRAINING SESSION

o Step 1:  Holding volume of a 120” diameter wheel has a volume of approximately 65 cubic feet

o (3.14 * Diameter 2 /4) * 10”/12”

o Step 2: Holding volume times maximum wheel speed of 20 rpm gives 1,300 cfm to purge this size wheel

o For < .045% a safety margin of at least 10% is required

So purge volume (no seal leakage) required to purge any 270 mm 10’ wheel is approximately 1,300 scfm regardless of the pressures across the airstreams (this determine purge angle) or airflow volume through the wheel

Calculating Purge Volume for Any Manufacturer Easily Done

SEMCO Technology Training Session 4

Purge Basics:

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o The purge volume required for a wheel is a function of the wheel diameter, depth and rotational speed.  It is not a function of the airflow handled or system pressure.

o Knowing the purge pressure, the purge angle can be adjusted so that the purge area at the resultant purge velocity provides the purge airflow needed

Without a Purge More Outdoor Air is Needed

SEMCO Technology Training Session 6

6.2.7 Outdoor Air Correction Factor (OACF). Consider the impact of OACF to ensure that the correctamount of outside air is introduced to meet the ventilation requirements for the building. Example: If theoutdoor air is being measured at the outside air inlet and the OACF = 1.1, the measured air volume should be110% of the required (ventilation design) Supply Airflow. This will ensure that the specified level of outdoor air is being introduced into the building (even with the OACF impact from the Air-to-Air Heat Exchanger).