What is ASHRAE 52?

ASHRAE 52 is one of the earliest official standards for evaluating the efficiency of air filter in HVAC systems. It was first introduced in 1968 by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE).
In 1992, the original ASHRAE 52 standard was updated to ASHRAE 52.1. This revision eventually led to the development of ANSI/ASHRAE 52.2, titled "Methods for Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size."

ASHRAE 52 Standard Characteristics

The ASHRAE 52 standard utilizes two primary methods to measure filtration efficiency: the "Arrestance Test" and the "Atmospheric Dust Spot Test (ADST)".
  • Arrestance Test: This method assesses the ability of low-efficiency filters to capture and retain larger dust particles.
  • Atmospheric Dust Spot Test (ADST): This test evaluates higher-efficiency filters by measuring color changes on a test filter, which indicate the average removal efficiency of fine dust particles.
Ashrae
While these testing procedures have been fundamental for decades, some inaccuracies and limitations have emerged in the original ASHRAE 52 standard, highlighting the need for continuous improvement in testing methodologies.

Some inadequacies of the standard ASHRAE 52

First, the ADST method uses "Atmospheric air" to check the efficiency of the filter. This use of atmospheric air creates a problem: outdoor air everywhere is different, that is, there is no stability and uniformity. Conduct dust inspection in different areas that distort the results.
The next inadequacy is the determination of filtration efficiency through discoloration without taking into account particle size, especially microscopic dust particles (fine dust). In addition, the ASDT method cannot determine the minimum efficiency of the filter, but only determines the maximum performance at peak conditions.

Distinguish between ASHRAE 52.1 standard and 52.2

When the ASHRAE standards committee addressed these inadequacies in 1992, they decided to keep the ASHRAE 52 Standard while creating a separate standard for the improvement method.
The old test methods are mentioned in ASHRAE 52.1 standard, while new instructions on filter testing are found in Standard ASHRAE 52.2.

ASHRAE 52.1

The ASHRAE 52.1 standard measures the following parameters:
  • Dust Spot Efficiency (ADST - Atmospheric Dust Spot Test): Measures the efficiency of the filter in removing atmospheric dust from the test air.
  • Dust Capture Efficiency (Arrestance Test): Determines the amount of total dust the filter can collect.
  • Dust Holding Capacity: Indicates the amount of dust the filter can hold until it reaches the final pressure differential. This capacity provides an estimate of the filter's relative lifespan.

ASHRAE 52.2

  • ASHRAE 52.2 measures fractional particle size performance (PSE) of the filter. This test helps determine the filter's ability to remove dust particles of different sizes (ranging from 0.3 microns to 10 microns).
  • ASHRAE 52.2 also introduces a new method for recording filter efficiency and establishes the Minimum Efficiency Reporting Value (MERV).
MERV is assigned to filters based on their minimum filtration efficiency at different particle sizes. The "MERV 1" rating is the least effective, while the "MERV 16" rating is the most effective.
  • The ASHRAE 52.2 standard does not consider the filter’s lifespan. Dust holding capacity is not a necessary parameter in the test report.
When determining the estimated lifespan of a filter, the ASHRAE 52.1 standard is still used.

Some main differences between ASHRAE 52.1 and ASHRAE 52.2 standards:

First, to improve the control of laboratory experiments, ASHRAE 52.2 uses laboratory-generated Potassium Chloride instead of atmospheric air, as in the ASHRAE 52.1 standard. This change prevents inconsistencies due to varying outdoor air conditions. The test apparatus is also improved, and the new method uses an optical particle counter to measure the efficiency of upstream and downstream filters.
Next, the ASHRAE 52.2 standard uses the Minimum Efficiency Reporting Value (MERV) to evaluate dust filtration efficiency.
To determine this value, the experiment considers 12 particle size ranges from 0.3 microns to 10 microns. The test method records the minimum efficiency curves of filters in 6 different stages.
By comparing these 12 particle size ranges with 6 different load stages, the tester can determine the minimum performance curve.
This derived minimum performance curve is the primary data product of the process. The curve also determines the MERV level by calculating the average filtration efficiency across three size bands. This helps contractors and consumers easily understand how air filters perform based on minimum requirements for specific particle sizes.

Comparison table of ASHRAE 52.1 and ASHRAE 52.2 standards

ASHRAE 52.1 ASHRAE 52.2
Substances used in the experiment

ASHRAE synthetic test dust (for dust holding and holding ability test)

Atmospheric dust (for dust point efficiency test)

 Polydisperse KCL solid spray
Particle size N/a 0.30 μm - 10 μm
Sampling tools Dust opacity meter Optical particle counter (OPC) or Aerodynamic particle counter
Air duct Straight windpipe Straight pipe/ U-shape tube, with HEPA filter
Wind in Outdoor air (to achieve dust point efficiency), not indoor air Indoor air or air
Wind out Exhaust gas to the outside or an indoor space completely isolated from the inlet, or circulating Gas discharged outside, indoors or circulating
Temperature N/a 10℃ ~ 38℃
Humidity N/a 20% ~ 65%
Results Obtained Head pressure difference, initial/average dust point performance in the atmosphere, dust holding capacity Head pressure difference, at least 75 effective values, MERV under the rated air flow
Weakness Not counting the filtration efficiency according to each particle size; the master obtains the dust dot efficiency of the filter. Cannot evaluate industrial air purifiers and air purifiers in the room such as ozone generators and ionizers.

Will ASHRAE 52.2 Completely Replace ASHRAE 52.1?

"Even with all the improvements of ASHRAE 52.2, it is still necessary to maintain the ASHRAE 52.1 Standard, at least for the time being," said Barney Burroughs, head of the ASHRAE Standing Standards Project Committee responsible for Standard 52.2 (speaking in 2002).
Mr. Burroughs mentioned that while most leading manufacturers are now testing their filters with the latest standards, many filters still in use within HVAC systems can only be evaluated using the old weight measurement technique.
He also noted that the older 52.1 standard includes some methods relevant to modern testing procedures, such as air filter loading, determining pressure drop (initial and final pressure differences), and device instructions.
Burroughs hopes that in the near future, ASHRAE 52 will incorporate valid aspects of 52.1. Once this integration is complete, Standard 52.1 and its atmospheric dust test will be phased out.
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