|Purpose||This is using the ASHRAE Epidemic Task Force's Building Readiness Guide for determine the Equivalent Outdoor Air to be used for determining time required for flushing between occupied periods. The intent is to perform the calculation to reduce the flush time from the prescribed 2 hours without a calculation.|
Building Readiness Guide
|Date||May 9, 2022|
|Link to File|
|IMPORTANT||This tool is intended to simplify the calculation process for determine the time required to achieve the recommended three air changes to achieve 95% contaminant reduction.|
Airborne transmission is not the only mode of transmission, therefore additional risk reduction strategies are required
Universal mask wearing will reduce the bioburden in the space but is not factored into this spreadsheet.
Read the DISCLAIMER at bottom of this worksheet
|USE OF THIS TOOL|
This Google Sheet is locked down. You can go to "File --> Download " and get an excel version that you can use and adjust.
The user must understand the following about the building, system, and spaces:
Space area and ceiling height OR volume
|Filter MERV or MERV-A Rating|
Filter installation gaps via an integrity test
|UVC Inactivation percentage|
|Supply air CFM or ACH|
|Outdoor air CFM of ACH|
|Portable Filter CADR|
If these terms are not clear, please reference the Building Readiness Guide (linked above)
You should only have to enter data in the cells with BLUE TEXT
This assumes an arrangement of the following unit, where the filter is the first thing the return air hits and then the UVC.
* There might be an In Room Fan HEPA Filter Unit
Abbreviations and Assumptions:
ACH_e is the air changes of equivalent outdoor air
ACH_oa is the air changes of outdoor air
ACH_f is the equivalent outdoor air changes due to filtration (best case installation of new filter)
ACH _e,c is the equivalent outdoor air changes due to the air cleaner after the effect of the filter
Q-__R is the recirculated air
Q-e is the equivalent outdoor air flow rate
Q-e,f is the equivalent outdoor air flow rate from the filter
Q-e,f+c is the equivalent outdoor air flow rate from the air cleaner (after it went across the filter)
Ez = zone air distribution effectiveness (From Std 62.1 Table 6-4)
eeta_f is the efficiency of filter
eta_c is the efficiency of the air cleaner
eta_T is the total efficiency of the series of devices
So, eta_T = 1 – (1-eta_f) * (1-eta_c), which allows us to state that
Q-e,f+c = Q-e,f + Q_e,c Q_R* [1-(1-eta_f)
Q-R * [1-(1-eta_f)*(1-eta_c)] = Q_R * eta_f + Q_e,c
Re-arranged it results in the following equation:
Q_e,c = Q_R * (1-eta_f) * eta_c
To make that ACH_e.c, multiple Q_e,c by 60 and then divide by space volume)
The goal is to determine equivalent air changes, so revise the equation to be:
ACH_e = ACH_oa + ACH_f + ACH_e,c
The following impacts are not being addressed here that result in a more conservative time:
• Deposition of particulate (PM2.5 is typically assumed to be about ½ air change of particle reduction)
• Temperature of the space and or change of temperature.
• Relative humidity of the space and or change of RH.
|MERV Rating||Droplet Nuclei Filter Efficiency||E1 Average (Note 1)||E2 Average (Note 1)||E3 Average (Note 1)|
Anticipated Distribution of Virus
|4||16%||10.3%||29.9%||11.9%||This information for distribution of virus is based on studies on Influenza A in addition to some recent studies on SARS-CoV-2 viral particle sizes in the air. |
The Influenza A studies are document in the ASHRAE ETF Building Readiness Guide. The limited studies on SARS-CoV-2 are not conclusive enough at this time to pinpoint exact viable virus particles that would align with the ASHRAE Standard 52.2 ranges. The SARS-CoV-2 studies, linked below, only has one consistent conclusion between those studies. The consistent item is that the majority of particles with viable virus is less than 5 microns in size.
Based on that information, we have update the Anticipated Distribution of Virus to be as shown in the table below. As always, you can easily be more conservative by having all of the particles in the E1 range and none in E2 or E3. When you edit those numbers, the calculations are updated.
New Values from SARS-CoV-2
Based on Influenza A Only (in v1.1)
Note - if you override these values it will change the Droplet Nuclei Filter Efficiency
|16||97%||95.0%||98.0%||98.0%||The intent is that the distribution between the filter size ranges can be adjusted by the User. For a conservative approach to this calculation, make the E1 be 100% and then the other two at 0% since this is the lowest performing particle size for all of the filters listed.|
|100||72%||34%||92%||86%||USER ENTERED FILTER DATA|
MERV-100 is User Entered Data for a filter that you have specific data or are using MERV-A values. Enter the ASHRAE Std 52.2 Test Results for the E1, E2, and E3 values.
If the system has no filter, choose the 100 and make the E1, E2, and E3 values all 0%.
Note 1: Filter data is based actual ASHRAE Std 52.2 test results for over 140 filters that are on the market. This data does take into account the degradation of the filters.
Filter efficiencies in this table assume a well installed and well sealed filter to achieve this performance. Based on your installation, you might use the 100 ROW and apply a leakage factor to reduce the efficiency of the filter.
|Zone Air Distribution Factor||1.0|
This factor is to include the impact of the room air distribution and the potential that the space is not well mixed.
he following table from ASHRAE Std 62.1. Select your system and then insert the Ez value in cell B59.