Author Archives: Administrator

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Reducing COVID-19 Exposure Risk from aerosol generating patient medical procedures

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The generation of aerosol particles from patient procedures posses great risk for exposure to COVID-19 to dentistry workers and employers (1). Other commonly performed medical procedures that generate aerosols or that create uncontrolled respiratory secretions, include (4):

  • open suctioning of airways
  • sputum induction
  • cardiopulmonary resuscitation
  • endotracheal intubation and extubation
  • non-invasive ventilation (e.g., BiPAP, CPAP)
  • bronchoscopy
  • manual ventilation
  • nebulizer administration (uncertain)
  • high flow O2 delivery (uncertain)

Aerosol particles in a treatment room may remain in suspension for up to 13 to 20 hours (99% to 99.9% deposition) after the procedure for particles between 0.3 to 10 microns (2). This posses a risk of exposure for any patient or personnel that enters the room where the procedure was previously performed if proper Environmental Infection Control measures are not put into place.

CDC Interim Infection Prevention and Control Recommendations for Patients with Suspected or Confirmed Coronavirus Disease 2019 (COVID-19) in Healthcare Settings (4) recommends the following:

“Once the patient has been discharged or transferred, HCP, including environmental services personnel, should refrain from entering the vacated room until sufficient time has elapsed for enough air changes to remove potentially infectious particles. After this time has elapsed, the room should undergo appropriate cleaning and surface disinfection before it is returned to routine use.”

The only way to minimize the risk of exposure from aerosol particles is to maintain the room negative pressure at all times, close the treatment room after the procedure is finished and the patient and personnel is no longer in the room and provide proper filtration for a prescribed duration.

Calculation of the Room Air Changes (ACH) and the elapsed time for aerosol particles count to be reduced to less than one percent of the initial concentration is necessary to properly size and select in-room air cleaners.

Current CDC guidelines (5) provides in Table B.1 the elapse time for removal of aerosol particles as follows:

Room Air Changes
Rate per Hour
99% Removal
Elapse Time (mins)
99.9% Removal
Elapse Time (mins)
* Most commonly design guidelines and codes prescribe rates for patient-care areas.

Notes to this table emphasize that these values apply for perfect mixing conditions of the air within the space. It also warns that perfect mixing does not usually occurs and that removal times will be longer in rooms or areas with imperfect mixing or air stagnation.

A study done by the US Environmental Protection Agency on In-Room Air Cleaners (2) shows that for a room with a 2:1:1 (L:W:H) aspect ratio with central furniture and an air cleaner in a corner at an angle, the mixing efficiency or air change effectiveness (ACE) can be as low as 44%. This means that the amount of air obtained from the above table Room Air Changes Rate would have to be multiplied by a factor greater than 2.25.

As we can see great care must be taken when using In-Room Air Cleaners to reduce the risk of exposure to COVID-19 from aerosol particles. Air distribution factors are important and can only be assessed by a competent professional engineer.


  1. COVID-10 – Control and Prevention, Dentistry Workers and Employers. US Department of Labor, Occupational Health and Safety Administration Website. Obtained May 29, 2020.
  2. Evaluation of In-Room Air Cleaners for Building Protection – Final Report. US Environmental Protection Agency, EPA/600/R-08/012, January 2008.
  3. Guidelines for Environmental Infection Control in Health-Care Facilities. US Center for Disease Control and Prevention, July 2019.
  4. Healthcare Infection Prevention and Control FAQs for COVID-19. US Center for Disease Control and Prevention, May 29, 2020.
  5. Interim Infection Prevention and Control Recommendations for Patients with Suspected or Confirmed Coronavirus Disease 2019 (COVID-19) in Healthcare Settings, Appendix B, Table B.1. US Center for Disease Control and Prevention, May 18, 2020.


Aerosol Generation during Dental Procedure Photo from ADS Dental System. The Author declares that there is no conflict of interest.

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New Engineering Articles

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Hello everyone!

It’s been a while but we’re back. Starting with the addition of some engineering articles that will be helpful for those of you that like to do their own excel sheets. The articles date back to when I started engineering and I experimented combining them with computer programming. You can find the section here.

We will be adding more resources and spreadsheets to the site in the near future, so be sure to check back soon!


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Calculating Hunter’s Curve using Excel

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Hunters Curve

Have you ever wondered how to convert from fixture units to flowrate? A curve fit on the code tables will not work satisfactorily! I have tried that myself. Here is the method as it was developed by Roy B. Hunter back in 1940. But don’t let this year set you back because even today’s code use his approach to estimate water flowrate demands for buildings.

Hunter’s approach to solve this engineering problem was statistical. This is the reason that a simple curve fit wont’t work. Hunter used a binomial distribution to estimate the probability that a given number of fixtures, m, out of a total of n, will be operating at an arbitrary instant of observation.

The equation he developed is as follows:


Hunter used a 1/tau value of 0.01 which means that the probability of m fixtures being open is 99% or greater.

Because the curve was used for different kinds of plumbing fixtures, a fixture weighting factor (w) had to be incorporated. In systems with predominantly flush tanks, the weight for a Flush Tank w=5, for Flush Valves w=10. To covert from fixture units to fixture number the following equation is used, n=FU/W.

In order to calculate the demand in gallons per minute, a factor q equal to the average volume rate of flow, in gallons per minute, to or from the plumbing fixture during actual operation is used. Again, in the case of flush tanks systems q=4 gpm, for flush valve systems q=27 gpm. The demand flowrate (GPM) is the product m x q.

Although Hunter’s formulation seems complicated, it is relatively easy to calculate using excel. Excel has built in statistical functions to calculate the binomial probability function. In order to calculate the flow rate according to the binomial probability function the following equation must be entered into a cell.


Because the binomial probability function works with discrete values of fixtures (ie. 1,2,3,4,5,etc.), using fixture unit weights will lead to a steeped probability function as it can be depicted in the stepped bell shape and stepped demand flowrate curve shown above.

The Binomial Probability function can be converted to a continuous probability function using the normal probability. In this case the mean of the normal probability is nq and the variance is nq*p(1-p) with p=t/T. The following equation was used to calculate the smooth Hunter’s curve above.


Research by Hunter and other’s determined that the values for t and T depending on the predominant fixture type. For Flush Tank systems t=60, T=300 (p=0.20); for Flush Valve systems t=3 and T=300 (p=.01).  In this regard, current codes do little to specify these parameters that are highly dependent on the building classification and the characteristics of the fixtures used. It is my personal opinion that International codes should provide guidelines on which of the two probability functions to use, and prescribe a value for the probability (1-tau). International codes should also prescribe the values of t and q based on fixture characteristics and values of T based on building classification.

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Psychrometry Calculations

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Don’t let Psychrometry get you down!

Type A Unit Configuration now up online. Save valuable time and energy with the use of this exclusive engineering tool. Check out our Shop for more spreadsheets and make sure to come back often – more formula solutions are being added on a regular basis.

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Grand Launch

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We’re up online!

Welcome to the Grand Launch of JLEngineering Calculations. The only place you’ll find all the tools you need to take your Mechanical Engineering career to the next level.

Solving Mechanical Engineering problems just got easier – one spreadsheet at a time.

Make sure to check in with us often, more spreadsheets will be added soon.

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