Computers: Model Magic

Computer modeling speeds the analysis of indoor air quality and ventilation.

by Michael J. Blotzer, MS, CIH, CSP

Models have been used for centuries to evaluate and refine designs before construction and production. Models help architects see how a building fits into the surrounding environment, evaluate interior space and identify potential construction problems. Engineers build and test models to help refine the design of ships, aircraft, automobiles and other complex products.

Computers greatly enhance the modeling process. Computers generate virtual three-dimensional models faster, cheaper and in more detail than actual physical models. Computer models are also more versatile. Designers and customers can "walk" through and around a virtual model to develop a better understanding of a design and its limitations.

Computers can do more than simply replace physical models. Computers bring mathematical models of the physical world to life to simulate how a design will behave in the real world. These models can quickly assess the impact of "what if" changes in new designs and also help track down problems in existing designs.

The National Institute of Standards and Technology's (NIST) Building and Fire Research Laboratory (www.bfrl.nist.gov) is a leader in developing computer models to improve public and worker safety and health. While much of the laboratory's work is research related, one application in particular, CONTAMW, has practical safety and health applications.

CONTAMW is an indoor air quality and ventilation analysis program designed to predict air flows, contamination concentrations and building occupant exposure to airborne contaminants. The airflow model includes air infiltration through walls, windows, and doors; air flows driven by mechanical ventilation systems; wind effects; and indoor and outdoor temperature differences.

Modeling a building begins with drawing the building floor plan on the CONTAMW "sketch pad". This floor plan consists of defined "zones" basically a room or space within a building with a uniform temperature and contaminant concentration. "Airflow paths," doors, windows and other building components that allow air to move between zones, are added next.

Defining the building heating ventilation and air conditioning (HVAC) system completes the model. An HVAC system can be built by drawing the actual duct work including detailed air handler specifications, or by using CONTAMW's Simple Air Handling System.

Predicting contaminant concentrations involves adding contaminant sources with a known or estimated generation rate. Then, based upon modeled air flows, CONTAMW models the dispersal of the airborne contaminants and occupant exposures. The software supports the impact of chemical reactions, adsorption, deposition and filtration on contaminant concentration.

"CONTAMW allows the analysis of large buildings with hundreds of zones. You couldn't do this without a computer," said Andrew Persily, Ph.D., NIST Indoor Air Quality and Ventilation group leader. While CONTAMW has broad applications, according to Persily, its major non-research application is building smoke control system design.

Smoke, not flame, is the primary killer in a building fire. Smoke travels rapidly throughout a building and it's critical that escape routes, such as stairwells, are protected so that occupants can escape to safety.

One way to keep smoke out of a stairwell is to maintain the stairwell at a slightly higher pressure than the rest of the building. CONTAMW helps engineers design a building's smoke control system so that it ensures effective protection under a variety of wind and HVAC configurations.

Indoor air quality expert Terry Brennan, Camroden Associates uses CONTAMW to evaluate building designs to identify and correct potential IAQ problems before construction. "I can look at the air flows throughout the building, including the mechanical flows, stack flows and wind-driven flows through ducts, walls, floors and around pipes, for different outdoor conditions. I can see that when it's 20 degrees F outside, the make-up air for the top five stories all passes through the floors below. I can try air tightening in some places and adding transfer grilles in others until ventilation is achieved. I can see what happens when someone opens a window.

"The contaminant source models allow me to see how contaminants move through a building," adds Terry. "I can do a virtual release test in a lab and see what happens. Based on the model, I can develop air tightness and air flow specifications that can be tested in the actual building. Working with architects and engineers, I can develop detailing that is likely to achieve the specs."

Terry also uses CONTAMW in his IAQ investigations. "Existing buildings have a great advantage. I can make numerous pressure and flow measurements under different conditions and calibrate my model. When all is said and done, I can have a model that reproduces many actual field measurements."

Models have limits, of course. "We have a fair degree of confidence in CONTAMW," states Persilly, "but the model is only as correct as the input data." And entering the correct data for accurate modeling is not trivial.

Detailed models require accounting for all important air flow paths doors, windows, duct work, wall penetrations, exhaust fans, water heaters, chimneys, mail slots and more. While NIST provides libraries of air flow path data to simplify data entry, it still can be a painstaking process.

Other program feature the ability to model control systems, input detailed weather information and create operating, occupancy and contaminant generation schedules improve model utility, but at the cost of increased complexity.

Fortunately, not all applications require the ultimate in accuracy.

CONTAMW can be used on a basic level to gain quick insight about field investigations. Ages ago, I investigated an IAQ complaint I felt certain was due to occupancy overload. I wish I had CONTAMW back then because it took less than 30 minutes to build a simple model to test my hunch and calculate carbon dioxide buildup due to occupancy over time.

The model of the building I developed was quick and dirty. I didn't bother to model the entire building, just the room and a second zone to represent the rest of the floor. The model took advantage of CONTAMW's simple HVAC design option, eliminating the need to include duct details. Carbon dioxide generated by room occupants was modeled with a single contaminant source. I scheduled room occupancy from 8 a.m. until 5 p.m. with an hour break for lunch. I scheduled the HVAC to run from 7 a.m. until 8 p.m.

The modeled carbon dioxide profiles were similar to actual field measurements. It was simple to tweak the model to evaluate the effect of reducing the number of occupants and increasing ventilation on carbon dioxide levels.

CONTAMW is available from NIST at www.bfrl.nist.gov/IAQanalysis/CONTAMWdesc.htm. The site also includes program documentation, data files and other information. A CONTAM user group is available at groups.yahoo.com/group/CONTAM.

Finally, folks attending this month's American Industrial Hygiene Conference and Exposition in Dallas can attend a Monday afternoon presentation on "CONTAMW, A Multizone Indoor Air Quality and Ventilation Analysis Software Tool."

Contributing Editor Michael Blotzer, MS, CIH, CSP is an occupational hygiene and safety professional, writer, and computer enthusiast who brakes for animals on the information superhighway. Mike can be reached by mail addressed to Occupational Hazards, by fax at (309) 273-5493, or by electronic mail at [email protected]

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