THE COMPLICATED CASE OF THERMAL COMFORT RISK ASSESSMENT

Anil Kapahi, PhD

As thermal comfort becomes top of mind for many architects and building owners, we look at the complexities of achieving it.

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Have you ever found yourself sitting in an auditorium or an office space and either needing a warm blanket or wanted to fan yourself? While it’s often an overlooked part of building code and design, thermal comfort is becoming top of mind for many architects and building owners.

What is Thermal Comfort?

Thermal comfort is complicated to determine, because one person’s hot is another person’s cold — just think of the thermostat wars which take place in offices. According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 55, thermal comfort is the “condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation” — essentially, it is making sure someone isn’t too hot or too cold. Thermal comfort drives from a primary requirement to keep the temperature of the innermost part of the human body in the range of 36-38 °C (96.8-100.4 °F)

The body achieves thermal comfort when natural heat loss equals heat produced by metabolism. It is essentially an energy balance within the body and can be summarized as:

Providing Thermal Comfort in Practice

A few months ago, we were asked to determine the thermal comfort for a multi-million-dollar performance theater. To add a layer of complexity, the theater was being designed to have moveable seating balconies that could be configured for specific types of events. Given the moveable nature of the seating balconies, the architect was looking to omit the HVAC ductwork and delivery of conditioned air to the patrons seated at these locations. Without ductwork, it was difficult to ensure adequate thermal comfort to the guests sitting in the balconies.

We proposed a methodology to evaluate the thermal comfort in the building without the ductwork at balconies to better understand the problem. This was done using six factors outlined by the ASHRAE 55 that were quantified using Computation Fluid Dynamics (CFD) and literature review. These factors consist of environmental factors including temperature, air velocity, relative humidity and mean radiant temperature as well as personal factors such as metabolic rate and clothing. These factors can be combined according to ASHRAE 55 guidelines to assess thermal comfort at different locations in the theater space.

A 3D CFD model was the ideal tool to quantify environmental factors at every location in the theater. In addition to the fluid flow model, auxiliary heat transfer models were used to represent the heat emitted by people, audio-visual loads and lighting in the theater. The model also incorporated the heat load due to solar radiation to ensure the thermal comfort assessment is comprehensive.

After analyzing the modeling results, we quantified the thermal comfort of the patrons that the architect and mechanical engineer could use to determine if they can omit the ductwork to the seating balconies.

Understanding how the HVAC environment impacts the thermal comfort allowed our client to remove the ductwork from the seating balconies. This allowed them to make an informed decision by keeping the flexibility of re-arranging the seating to allow for different configurations in the performance theater without compromising with the comfort of patrons. This idea of assessing thermal comfort for a space can be applied to various building occupancies such as airports, indoor stadiums, concert halls etc. At the end of the day, the main purpose of a building is to make sure people inside it are comfortable and having the tools and expertise to assess it at the design stage is invaluable.

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