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©2017 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 59, no. 11, November 2017

By Aleksejs Prozuments, Ph.D.; Anatolijs Borodinecs, Dr.Sc. Ing, Member ASHRAE

About the Author
Aleksejs Prozuments, Ph.D., is a researcher at Riga Technical University in Riga, Latvia, and currently on an exchange program as a research fellow at the University of Washington in Seattle. Anatolijs Borodinecs, Dr.Sc. Ing., is a professor at Riga Technical University in Riga, Latvia.

Supply airflow rate and supply air temperature can be varied to maintain the room temperature setpoint according to the air temperature conditions outdoors and heating loads indoors. Increasing the supply air temperature and decreasing supply airflow rate can reduce energy consumption. However, it causes insufficient air circulation, leading to risk of low indoor air quality. Increasing the supply airflow will improve air circulation and mixing effectiveness within the room, but it will also increase energy consumption, so there is a need to find an optimal correlation between air mixing effectiveness and energy efficiency.

The objective of this study was to identify the correlation between the minimum airflow and supply air temperature that would maintain satisfactory thermal comfort in the occupied zone when introducing warm air into the room. The study was conducted in the full-scale test chamber that represented a typical single office room (Figure 1). Twenty-four air velocity and temperature measuring probes were positioned at different levels and locations of an occupied zone. A supply air diffuser prototype with a perforated faceplate was integrated into the ceiling and tested to see how the warm air is distributed within the test chamber. Numerous supply air temperature and flow rate variations were analyzed during the study:

  • To see how each variation affects the air-distribution pattern; and
  • To find the best matching supply air temperature and flow rate balance to prevent the risk of air mass stratification; and to ensure good indoor air quality.

 

Background

In mixing ventilation systems, indoor air quality is highly dependent on air mixing effectiveness. When supplying cool air into the room, there is a high risk of local cooling and draft. Thus, it is necessary to control the air distribution to prevent excessive air velocity and draft rate occurrence in the occupied zone. When supplying warm air, on the other hand, there is a risk of air stratification in the room when a layer of fresh and heated air mass builds up right below the ceiling; whereas a layer of stuffy and relatively cooler air mass stays in the occupied zone.

Often, there are convectors, radiators or space heaters used for heating the office rooms, instead of airborne heating via air terminal devices. In this study, research on air stratification was carried out and possible ways to prevent it were analyzed, when warm air is supplied via air supply diffusers from the ceiling.

In heating ventilation systems, air mass stratification can occur due to the density difference between the warm supply air mass and relatively cooler room air. Without sufficient airflow that forces the supply air to reach the humans’ occupancy zone and thus to ensure a good mixing effectiveness within the space, air mass stratification is a common problem in heating ventilation. According to Jurelionis, et al., combined air heating and ventilation systems often are used in low energy buildings. However, operating these systems in the heating mode increases vertical air temperature gradient in rooms and can have negative effect on indoor air quality.

Due to the buoyancy effect, the layer of warm supply air builds up in the upper part of the room and remains at the ceiling, whereas in the human occupancy zone the air temperature remains virtually constant. This is particularly common in traditional heating ventilation systems that are not designed properly. What’s more important is that the air stratification fails to satisfy human comfort and presents a significant failure in maintaining indoor air quality, which can lead to CO2 levels rising above the acceptable limit. Given that in traditional ventilation systems, exhaust terminal devices and exhaust grilles are placed in the upper part of the room, the risk of short-circuiting increases.

 

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