Chilled beams are gaining popularity in North America, so engineers are establishing better models and methods for their performance. Terminal unit geometry of active chilled beams is one area where researchers are improving efficiency.
As chilled beam systems continue to take root in facilities vying for greater energy efficiency, scientists are learning more about how valuable they are and how to best optimize them. Two recent studies identify the performance of each type of system – passive and active chilled beams.
A Model for Passive Chilled Beam Performance
A study from the Ray W. Herrick Laboratories at Purdue University’s School of Mechanical Engineering sought to evaluate the performance of passive chilled beam systems compared to conventional systems. While passive chilled beam systems can reduce cooling requirements, improve comfort, increase energy efficiency and reduce ductwork, this evaluation has been difficult in the past.
“Passive chilled beam modeling is challenging because they have a complex geometry and the primary heat transfer mechanisms of radiation and natural convection are strongly coupled to the space characteristics and thermal conditions,” explain the researchers.
One way the Purdue team has developed modeling is to “consider both the chilled beam and its surroundings, since the performance of passive chilled beams are strongly coupled to the specific characteristics and conditions within the spaces.”
Using the model, the research team found that radiation cooling of a passive chilled beam system provided total energy savings of 10-21%, depending on the system configuration. These results were found using the weather conditions of the Indiana-based lab.
The researchers established four key conclusions from the model, including:
- Energy savings of up to 12% are possible using a passive chilled beam system under Midwest weather conditions compared to a traditional air system only controlling air temperature.
- A separate chiller with a higher chiller water operating temperature can provide an additional 11% in energy savings.
- Thermal comfort improvement with passive chilled beams can be achieved with a reduced relative air speed rather than the increased radiation cooling effect.
- Radiation cooling was calculated as 5-7% of the cooling capacity for the configuration in this study, which is essentially inconsequential in terms of thermal comfort estimation.
Optimizing Active Chilled Beam Systems
An article in Applied Thermal Engineering examines configurations of active chilled beams to find which unit geometries provided the most efficiency.
One of the key metrics for this study is the entrainment ratio (ER), which is “the ratio of the mass flow rate of room air induced to the mass flow rate of primary air supplied,” according to the study. As an index for active chilled beam systems, it identifies the efficiency of the terminal unit, as well as the overall energy performance.
Using this index, the researchers were able to confirm that terminal unit geometry increases entrainment performance.
“Induction can be enhanced by locating the nozzles as well as the induction kernel closer to the center of the terminal unit,” write the researchers. “This modification makes the space inside the terminal unit more effectively used for air mixing and entrainment.”
Furthermore, the researchers studied mixing chamber nozzles, finding that the optimal nozzle length is between 60 and 80mm. With a 70mm nozzle, they were able to increase the ER by 30%, proving that the nozzle can dictate other geometric characteristics of the terminal unit.