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Don't Turn Active Beams into Expensive Diffusers

©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 4, April 2012.  

By Andrey Livchak, Ph.D., Member ASHRAE; and Chris Lowell, Member ASHRAE

About the Authors
Andrey Livchak is vice president of engineering for Halton Group Americas. Chris Lowell is managing director of Halton Indoors Americas.

Active chilled beams have been used for more than 20 years. The term “active chilled beam” became an oxymoron, with active beams being used for cooling and heating. Now, they are called “active beams” or simply “beams.” Beams are gaining popularity in North America and are being designed with higher airflows to match increasing space loads.

Beam designs with primary airflows significantly exceeding space latent load and minimum ventilation requirements are also driven by engineers’ attempting to reduce system first costs and total number of beams. Unfortunately, this approach compromises the system’s energy performance and diminishes advantages of active beam systems over all-air systems. This often leads to active beams being used as expensive diffusers.

This article will help engineers gain fundamental knowledge about what parameters affect active beam performance and introduce new criteria for beam selection.

This article is for the HVAC engineers who are familiar with chilled beams and have used them in design practice. For readers who want to know more about chilled beams, please refer to the prior publications in this journal and references at the end of this article.

Primary air in active beams (Photo 1) is supplied into a mixing chamber through rows of nozzles. Negative pressure that is created in the mixing chamber facilitates induction of room air through the cooling coil. Induced air, cooled by the cooling coil, mixes with the primary air. This mixture of recirculated cooled air and primary air is supplied to the space. In an optimum design, primary airflow is intended to satisfy space outside air requirements and dehumidification to avoid any condensation on beams’ surfaces. The cooling coil is used to compensate for space sensible load only. Primary air is always cooled and dehumidified before it enters a beam.

Designing Chilled Beam Systems

When first introduced in Northern Europe, the design objective for active beam systems was to separate ventilation load from space sensible load and handle space cooling and dehumidification with minimum airflow. Water is a more effective media than air to transport energy due to its higher density and specific heat. One unit volume of water can carry about 3,500 times more energy compared to the same volume of air.

Already high space loads in the U.S. are often further overestimated by design programs not accounting for transient heat transfer effect, as well as the tendency of engineers to put a “safety margin” on top of the estimates, resulting in HVAC systems designed with oversized cooling capacity. In active chilled beam applications, this leads to beams designed to operate with excessive airflows. As a consequence, the active chilled beam often works as an expensive diffuser, with the water valve shut and all cooling provided by primary air. Indeed, beam cooling output is controlled by either a mixing valve, regulating water temperature in the coil, or by an on-off valve modulating water flow through the coil. This valve closes when space thermostat setting is satisfied. When the system is oversized and primary air provides sufficient space cooling, the water valve stays closed. We did see installations where all of the control valves on active beams were closed throughout the entire summer.

Active beam total cooling capacity is the sum of cooling capacity provided by the primary air and the beam coil.
                                          P = Pa + Pw                                          (1)
Cooling capacity provided by the primary air is calculated using the following equation:
                                  Pa = mp × cpa (tp – tr)                                  (2)
Assuming primary air is supplied at 55°F (12.8°C) and space temperature is maintained at 75°F (23.9°C), the primary air provides about 22 Btu/h (6.45 W) of cooling per cfm of primary air (10.4 W per 1 L/s). Figure 1 demonstrates contribution of air (Pa) and water (Pw) to the total cooling capacity of an active beam (P) as a function of primary airflow. As the primary airflow increases, the water contribution to the total beam cooling capacity drops and the air contribution in total beam cooling capacity increases. This chart is representative of a beam designed to operate at fairly low primary airflow. There are chilled beam systems operating at 20 cfm per linear ft of beam (31 L/s·m) and higher with primary air contributing 60% or more to the total beam cooling output.

C. Wilkins and M. Hosni1 demonstrated that plug loads are overestimated for office buildings. This, along with added safety design factor for HVAC equipment, often results in the air-conditioning systems operating only at 80% capacity on a design day. As we mentioned previously, most active beams are designed as constant air volume systems with water in the coil providing space temperature control. Let’s see what happens to an office space with an active beam sized with primary airflow to cover 60% of total cooling load. Assuming 20% safety margin for extra cooling capacity, this leaves only 28% (100% – 1.2 × 60%) for cooling output adjustment via cooling coil. This is certainly not enough to adequately respond to a variable load in the space in the intermediate season. As a result, the building will be overcooled in summer, thermal comfort compromised and overall HVAC system energy consumption increased.

With that being said, we don’t want to underestimate benefits of active beams. When properly applied, it is an energy-efficient, low maintenance and comfortable system. Our recommendation is to design active beams to operate at minimum primary airflows. If that is not possible, use a variable air volume (VAV) beam system, which is described later in this article.

Designing Beams for Minimum Primary Airflow

As concluded earlier, the most efficient chilled beam system is the one that operates at minimum primary airflow and satisfies space sensible load primarily by using the cooling coil. The most efficient, by cooling performance, active beam is the one that provides the highest cooling output at minimum primary airflow per unit length of beam. Let’s define a parameter that represents this important performance of an active beam and call it coil output to primary airflow ratio (COPA).

Citation: ASHRAE Journal, vol. 54, no. 4, April 2012

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