Tuesday, November 22, 2011

Active Chilled Beam Design - Occupant Comfort

Active chilled beam systems offer design consultants an effective means to create a comfortable space while offering numerous opportunities for energy savings. There are several critical aspects of the design that must be addressed to ensure occupant comfort in addition to meeting the load requirements in the space. These include primary airflow supplied to the beams, placement and sizing of the beams.

The first tem that should be addressed in design of a chilled beam system is the volume of primary air supplied to the space. In chilled beam systems the primary air is the only source for both ventilation air and removal of latent heat gains in the space.

In order to determine the primary air requirements for the space in an active beam system, two separate items need to be considered. The minimum ventilation rate for the space must be compared to the amount of conditioned air required to maintain control of the humidity level in the space. Guidelines for determining the minimum ventilation requirements are given in ASHRAE standard 62.1-2010. The required flow ate to meet the latent load is determined by the following equation:

It follows that the required flow ate to maintain control of the humidity will rapidly increase as the difference between the room air and primary air humidity ratios decrease. As a result, designs seeking to maintain relatively low humidity ratios will need a high primary flow ate if the dew point temperature of the supply air is close to the design dew point in the room. Criteria for maximum dew point temperature in the space is set in ASHRAE Standard 55-2010; for a room design temperature of 75BF the maximum relative humidity is 63.5%. The larger of the two flow rates calculated will dictate the requirements in the space.

Once the required supply flow ate has been determined the beam throw pattern and unit size can be selected.

While there are many factors affecting the size/length of the chilled beam, the appropriate throw pattern should be determined based on multiple room considerations, e.g., shape/layout, intended use of the space, and windows.

In open office spaces as well as internal offices two-way or four-way beams are typically used. The flexibility provided by two-way and four-way beams, due to multiple sizes and nozzle configurations, allow them to be appropriately applied in most applications. One-way beams are typically used in perimeter zones and small spaces such as individual offices and hotel rooms.

After the throw pattern has been decided, placement of the beam within the space can be determined. Active chilled beams, because of their design, share throw characteristics with conventional slot diffusers. Placement and orientation of active beams is critical for thermal comfort due to long throw values associated with active beams.

In open office plans t is typically more cost effective to use several longer beams that are installed parallel in space, instead of numerous smaller beams the length of the module division. However in an open office the number and size of beams used will be determined by balancing the cost per beam, cost of air side operating pressure, and water side pumping power to achieve optimum energy efficiency. When applying two-way and four-way beams in small offices and individual offices the recommended location is directly above the occupants. This will result in the lowest velocities within the occupied space. It is also recommended that two-way beams are installed lengthwise in the space. This will allow for the use of longer beams, reducing the cooling requirements per linear foot which will in-turn lower total air flow per foot and the resulting velocities in the space ensuring occupancy comfort. If placement is required near a wall use of one-way throw beams are recommended. One-way beams can also be effectively used in perimeter zones for cooling applications; however they should be supplemented with baseboard heating to address window loads during the heating season. Two-way beams can be effectively applied in perimeter zones for both heating and cooling. Care must be taken if two-way beams are installed parallel to windows. In intermediate seasons when internal cooling is required and window surfaces are cool an acceleration of the air can occur in the space creating drafts and potential discomfort.

Once the primary air requirements, type, and orientation of the active beams in the system have been determined an iterative approach should be used to determine unit size/length and final placement of the beam based on operating conditions. In addition to providing latent cooling, supply air will deliver sensible cooling. Location for final placement should take into consideration the allowable average air speed in the occupied space in accordance with ASHRAE Standard 55-2010. Accounting for the air side sensible capacity will allow for reduced capacity requirements of the water coils in the beams.

Designing with this in mind will reduce airflow requirements per linear foot, which will help to meet the requirements for thermal comfort. When placing two beams in the same space as shown in Figure 2, care must be taken to ensure that the colliding air streams do not result in velocities over 50 fpm causing discomfort. A general guideline to achieve air velocities of 50 fpm or less in the occupied space is to ensure the velocities of colliding (V collisions) airstreams are below 100 fpm. If velocities at the point of collision are greater than 100 fpm, the distance from the ceiling for the airflow to slow to 50 pm is noted in the equation below:

Buildings and systems utilizing active chilled beams can achieve significant reduction in energy usage through optimization of the active beams selected, and the equipment selected for providing primary air and chilled water to the units. While savings may be obtained through reduced energy usage, the cost of reduced productivity resulting from worker discomfort can quickly erode the realized energy savings. Therefore it is critical that the appropriate considerations must be made to ensure thermal comfort is maintained in the space.

Matt McLaurin - Senior Design Engineer

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