Chilled Beam Application & Control Hot and Humid Climates

David Schurk, DES., LEED AP. Director of Texas Healthcare Accounts Automated Logic Corporation

Overview

It is a misconception that chilled beam technology cannot be utilized in hot and humid climates. With proper design and control, chilled beams can be a viable alternative to traditional HVAC systems, regardless of location. This engineering bulletin will provide a basic overview of chilled beam application and will discuss some of the methods utilized to ensure trouble free operation. Also discussed will be first cost and energy savings benefits that may be realized when chilled beam systems are employed as well as ways to measure and verify system performance in order to optimize results

Concepts and Benefits

Chilled beam systems use water as well as air to transport thermal energy throughout a building. The chilled beam includes a hydronic coil which provides heating or cooling to the space. Either 2-pipe or 4-pipe designs are available. The benefit of the 4-pipe configuration being that some zones can receive cold water for space cooling while other zones simultaneously receive hot water for space heating.

Chilled beams come in both active and passive configurations, both require the building ventilation and latent loads be decoupled and addressed separately. Dry, conditioned air is supplied to the space to handle these loads, as well as offsetting some of the space sensible load. When conditioned primary air is supplied directly to the chilled beam itself, the device is called an “active” chilled beam (ACB). The primary air travels through nozzles in the beam where its velocity is increased, inducing additional room air through the beams coil. This induced air mixes with the primary air and is discharged back into the space through slots along the beam. 

Passive chilled beams (PCB) are not directly supplied with primary air and rely completely on natural convection to provide their sensible capacity. They will not be specifically addressed in this paper as they have limited application in hot and humid climates although they can be used when needed to supplement a load requirement where an active beam falls short.

Chilled beams are ideal for applications with high space sensible cooling loads and should be installed where the tightness of the building envelope is adequate to prevent excessive moisture transfer. Space moisture gains due to occupancy and/or processes should also be moderate. Successful installations of chilled beam systems have included the following applications, regardless of local climate.

As a “decoupled” system, where hydronic-based heating or cooling devices are integrated with the primary air ventilation system, this allows for individual optimization of all heating, cooling and ventilation functions with unique opportunities for savings in energy, space and maintenance, such as:

• Energy-Efficiency: The ability of water to carry a larger quantity of Btu’s per unit volume or mass as compared to air allows a reduction in the energy used to transport an equivalent amount of heat. The primary airflow required for the ACB serving a space can be up to 60%-70% less than for a conventional VAV system at design cooling conditions, with energy savings realized through fan horsepower reductions. Limiting the sensible cooling capacity of the chilled beam during low load situations can eliminate the requirement for inefficient reheat while still maintaining comfortable room temperatures. Higher chilled water supply temperatures used with chilled beams (typically 58°F-60°F) will provide chiller energy savings and can increase the run-hours of water-side economizers or fluid coolers, if used.
  
• Improved IAQ: Air delivered through the beam at ratios of 1-part primary and 2 to 4 parts (induced) room air increase air-change rates and promotes mixing of the space and ventilation air. Chilled beam systems can be supplied with 100% outdoor air (to satisfy ventilation requirements) which could then be directly exhausted, eliminating pollutants that would otherwise be transported through return ductwork or between air distribution zones. This leads to improved IAQ and thermal comfort.
• Improved Acoustics: Chilled beams do not include fans, therefore may not require that any major noise generating devices be located near the occupied space. When chilled beams are designed with acoustic sensitivity in mind they can easily contribute to <40NC design sound levels.
• Reduced First-Cost: The increased heat transfer properties of water allow the air transport system to be reduced in size and cost. The simplicity of chilled beam systems can add to these savings. Chilled beams require minimum installation space and clearance, which reduces interstitial heights and floor to floor measurements. If properly accounted for, this will lead to a reduction in building materials or allow for increased ceiling height. A building with chilled beams will allow for smaller mechanical rooms (or complete elimination) through less floor space requirements due to downsized air handling equipment. 

 

• Combined Services: Multi-service (or integrated) chilled beams can incorporate other space services into the linear enclosure associated with the device. For example, lighting fixtures and controls, speakers, occupancy sensors and smoke detectors can be fitted within the beam at the factory and delivered to the job site as a single element, reducing coordination issues and saving installation time. Primary air ductwork, CHW piping, data cabling, etc. can all be routed above beams to provide additional space savings.
• Reduced Maintenance Costs: Chilled beams are sensible cooling devices and when controlled properly will not promote the formation of condensate which can lead to bacterial and mold growth. They do not require drain pans which require cleaning (although some manufactures do offer a drip-tray option). They do not contain fans or filters to maintain and require only simple periodic service including vacuuming of the dry coil

Upgrade & Retrofit of Existing “Induction Units”:

No conversation on chilled beams would be complete without mentioning the opportunity for their use in retrofitting older induction unit installations. Very popular in the 1950s and 1960s, induction units were used in large buildings where space was at a premium and the small primary air ductwork used with the induction system was an advantage in reducing mechanical space requirements and floor heights. Induction units were typically mounted on the floor (up against an outside wall) in the building’s perimeter zones and then concealed under enclosures built to suit the size of the units selected.

     
The energy crisis of the 1970s forced induction systems into disfavor. There were energy cost concerns as the old induction units required very high inlet static pressure (often 1.5 – 3.0 in. w.c. or more) which imposed a significant fan energy penalty. Old induction units produced relatively high noise levels and because they were located in the occupied space, provided poor air distribution patterns which promoted drafts. These units were often pneumatically controlled and offered none of the comfort and efficiency advantages of a modern building automation system (BAS). They could also be a constant headache for maintenance personnel.

Existing buildings which utilize induction units have some real infrastructure issues when it comes to renovation. Small, high velocity primary OA risers, existing piping and relatively low floor to floor height can make the conversion to a more traditional HVAC system cost prohibitive or impractical. Many such buildings have been abandoned in favor of new construction, while there are still thousands of these older buildings now in need of updating. Rather than simply replacing the old induction units with like units or renovating the entire HVAC system, there is often an opportunity to significantly improve the performance of these old induction system through the use of new chilled beam technology while re-using much of the building’s existing infrastructure (ductwork, piping, etc.).

[an error occurred while processing this directive] Considerations in Hot & Humid-Climates:

Humidity becomes an issue if the surface of any cooling coil or unit panel dips below the surrounding air’s local dewpoint. When the air in contact with this cold surface falls below its dewpoint temperature, there is a certainty of condensation forming. 

With chilled beams (and other A/C devices) comes a challenge to prevent condensation formation on any cool surfaces. This is addressed by the primary air system which supplies dry air to the beam to handle the space latent and ventilation loads and will limit the indoor dewpoint temperature, typically below 55°F. How dry this air must be depends on the quantity of primary air delivered, as well as the load in the space. Chilled water, supplied to the beam to handle space sensible loads, should be provided at temperatures above the local dewpoint so as not to promote the formation of condensate. Chilled water temperatures are typically delivered at 58°F-60°F and when properly controlled will keep beam surface temperatures elevated above the local dewpoint. 

There are factors which help provide relief in situations where a surface may momentarily dip below the local dewpoint temperature. A space will generate a specific latent load (from internal sources) which is then available to accumulate as condensate on any cooled surface. When this moisture is spread across the entire surface area of a coil or panel there is a limited amount of condensate that can form. Even if cool surfaces in the space are left uncontrolled, they are likely to be only slightly below the space dew point which also limits the amount of condensate that will result.

Penn State Professor Stan Mumma, PhD., P.E., and Fellow ASHRAE (2002) performed various studies on chilled ceiling panels (the chilled beam’s predecessor). In these studies the surface temperature of the chilled ceiling panels were reduced, or space latent loads increased substantially beyond their design perimeters. His findings conclude that the formation of condensation in environments with chilled ceilings is a slow process and one that can be avoided by sound design and control. A review of his work will help provide proper perspective to the problems and risks associated with the installation of any cooling device that will come in contact with air.

• It should be noted that Houston, TX does not have a patent on humidity.  The University of Miami regularly uses chilled beams and the ASHRAE dehumidification design dewpoint for Miami is actually higher than Houston (Bush).

Control Strategies:

Primary air flow rate can be controlled by a fully self-contained volume flow limiter (VFL) which requires no power or control connections and may be field set to maintain a volume flow rate to the beam. VFL’s are recommended for use on beams fed by an air handling unit that is also supplying VAV terminals. The VFL compensates for system pressure changes to maintain the beam’s design airflow rate.

Room temperature control is primarily accomplished by varying the water flow rate or its supply temperature to the chilled beam coil in response to a zone thermostat signal. Modulation of the chilled water flow rate typically produces a 7°F to 8˚F swing in the beam’s supplied air temperature, which affects a 50 - 60% reduction in the beam’s sensible cooling rate. This is usually sufficient for the control of interior spaces (except conference areas) where sensible loads do not tend to vary significantly. If additional reduction of the space cooling is required, the primary air supply to the beam can be reduced. In any case, modulation of the chilled water flow rate or temperature should be the primary means for controlling room temperature as it has little or no effect on space ventilation and/or dehumidification. Only after the chilled water flow has been discontinued should the primary airflow rate be reduced. Note: The chilled beam along with its primary air and water transport systems can be slightly oversized when there is a concern of peak load variations above design. 

As long as the space dew point temperature can be maintained within a reasonable range (+/- 2°F ) and the chilled water supply temperature is at (or above) this value, condensation will be avoided on chilled beam surfaces.  Should there be periods when room humidity conditions drift or rise above design, and a dewpoint sensor detects condensate formation (or the potential), typical control action would be to modulate flow to the beam or reset the chilled water supply temperature (higher) in order to reduce beam capacity and increase surface temperatures. An alternative to this is to simply shut off the CHWS to the zone and allow the conditioned primary air (if sized with excess capacity) to assist in returning the space to its proper humidity level. This is not recommended in humid climates as thermal control may be lost resulting in a space that can’t be occupied comfortably.  

Control Sensors:

Preventing the formation of condensate on chilled beam surfaces must be addressed, especially in hot and humid climates. Proper system design combined with measurement and control of space humidity will help ensure satisfactory performance. The following will discuss a few of the more common control sensors used to help make sure requirements are met.

• Monitor Local Temperature and Dewpoint: The use of a high quality space mounted temperature sensor and relative humidity sensor (located on the face of the beam, in the zone or in the return duct) can provide great results. It is important to measure humidity where the risk of condensation is highest. Responsiveness to local humidity spikes is a key requirement that goes into the selection and placement of these sensors. It is also advisable to provide additional dew point or humidity monitoring throughout the building in case a local sensor fails. All sensors must remain easily accessible.

 

• Monitor Moisture on CHWS Pipe: A sensor can be installed which will read the coldest piping location in the zone. This allows for the detection of condensation directly at its “worst case” location. When moisture is detected the zone water flow is shut off and will not be restored until the moisture has been evaporated. This can also signal an increase in primary airflow to assist in returning the space to acceptable humidity conditions.
  

 

• Monitor Outdoor Conditions: In certain situations it may be acceptable to base control actions on indoor and outdoor air temperature and humidity. Knowing these values, along with the latent load in the building (when infiltration is predictable) allows the BAS to calculate the condensate potential. This is not an option we recommend in hot and humid climates as primary control is not based on actual space thermal measurements.
  

In addition to the more traditional sensing devices described above, a variety of high-performance sensors are available when more demanding requirements are presented. These include Impedance Dew Point Sensors, Chilled Mirror Sensors, and Dark Spot Optical Hydrocarbon Dewpoint Detection.

Monitoring & Measurement

What gets measured gets done and what doesn’t get measured gets ignored. Maintaining local dewpoint temperature may be the single most important factor in ensuring trouble free operation. But as mentioned earlier, there are additional factors which play a role in providing indoor environmental quality and comfort. No matter how good the design, control and  installation of the chilled beam system is, it is all for naught if “real-life” performance is not measured and verified in order to assure that the desired results are delivered.

Facility managers need to know at a glance whether or not they are maintaining critical space dewpoint levels and comfortable and healthful conditions in their buildings. By programming space sensor readings into a Building Automation System, a facility manager can easily generate a measurement tool that provides a numeric “score” of how well the buildings HVAC system is performing. These readings can be given a grade between 0-100 (based on how close they are to set point) and rolled up into one overall building or zone grade. “100” would be perfect and any numeric score below this would indicate degradation in performance.

Automatic Logic’s Environmental Index (EI) doesn’t require a facility manager to manually review individual temperature, humidity or CO2 readings to determine the HVAC systems compliance to comfort set points, nor does it require manual calculations be performed to score the results. Space conditions are automatically graded, color coded and displayed in real time on a thermograph of the building floor plan which allows a viewer to instantly recognize any areas of concern. 

Any condition that can be read and sent to the EI (temperature, humidity, CO2, dewpoint, etc.) can be scored and indexed for the operator to visually keep track of. By using this color format, the ALC Environmental Index is intuitive and as easy as obeying a street traffic light.

It is vital to be proactive to potential humidity issues which will result in the loss of space control, with one quick glance at the indexed floor plan a facility manager can make an instant assessment of space dewpoint and other thermal comfort conditions throughout the building. Should an unacceptable condition be detected immediate action can be taken in order to remedy the situation before it becomes a problem. Environmental Indexing can provide up to the minute trending, recording and notification of all thermal (and even energy) related information, providing owners or facilities managers with measurement and verification that documents HVAC system performance. EI can even be utilized in existing buildings where sensors and controls are already in place.

[an error occurred while processing this directive] Implementation:

The key to a successful chilled beam installation starts with a design that provides fulfillment of the buildings heating and cooling needs. Detailed analysis of sensible and (particularly) latent loads tied to internal sources, ventilation and infiltration must result in equipment and transport systems that are sized and installed to handle loads so proper environmental set points can be maintained. The BAS system and control strategy implemented will result in a system that will realize the design intent. Measurement and verification will ensure trouble free operation and that the building achieves optimum environmental comfort and energy performance.

Chilled beams may require a bit more design consideration than other, typical HVAC systems but as long as important details are not overlooked, they can be successfully applied in any climate. Superior performance comes from combining the experience and expertise of all parties involved in the building’s design.

Please feel free to contact us for additional guidance and support on chilled beams, or any other building application and control requirement.

AutomatedLogic Houston
David N. Schurk, DES., LEED AP.
Director of Healthcare Accounts
I can be contacted at davids@uescontrol.com or by phone at 281-702-3503

“To meet our energy challenge requires the most important energy of all….human creativity”

References:
1.    Automated Logic Control System Design and Application Manual.
2.    Automated Logic WebCTRL Applications Manual.
3.    TROX Chilled Beam Design Guide.
4.    Chilled ceilings, addressing the concerns of condensation, Mumma, SA (2002).
5.    Ceiling radiant cooling panels, Mumma, S.A (2006.)
6.    Price Engineers HVAC Handbook.
7.    DADANCO, Breathing Life Into Your Buildings.

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