application issues for chilled beam technologies

Application Issues for Chilled Beam Technologies

Boggarm S. Setty, P.E., ASHRAE Fellow

ABSTRACT

Recently, many discussions and papers relating to chilled beams have been published. Chilled beams usage has become rampant along with Dedicated Outside Air Systems (DOAS). The paper discusses the design issues relating to filtration, induction effect, certification and energy consumption. In most cases, only the energy savings relating to reduced air qualities may not be real. The paper discusses the application issues for chilled beam technologies, its usages and cost effectiveness.

GENERAL

Chilled beam technologies rely on direct heat transfer within a space by circulating liquids (usually chilled or hot water) through a heat transfer surface or coil that in-turn cools space through radiation and convection. In commercial applications, in order to maximize cooling effects and to control moisture, chilled beam solutions can feature a separate ventilation (Dedicated Outside Air, DOA) system, to accommodate not only code required air exchange rates, but to provide a means to pressurize and dehumidify space conditions: Such configurations are often referred to as an Active Chilled Beam (ACB) system. There are issues to consider in applying these systems.

BACKGROUND

Chilled beam technologies are not that new. Today’s ACB systems are adaptations of the floor and ceiling mounted induction units used in the mid-1900s. Like the older induction units, coils are included to further temper induced air. Rather than being only a heating coil, the ACB system offers either 2-pipe or 4-pipe cooling and heating capabilities. The older induction technology became less favored in the late 1960’s - 1970 due to energy efficiency, maintenance issues, and high initial cost. Early induction air units were often associated with high-pressure air supplies, which resulted in significant fan energy use. It was also the case that surface temperatures within induction units could allow condensation on exposed surfaces, and such also required regular maintenance to keep primary air nozzles clean. Induction systems were also relatively expensive due to required high-pressure ductwork construction and costly components. By the mid-1970’s, the use of such systems virtually stopped with the introduction of lower cost and more efficient Variable Air Volume (VAV) system technologies.

NEW TECHNOLOGIES

The recent resurgence of interest of ACB technologies is based upon assessments that such systems are more energy efficient: It is argued that because the majority of space conditioning energy is moved in water, not air, there are inherent energy savings. Care must be taken in relating to performance expectations, as many assessments do not take into consideration water flow (pump) energy use and its associated cost. Also, the energy needed to induce air movement in the space is still felt back at the primary supply’s air handling unit – in this case, a DOA ventilation unit.

Current chilled beam technologies utilize linear horizontal air grilles, located at the ceiling plane, with air supply

LV-11-C061

©2011 ASHRAE. THIS PREPRINT MAY NOT BE DISTRIBUTED IN PAPER OR DIGITAL FORM IN WHOLE OR IN PART. IT IS FOR DISCUSSION PURPOSES ONLY AT THE 2011 WINTER CONFERENCE. The archival version of this paper will be published in ASHRAE Transactions, Volume 117, Part 1.

1

delivered across the ceiling. Primary air supply “outlets” within the chilled beam do require less pressure than their induction unit predecessors. Induction Nozzle Pressure (INP) is often referenced as low as 0.5 – 0.75 inches WG, but may require higher values. This combination of ventilation air delivery and water energy movement provides the cooling and/or heating energy required for space conditioning.

Such technology applications are perhaps considered in comfort space-conditioning applications that have minimal

latent loads and continuous operation. Deep row coils, which are normally associated with high latent load control, are not supported by current ACB unit technologies, requiring the consideration of supplementary dehumidifying equipment when addressing high latent loads.

Existing (older) buildings have additional challenges associated with heavy infiltration loads for spaces with envelope

or entrance elements. In these and other high latent load situations, chilled beams could have issues with temperature and humidity control, and resulting condensation.

This paper specifically compares two air conditioning system applications: a conventional VAV system with Dedicated

Outdoor Air (DOA) and perimeter fan coil units, against an Active Chilled Beam (ACB) system. To be sure, there are numerous variations in system configurations that can be associated with such systems. Also, there can be many space load conditions besides the ones assumed; so this paper’s analysis is not intended to offer a definitive comparison of the two technologies, but to show possible performance issues. To be somewhat definitive, our analysis did consider three different city locations to address climate impacts, and different INPs were also considered.

ACTIVE CHILLED BEAMS

Chilled beams can be characterized as in-room terminal units. Because passive chilled beams do not provide for ventilation, they can’t directly be compared an alternative to a VAV system. As an ACB system provides for ventilation, it can be directly compared in function and performance as an alternative to a VAV system. Figure 1 represents a 100 percent DOA system that provides primary air to ACB terminals. In this particular case, the DOA - ACB unit generates 4.5” WG static pressure to provide an induction ratio of 3-4 at the terminal.

Figure 1 Integrated DOA - Active Chilled Beam System

2

Figure 2 Active Chilled Beam Terminal

As shown in Figure 2, an active chilled beam consists of a cooling/heating (i.e., two-pipe or four pipe) finned coil and a

primary air supply (Ventilation air) duct through the length of the beam with nozzles that induce room air across the coil and diffuses it back into the room. Some ACB terminals may also include illumination functions. Note there are no filters to clean secondary air.

CHILLED BEAM DESIGN ISSUES

Leakage

Air and moisture leakage through a building’s envelope can be high in both new and existing buildings. In older buildings, the infiltration rate can be particularly high, likely to exceed 1.0 – 1.5 cfm/sf of wall area. In such cases, significant rates of primary airflow through the ACB terminals would be needed to pressurize the building and to impede water vapor transfer (i.e., water vapor transfer through infiltration). Chilled beams must address infiltration loads when considering both heating-cooling operations, especially latent loads.

High Occupancy

Areas of potentially high occupant density must also be carefully addressed. The magnitude of the changes in sensible and latent loads due to varying occupancy loads and increased ventilation requirements can be significant for spaces; such as conference rooms, courtrooms, auditoriums and training rooms. In particular, during periods of high occupancy, the latent cooling demand can dramatically increase. Subsequently, a larger primary airflow rate, at a lower dew point, could be needed for both ventilation and moisture control. If the primary air flow is limited to a normal ventilation rate, additional latent cooling will be required for unusual conditions, often requiring auxiliary air-conditioning equipment. Without such latent load control, surface temperatures within the chilled beam may go below dew-point conditions, causing moisture condensation, and potential dripping into the occupied space.

Space Air Distribution

Zone or space air distribution is critical to proper ventilation and occupant comfort. Chilled beam air delivery must be designed such that necessary minimum static pressure is available to maintain sufficient air-circulation (e.g. having an induction ratio of at least 3 to 4). In many cases, chilled beam manufacturers require 1/2” and 3/4” WG which are required to create the needed induction effect in order to create proper air distribution.

Brake-horsepower

The difference in total brake-horsepower (BHP) between system comparisons must consider supply and return air fans and heating hot water and chilled water pumps. Power related calculations must also address the rated airflow static pressure drops and water pressure demands of terminal units and their impact to BHP needs. The considered VAV and the ACB

CEILING

HEATING/COOLING COIL

INDUCTION NOZZLE

3

systems have different configurations and size requirements for these system components, and it is here that oversight may occur by not considering all involved components or their required operating schedules. In addition, for both systems both DOA fans and involved system water pumps must be scheduled to run year round if continuous pressurization and humidity control is to be achieved. In some assessments, fans and pumps for chilled and hot water circulation are assumed to be “off” during unoccupied periods: While such an operation can cause discomfort for the start-up of a VAV system, it can cause moisture damage for an ACB system should high humidity conditions exist.

Response Time

The time delays and expected frequency of load changes in each zone and exposure also need to be considered, along with the energy waste due to switch-over. For an ACB system, the heat loss or gains involved are often not considered, nor is the possible moisture damage impact of restart-up.

Churn Rate

The occupancy churn rate (i.e., number of times per year that workstations or spatial requirements are changed) can also cause start-up moisture problems for an ACB system. Time impacts can be significant when considering burden of relocating both water piping and primary air ductwork. Modifying an ACB system can result in additional time delays for drain-down, refilling and purging, and water leak corrections. Additionally, flexibility may be encumbered by the designed beam terminal sizes, refitting drain pans, and relocating controls.

Reliability and Control Systems

Reliability and longevity of additional control systems can also be a problem for chilled beam systems. As with other hydronic terminal units, e.g. Fan Coil Units (FCUs), chilled beam terminals have “self-contained microprocessor controllers.” Possible condensation and accessibility issues may increase maintenance costs.

Water Damage

Disruptions to occupants can be a major issue if water leakage occurs. As hot and chilled water supply and return lines will typically be circulated through several chilled beams in a zone or area, the disruption to one terminal involves a large area. In this context, consider that the ACB distribution of piping spans 70 to 80 percent of the ceiling area, as opposed to a VAV systems more localized VAV box locations.

Building Pressurization

As addressed earlier, building pressurization and dehumidification should be considered for both systems. While a comfort and (worse case) a mold control issue could present itself for a VAV application, ACB systems can cause liquid moisture damage if allowed to operate below dew-point conditions. As such any start-up regiment for an ACB system must involve an early drying-out of building air by the early and separate operation of the DOA system, with chilled water going to ACB terminals only after space dew-point temperatures have been lowered below ACB terminal temperatures.

Filtration

VAV systems provide the necessary 10 MERV filtration through the building’s DOA and VAV air handling unit filters. However, for an ACB system, only the primary air is filtered: Induced secondary air at terminals is not filtered. As such, an ACB system may be difficult to achieve the necessary 10 MERV filtration performance without auxiliary space filtration units.

4

An additional maintenance issue for an unfiltered ACB terminal is that the chilled beam/cooling coils will pick up more dust/lint and mold particles from the occupied space than comparable terminal units: Such could cause additional cleaning efforts to protect occupant health.

Certification

VAV systems and their components can receive certification by a third party, such as the Air Conditioning, Heating, and Refrigeration Institute (ARHI), relating to its performance. However, for ACB systems, certification is typically available only through the manufacturer.

Chilled Water Temperature

Chilled water temperature can be adjusted to save energy by allowing it to rise during part load. Also full cooling chillers can be used without any mechanical cooling to save energy.

Further Operation and Maintenance Issues

As discussed earlier, to avoid condensation on chilled beam surfaces, the air dew point temperature in each space must be maintained below the coldest surface temperature of the chilled beams. Operations and maintenance staff must accurately and constantly monitor the indoor air dew points and chilled beam surface temperatures in each separate space. On a rise in room dew point temperature, swift action by operational staff is needed to prevent condensation by quickly reducing the room dew-point temperature or warming the supply water temperature to the chilled beams.

As such, design of a chilled beam system must include the ability of the operations staff to be informed, under all operating conditions, that building pressurization is maintained.

Related to operations and maintenance, the design of a chilled beam system must specifically provide for ease of

regular inspection and maintenance of valves, piping, insulation, terminal unit connections, and terminal heat transfer surface areas of the chilled beams. An ACB design must also include concise maintenance procedures, and inspection schedules.

Design of an ACB system must be then reconciled with the anticipated expertise of the operations and maintenance

staff available to the building. The design decision-making process must work with owners to insure required operations and maintenance training.

COMPARISON OF VAV AND ACTIVE CHILLED BEAM DESIGN FEATURES

The considered ACB system was described earlier and is shown in Figure 1. The ACB system uses 3,000 cfm ventilation primary air to induce a total air movement of 10,000 CFM. Each ACB terminal features a 4-pipe system (heating/cooling) coil.

The compared VAV system alternate consists of a “Dedicated Outdoor Air (DOA) unit, connected to a floor-based

VAV Air Handling Unit (AHU). A perimeter FCU system is also provided for perimeter zones. This integrated DOA and VAV system is fairly conventional where precise ventilation control is needed. Refer to Figure 3.

Involved DOA systems must typically be capable of providing air at a minimum of 50°F dew-point, considering both

the latent loads generated in the space and the loads from the outside air. In certain cases, recovery heat transfer could also be used, thereby occasionally satisfying ventilation needs with reduced tempering.

5

Figure 3 Integrated DOA - Variable Volume System

For the comparison of system performance, a 10,000 square foot office was modeled using an hourly building energy use program. For the VAV system, a 10,000 cfm system provides heating/cooling with a 20% ventilation air from the DOA unit. Air is filtered to 10 MERV. Supply air ducts run in the ceiling spaces and connect to approximately 10 VAV boxes. FCUs are provided to accommodate building skin loads and perimeter zone space conditioning.

MODELING ASSUMPTIONS

The simulated building is a simple rectangle, as shown in Figure 4, representing a typical office space facility.

Figure 4 Modeled Space

The following input data serves to further define assumptions within the model.

• Lighting: 1W/ft2; 80% load to space; Schedule per 90.1-2007. • Misc. Loads: 2 W/ft2. • People: 143 ft2/person; 250 Btu/h sensible and 200 Btu/h latent per person; Schedule per 90.1-2007. • Ventilation: Per 62.1-2007 with 5 cfm per person and 0.06 cfm/ft2; Schedule per 90.1-2007. • Thermostat: Cooling set-point at 75/81 °F and heating set-point at 72/64 °F; 50% RH. • Ceiling/Floor: Adiabatic. • Wall U-value: U-0.064 per 90.1-2007. • Windows: 33% WWR on all exterior walls; U-0.55 and SC-0.46 per 90.1-2007. • Height: Floor-to-ceiling height of 10 ft. • Infiltration: 0.3 ACH for perimeter zones; 0.1 ACH for core zone. • Economizer (air-side): Dry bulb, 65 °F. • Minimum Room Relative Humidity: 30%. • Chilled Water Reset: per Appendix G. • Supply Air Reset: for VAV system. • Static Pressures for VAV: 4” supply; 2” return. • Static Pressures for Fan Coil: 0.5” supply. • Static Pressures for DOA for Fan Coil: 1.5” total static including supply fan and ret./exh. fan. • Air-to-Air Energy Recovery: Total-energy wheel at 74% sensible effectiveness and 71% latent effectiveness.

6

• Chilled Beams Heating: 4-pipe. • Static Pressure Drop in the Beam: 0.5”or 1” or 2” or 2.5” simulated. • Static Pressures for ACB: Supply 4.5” WG is assumed. • Sensible Cooling Capacity for Chilled Beams: 40.27 Btu/h per cfm of primary air flow. • Auxiliary Cooling Coil Losses to Plenum for the Chilled Beams: 5%. • BHPs assume 90% motor efficiency.

RESULTS

Computer simulation results are presented for three different cities representing three different climate zones: • Billings, Montana (Zone 6B) • Los Angeles, California (Zone 3B) • New-York, New York (Zone 4A)

Each set of findings is grouped to reflect these climate issues, but also to consider different ACB terminal INP conditions (i.e. 0.5, 1.0, 2.0, and 2.5 inches WG).

EUI** Supply Fan Ret/Exh Fan Ventilation CHW Pumps HW Pumps (kBtu/SF/Yr.) BHP (MBtu) BHP (MBtu) BHP (MBtu) BHP (MBtu) BHP (MBtu)

Billings 0.5” ACB INP*

VAV 65.9 10.5 84.3 4.2 27.8 0.7 6.6 1.3 2.8 0.2 0.3 ACB 70.3 9.0 122.5 5.3 79.2 / / 1.0 2.2 0.1 0.3 Difference -4.4 +1.5 -38.2 -1.1 -51.4 / / +0.3 +0.6 +0.1 0.0


VAV 65.9 10.5 84.3 4.2 27.8 0.7 6.6 1.3 2.8 0.2 0.3 ACB 71.8 10.0 134.8 5.3 79.7 / / 1.0 2.2 0.1 0.3 Difference -5.9 +0.5 -50.5 -1.1 -51.9 / / +0.3 +0.6 +0.1 0.0


VAV 65.9 10.5 84.3 4.2 27.8 0.7 6.6 1.3 2.8 0.2 0.3 ACB 74.9 11.7 159.9 5.3 81.0 / / 1.0 2.3 0.1 0.3 Difference -9.0 -1.2 -75.6 -1.1 -53.2 / / +0.3 +0.5 +0.1 0.0


VAV 65.9 10.5 84.3 4.2 27.8 0.7 6.6 1.3 2.8 0.2 0.3 ACB 77.0 12.7 173.2 5.4 82.0 / / 1.0 2.3 0.1 0.3 Difference -11.1 -2.2 -88.9 -1.2 -54.2 / / +0.3 +0.5 +0.1 0.0

L.A. 0.5” ACB INP*

VAV 56.6 8.4 81.3 3.5 29.7 0.6 6.6 1.3 6.5 0.1 ~0 ACB 66.9 8.0 107.8 4.7 71.4 / / 1.0 3.5 0.1 0.2 Difference -10.3 +0.4 -26.5 -1.2 -41.7 / / +0.3 +3.0 0.0 -0.2


VAV 56.6 8.4 81.3 3.5 29.7 0.6 6.6 1.3 6.5 0.1 ~0 ACB 67.9 8.7 118.5 4.7 71.7 / / 1.0 3.5 0.1 0.2 Difference -11.3 -0.3 -37.2 -1.2 -42.0 / / +0.3 +3.0 0.0 -0.2





New York 0.5” ACB INP*

VAV 56.4 8.6 74.4 3.2 27.0 0.7 6.6 1.3 3.8 0.1 0.1 ACB 67.5 7.8 107.7 4.6 71.3 / / 1.0 2.5 0.1 0.3 Difference -11.1 +0.8 -33.3 -1.4 -44.3 / / +0.3 +1.3 0.0 -0.2

7


VAV 56.4 8.6 74.4 3.2 27.0 0.7 6.6 1.3 3.8 0.1 0.1 ACB 68.8 8.7 118.5 4.6 71.9 / / 1.0 2.5 0.1 0.3 Difference -12.4 -0.1 -44.1 -1.4 -44.9 / / +0.3 +1.3 0.0 -0.2





ACB INP = Active Chilled Beam Induction Nozzle Pressure. **EUI = Energy Use Index, (kBTU/GSF/Yr.) As can be seen, for the systems defined, the VAV system proved to be more energy efficient than the ACB system.

Considering the earlier described characteristics of the two systems, the following Comparison Table results…

Comparison Table

Features VAV System with DOAS

Active Chilled Beam with DOAS

Filtration Yes No EUI Low High

Operation and Maintenance Low High Controllability Yes Difficult

Comfort Yes - Churn Yes Difficult

CONCLUSION

Potential limitations of the chilled beam have been identified within this paper. Some application conclusions are summarized as follows:

1. Operational limitations, especially those related to the potential of a colder-than-dewpoint surface temperature located within the building, can create the possibility of condensation drips into occupied space.

2. The potential for disruption to tenants is a major concern for any modification to refit space, or repair ACB terminals.

3. There is an inability of chilled beams to effectively condition high-occupancy load spaces or other spaces with high latent heat gains. This limitation could also apply to accommodating buildings with high infiltration rates, especially older buildings.

4. There is a lack of nationally-recognized standard certification programs for rating the capacity and performance of the chilled beams or radiant panels.

5. Secondary air is not filtered and could create long-term unhealthy conditions without proper system maintenance.

6. In all cases, modeled energy use of the VAV system is less than the ACB system. 7. In considering an ACB system, the owner and design team must work together to assure adequate operating

staff training.

8

application issues for chilled beam technologies

Documents