Debunking Myths of Active Chilled Beams: What You Thought You Knew But Were Wrong, Part 1

Chilled beams offer building owners greater energy efficiency, reduced building first cost, reduced day-two maintenance, and improved indoor air quality. Unlike conventional mixed-air systems that use air as the heat transfer medium for the total load of a conditioned building, “decoupled” chilled water systems split the latent and sensible loads and drive local interior zone sensible loads directly to chilled water supplied through coils mounted in coil casings called chilled beams. Since water is a denser heat transfer medium than air and can hold 600 times more Btu per pound of heat transfer medium, it requires significantly less volume of water than air to maintain space temperature. Energy efficiency is gained by reducing fan horsepower that would otherwise be required to remove sensible heat from a building using an air-side HVAC design. Chilled beam systems are typically coupled with either a dedicated outdoor air system (DOAS), standard chilled water air-handling units, or energy recovery units in order to control latent loads. The chilled beams then address space sensible loads. Consequently, by “decoupling” the sensible and latent load, a building owner can see 20%-40% or more in energy savings when compared to an ASHRAE 90.1-2013 baseline system. There are two types of chilled beam designs:

Debunking Myths of Active Chilled Beams:

Passive Chilled Beams (PCB)
Active Chilled Beams (ACB)
The Condensation Test Executive Summary
The Test
Setup & Procedure
Temperature & Humidity Data
11:10 a.m. Start of the Test
Conclusion

Passive Chilled Beams (PCB)

Passive beams house horizontally mounted chilled water coils in sheet metal enclosures located at the ceiling level. Chilled water is circulated through the coil where heat, local to the beam, is removed by conduction to the beam’s chilled water loop. Convection causes the cooler air to drop to lower levels of the conditioned space, allowing for thermal stratification to occur.

Active Chilled Beams (ACB)

Similar to passive chilled beams, active chilled beams (ACBs) house a chilled water coil mounted horizontally but in the bottom tier of a two-tiered sheet metal enclosure. Unlike passive beams, which work in spaces designed to maintain a state of thermal stratification, active beam systems are designed to create a thoroughly mixed-air environment to maintain uniformity of temperature throughout the cubic volume of space. To do so, active beams need to deliver primary air at a high velocity through nozzles located in a sheet metal divider plate between the two plenum tiers. Air, injected at high velocities through these nozzles located along both sides of the coil, creates a negative pressure zone above the coil, allowing room air to be induced through the beam coil where sensible heat energy is transferred from the room to the beam coil by conduction. Primary air is injected at a high velocity to the space to maximize the amount of room air being induced into the supply air jet, creating a uniform mixed-air environment to maintain uniformity of the temperature defined by the space thermostat throughout the cubic volume of space.

Advancing chilled beam technology, however, can be challenging. As engineering design professionals try to promote this proven HVAC high-performance solution, design engineers are often met during design meetings with questions from clients who challenge the decoupled hydronic concept and often field the following questions?

With ACBs, won’t it “rain” in the space?
Will the load be able to be maintained?
What is the response time of an ACB system compared to a variable air volume (VAV) system when reacting to increased loads in the space?
What happens when you introduce a large latent load to a space, such as a conference room or waiting room, all at once?

Although chilled beams have many advantages over traditional air-side systems, many projects do not advance chilled beam systems into HVAC designs due to these various concerns. One major cause for not applying the technology shared by owners and design teams alike is the need for maintaining building humidity control. If such a building or space should lose control of moisture levels and dew point exceeds the entering chilled water temperature, it’s feared condensation will rapidly occur on the chilled beam coils, causing beads of moisture to form and drip to the space below. However, there is a common misunderstanding as to how long it takes for condensation to actually form to a point where it will begin to drip into the space once space dew point rises above an ACB coil’s entering chilled water temperature.

Engineers from SmithGroup teamed up with Dadanco, a chilled beam manufacturer, and Varitec, a southwestern manufacturer’s representative, to prove how long it takes condensation to form on beam coils and piping before it becomes a concern for an owner. If this time span is considerable, it can then be demonstrated that a properly designed control system will respond at a fast enough rate to shut off chilled water loops serving such zones or revert to a water temperature reset strategy to prevent condensation from forming to a degree that it would become a concern.

Over the next two articles, we will demonstrate facts regarding the operation and performance of ACBs. The first article will look at what happens when ACBs are delivered at a chilled water temperature below a room’s air dew point temperature. There is a perception chilled beams will immediately “rain” moisture when the room air dew point exceeds the ACB chilled water temperature. This article will reveal findings from laboratory tests that measured the rate of condensation formation on ACBs when room dew point exceeds the ACB chilled water supply temperature.

The Condensation Test Executive Summary

Team members from SmithGroup Inc., Varitec, and Dadanco collaborated to develop parameters and procedures used to test and understand the length of time a typical ACB forms condensation on the coil. Once the length of time is known, we can prove or disprove whether condensation becomes an issue when utilizing ACBs in design. The test was performed by the Dadanco team in Dadanco’s environmental test chamber. Data from test results show it took 30 minutes for evidence of condensation to form on the beam coil inlet. It took another 3 ½ hours for condensation to form on the finned area of the coil and 18-22 hours before any condensation accumulated enough to fall from the coil.

The test results clearly illustrate, should space dew point exceed ACB supply chilled water temperatures, that ACBs can operate at or somewhat below room dew point for longer periods of time than presumed before condensation occurs at a level to be of concern. Furthermore, it proves that it takes many hours before condensation water droplets become large enough to fall from wetted surfaces, confirming humidity control strategies, when properly defined, have more than adequate time to respond to rapid latent load swings to prevent condensation.

The Test

A typical 10-foot-long ACB was installed in the center of Dadanco’s environmental test chamber to observe the process of condensation forming on the chilled water coil over a period of several hours (Figure 1). Insulation was not installed at the piping connections to allow for visual inspection of moisture buildup. The test parameters were as follows:

FIGURE 1. The active chilled beam condensation test was conducted at Dadanco’s environmental test chamber.

Room Condition: 75°F/50% RH (55° dew point) – held constant for the entire test;
Entering Chilled Water Temperature: 53.5° (1.5° below the room’s dew point temperature);
Chilled Water Flow: Constant gallons per minute (gpm); and
Primary Air: Constant volume of air at constant temperature.

Setup & Procedure

It was essential for Dadanco’s lab to be at a steady state prior to the test. For temperature and humidity to stabilize, test chamber controls were programmed to operate the air handler and humidifier overnight. Furthermore, a total of five heat sources (called DIN-men) were used to simulate the thermal load of occupants within the space. Each DIN-men is designed at 200 watts output. During this time, chilled water was supplied at 58°, or 3° above space dew point, a common ACB design practice to prevent condensation buildup on the beams. Room air dew point at the chilled beams was calculated from room dry bulb temperature and room RH. A pair of temperature and RH sensors were located at each end of the beam, approximately level with the face of the ACB. The two temperature sensors were averaged for room temperature, and the two RH sensors were averaged for the RH used in the dew point calculation. One of these sensors was used as a reference temperature to control DIN-men to simulate occupant heat output. One of the RH sensors was used as the reference temperature to control the humidifier output. The entering chilled water temperature was measured by a temperature sensor inserted into a thermal well in a valve that is connected to the chilled beam coil.

On the scheduled day, testing began at 11:10 a.m., when the chilled water supply temperature was reduced to 53.5°, or 1.5° below space dew point, dropping to below dew point at 11:32 a.m. Photos of the ACB were taken at six timed intervals throughout the first day, beginning at noon and ending at 5:10 p.m. Photos were taken to record condensation formation and buildup where it occurred. After the 5:10 inspection, the test was left running overnight under the same conditions in order to observe the level of condensation present on the beam and to see if water droplets formed large enough to drop to the floor.

Temperature & Humidity Data

Table 1 shows the readings at each of the two sensors as well as the averages of the measured room temperature and RH. The entering water temperature and the calculated room dew point are also listed. The most important factor in moisture formation on piping and coil surfaces is the difference between the entering chilled water temperature at the cooling coil and the room dew point. On average, the entering water was approximately 1.7° below the room dew point temperature.

TABLE 1. The readings at each of the two test sensors as well as the averages of the measured room temperature and room RH.

11:10 a.m. Start of the Test

12 p.m. (approximately 1 hour): There was no visible condensation on the coil within the chilled beam, but some small droplets had begun to form on the uninsulated chilled water inlet. This is always the first location where condensation will form, because the water is coldest and there is no air movement over the pipe (Figure 2).

FIGURE 2. 12 p.m: No condensation on the coil within the ACB, and small droplets have started forming on the chilled water (CHW) piping at the inlet to the ACB.

1 p.m. (2 hours): There was more condensation on the supply connection, and some fine beads had formed on the first return bend of the coil. There was no condensate on the finned area of the coil, and no condensate had fallen (Figure 3).

FIGURE 3. 1 p.m: More condensation is forming on the supply piping, and fine beads have formed on the first return pipe bend of coil.

2 p.m. (3 hours): There was no substantial visible difference from 1 p.m. (Figure 4).

3:30 p.m. (4.5 hours): The first visible condensation on the coil inside the chilled beam casing had formed only small amounts on the beginning of the first tube in coil. There was still no visible condensation on the fins. The size and quantity of beads on the supply connection and return bends had increased noticeably.

FIGURE 4. 2 p.m: No substantial visible difference from 1 p.m.

4:25 p.m. (5.5 hours): The size of beads on the coil connection and return bends increased, but there were no other visible changes on the coil inside the chilled beam casing.

5 p.m. (6 hours): There was no significant difference between 4:25 and 5 p.m. There was significant condensate on the supply connection and first return bend, and there was a small amount of condensate on the first tube of the finned area but no visible condensation on the fins. No condensate had fallen (Figure 5).

FIGURE 5. 5 p.m: Condensate beads are forming on the coil tubes of the finned area. No condensation is present on the fins. No condensate has fallen.

9:15 a.m. (next day – 22 hours): After running overnight at the same conditions, significant condensation was visible on the coil fins around the first tube of the coil (where water is coldest), and some droplets of water fell from the coil to the floor. At what time the droplets began to fall was not recorded.

Conclusion

Test results show that while condensation does occur when room dew point/humidity levels rise above chilled beam supply water temperature, the process of condensation at the beam supply/return coil connections and coils/fins occurs at a much slower rate than is often presumed. The team’s test clearly demonstrates that it takes approximately 30 minutes for a conditioned space operating below typical design conditions (typical chilled water temperatures of 2°-3° above room design dew point) for any evidence of condensation to become apparent at the supply coil inlet to the ACB. Furthermore, it shows that it takes approximately four hours for condensation to form on the coil/fins inside the ACB casing, and a period of 18-22 hours lapsed time before water droplets reached the necessary size and density to fall to the floor.

This team is not recommending that ACBs be designed to operate outside or below proper design set points and conditions. On the contrary, it’s important to accurately calculate latent loads and to design the primary air volume and conditions required for dehumidification capacity in order to keep conditioned zones served by ACBs at a dew point below the chilled water supply temperature. However, the testing demonstrates that condensation formation and water droplet buildup is a very slow process. These are very important facts to consider when ACB systems are a potential project solution.

Humidity control strategies can be effectively utilized in ACB systems. Two such strategies are either a chilled water temperature reset mode, where chilled water supply temperature is elevated to a level above space dew point, thereby preventing condensation from occurring, or a second option of shutting the chilled water valve at the ACB or ACB zone valve. These control strategies can respond in a time frame that would prevent any moisture buildup to occur when water enters an ACB below space dew point. Insulating the coil piping supply will expand the window of condensation, allowing controls even more time to respond.

Any given project has multiple system options to be evaluated during design. This team’s testing and analysis debunked one of the main myths regarding the rate of condensation formation on ACBs and now gives project teams, design engineers, and clients an alternate system solution to evaluate during design.