Chiller action

With the demand for more energy efficient air conditioning systems rising, new technologies are entering the market. Thomas Michaelsen and Ian Leake outline the benefits of multi-service chilled beams.

Trox Middle East LLC, ANALYSIS, MEP

With the demand for more energy efficient air conditioning systems rising, new technologies are entering the market. Thomas Michaelsen and Ian Leake outline the benefits of multi-service chilled beams.

When it comes to ensuring thermal comfort conditions in offices, conference rooms and meeting rooms as well as the best air quality possible, architects and specialist planners are provided with a wide range of ventilation and air conditioning systems to choose from. Among these, air-water systems are continuously growing in importance.

Chilled beams are particularly suitable for installation in new buildings and modernisation projects due to their compact design, the minimal investment and operating costs involved and their adaptability to the wishes of architects and building owners.

Passive or active systems?

First, a distinction can be made between the various designs and features of chilled beams in passive and active systems. The term passive means that the secondary air is cooled convectively ie it is unforced.

Active systems, on the other hand, use fans to bring pre-conditioned outside air into the room and cool the secondary air by means of induction. The induced air is guided through the chilled water coils in the beam.

For Middle East applications, passive chilled beams are unlikely to be suitable due to the high capacities needed. Active chilled beams should be considered for applications as these can take care of higher loads.

There are two fundamental differences between active and passive chilled beams. Firstly, active chilled beams are specifically used to induce a defined amount of conditioned outside air into the room.

Secondly, in active chilled beams the warm ambient air no longer flows into the beam from above as is the case in passive chilled beams. Rather, the warm air is absorbed into the beam through the bottom of the unit.

In a central ventilation device, filtered outside air is delivered as primary air to the active chilled beam via an air duct system. The primary air flows under high pressure into the chilled beam through a number of nozzles in the left and right air chambers.

The high velocity of this primary air results in negative pressure in the central chamber of the chilled beam, which ensures that warm ambient air is absorbed (induction) into the chilled beam through the bottom of the device (see figure 2, opposite).

This secondary air flows through the heat exchangers arranged on the left and right and is thereby cooled. The cool secondary air is subsequently mixed with the primary air, which is then discharged into the room with a direction of airflow that is nearly parallel to the ceiling.

As such, depending on the size of the nozzles chosen, the device has an induction ratio of approximately 1:4, which means that for every 1m3 of primary air, approximately 4m3 of ambient air is induced into and cooled within the chilled beam.

Because active chilled beams absorb secondary air from below, they can also be installed in suspended ceilings. This is particularly advantageous for modernisation and retrofit projects that already have suspended ceilings.

Chilled beams in application

Because of the effects described above, the use of active chilled beams can also establish a desired and stable room airflow that is significantly less sensitive to thermal disturbances in comparison to passive chilled beams.

The dimensions of an active chilled beam have a significant influence on its overall cooling capacity. The standard width of chilled beams ranges from 300 to 700mm, with an overall height of 120 to approximately 300mm.

Supply air volume flow rates ranging from 80 to 400m/h per metre of beam length result in cooling capacities of up to 185W/m2 depending on the layout and spacing.

This also provides for adequate fresh air quantities to ensure a pleasant working environment.

To adjust the cooling capacity and the supply air volume flow rate to meet the current requirements in the room, two parameters control the performance of an active chilled beam:
 

  •  water-side control, which modifies the flow and temperature of the chilled water;

 

  • air-side control, which modifies the volume of primary air. This in turn controls the amount of induced secondary air.

Chilled beams can be designed for a specific project and tailor-made to meet the needs of the building owner and architect.

There has been a noticeable trend towards multi-service chilled beams, the objective being to integrate functions such as lighting, sprinkler systems, electrical cabling, smoke detectors and presence detectors, along with ventilation and cooling functions.

Middle East application

While chilled beam technology is a relatively new concept in the Middle East, it has been successfully used for many years throughout the world, including countries that have aggressive climates with high temperatures and humidity.

A common concern is that of condensation occurrence and possible subsequent water damage. It should be remembered that the system is designed to run dry, as opposed to say a fan coil system, which is deliberately designed to run wet.

The design of a chilled beam system is such that several safeguards are built-in to ensure that condensation problems do not occur.


Firstly the water system layout: according to tests carried out by CIBSE, significant misting only occurs when chilled water is supplied at 2°C below the room air dewpoint. Tests were carried out using chilled water at a temperature of 14°C and with a dewpoint of 16°C.

This equates to: 22°C/70% relative humity (rh), 24°C/60% rh or 27°C/50% rh. When water temperature is designed to be controlled at 1°C above dewpoint there is a 3°C safety margin, which allows a significant margin for error and changed conditions.

As a secondary precaution, the cooling coils are mounted vertically, allowing a drip tray to be fitted underneath to contain any water that might occur due to unforeseen circumstances. The drain tray is designed to contain the moisture for some time and will evaporate once the system has returned to normal operation.

Considering the above mentioned layout criteria, this can only happen when much more moist outside air is directly guided into the room than through normal infiltration (= failure).

Thus the drip tray is not connected to a condensate extract system (because, as mentioned above, the coils are run dry and are not condensate producing under design conditions, with a high security factor of 3K difference in room air dewpoint), so no pump or drainage system is needed.

Further security is provided by the installation of moisture or leak sensors on the supply water pipe to a zone and the drain tray below. These will trigger either an increase in the chilled water supply temperature or full shutdown of the chilled water supply in the event of a major problem.

At that time a degree of cooling will continue by the continued supply of conditioned primary air.

A major benefit of a chilled beam system is the reduction in the need for maintenance. There are no fans or motors, no moving parts within the occupied zone, no condensate pumps, no filters and no consumables.

All parts are easily accessible and it is only recommended that the coils, which are self-cleaning due to their wide fin spacing, are checked and cleaned with a standard household vacuum cleaner every four years.

Energy benefits

A chilled beam system provides an energy efficient solution. This is because the specific thermal capacity of water to transport energy is roughly 10 times that of the capacity of air. For example, a 28mm water pipe can transport the same amount of energy as 450x450mm duct.

In general an air/water system provides an energy transportation saving of around 33% over an all-air system, while an all-water system offers a saving of around 40%.

This saving equally applies to a fan coil system, however it should be remembered that with such systems the heat produced by the FCU motors must also be absorbed by the air cooled by the chilled water coil.

This equates to a net energy saving by the chilled beam even when considering the higher pressure to be provided by the air handling unit fan and the return air system necessary in some layouts.

There are possible additional energy savings due to the fact that chilled water is only needed at a temperature of say 14°C as opposed to 6 or 7°C for other systems.

This saving will of course depend on how the chilled water is obtained and does not apply if district cooling is being used, which in turn has its own advantages.

Specific costs analysis has shown that the capital cost for a chilled beam solution can be cost neutral compared to a typical fan coil system.

Thomas Michaelsen and Ian Leake are technical director and sales director of Trox Middle East.

Help at hand for chilled beam selection

Trox Middle East is currently involved in the design of a number of prestigious projects within the Middle East region, where the decision has been taken to adopt chilled beam technology in order to provide a quiet and comfortable environment.

Trox has successfully installed exposed multi-service chilled beams within its own premises in Dubai. Additionally the firm has installed in-ceiling chilled beams within its Air Flow Studio, where it can carry out inhouse testing and provide accurate measurements of all relevant air and water parameters, plus provide mock-ups for clients that wish to explore the benefits of using such a system.

Trox can also provide a CPD seminar that is accredited by both the Chartered Institution of Building Services Engineers (CIBSE) and the Royal Institute of British Architects (RIBA), where the subject of chilled beam systems can be discussed in full detail. This can help to decide whether a chilled beam solution is applicable for any particular project.

RELATED LINKS: Cool thinking for the future, Intelligent operations, Cool climates

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