Achieving the balance with chilled beams

Much of the discussion around the advantages of chilled beams in indoor thermal comfort is defined by two different independent bodies. CIBSE provides guidance on thermal comfort criteria, and British Standards lays down various comfort standards, of which BS EN ISO 7730:2005 is the most significant. This standard defines the parameters determining thermal comfort, and explains how these can be interpreted.

 

The comfort conditions are of course governed by how the space will be used. BS EN ISO 7730:2005 does not strictly prescribe compliance requirements but it does provide categories for guidance. For example, in the UK, common practice for office spaces has been to design to achieve a maximum local mean air velocity of 0.25 m/s, which could be interpreted to achieve Category C prescribed by the standard. However if looked at in more detail, velocity on its own is not enough to determine if comfort conditions are achieved. Draught rating is actually the main determinant, which is also a function of turbulence intensity. So the lower the turbulence intensity, the higher the velocity that could be achieved whilst complying with a prescribed category.

These indices are used by Trox when measuring indoor thermal comfort in our air distribution laboratories, during development of new products, and when testing system designs on behalf of customers. We go beyond these parameters however (as do many of the consultants who specify our equipment) and our software allows us to calculate other parameters including PMV (Predicted Mean Vote), PPD (Percentage of People Dissatisfied), Tu% (Turbulence Intensity), DR (Draught Rate) and PD (Percentage Dissatisfied). We can also alter the values for metabolic rate and clothing to suit the application.

There are also, of course, minimum ventilation requirements as detailed in BS EN 13779:2007 and BS EN 15251:2007. Similar to ISO 7730, EN 15251 prescribes different categories for occupant satisfaction, for example to comply with category one, the minimum ventilation rate for diluting emissions (bio effluents) from people is 10l/s per person.

So how can best practice in chilled beam design assist in this process?

Understanding operation

Typically a passive chilled beam will provide a downward flow of cooled air so, if installed directly above a work station, the cooling output should be limited so that the targeted draught ratings (velocity and turbulence intensity) in the occupied height are achieved. It is also important to design the supplementary ventilation system to provide the fresh air prescribed by the standards, and to deal with the latent loads, in such a way that the natural convective operation of the passive chilled beams is not disrupted. This is best achieved with an upwards displacement ventilation strategy, with the supply air introduced into the space at low level. Trox’s expertise, as a supplier of both systems, can provide designers with a detailed understanding of how both systems operate, coupled with the capability to empirically assess and demonstrate an application. This is particularly useful for systems that operate in extreme conditions or serve critical environments.

In applications where high solar gains are present (e.g. refurbishment of old buildings), operating passive chilled beams at higher outputs at the perimeter, without causing discomfort in the occupied zone, is also a useful strategy. This enables comfort conditions to be maintained in the internal occupied areas (detailed in Table 11 of BS EN 13779 2007) with downsized components and lower air change rates. As seen in Figure 1, hot air, from the vicinity of the glazing, rises and hits the coil of the passive chilled beam and moves downwards through the coil to provide cooled air. A cavity barrier prevents warm air ‘overshooting’ the beam. The result is the delivery of cooled air into the space without causing discomfort in the occupied space. Utilising the up currents on the inside of the glazing actually enhances the heat transfer performance of the beam, and its proximity to an external façade helps prevent interference with comfort conditions within the defined boundaries of the occupied zone.

Flexibility of installation

In applications requiring delivery of ventilation and sensible cooling from one device, active chilled beams may be used. These can deliver high cooling outputs due to their air distribution characteristics, and are typically able to ventilate spaces with large thermal loads without generating draughts. They also offer a high level of flexibility of installation to suit the shape, dimensions and layout of occupied spaces.

Active chilled beams are ceiling mounted induction units which, with the higher discharge velocity through their purpose built nozzles, create a drop in static pressure within their mixing chamber. This induces the recirculating room air through a finned heat exchanger, and discharges the mixture of conditioned primary and secondary air through a slot diffuser horizontally into the space. The horizontal discharge and high entrainment of room air helps achieve low velocities and high levels of comfort at the occupied height (i.e. discharged air moves across the ceiling, before descending into the room at the room boundaries).

To achieve an optimum combination of thermal output and air distribution, however, the minimum ceiling or installation height should generally not be below 2.60m. In case of ceiling or installation heights up to 3.80 m, the supply air will reach the occupied zone without taking special action in the cooling mode. As these are forced convection devices (unlike passive chilled beams), it is also possible to use a 4 pipe heat exchanger and heat as well as cool with active chilled beams. However in order to prevent vertical temperature stratification, and short circuiting of recirculating air, the off-coil discharge temperatures should be maintained at a reasonable level. The rule of thumb for achieving this is limiting the low temperature heating water supply to 45°C maximum.

Humidity control

Chilled beams, both passive and active, are sensible cooling devices that have to be operated above the dew point temperature of the room air to prevent condensation. This means maintaining the relative humidity within the conditioned space within a reasonable tolerance to achieve a design dew point temperature, and supplying chilled water to the chilled beams above this value. Typically room air set points are 24°C dry bulb, and 50% RH, which equals a dew point temperature of approximately 12.9°C. To compensate for tolerances of control, and provide a safety margin in relation to this value, we recommend supplying the chilled beams with chilled water approximately 1K above the dew point. This typically equals to 14°C. To achieve an optimum thermal performance, mean water temperatures are usually designed to achieve 15.5°C, which equals to a water return temperature of about 17°C.

Close control of relative humidity within the space is paramount for designing a chilled beam system, and this can only be achieved (especially in the UK climate) with a central mechanical air handling unit plant, that carries out the necessary dehumidification of the fresh ventilation air. It is crucial that the fresh air supply is reheated above the room dew point, to prevent cold surface temperatures on the plenums of active chilled beams and avoid condensation.

Chilled beam systems should therefore be complemented with the correct AHU equipment. The ventilation system efficiency no longer becomes a component level issue, and Trox as a provider of the entire system (chilled beams and AHUs inclusive), is in a unique position to optimise the selection of the ventilation equipment with regards to achieving lowest specific fan powers (optimised by Trox within the AHU and downstream of the AH
U including all of the ventilation components that would be used in an active or passive chilled beam system, for example chilled beam selections optimised for pressure drops and nozzle configuration, displacement diffusers, volume flow controllers that include CAV and VAV boxes and attenuators); maximum heat recovery (minimum or no reheat requirement achieved via hygroscopic or conventional thermal wheels, plate heat exchangers or run-around coils ); and effective dehumidification.

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