Select Committee on Health Written Evidence


  Extract from the PhD thesis of Dr Andrew John Geens entitled "A Critical Appraisal of the use of Displacement Ventilation in Commercial Buildings" May 2005.

  The main aim of this thesis has been to investigate the effectiveness of displacement ventilation in providing good indoor air quality when used in conjunction with supplementary cooling devices, and to assess alternative techniques for dealing with higher heat loads by displacement ventilation alone. This chapter presents the conclusions that may be drawn from the results obtained, and recommendations for further research that will strengthen and build on these conclusions.

9.1  Conclusions

  1.  When a chilled ceiling device is used to provide supplementary cooling in conjunction with a displacement ventilation system, the resulting downward convection plumes will disrupt the buoyancy driven displacement flow, reducing ventilation effectiveness and hence indoor air quality. Further, if the chilled ceiling devices operate following a period of operation of the displacement ventilation alone, the displaced "contaminated" air is "dumped" or re-circulated back into the occupied zone.

  2.  The "dumping" effect described in 1 above is most pronounced when the chilled ceiling device is of the chilled beam type. The chilled panel does not produce such a strong effect, and with care over the construction of the ceiling this effect can be minimised. It is unlikely in practice that it can be eliminated completely and it is concluded that displacement ventilation cannot be achieved in practice where ceiling mounted supplementary cooling devices are used. Systems currently in use and described as displacement ventilation with ceiling mounted supplementary cooling devices are in fact mixing ventilation systems delivered via displacement ventilation diffusers with ceiling mounted supplementary cooling devices.

  3.  Textile diffusers show great potential for use as displacement ventilation supply diffusers increasing the number of buildings that can benefit from displacement ventilation. However, care is required in their design to avoid problems of noise generation. The high volume flow rates achievable with the use of textile diffusers introduces the concept of a "high flow" displacement ventilation system, in contrast to the "standard flow" displacement ventilation currently utilised for commercial building applications.

  4.  When used for displacement ventilation, textile diffusers can be used for cooling loads up to 50 W/m2 without the assistance of supplementary cooling devices. At this load, using PPD/PMV as indicators, thermal comfort is comparable with a conventional displacement ventilation system assisted by a supplementary cooling system in a 40:60 ratio.

  5.  The demonstrated ability of the textile diffuser to create a relatively robust displacement flow regime at high volume flow rates makes it particularly suitable for applications where strong odours or levels of contamination are produced, and where segregation of the occupants from the contaminants is required.

  6.  The use of a textile diffuser for displacement ventilation significantly reduces the size of the draught zone in front of the diffuser. This increases the effective useful floor area, making the textile diffuser a better diffuser option even when high flow rates are not required, or when supplementary cooling devices are going to be used.

9.2  Further work

  The use of textile diffusers for displacement ventilation for cooling loads up to 50 W/m2 has been demonstrated to have certain merits over the use of other diffuser types requiring supplementary cooling devices in the ceiling. However these conclusions have been based on physical modelling in one test room facility, using instrumentation to measure temperatures and velocities. These results have been converted to standard comfort indices for comparison with other systems.

  The work has clearly demonstrated the technical feasibility of this application. However, the perceived benefits, namely good air quality and low energy consumption whilst maintaining thermal comfort, are deduced rather than measured. This means that there is considerable scope for further work to remove uncertainty in these areas.

  The performance of the system in use in an operational building needs to be monitored and assessed. A range of monitoring techniques will be required to address the diverse criteria of thermal comfort, air quality and energy consumption.

  The question of thermal comfort of the occupants requires objective and subjective treatment. The objective analysis for comparison with Standards can be provided by the same physical measurement techniques used in the experimental test room, as all the equipment is readily transportable. The subjective analysis to ensure occupant satisfaction can be achieved with the use of questionnaire or interview based surveys of the occupants of a building using this system. The occupants can also assess air quality as part of an occupant survey, particularly where contaminants are readily discernible by the occupants as is the case in buildings where smoking is permitted.

  Most office buildings do not have readily detected contaminants, reducing the effectiveness of occupant surveys in assessing air quality. In these buildings, instrumentation to monitor a contaminant level such as carbon dioxide can be used to assess the ventilation effectiveness. Alternatively tracer gas techniques may be used.

  The issue of energy consumption can be addressed by analysing the actual energy performance of a building in use over a full 12 month period. Alternatively, computational modelling techniques are available to simulate the operation of a building in terms of the energy performance of the environmental systems.

  The only installation currently identified as using this technique is serving a conference facility with a restaurant area as identified in Section 7.8. In the absence of any other opportunity this building could be used as the basis for this proposed monitoring work. The air supply rates for this building are higher by a factor of approximately two than those assessed during the experimental work for this thesis, presenting an opportunity to re-appraise the cooling performance of the technique. As the building operator allows smoking in the building, it also presents a good opportunity for assessing the segregation of smokers and non-smokers. This could lead to applications in leisure and retail buildings where customers smoke and staff need protection from the smoke.

  With the possibility of use in leisure and retail buildings there is also scope for incorporating the textile diffusers directly into the decorative finishes of the occupied spaces such as within bar frontages or under bench seating.

  However, the main application for displacement ventilation is currently in commercial office buildings. Particularly in respect of thermal comfort, the nature of occupancy is significant and monitoring in other types of building would not be reliably transferable. It is therefore important to have a textile diffuser system installed and monitored in an office building.

  Although the use of comfort indices indicated comparable performance for the textile diffusers, the temperature gradient in the room was on the limit of acceptability. By conducting the tests in a test room set up for testing chilled ceilings, the optimum performance of the system was not established, with the test room scenario probably being the worst case. It is likely that the best comfort conditions will be established when the system is used without a suspended ceiling, taking advantage of the thermal mass of the ceiling, with or without the use of night cooling. This needs to be examined, ideally in an operational building rather than a test facility, where the thermal mass may be difficult to model.

  The experimental work has addressed cooling loads of the nature and magnitude of typical internal casual gains (machinery, occupants and lighting) for a commercial office application. No account has been taken of the problem of direct or indirect solar gain. The nature and magnitude of these gains and their impact on the occupied space can be significantly varied by the design of the fabric of the building. Further work is required to assess the relative merits of dealing with the issue of solar gains in the design of the building, ie shading and thermal mass or dealing with them through the use of air conditioning systems. There may be scope for dealing with these additional gains with the use of a "high volume" displacement ventilation system, when some care is taken to minimise solar gains, particularly through glazing.

  Finally, further physical modelling is required on a fine measurement grid to establish the nature of the discharge profile in close proximity to the diffuser, and hence the size of the draught zone for the diffuser.

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