Code of Practice COP 17/99 DISPLACEMENT VENTILATION AND STATIC COOLING DEVICES Tariq Abbas
Code of Practice COP17/99 DISPLACEMENT VENTILATION AND STATIC COOLING DEVICES Tariq Abbas The Building Services Research and Information Association Old Bracknell Lane West, Bracknell, Berkshire RG12 7AH Tel: + 44 (0)1344 426511 Fax: + 44 (0)1344 487575 e-mail: bsria@bsria.co.uk www.bsria.co.uk
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ACKNOWLEDGEMENTS ACKNOWLEDGEMENTS This work was part funded by the Department of the Environment, Transport and the Regions, under the Partners in Innovation scheme. BSRIA acknowledges the financial support of the Department of the Environment, Transport and the Regions and would like to thank the following sponsors for their contribution which has led to the production of this Technical Note. Department of the Environment, Transport and the Regions. Hoare Lea and Partners Comyn Ching and Co. (Solray) Ltd Halton Products Ltd Gilberts (Blackpool) Ltd University of Glamorgan Flomerics Plc SAS Holdings Stifab Airteknik Klima-Therm Ltd The research project was undertaken under the guidance of a project steering group drawn from industry representatives and BSRIA staff. Particular acknowledgement is given to Peter Jackman for his help in producing this publication. The industry representatives on the Steering Group were: Terry Wyatt Nigel Atkinson Mike Beeton Mark Seymour Maurice Herman John Mawdsley Andrew Geens Helen Sutcliffe David Stanton Consultations were undertaken with the following industry experts: David Arnold Alan Green Robert Cohen Terry Wyatt Maurice Herman Mike Holmes Nigel Atkinson Troup, Bywater and Anders Ltd Trox (UK) Ltd Halcrow Gilbert Associates Hoare Lea & Partners Comyn Ching & Co (Solray) Ltd Ove Arup & Partners Halton Products Ltd This document is published with the Department s consent, but the views expressed therein are those of the steering group and are not necessarily accepted or endorsed by the Department. Every opportunity has been taken to incorporate the views of the editorial panel, but final editorial control of this document rests with BSRIA. Displacement ventilation and static cooling devices BSRIA COP 17/99
EXECUTIVE SUMMARY EXECUTIVE SUMMARY The characteristic of a displacement ventilation system is the provision of fresh cool air directly into the occupied zone at a speed and temperature that does not cause discomfort. Once the air enters, it spreads across the floor forming a reservoir of fresh cool air. The movement of air within the space is primarily buoyancy driven such that heat sources (particularly people) generate vertically rising thermal plumes which entrain surrounding air. The reservoir of fresh cool air at floor level flows to replace that which was warmed and lifted into the thermal plume. The overall result is a vertical stratification of temperature and pollutant concentration such that the best conditions are achieved and maintained in the occupied zone. Displacement ventilation systems are a viable alternative to conventional air conditioning and ventilation systems and offer the potential for better internal conditions alongside significant energy savings. However, these achievements will only be realised through good design practice, which in turn relies on a comprehensive knowledge of the design issues. This document presents a detailed description of how displacement ventilation systems work, applications where it can be used, design issues, calculation procedures, system performance in terms of temperature, air speed, thermal comfort and indoor air quality as well as control, installation and maintenance. The aim of this document is to recommend good, accepted practice for the design and implementation of displacement ventilation systems with and without static cooling devices and is based on previous and current research as well as the views of consultants and manufacturers considered to be experts in this field. Displacement ventilation and static cooling devices BSRIA COP 17/99
CONTENTS CONTENTS GLOSSARY...i 1 INTRODUCTION... 1 2 DESIGN CONCEPT... 3 2.1 How displacement ventilation works... 3 2.2 How chilled ceiling panels work... 3 2.3 How passive chilled beams work... 4 2.4 How combined systems work... 5 3 APPLICATIONS... 6 3.1 Main application areas... 6 3.2 History of use... 6 3.3 UK today... 6 3.4 Forecast... 6 4 DESIGN CONSIDERATIONS... 7 4.1 Room height... 7 4.2 Floor void plenum chambers... 7 4.3 Heat loads... 8 4.4 Vertical temperature gradient... 8 4.5 Pollutants... 9 4.6 Varying loads... 9 4.7 Downdraughts... 9 4.8 Infiltration... 9 4.9 Draughts... 10 4.10 ATD selection... 10 4.11 Aesthetics... 13 4.12 Partitions and obstructions... 14 4.13 Perimeter conditioning... 15 4.14 Noise... 15 4.15 Extract positioning... 16 4.16 Lettable floor space... 16 4.17 Vertical radiant asymmetry... 16 5 DESIGN PROCEDURE - THEORY AND EXAMPLES... 17 5.1 Thermal comfort based... 17 5.2 Example of thermal comfort-based calculation... 18 5.3 Indoor air quality based... 18 5.4 Example of IAQ based calculation... 20 5.5 Alternative recommendation... 21 5.6 Example using alternative approach... 21 5.7 Displacement ventilation with static cooling devices... 21 6 PERFORMANCE... 22 6.1 Experimental set-up and conditions... 22 6.2 Temperature... 23 6.3 Speed... 24 6.4 PPD... 24 6.5 Concentration... 25 6.6 Ventilation effectiveness... 26 Displacement ventilation and static cooling devices BSRIA COP 17/99
CONTENTS 7 INSTALLATION/MAINTENANCE... 28 7.1 Plenum chamber... 28 7.2 Static cooling devices... 28 8 CONTROLS... 29 8.1 Displacement ventilation... 29 8.2 Static cooling devices... 30 NOMENCLATURE... 31 REFERENCES... 32 FIGURES Figure 1 Displacement ventilation concept... 3 Figure 2 Typical chilled ceiling design... 4 Figure 3 Typical passive chilled beam design... 4 Figure 4 Combined systems... 5 Figure 5 Flat profile... 10 Figure 6 Rectangular and triangular section... 11 Figure 7 Semi-cylindrical for wall alignment... 11 Figure 8 Quarter-cylindrical for corner alignment... 11 Figure 9 Cylindrical... 11 Figure 10 Circular and rectangular floor diffusers... 12 Figure 11 Induction air terminal... 12 Figure 12 Free-standing stainless steel air terminals... 13 Figure 13 Chilled beam... 14 Figure 14 New Guardian Royal Exchange Restaurant... 14 Figure 15 Royal Society of Arts Lecture Theatre... 15 Figure 16 Typical variation of room air temperature with height... 17 Figure 17 Heat source located near a wall... 19 Figure 18 Convective plume around a cylinder... 20 Figure 19 Variation of mean temperature with height... 23 Figure 20 Non-dimensionalised temperature variation with height... 24 Figure 21 Variation of mean speed with height... 24 Figure 22 Variation of mean PPD with height... 25 Figure 23 Variation of concentration with height... 25 Figure 24 Conventional ventilation effectiveness applied to mixing ventilation systems... 26 Figure 25 Ventilation effectiveness for displacement ventilation systems... 26 Figure 26 Effectiveness of entrainment in the human boundary layer... 27 Figure 27 Hinged drop-down chilled ceiling panel system... 28 TABLES Table 1 Outline of content... 2 Table 2 Main applications... 6 Table 3 Design options... 7 Table 4 Calculation table... 18 Table 5 Summary of findings... 22 Table 6 Mock-up tests and load levels... 22 Table 7 Conditions over all tests... 23 BSRIA COP 17/99 Displacement Ventilation And Static Cooling Devices
GLOSSARY OF TERMS GLOSSARY OF TERMS STRATIFIED PASSIVE (BEAMS) DOWNDRAUGHT REVERBERATION CHAMBER GENERATED NOISE ENTRAINMENT COANDA EFFECT Comprised of successive layers Not involving forced air flow Descending air caused by local cooling, especially from windows A sound measuring room specially designed to facilitate the production of diffuse sound fields Noise that results from turbulent air flow fields which become concentrated into a discrete region Entrainment of air is the process by which air is drawn along a flow path by virtue of viscous shear forces. This term is synonymous with induction. Specifically: Tendency for an air jet to attach itself to a solid convex body as a result of the pressure variation that is created perpendicular to the curved streamlines, named after the Rumanian engineer Henri Coanda. This phenomena results from a force acting towards the centre of curvature. Colloquially: It is often misquoted as the reason for wall jet attachment, which has nothing to do with curved surfaces. The Coanda effect here refers to the situation where one side of a jet is close to a large solid surface. The supply of air for entrainment on the wall side of the jet is restricted, resulting in a partial vacuum. Hence, the jet remains attached to the solid surface. We shall continue to refer to this phenomenon as the Coanda effect in this document for reasons of continuity only. TREATED SPACE ATD IAQ STRATIFICATION HEIGHT PNC A volumetric portion of the enclosure starting from floor level and ending at a specific height. This specific height is the point where air entering the volume is equal to the amount of air leaving. The treated space is usually designed to be a volume that at least encompasses the occupied zone. Air Terminal Device Indoor Air Quality Height to top of treated space. Preferred Noise Criteria. Refers to a set of rating curves derived in the USA to assess the acceptability of HVAC plant noise BSRIA COP 17/99 Displacement ventilation and static cooling devices i
INTRODUCTION SECTION 1 1 INTRODUCTION The aim of any ventilation system is to provide air to an indoor space in an efficient manner so as to create a comfortable and clean indoor climate for the occupants. Generally, there are two ways to ventilate a space. The traditional option is known as a mixing ventilation system which is based on the philosophy of continual dilution of stale warm air from a fresh supply. The ventilation air may be provided by an air conditioning system in which the air will typically be supplied at 12 C to 14 C. To encourage mixing, the air is supplied at high velocity and with a high degree of turbulence. A feature of this system is that conditions within the enclosure are considered to be predominantly uniform. The second method, namely displacement ventilation, intends to keep new and stale air separated and involves introducing the air at low speed directly into the occupied zone at a temperature usually slightly lower than the room temperature. A feature of this system is that the supply air spreads across the floor forming a reservoir of fresh cool air. This reservoir of air is in contact with heat sources (particularly people) and gets warmed so that it is lifted directly up and along the person and some is breathed en route to the plume that develops over the head, rising to a height of some 2 m to 2.7 m above the head. The warm stale air is extracted at the highest point. As a result, a temperature and contamination stratification of the air exists within the enclosure. The restriction to the supply air temperature limits the cooling capacity of the ventilation system and static cooling is often used to augment this. The comparative merits of both systems have been the subject of considerable debate in recent times [Ref 1] primarily due to the gain in popularity of displacement air systems. The reason for this change probably lies in the drive to design buildings that consume less energy whilst maintaining good indoor air quality and thermal comfort. The aim of this document is to recommend good, accepted practice for the design and implementation of displacement ventilation systems with and without static cooling devices and is based on previous and current research as well as the views of consultants and manufacturers considered to be experts in this field. It is an advisory document not intended to provide objective criteria by which compliance may be judged. The code of practice is not intended to reflect any single viewpoint, but highlights current thinking on the application of displacement ventilation systems with and without static cooling devices. BSRIA COP 17/99 Displacement ventilation and static cooling devices 1
SECTION 1 INTRODUCTION An outline of the various sections is provided in Table 1 below. Table 1 Outline of content Chapter Title Issues covered 2. Design Concept Describes the operational principles of displacement ventilation systems and static cooling devices. 3. Applications Describes the main application areas and presents a brief overview of the past, present and future for displacement air systems. 4. Design What should be considered before and after selecting a Considerations displacement air system. 5. Design Presentation of design calculations with examples. Procedure 6. Performance What should be expected from a displacement system with and without static cooling devices in terms of IAQ and thermal comfort. 6. Controls How these systems can be operated. 7. Installation & How to keep these systems running. Maintenance 2 Displacement ventilation and static cooling devices BSRIA COP 17/99