value engineering of car park ventilation system

8
1 Value Engineering of Car Park Ventilation Systems Ross Edward Bates Catherine Simpson Building Simulation Ltd Summary The use of Computational Fluid Dynamics (CFD) is becoming more widespread within the building services industry. This paper is about the use of CFD for car park ventilation design. Whilst it is important that the simulation engineer is able to use the CFD tool well, it is also important that the design team understand the sensitivity of input data. This paper explores the use of CFD to develop car park ventilation schemes and discusses the uncertainty of input data as well as acceptance criteria in order to demonstrate compliance with Building Regulations. A range of case study material is used to demonstrate the application of CFD and the issues that can be addressed. Acknowledgements I would like to thank Roberts & Partners, Lend Lease and The Berkeley Festival Waterfront Company Ltd for their kind permission to use the case study material within this paper. Introduction and Background Planning requirements often mean extensive car parks to proposed offices, residential and retail developments. The ventilation of these car parks can present a significant cost to the developer in terms of capital expenditure for plant, energy consumption and maintenance, as well as the implications for the programme if extensive ductwork and plant has to be installed. The use of Computational Fluid Dynamics (CFD) is most effectively used at early design stage: ‘prevention is better than cure’, but can be used as a tool for solving existing problems. An excellent example of using CFD effectively at the design stage is the Bluewater Retail Development, detailed in the case studies section. The specific objectives of a project can vary considerably. The key objectives of the simulations are normally to ensure that the distribution and concentration of carbon monoxide (CO), is in accordance with Building Regulations. The Building Regulations comprise a number of Approved Documents. These Approved Documents contain design options which if adopted, the scheme is ‘deemed’ to comply. These documents also contain provisions which enable alternative designs solutions to be considered, provided that it is demonstrated that the designs meet the Building Regulations. Compliance with Building Regulations can be demonstrated using computer simulation. The Building Regulations are produced centrally by Government, however their interpretation and enforcement by building control officers can vary from district to district.

Upload: mitasyahu

Post on 19-Oct-2015

36 views

Category:

Documents


1 download

DESCRIPTION

Cost Effecitive ACMV design for Basement Ventilation Systems

TRANSCRIPT

  • 1Value Engineering of Car Park Ventilation Systems

    Ross Edward BatesCatherine SimpsonBuilding Simulation Ltd

    Summary

    The use of Computational Fluid Dynamics (CFD) is becoming more widespread withinthe building services industry. This paper is about the use of CFD for car park ventilationdesign. Whilst it is important that the simulation engineer is able to use the CFD toolwell, it is also important that the design team understand the sensitivity of input data. Thispaper explores the use of CFD to develop car park ventilation schemes and discussesthe uncertainty of input data as well as acceptance criteria in order to demonstratecompliance with Building Regulations. A range of case study material is used todemonstrate the application of CFD and the issues that can be addressed.

    Acknowledgements

    I would like to thank Roberts & Partners, Lend Lease and The Berkeley FestivalWaterfront Company Ltd for their kind permission to use the case study material withinthis paper.

    Introduction and Background

    Planning requirements often mean extensive car parks to proposed offices, residentialand retail developments. The ventilation of these car parks can present a significant costto the developer in terms of capital expenditure for plant, energy consumption andmaintenance, as well as the implications for the programme if extensive ductwork andplant has to be installed.

    The use of Computational Fluid Dynamics (CFD) is most effectively used at early designstage: prevention is better than cure, but can be used as a tool for solving existingproblems. An excellent example of using CFD effectively at the design stage is theBluewater Retail Development, detailed in the case studies section.

    The specific objectives of a project can vary considerably. The key objectives of thesimulations are normally to ensure that the distribution and concentration of carbonmonoxide (CO), is in accordance with Building Regulations.

    The Building Regulations comprise a number of Approved Documents. These ApprovedDocuments contain design options which if adopted, the scheme is deemed to comply.These documents also contain provisions which enable alternative designs solutions tobe considered, provided that it is demonstrated that the designs meet the BuildingRegulations. Compliance with Building Regulations can be demonstrated usingcomputer simulation. The Building Regulations are produced centrally by Government,however their interpretation and enforcement by building control officers can vary fromdistrict to district.

  • 2Value Engineering can identify potential savings in capital, maintenance and energycosts without any adverse effect on performance. Computer simulation is therefore,becoming an essential value engineering design tool. For example, a designer mayquestion why 6 air changes per hour (ACH) should be provided when 4 ACH willaccomplish the desired result. The benefits of simulation are that the proposed designscan be tested against the acceptance criteria contained within the Building Regulationsbefore any financial commitment is made. This is illustrated by the Three Waters car parkin Madrid, which is discussed in the case studies section.

    The acceptance criteria within the Building Regulations adopt the Health and SafetyExecutive, EH40/2000 Occupational Exposure Limits 2000 1. The main two criteria aredetailed below:

    1. Long Term Exposure Limit (8 hour time weighted average (TWA) reference period)The long-term exposure limit states that over an 8-hour period a person can beexposed to no more than 30 parts per million (ppm) of carbon monoxide. Thisapplies to both employees and members of the public.

    2. Peak Levels - Short Term Exposure Limit (15 minute reference period)For car parks the carbon monoxide levels are to remain below a designated peakvalue. At present the regulation is that For a 15 minute period the level of carbonmonoxide is not to exceed the peak exposure of 200 ppm.. This peak figureincludes the appropriate background or ambient carbon monoxide level.

    Modelling Parameters and Techniques

    The car park models are assessed using a three-dimensional, steady state simulationwhich calculates air movement in terms of mass, velocities, temperatures and carbonmonoxide concentrations in all three dimensions (x, y, z). The input data for the modeloften necessitates a number of assumptions about key variables, particularly withinnovative designs. It is important, therefore, that these assumptions are representativeof the problem and this is usually based on experience from previous models. Sensitivity tests have shown that the following variables can significantly influence thesimulation results:

    Obstructions to Airflow

    This includes items such as, surrounding buildings, local topography, parked vehicles,pedestrian access cores, lifts and stairs. Depending on the design, these obstructionscould influence the air movement patterns and hence the distribution of carbonmonoxide.

    Ambient conditions

    The simulations are usually run for what is considered to be a worst-case scenario. Thiscould be a calm or a windy day depending on the car park design. The calm dayscenario relies on stack effect for ventilation and is normally run for a summer designday. The high ambient temperature reduces the amount of buoyancy experienced by thehot exhaust gases, therefore minimising air movement and reducing carbon monoxidedilution.

  • 3If the effect of wind driven ventilation is to be considered, then wind pressure coefficientdata from a wind tunnel test could be used or in the absence of this, generic windpressure coefficients could be used with extreme caution. Generic wind pressurecoefficients should only be used to compare options rather than to establish absolutevalues, as they do not take account of the site location or the building design.

    Vehicle Movements

    In order to determine the levels of carbon monoxide emissions it is necessary todetermine the number of vehicles that are moving at any one time within the car park.This can be acquired from a vehicle movement profile if a traffic study has beenundertaken or from a case study on a similar car park. The peak figure is usually 10% to15% of the total capacity of the car park.

    Having established how many vehicles are moving at any one time, the type and durationof movements can be considered: for example are vehicles idling, stationary in a queue,manoeuvring into/out of parking bays or accelerating. The distribution of vehicles willdetermine the location of emissions; alternatively in the absence of sufficient information,a uniform distribution may be assumed.

    Vehicle Emissions 2

    The type of fuel and the age of the vehicle will determine the composition of the emissions.The amount of carbon monoxide varies depending on whether an engine is warm or cold: forexample the carbon monoxide from a warm petrol engine is in the region of 46.22 mg/s andfor a cold engine 136.15 mg/s. The exhaust gas temperature affects the emissions fromvehicles with catalytic converters that are only effective when the engine is warm.

    The Duration of Occupants in the Car Park

    This can either be based on a study for an existing car park or on reasonable estimatedfigures specific to the site. For example; 5 minutes to drive in and park, 5 minutes to collectbelongings and walk out of the car park, 5 minutes to walk back to the car and loadshopping, 5 minutes to drive out of the car park and, say, 5 minutes spare. This gives a totaltime of 25 minutes exposure period. This figure would then be used to calculate the 8-hourTWA exposure limit.

    Other important variables amongst others to be considered include, all natural ventilationopenings, the type of site and its geographical location and the appropriate backgroundcarbon monoxide level.

    The range of detail incorporated into the computer models depends on the time and feesallowed for analysis as well as the amount of information available. To avoid extensivecosts of detailed models at the early design stage prior to design confirmation, simplificationsto the model can be made. For example, generic models can be built to represent a numberof different design proposals. These results are used to compare options but not to provideabsolute values to assess compliance with Regulations. The design option selected will bedeveloped in more detail to assess compliance with Building Regulations. An example ofgeneric modelling is Heron City, Lisbon which is contained within the case studies.

  • 4Case Studies

    The CFD models, as well as allowing the effectiveness of the proposed design to bedetermined, can also be used to examine possible solutions to any problems identified in theanalysis. The following case studies demonstrate how CFD was used to fine tune originaldesign concepts.

    Three Waters Madrid

    This project was for an underground car park beneath a new shopping centre in Madrid. Afully ducted ventilation system was the approved design contained with the Spanish BuildingRegulations. However, this would be costly to install and would impact on the developmentprogramme. Therefore, the purpose of the simulation was to demonstrate that a non-ducted,mechanical system would also meet the Spanish Building Regulations.

    Figure 1 above indicates the position of the ventilation components and their flow directionfor the two schemes. The Fully ducted scheme had a uniformly distributed mechanicalextract with make-up air drawn in via the entrances and exit points. The non-ducted schemecombines a central ducted supply, with extracts positioned at the perimeter, make-up air wasagain drawn in via the openings provided for the entrances and exits.

    Non-ducted scheme

    Fully-ducted scheme

    Mechanical ExtractMechanical Supply

    Natural Make-up Air

    FIGURE 1

  • 5Figure 2 above shows a CFD plot for each of the schemes, it can clearly be seen that theproposed non-ducted scheme performs better that the alternative fully-ducted scheme.

    Gunwharf Quays Portsmouth

    This was a naturally ventilated car park comprising two basement sections linked by a splitsubterranean level. This resourcefully utilised a number of different natural ventilationopenings such as louvres, vehicle entrance and exit points, ventilated doors, roof seatedvents and hit and miss boarding. For a naturally ventilated car park design, the requirementsstate that each level of the car park should have a ventilated free area not less than 2.5% ofthe floor area at that level, of which at least 1.25% should be equally provided between twoopposing walls3. In this case the ventilation openings were not equally provided betweentwo opposing walls, however, the simulation demonstrated that the alternative designcomfortably satisfied Building Regulations.

    Figure 3 overleaf illustrates the car park design and indicates the position and dimensions ofthe natural ventilation openings.

    Fully-ducted scheme

    Non-ducted scheme

    Exhaust COConcentration

    (ppm)

    220190160130100704010

    220190160130100704010

    FIGURE 2

  • 6Figure 4 below illustrates two CFD plots from the simulation, taken 1m above the floor level.The upper plot shows the distribution of carbon monoxide and the lower plot shows airspeed. These plots show that there is a greater concentration of carbon monoxide in the lefthand side of the car park due to fewer ventilation openings in this location. The airmovement is multi-directional, the speed is within a comfortable range for people using thecar park.

    0.320.290.250.210.180.140.110.070.040.00

    Speed(m/s)

    1581411329788705335170

    ConcentrationCO (ppm)

    FIGURE 4

    Key Item Size (Number) (m) % FA Actual FA (m)Entrance Ramp 6 x 2.5 100 15.00Seat Vents 0.9 x 0.9 x 16 80 10.37Vented doors 2.1 x 0.9 x 8 40 6.05Hit & Miss Boarding 1.4 x 136 40 76.16Louvres - Type: 1 3.253 x 1.06 x 31 50 53.45

    2 4.038 x 1.06 x 6 50 12.843 3.420 x 1.06 x 3 50 5.444 4.360 x 1.06 x 15 50 34.665 1.660 x 1.06 x 1 50 0.88

    TOTAL FA (m) 214.84

    FIGURE 3

  • 7Heron City Lisbon

    At early concept stage a series of generic models were tested to formulate an appropriateventilation strategy. The following three strategies were compared; natural ventilation,central mechanical extract with natural ventilation intake, central mechanical supply withnatural exfiltration. The generic model maintained the free areas of perimeter openings, floorareas, volumes and the aspect ratio of the car park whilst omitting unnecessary detail.

    Figure 5 above displays the carbon monoxide plots for the three strategies. The simulationsshow that the design with the central supply is most effective at removing the carbonmonoxide. However, due to the intended construction of open recreation and eating areasabove the car park perimeter, the central supply solution could create unpleasant odours.With this consideration, the central extract design was deemed the more appropriatescheme.

    Bluewater Kent

    Bluewater is the largest and one of the most innovative designs of its kind in the UnitedKingdom, comprising 154,000m of shopping and leisure facilities. Lend Leases conceptwas to create a natural fresh environment throughout the complex, inclusive of the carparking facilities for over 13,000 cars. The original designs detailed mechanical systems,however a wind tunnel model supplying the external pressure coefficients was combined witha CFD model, which lead to a number of design modifications and demonstrated that themajority of the car parks could be naturally ventilated.

    Natural Ventilation Scheme

    Central Supply Scheme 2 ach

    Central Extract Scheme 2 ach

    210187163140117937047230

    ConcentrationCO (ppm)

    FIGURE 5

  • 8Localised CO detectors combined with fan-assisted ventilators were introduced to areas thatthe computer simulation showed could potentially exceed CO limits. This was an excellentexample of Value Engineering of car park ventilation systems and provided considerablesavings particularly in construction, operation, maintenance and programme as well assignificant environmental benefits.

    Figure 6 above is a site photo of Bluewater, illustrating the fan-assisted ventilators on the topdeck, the CO sensors are positioned directly below the ventilators on the lower deck.

    Conclusion

    The use of Computational Fluid Dynamics (CFD) as a tool for value engineering cansignificantly benefit new and existing developments by avoiding unnecessary capitalexpenditure, reducing construction time, and providing reductions in energy consumption,CO2 emissions and maintenance.

    Many modern buildings are complex in shape and design and therefore they require anengineered solution. The majority of simulations undertaken for car parks are required toshow that the proposed design complies with the appropriate part of the BuildingRegulations. However, it must be borne in mind that many of the key variables areapproximations and therefore it is important that the sensitivity of the results to anyassumptions made is fully appreciated.

    The case studies have helped highlight that substantial benefits can be obtained from CFDsimulations of early design proposals. The benefits are greatest when designs are tested atan early stage which allow designs to be fine tuned as opposed to a late stage re-designwhich can be costly.

    It must be remembered, that simulation results, no matter how comprehensive, are not aguarantee of performance. However, used correctly, CFD is an invaluable tool for theprofessional engineer to realistically appraise design options.

    References

    1 Health and Safety Executive, EH40/2000 Occupational Exposure Limits 2000.

    2 Figures from a client, based on data provided by the AA (Automobile Association) 1995.

    3 Taken from: Building Regulations, B3 Special Provision, Section 12, Naturally Ventilated 12.6.

    FIGURE 6

    SummaryAcknowledgementsIntroduction and background

    Modelling Parameters and TechniquesCase studiesThree Waters - MadridGunwharf Quays - Portsmount

    Heron City Lisbon

    Blue Water - KentConclusion