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Car Park Ventilation System Design - Fan Quantities

Car Park Ventilation System Design - Fan Quantities

Description

For the purpose of estimating costs, the steps on the following pages may be bypassed. Allow 5N of thrust per 100m2 of car park floor area to approximate the number of fans required.
Estimating fan quantities
The following steps are sufficient to create an initial impulse ventilation system design. A Computational Fluid Dynamics (CFD) analysis is often required to prove and further refine the design. Fans may need to be re-orientated, or in some cases, added or removed. An impulse ventilation system can be tailored to suit virtually any car park. Before considering fan locations, the system layout will need to be identified. Refer to the previous section for information relating to system layouts and their suitability for particular car parks.
Step 1 - Assessing Car Park Geometry
First identify the supply and exhaust points in the car park. A system that complements the natural air path and is able to circulate or move air effectively within the car park should be chosen. Certain layout features may assist the effectiveness of a particular layout as shown below:
(a) ‘Natural air path’
carPark Natural air path - prefered Figure 6(a). Preferred natural air path
carPark Natural air path - to be avoided Figure 6(b). Natural air path to be avoided
  • For ‘Linear Flow Systems’, supply and exhaust points should be spaced across the length of a car park.
  • ‘Circular Mixing Systems’ are more tolerant of closely placed supply and exhaust points, but it is advisable to have a good amount of separation.
  • Supply air points should include access ramps to outside.
  • The impulse ventilation system layout should complement the natural air path from supply to exhaust points.
(b) Ceiling features
To make the system more effective, position JetVent Fans in-line with supporting ceiling beams as illustrated in Figure 7(a). If this is not possible, the system becomes less effective and more fans may be needed.
JetVent ceiling beam parallel Figure 7(a). Fan’s air movement parallel to beams is most effective
JetVent ceiling beam perpendicular Figure 7(b). Fan’s air movement perpendicular to beams is less effective
(c) Vertical clearance
Sufficient vertical clearance ensures maximum flexibility in system design. JetVent Fans may be recessed between ceiling beams to minimise the height of the system.
JetVent vertical clearance Figure 8(a). Sufficient clearance
JetVent vertical clearance -insufficient Figure 8(b). Insufficient clearance
(d) Obstructions
If there is no option and the JetVent Fans must blow across ceiling beams, they have to be positioned a sufficient distance away from the obstruction as illustrated in Figure 9(b). A horizontal distance eight times (8x) the height of the obstruction is generally sufficient. Nozzles on the JetVent units are specially designed and angled downwards for this purpose.
Car Park - Obstruction too close Figure 9(a). Obstruction too close
Car Park - Obstruction too close Figure 9(b). Obstruction out of the way
(e) Clashes with other services
Place mechanical service components, such as sprinklers, signs and pipework out of the JetVent’s discharge pattern area. Examples of these clashes are shown below.
carPark Natural air path - prefered Figure 10(a). An example of how to avoid clashes with pipe-work
carPark Natural air path - to be avoided Figure 10(b). Jet fan layout in relation to sprinkler heads
Step 2 - Identify Fan Selection and Spacing
Table 5 shows the maximum and recommended spacings between JetVent Fans for different levels of fan thrust. These spacing distances are guidelines for fans placed in series. When using these spacings, air velocities in most of the ventilated areas should be greater than 1m/s. Analysis will determine whether this is achieved in a particular car park design. In some ideal cases, designs using the maximum distances have been effective.

Fan thrust depends on the operating speed of a particular fan unit and its thrust rating. See tables one to four for the thrust ratings of various fan models at different speeds.
Operating
fan thrust
Recommended fan to fan
spacing distance
Maximum fan to fan
spacing distance
Approximate
coverage area
50N 45m 60m 100m2
28N 34m 45m 560m2
25N 30m 40m 500m2
19N 23m 30m 380m2
12N 12m 20m 250m2
Table 5. Fan spacing and coverage
Note that using fewer higher rated JetVent fans generally makes the system more cost effective than using more lower rated fans. However, to effectively ventilate car parks with unusual or irregularly shaped geometries, selecting more fans with smaller thrust ratings may be necessary.
CarPark Two 25N fans Figure 11(a). Two 25N fans
CarPark Two 50N fans Figure 11(b). One 50N fan
Step 3 - Design Example
For the purpose of estimating costs, the steps below may be bypassed. Allow 5N of thrust per 100m2 of car park floor area to approximate the number of fans required. Fans should be placed in the laneways with the air blowing along them. This will ensure that the high air velocities close to the outlet nozzle do not significantly disturb pedestrian traffic as velocities will be lower at the lane edges. Also, ensure that the throw pattern of the selected fan is long enough to reach the next impulse fan.
Figure 12 shows an example of how fans are sized and placed based on floor area. [Download PDF]
Figure 12. Fan sizing and placement example
Design parameters;
  • Based on 5N per 100m2 floor area, minimum total fan thrust = car park floor area x 5N/100m2
    = 1920m2 x 5N/100m2
    = 96N
  • Minimum thrust criteria can be achieved with 4 x JISU-CPC-50N fans on low speed (operating thrust 25N), total fan thrust= 25N x 4= 100N.
  • Fans are spaced within the 30m spacing recommendation for 25N thrust.
  • Final fan to wall spacing under 40m maximum spacing guide lines for 25N fan thrust. This is because the exhaust point is an area of low pressure, making it likely to enhance the fan throw distance.
Placement of CO sensors:
Because the guidelines for positioning CO sensors in AS/NZS1668.2:2012 is based on a ducted system, we propose that the following guidelines be used as a starting point for their placement in a jet fan system.
  • No part of the enclosure shall be greater than 25 metres from a sampling point. (A 50 metre diameter circle around a CO sensor can show coverage areas).
  • Additional detectors shall be installed in areas where people may congregate within the car park and are not within separately ventilated areas.
  • The most practical mounting position for a CO sensor within a car park is the support columns.
  • CO sensors will be more effective if placed in areas where CO levels are likely to be high. Eg. Placing a CO sensor in front of a fresh air intake is not likely to be effective.
If jet fans are placed in each laneway of a car park and the recommended low speed jetfan spacings are followed, the above guidelines can be achieved by using one CO sensor per jet fan and providing additional CO sensors at congregation points. Therefore, for this car park we would suggest installing 5 (4 + 1) CO sensors for good coverage.
Step 4 - Calculate Supply & Exhaust Rates
Section 4 of AS/NZS1668.2:2012 details requirements for ventilating car parks. In particular, sup-sections 4.4.3 and 4.4.4.2 of the standard outlines calculations of exhaust rates. In any of the two cases below, the exhaust air flow rate for a car park is taken as the greatest of the following calculations:
Calculation of exhaust air flow rates
Car parks with more than 40 spaces Car parks with less than or equal to 40 spaces
a) 2000 x F x T Litres/s (minimum air quantity for one operating car) a) 2000 Litres/s
b) 0.85 x P x (100 x n1+ n1 x d1 + n2 x d2 ) x E x T b) 2.5 x A Litres/s
c) 2.5 x A Litres/s (minimum air quality based on area of car park) c) 400 n1 x P Litres/s
Supply air flows should range from 75% to 90% of the exhaust air quantity. This is based on the pressurisation of the car park, which should be 12Pa maximum.
Reference information for the calculation of exhaust air flow rates in the table above:
A = the area of the zone or level, in square metres
d1 = the average driving distance, in metres, within the zone or level under consideration for the exit of a car parked there (see Clause 4.4.4.1)
d2 = the average driving distance, in metres, within the zone or level under consideration for the exit of a car whose exit route passes through the zone or level under consideration, but excluding any part of the exit route designated as queuing areas and ventilated in accordance with Clause 4.6 (see Clause 4.4.4.1)
E = the staff exposure factor (E)
F = the staff usage factor (F)
n1 = the number of parking spaces in the zone of level under consideration (see Clause 4.3.2)
n2 = the number of parking spaces situated in other parts of the car park, having exit routes passing through the zone or level under consideration
P = the parking usage factor (P)
T = the vehicle type factor (T)

If the car park has significant queuing areas for vehicles, refer to section 4.6 in AS1668.2:2012
Appendix
AS/NZS1668.2 - 2012 Calculation Factors
Parking Usage Factor (P)
Use of car park Parking
usage
factor (P)
Residential 0.3
Commercial 0.5
Retail/food and drink services 0.7
Entertainment/sports centres 1.0
Vehicle Depots 2.4
Vehicle Type Factor (T)
Use of car park Parking Vehicle type
factor (T)
No special vehicle
population
1.0
Diesel vehicles 2.4
LPG vehicles 1.0
CNG vehicles 1.0
Electric powered vehicles 0.1
Motorcycles 0.25
Staff Usage/Exposure Factor (E & F)
Parking procedure Staff exposure factor (E) Staff usage factor (F)
No special procedures (self-parking), any staff in separate enclosure ventilated in accordance with Clause 4.2.2 1 1
Self-parking stack parking, any staff in separate enclosure ventilated in accordance with Clause 4.2.2 1 1 + 0.1 x No. of car spaces without immediate access to driveway
No special procedures (self-parking), staff located in car parking enclosure 1.8 2
Self parking stack parking, staff located in car parking enclosure 1.8 2 + 0.25 x No. of car spaces without immediate access to driveway
Attendant parking no stack parking 1.8 2.5 x No. of attendants
Attendant parking stack parking 1.8 3.5 x No. of attendants
Mechanical stack parking 1.8 2 x No. of car engines
operating at any one time
References
  • Australian Building Codes Board 2013, Building Code of Australia Class 2 to Class 9 Buildings Volume One, ABCB, Canberra.
  • British Standards Institution 1999, Fans for general purposes. Methods of noise testing (BS 848-2 – 1985), British Standards, London.
  • British Standards Institution 1999, Fans for general purposes. Performance testing of jet fans (BS 848-10 – 1999), British Standards, London.
  • Federation of Environmental Trade Associations (FETA) 2007, CFD Modeling for Car Park Ventilation Systems, Federation of Environmental Trade Associations, Berkshire.
  • Standards Association of Australia 1999, Methods of test and rating requirements for smoke-spill fans-(AS 4429 – 1999), Standards Australia, Sydney.
  • Standards Association of Australia 1998, The use of ventilation and air-conditioning in buildings Part 1: Fire and smoke control in multi-compartment buildings (AS 1668.1 – 1998), Standards Australia, Sydney.
  • Standards Association of Australia 2012, The use of ventilation and air-conditioning in buildings Part 2: Ventilation design for indoor air contaminant control (AS 1668.2 – 2012), Standards Australia, Sydney.


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