fans in fire safety

FANS IN FIRE SAFETY

FUME AND SMOKE CONTROL IN ENCLOSED CAR PARKS

(DUCTING OR JETFANS?)

By J. A. Wild – C.ENG; F.I.MECH.E

In Association with …

CONTENTS SUMMARY INTRODUCTION ENCLOSED CAR PARKS 1.0 . Ventilation Requirements 1.1. Vehicle Fume Extract 1.2. Fire Smoke Extracts 1.3. Jetfan Systems SMOKE CONTROL 2.0. Background 2.1. Controlling Air Velocities

FANS 3.0. Fan Specification CONCLUSION

SUMMARY The Flakt Woods Group have ventilated road tunnels using impulse ventilation with Jetfans for over 30 years, and there systems are operating in many countries. Flakt Woods are truly world leaders in this field. Recently they have begun to apply their vast experience with Jetfans, to solving the problems of ventilating enclosed and underground car parks. In partnership with the smoke control specialists, Fire Engineering Associates Ltd of Trafford Park, Manchester, they now offer a complete package, from the design to commissioning of car park ventilation. Through the package they can now offer, the strength of Flakt Woods Group with the flexibility of Fire Engineering Associates Ltd. Jetfan at Fremlin Walk, Maidstone, Kent

INTRODUCTION Over the last 3 to 4 years, impulse ventilation using Jetfans, has been replacing the more traditional method of ducted extract, in providing for the ventilation of enclosed and underground car parks. The ventilation of a large enclosed space, within a building is usually achieved by supplying the treated air at a high level, into the space via a system of distribution ductwork, terminating at a number of small grilles or diffusers. The airflow out from these diffusers induces air movement within the space many times that discharging from the grill itself. This ensures an even distribution, throughout the space, by mixing the ‘new’ air with the ‘old’. Because the incoming air is being controlled, the position of the air extraction point is relatively unimportant. Car park ventilation systems over the past 20 years have used a system of extract ducting, connected to exhaust fans sized to provide usually, an air change rate within the car park. We loose any control over the incoming air, which is allowed to enter freely, in the main, down the vehicle entry and exit ramps. Impulsive ventilation uses a number of small Jetfans suspended from the car park ceiling, replacing the ductwork (Fig 1). The system retains the main exhaust fans to provide the air change rate etc, whilst the Jetfans create and control the air movement within the space. They act in much the same way as the diffusers in a normal ventilation system, allowing us to regain some control over the incoming air. The only proviso being that there are sufficient Jetfans in the design to achieve this. Figure 1. Jetfan in Car Park

ENCLOSED CAR PARK 1.0 - VENTILATION REQUIREMENTS The ventilation system for an enclosed car park has two functions:

To remove vehicle exhaust fumes, mainly carbon monoxide, during normal car park usage.

To remove the smoke in the event of a fire. The design specifications covering these two functions are set out in detail in Approved Documents B and F of the Building Regulations of England and Wales, and similar recommendations are contained in the technical documents for Scotland. These are:

6 air changes per hour, or a ventilation rate to maintain the CO level less than 50 parts/million for the fume control function.

10 air changes per hour, or a ventilation rate based on the requirements in paper no. BRE 368 (Ref. 1) for the fire smoke removal.

The total ventilation duty to be divided between two fans These fans to be capable of surviving 300o for 1 hour The exhaust ductwork system to have extract grilles – 50% at high

level and 50% at low level. Consideration is to be given to the supply of replacement air.

Number One Deansgate, Manchester Jetfan at

1.1 VEHICLE FUME EXTRACT To discuss the differences in performance between a ducted system and a Jetfan system in this mode of operation, we can look at a typical example. A car park, 4000m2 in area and 3m high, is ventilated by a ducted extract system designed to the requirements of Approved Documents B and F. The ventilation duty of 20m3/sec- (6 air changes/hour), is divided between two fans, with 100 extract grilles, (50 at high level and 50 at low level), placed evenly throughout the car park. Replacement air intake is via the vehicle ramps at the top left hand corner of the car park. The extract ductwork is then replaced by Jetfans with the two main extract fans located along the right hand sidewall. Both these systems were modelled on a Computational Fluid Dynamic (CFD) programme, and their respective performances predicted and compared. The results are illustrated on the CFD charts below. FIGURE 2. CFD PERFORMANCE COMPARISONS

Inlet (Via ramp)

Air speed Air quality

Extractgrilles


Jetfans

Inlet (Via ramp)Extract fans

Jetfans

Ducted system

Jetfan system

Inlet (Via ramp)


Extractgrilles


Jetfans


Jetfans

Ducted system

Jetfan system


Extractgrilles


Jetfans


Jetfans


Jetfans


Jetfans

Ducted system

Jetfan system

FIGURE 2B

FIGURE 2A

DUCTED SYSTEM - FIGURE 2A VELOCITY PROFILES – High velocity areas (over 0.5/sec), in the car park are shown in RED. Low velocity areas (zero) in BLUE. The ducted extract system chart shows a high velocity eddy current around the outside walls created by the replacement air entering the car park down the vehicle ramp in an uncontrolled manner. This results in a relatively stagnant, zero velocity, and area at the centre of the car park. AIR QUALITY PROFILES – Here the areas of high air change rate (over six) are in BLUE. The areas of low air change rate (zero to three) are in RED. The air quality result for the ducted systems mirrors that of the velocity profile. It predicts that the air at the centre of the car park will be more than 20 minutes old, suggesting an air change rate of less than 3 per hour at best. This in spite of the 100 grilles evenly spaced throughout the car park.

JETFAN SYSTEM – FIGURE 2B Using Jetfans, the improvement in the distribution of the airflow throughout the car park is obvious from the previous diagrams. The air velocity down the car park, from the air inlet to the extract points is much more even, and the air quality diagram now predicts 6 air changes per hour throughout. By providing better control over the incoming air, the Jetfan system results in an improved overall performance of the ventilation system when operating in Vehicle Fume Extract mode, compared with the ducted system. 1.2 – FIRE SMOKE EXTRACT DUCTED EXTRACT SYSTEM (APPROVED DOCUMENT B) – Hot smoke, from a car on fire will rise under buoyancy, to the ceiling of the car park, flow out along the ceiling in all directions forming a stable layer under the ceiling. Without ventilation, this layer will deepen until it fills and smoke logs the car park. With the ducted extract system, having grilles at 50% high level and 50% low level, only the high level grilles will be effective in exhausting the smoke. The system can only be 50% efficient – even though the ventilation rate has been increased to 10 air changes per hour. By the time the lower grilles become effective the car park will be smoke logged. (See Fig. 3). FIGURE 3 – FIRE SMOKE CONTROL - DUCTED METHOD

In addition, the amount of smoke produced by a fire is a function of the size of the fire and not the size of the car park.

The mass of smoke produced by a car on fire is calculated using the simplified expression – M= 0.19PY3/2Kg/sec. Where:

M = mass of smoke produced Kg/sec. P = perimeter of vehicle metres. Y = height of the smoke layer above ground.

Having calculated the mass flow, the volume extraction rate can be determined. A ventilation system designed to provide 10-air changes/hour will only provide the correct level of smoke extraction at a particular car park size. Table 1 illustrates these points: TABLE 1 – SMOKE EXTRACT RATES

CAR PARK SIZE M2

5 AC/HOUR M3/SEC

10 AC/HOUR M3/SEC

SMOKE PRODUCTION

M3/SEC 1000 4.17 8.34 12.6 2000 8.34 16.68 12.6 3000 12.5 25 12.6 4000 16.68 33.36 12.6 8000 33.36 66.72 12.6

Car park height 3.0m Smoke layer height 2.25 Fire parameter 12.0 Fire size 3.0mw Radiation losses 25%

Only in car parks above 3000m2 will there be sufficient ventilation to remove the smoke produced. Car parks below this size would smoke log. Above 3000m2 the system would increasingly be oversized and expensive. Approved document B allows the option of calculating the ventilation rate for smoke extraction from the fire size which, helps to deal with this potential problem.

1.3 – JETFAN SYSTEMS Replacing the extract ductwork with Jetfans has an immediate advantage of making the extract system 100% efficient. With the extract points at high level, all car parks down to about 1500m2 in area will be provided with sufficient ventilation to prevent smoke logging (see Fig 4). FIGURE 4 – FIRE SMOKE CONTROL – JETFAN METHOD

Smoke extract, using the ducted system can only be regarded as a smoke clearance system. The correct design and operation of an impulse system ensures that the smoke is allowed to move efficiently towards the extract location, providing smoke free routes for escape and fire fighter access. So again the Jetfan system provides a superior result.

SMOKE CONTROL 2.0 – BACKGROUND By controlling the incoming replacement air, Jetfan systems provide more effective ventilation of an enclosed car park than the ducted extract system, in both fume control and smoke clearance modes. This system can be taken a stage further, and the Jetfans can be used to provide a degree of smoke control in the car park, preventing the smoke from a vehicle on fire from spreading to unaffected areas of the car park. Klote’s first principle of smoke control states; “An air velocity of sufficient magnitude can control smoke movement”. (Ref 2) This principle is employed to control smoke movement in road tunnels. It also forms the basis for the velocity criteria in pressurisation systems used in high-rise and multicompartmentalised buildings. In road tunnels, the Jetfans are used to establish an air velocity, equal to the smoke velocity, from one direction along the tunnel (see Fig. 5). This prevents the smoke flow in that direction. FIGURE 5 – SMOKE CONTROL IN ROAD TUNNELS

Heselden first calculated the velocity of smoke flow from vehicle fires in tunnels (Ref. 3). A 3 Megawatt car fire in a tunnel 10m wide by 5m high would produce a smoke velocity of 1.3m/sec.

Smoke from a similar fire in a car park, will flow away from the fire unrestricted in all directions. As the smoke layer perimeter increases, it’s velocity will slow until it either stops for lack of buoyancy, or it finds the outer walls of the car park. Table 2 details the results of applying the tunnel calculations to car fires in enclosed car parks. TABLE 2 – SMOKE VELOCITIES FROM CAR FIRES

DISTANCE FROM FIRE (METRES)

SMOKE VELOCITY (METRES/SEC)

3 1.52 16 0.89 32 0.7 80 0.5

2.1 – CONTROLLING AIR VELOCITIES The required ventilation rate for fire smoke control in a 4000m2 car park, at 10-air changes/hour, would be 34m3/sec. With all the Jetfans running, this air would flow, more or less evenly through the car park, from inlet towards the main extract fans, at high level in the car park. (Fig 6), illustrates the pattern of air movement around a jet profile (Ref. 4). Air entraining into the jet from the lower ‘occupied zone’ (up to 1.8 metres high), is shown to be moving in the opposite direction to the jet itself. FIGURE 6 – AIR MOVEMENTS AROUND JETS

In a 3m high car park, the resulting air velocity for smoke control would flow in the upper 1.2m of the car park elevation. This is at the same height as the advancing smoke layer. The actual air velocity for smoke control now depends on the width of the car park, or smoke zone across the direction of airflow. Table 3 details the resulting controlling air velocities at varying smoke zone widths. TABLE 3 – CONTROLLING AIR VELOCITY AT VARYING SMOKE ZONE WIDTHS

SMOKE ZONE WIDTHS (METRES)

CONTROLLING AIR VELOCITY (METRES/SEC)

10 2.8 20 1.4 30 0.95 50 0.57

Comparing these air velocities in Table 3 with the smoke layer velocities in Table 2 shows that if the required ventilation rate of 34m2 /sec is concentrated along a 20m smoke zone, then there is the potential to stop the smoke layer approximately 5 metres from the fire. This would ensure that the major flow of the smoke would be towards the extract points, and not into parts of the car park unaffected by the fire. Selective running of the Jetfans, illustrated schematically (Fig 7) can achieve this and thereby provide a degree of smoke control in the car park. This option is not available with a traditional extract ducted systems. FIGURE 7 – SMOKE CONTROL

Extract fans (10ac/h) 34m3/s

Inlet air, Via

50m

Jetfan Off

OnJetfan

Smoke Control Zone

Wm-20m FIRE

80m

Work in this area is ongoing, but tests already carried out suggest that by using Jetfans in this way a degree of smoke control can be achieved, providing that there are sufficient Jetfans incorporated in the design to allow for selective running. This is the same requirement as that necessary to prove even distribution during the normal fume extract mode. FANS 3.0 – FAN SPECIFICATION Approved Document B specifies that fans used to ventilate enclosed car parks should be capable of surviving 300o for 1 hour. This is a wise precaution and one, which need not cause financial concern. Today, fans designed and produced to meet this requirement are a little more expensive than those, which operate up to 50o. However, from the 1st April 2005, all high temperature smoke control fans must comply with the European Product Directive. From that date it has been mandatory throughout the EU that such fans are tested to EN12101 – Part 3 are CE Marked and issued with a certificate of conformity. Under EN12101-3 the category 300c/1 hour does not exist, hence the fans supplied should be rated 300c/2 hours in accordance with EN12101-3. Any smoke venting scheme, car park or other, not using CE marked fans should be rejected. Verbal assurances are insufficient, and a certificate of conformity must be produced.

CONCLUSION There is little doubt that impulse ventilation using Jetfans offers a better solution to the dual problems of ventilating enclosed and underground car parks than provided by the more conventional ducted extract system. Several advantages arise:

Improved air and velocity distribution for the vehicle fume extract mode

Increased efficiency in the smoke clearance mode. Possibility of improved smoke control to keep whole areas of the car

park away from the fire, smoke free. Distribution ductwork removed from the car park area – eliminating

damage and providing a clearer internal appearance. Possible freeing up of more car park spaces. Quieter/lower powered extract fans. Easier/lower cost installation. Possible reduction in capital and running costs.

REFERENCES

1. H.P. Morgan and others – Design Methodologies for Smoke and Heat

Exhaust Ventilation – BRE 368. 2. J. H. Klote – An Overview of Smoke Control Technology – National

Bureau of Standards. USA Paper NBSIR87-362C 3. A. J. M. Heselden – Studies of Fire and Smoke Behaviour Relevant to

Tunnels. – Paper No. CP66/78 4. B. B. Daley – Woods Practical Guide to Fan Engineering – 3rd Edition

78.

David Fenlon – Director Tel – 0161 8727760 Fax – 0161 8727740

Email – [email protected]

Bryn Maddick – Director Tel – 0161 873 8482 Fax – 0161 8728483

Email – [email protected]

REF: FEA 001 COPYRIGHT: The copyright of this technical document is vested in Fire Engineering Associates Ltd. It shall not be replicated without the permission by anyone for any purpose.