Impulse ventilation for carparks

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VENTILATION of covered car parks is essential for removing harmful vehicle exhaust fumes and providing assistance to fire fighters by clearing smoke in the event of a fire.

Ventilation of car parks using impulse fans has been in use in Europe for more than 10 years and is becoming increasingly popular in the UK. This innovative approach provides major benefits in terms of reduced installation and running costs as well as smoke control.

Principle
The system of impulse ventilation is based on a number of small high velocity fans in place of the distribution ductwork. Impulse fans operate on well proven tunnel ventilation principles, producing a high velocity jet which thrusts against the air in front of the fan imparting momentum to all the surrounding air through entrainment as it diffuses. The volume of entrained air is significantly greater than the air quantity passing through the fan. The impulse fans are carefully positioned to direct the airflow towards the main extract fan intake points. The main extract fans are sized to provide the required flow rates, but given the reduced need (or complete elimination) of intake-scavenge ducting within the car park area, do not have to overcome the system resistance inherent in a fully ducted ventilation system.

Impulse fan performance is rated in terms of the thrust developed by the fan, which is the product of the mass flow rate times the change in velocity, ie. volume flow rate times the air density times the fan outlet velocity, and is measured in Newtons (N).

Figure 1. Impulse fans running showing air entrainment.

Requirements in the UK Standard BS7346-7
In the UK, much work is currently taking place on development of a Standard, BS7346-7: Code of Practice, for covered parking areas for cars.

The draft standard is primarily concerned with the removal of smoke and heat, but also deals with vehicle emissions due to the dual use of the ventilation systems, and specifies that ventilation to the car park may be provided in any one of three ways:

• Natural ventilation.
• Ducted mechanical ventilation.
• Impulse ventilation.

Figure 2. Illustration of a traditional ducted system versus an impulse system.

Application
The major differences between traditional mechanical and impulse ventilation systems are:

• The distribution ducting used in traditional systems is replaced by a number of small impulse fans to direct the airflow across the car park.
• Without the distribution duct resistance, smaller exhaust and supply fans and/or motors can be used.
• For impulse systems with smoke control design objectives, the airflow rates are calculated based on a design fire size rather than the prescribed number of air changes per hour.

Case study – Delft, Holland
Comparison based on a 5000m² car park designed for smoke clearance to achieve 10 air changes per hour.

• Ducted system - Installation costs including ductwork, extract fans, fitting, controls and cabling: 145K Euro (A$259K)
• Impulse system - Installation costs including extract fans, impulse fans, fitting, controls and cabling: 71K Euro (A$127K)
• Ducted system - Extract system resistance: 450 Pa ? Impulse system - Extract system resistance: 200 Pa
• Ducted system – Installed power: 60 kW (2 x 30 kW extract fans)
• Impulse system – Installed power: 36.6 kW (2 x 15 kW extract fans + 12 x 0.55 kW impulse fans)

This example shows the impulse system will cost 51% less to install and consume some 39% less power than that of the ducted system.

Suggested benefits of using Impulse Systems

• Elimination of bulky distribution ductwork within the car park.
• With less ductwork the lower system resistances can result in reduced overall power consumption being achievable.
• When used in conjunction with CO sensors, further energy savings can be obtained by selectively operating fans in polluted areas.
• Improved air quality can be achieved by more effectively mixing the air.
• Greater flexibility in installation and operation avoids the problem of stagnant areas.
• Effective smoke control by limiting the spread of smoke and directing the smoke flow.
• Less ductwork means a safer, lighter environment with better security due to increased visibility.
• Scope for reduced installation and overall construction build costs over traditional ducted systems.

Other considerations for using Impulse Systems

• Each impulse fan requires individual wiring.
• There may be restricted height in the relatively small footprint area below the impulse fans. The fans are normally attached directly to the ceiling and carefully positioned between the down stand beams to provide maximum clearance.
• Noise can be a disadvantage for single (high) speed fans, however, impulse fans are normally operated at reduced speed for general ventilation requirements.
• Design for these systems requires investment in, or access to, CFD (Computational Fluid Dynamics) modelling software and capabilities.

Fan types used for Impulse Ventilation

Axial Jet Fans - are available in various options including unidirectional, truly reversible, single speed or multi-speed.

Centrifugal Jet Fans - backward curved centrifugal and mixed flow fans that provide a high velocity laminar airflow in a reduced height profile to help overcome problems caused by obstructive structural beams and low overall ceiling heights.

Testing
In the UK, impulse fans must be tested in accordance with BS848-10: Performance testing of jet fans and BS EN 12101-3 : Specification for powered smoke and heat exhaust ventilators (Class F300) to verify suitability for operating at 300 °C for a period not less than 60 minutes.

Controls
In simplest form, the impulse and extract fans run at low speeds to provide general ventilation and at full speed to provide smoke clearance. This is a cheap and simple option to install, commission and maintain. The down side is that the fans must operate continuously, regardless of the traffic levels, resulting in wasted energy.

A more sophisticated control system would involve a CO monitoring system (for ventilation mode) and incorporate a fully addressable fire detection system able to indicate the location of the fire on the main control panel. In general ventilation mode, the CO monitoring system will vary the ventilation rate of individual fans to ensure that the CO concentration is maintained within safe levels. This automatic-variable control will result in reduced energy costs. In the event of a fire, the ventilation system would switch to smoke extract mode and operate the impulse and extract fans based on the location of the fire source and the smoke control system design parameters.

CFD Modelling
CFD (Computational Fluid Dynamics) is now being increasingly used in the design of car parks for modelling the carbon monoxide distribution and concentration as well as the thermal gradients and smoke densities in the event of a fire. Commercially available design packages facilitate detailed analysis of airflow in car parks, taking into account the often complex geometry of awkwardly shaped individual buildings as well as variations in vehicle movements and local weather conditions.

Conclusion
The use of Impulse Ventilation for Car Parks has a proven track record in Europe for providing benefits in terms of installation and running costs as well as providing protection to life and property in the event of a fire. The inclusion of impulse fans in the new Draft British Standard BS7346-7 signifies that Impulse Ventilation is becoming the norm for Car Parks in the UK and other countries.

This article is taken from a technical paper delivered by A. Macklin (Elta Group Ltd. UK) during a visit to Australia and NZ in February 2006. Copies of the paper and further information are available from Fantech and its distributors.

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