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.

Impulse ventilation has been used in Europe for more than 10 years and while being effective it offers substantial savings in power consumption and installation costs.

In an actual study of a 5,000m2 car park (the Deft Holland Case Study) the installed power for a conventional system was 60kW compared to 36.6kW for an impulse system having a major saving in energy of 23.4kW. If it is assumed that the fans operate for 2 hours/day 300 days/year the impulse system would save 14,040 kWh’s per year this equates to a saving of 14 tonnes of greenhouse gas per year or approximately 4 tonnes of carbon per year.

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 encourage air movement throughout the car park and towards the 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, (i.e. volume flow rate times the air density times the fan outlet velocity) and is measured in Newtons (N).

Requirements in the UK Standard BS7346-7

In the UK a new Standard has been developed, BS7346-7such as Code of practice on functional recommendations and calculation methods for smoke and heat control systems 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.

They are:

Natural ventilation
Ducted mechanical ventilation
Impulse ventilation

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.

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 modelling software and capabilities

Fan types used for Impulse Ventilation:

Axial Jet Fans: They are are available in various options including uni-directional, truly reversible, single speed or multi-speed.

Centrifugal Mixed Flow Jet Fans: They are 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 when used for smoke control to 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 its simple 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 in the car park is maintained within safe levels. This automatic-variable control will result in reduced energy costs. In the event of a fire, if used as part of the smoke control-clearance system 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

Computational Fluid Dynamics (CFD) 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.

ConclusionThe use of impulse ventilation for car parks has a 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 Feb 2006. Further information is available from Fantech and its distributors.

Architecture & Design Team

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