Below Ground Smoke Control Systems
Impulse v Induction
Architects, Consulting Engineers and Regulators alike can be forgiven for being confused by the options now being promoted for use in Impulse Ventilation Systems for the ventilation of basement and enclosed car parks.
Since the launch of impulse ventilation into the United kingdom in 2000 by PSB-UK Limited, this type of system has caught the imagination of the industry and now impulse ventilation is used in over 90% of car park that require mechanical ventilation.
During the last three years, PSB has developed many new products and system design techniques to improve safety, aid building design and reduce energy consumption. The new products include the Axial type of impulse fan and the Centrifugal type of induction fans. A number of companies are now marketing this type of product, but without a full understanding of their application, there are many pitfalls, with potentially significant consequences, awaiting the individual responsible for the design of the ventilation system.The industry is asking when should the Impulse fan be used and when should the induction fan be used and what are the relative benefits and disadvantages of each.
These two ranges of fans are basically designed to provide the same functions and these are:
- Influence the direction of airflow within the car park to ensure that all areas are efficiently ventilated
- Induce air turbulence during daily ventilation to ensure efficient mixing and dilution of pollutant gases with fresh air
- Manage the flow of hot smoky gases in the event of fire
So, what are the relevant benefits and disadvantages of these products?
Performance of a Impulse Fan
The performance of both the impulse and induction fans are measured in terms of thrust, usually expressed in Newtons. The level of thrust will determine the amount of air that the fan is capable of moving, not only the air passing through the fan, but the amount of air “induced” into the jetstream created at the fan outlet.
System Effects
The quantity of air movement created by the jetstream can be up to 8 times that passing through an impulse fan developing 50N of thrust. This level of air movement will depend on the useable thrust which will be affected by the resistance to airflow created by the obstructions within the car park, such as downstand beams, containment and ducting. The amount of resistance caused by the obstructions is taken into account in the design of the system and the selection of the number of fans.
One further consideration that must be taken in account is the number of impulse fans operating. Obviously, the greater the number of Impulse fans that are operating, the greater the volume of air that will be moved, therefore, care must be taken to ensure that the main extract fans will be of sufficient capacity to extract that volume of air otherwise the car park, in effect, will become pressurised. In such a situation for example, in a fire situation, smoke could be forced into escape and fire fighter access routes. This is particularly important when considering the use of the larger 100N Centrifugal type of fan.
A further consideration when using the larger of the centrifugal fan is the impact on sprinklers if they are used. Due to the width of the discharge nozzle of the 100N centrifugal fans, typically 1200mm, the high velocity discharge, in the order of 25m/s, will have an adverse affect on sprinkler patterns. In such cases, it is much better to use a larger number of smaller fans since, with the large nozzle size, spread of the high velocity air will impact on sprinklers even if the fan is located centrally between sprinkler heads; it must be remembered that the performance of a jet fan is based on the air induced into the jetstream from the surrounding area and the induced air will also impact on the sprinkler pattern. The new BS7346: Part 7: 2006 makes reference to this aspect of impulse ventilation systems.
Relative Benefits of the Two Types
Each of the two types of fan has its relative benefits which are summarised below. The axial type of fan is largely superior in terms of system performance and efficiency, whilst the centrifugal type of fan provides benefits in terms if space, although this is mainly restricted to the smaller of the two fans.
The Axial Impulse Fan
The axial fans tend to be more flexible and more efficient than the centrifugal type of jet fan. This is borne out by the following:
• Lower power requirement, 1.1KW as opposed to 1.4KW for the 50N fans and 2.2KW against 2.6KW for the 100N.
• Improved directional control for jetstream giving greater control of smoke in fire conditions
• Lower system losses with less jetstream contact with soffit
• Quieter in operation
Centrifugal Jet Fan
These fans tend to give greater benefits when height of car parks is restricted, particularly in the case of the smaller centrifugal fan.
• Saving in height of 180mm on the 50N fan and 120mm on the 100N.
• Compact assembly for the smaller 50N fan
The larger motor capacity necessary in the centrifugal fan is not great for individual fans, though of course this will be more significant for the overall system, but the loss of efficiency is caused by the compactness of the fans. Due to the restricted space, there is not a full scroll which normally forms part of the centrifugal fan construction and this creates much higher dynamic losses within the fan.
Further losses are created at the discharge due to the greater proportion of the discharge air being in contact with the soffit caused by the coanda effect, and this will also cause greater losses due to more significant contact with downstand beams and other obstructions.
Summary
Both types of fan have their relative benefits in terms of enabling flexibility of application. Correctly engineered, the systems will function reliably and efficiently. What is vital is that the system is designed and installed in an informed manner.
Now, a new standard has been published, BS 7346: Part 7: 2006 which provides in depth guidance on the design, installation and commissioning of car park ventilation systems. This is an extremely useful guide and covers the use of impulse ventilation systems.
