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Ductwork: Why ductwork designers opt for an all-round solution

Geoff Lee, joint managing director of CCL Lindab, explains why circular ductwork offers designers and installers a better deal than rectangular systems - and it is not just that it can be up to 50% cheaper.
Ductwork: Why ductwork designers opt for an all-round solution
DESIGNERS throughout the UK are increasingly opting for the considerable advantages that circular ductwork offers over rectangular systems.

Those advantages are not hard to see - they include greater energy efficiency, lower installation costs and shorter assembly times.

Leakage is the key issue. If there is one thing any kind of duct is required to be, it is airtight. Circular ducts are simply more airtight than the rectangular type. Making sure the ductwork is leak-free saves energy in terms of fan power and helps meet the increasingly stringent requirements for good indoor air quality.

Most modern ventilation systems struggle to achieve airtightness to class B so the energy saving potential is enormous. It has been calculated that if ducting systems across Europe could all be uprated to just class C the annual energy saving would be equivalent to the annual output of three nuclear power plants!

Even better performance can be achieved with Lindab's pre-sealed ductwork system - LindabSafe - which is three times more airtight than class C. Imagine the long-term energy savings that could be possible if this type of ducting were more widely specified.

Easy connection

One of the reasons circular ducts are more airtight is that sections are easier to connect. Joining two spiral wound ducts requires only a single fitting rather than the complete flanging system required for rectangular sections.

Circular ductwork isn't just easier to connect, it is easier to handle in every respect and that's one of the principal reasons behind its contribution to lower installation costs. There are others, however:

· Factory manufacture is highly automated and subject to strict quality control procedures.

· Less insulation material is required because of the shorter circumference of a circular duct as opposed to the rectangular type.
Ductwork is more accessible and easier to lag.

· The number of duct hangers required is less.

· Delivery time is shorter which makes the circular duct perfectly suited to the fast track building programme. Standardisation of sizes means that a comprehensive range of ducts and fittings can be kept in stock while rectangular and flat oval ductwork must always be tailor made for each individual project. Moreover, when it gets to site circular ductwork is a far more flexible friend. It can be cut to exact lengths and adjusted on site in a way that rectangular sections would never permit.

· Once installed circular ductwork is easier to clean.
With all this in mind it is hardly surprising that installation time for a circular ductwork system can be only a third of that required for a single rectangular equivalent.

Better airflow

In some cases cost benefits like these give the designer the option of using two or more circular duct runs, rather than a single rectangular duct - with all the inherent advantages in better airflow control, simplified air balancing and more flexible fire zoning.

All this is possible because circular ductwork usually requires less space than rectangular section units for a similar pressure drop.

Weight is also reduced which makes for easier on site handling and the number of operatives required can often be reduced. One individual is perfectly able to install circular ductwork systems up to 200mm diameter on his own.

Lightness, by the way, is not achieved at the expense of strength.

Circular ductwork is normally made from a 137mm steel strip, which is seamed and formed into a circular cross section. This method of manufacture is well proven in achieving high levels of rigidity with little need for additional stiffening.

Pressure drop is reduced. In a typical system comprising straight sections and bends and air distribution diffusers, pressure drop is often lower than for a similar rectangular system.

Circular ductwork is also less prone to noise breakout which can be a major problem in modern high velocity systems.

Tests have shown circular ductwork is better at reducing the level of penetration of low frequency noise through the duct wall.

The combination of all these advantages, in terms of easy handling, low noise and improved system performance, means that circular ductwork can often be installed for 50% less than an equivalent rectangular system.

With the current market drive for cost efficiency, it is hardly surprising that savings like these are causing so many specifiers to review the traditional approach.

Circular ductwork now accounts for nearly 50% of the UK total - and the proportion is growing fast.



Acoustic flexible ducting joins CCL Lindab range

CCL Lindab has introduced a range of acoustic flexible ducting which combines noise reduction capability with the proven advantages of the company’s Tecflex concept.

The latest Tecsonic range has been designed to meet the growing demand for lower noise levels in the working environment and the ultra quiet conditions required in theatres, libraries and law courts etc.

The acoustic flexible ducting will be available in two types, Tecsonic and Tecsonic S. Both are tough yet highly flexible and will permit bends of half D radius to be formed. They offer a safe and cost efficient method of distributing air from the main duct to VAV boxes, air terminal devices etc, while, at the same time, significantly reducing duct-borne noise.

Standard Tecsonic acoustic flexible ducting comprises an inner core constructed from multi-ply, aluminium polyester laminate, supported by an encapsulated, high tensile steel wire helix at 35mm pitch. This tough, flexible core features regular, closely-spaced micro-perforations which enhance the acoustic performance of the ducting, while virtually eliminating any possibility of fibre migration in the air stream.

The inner core construction is wrapped in a 25mm thick blanket of high density glass fibre, overlapped to ensure consistent acoustic insulation along the length of the ductwork.

For applications such as hospital operating theatres and clean rooms there is Tecsonic S. It is made in a same way as the standard Tecsonic ducting but with the addition of an acoustically transparent inter-liner between the inner core and the glass fibre layer. The extra barrier stops even the smallest fibres passing into the duct without significantly impairing the performance of the sound absorbing layer.

Both models are available in diameters from 102mm to 508mm and are suitable for air velocities up to 3.0m/s.

CCL Lindab T:01604 788350
1 February 2007

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