With the launch of the updated version of CIBSE Guide C, Building Services Journal discusses the sources for section 4 covering flow of fluids in pipes and ducts.
In answer to a simple question as to what could possibly be new in Guide C, the simple answer would have to be:
  • the pre-calculated tables of pressure loss through pipework have been extended from 48 pages to 68 pages by the addition of several more recent thermo-plastic pipes
  • one page of data on pipework fittings has been extended to six pages
  • two pages of data on ductwork fittings has been extended to 16 pages
  • new data is included for pressure loss factors for laminar flow in elbows of pipework

In extending the data considerable care has been taken not to over-complicate the Guide. We are fully aware that for his day-to-day calculations, the engineer requires a simple-to-read table. In this, the previous Guide, with just two pages for ductwork fittings, was admirably simple, but alas over-simple. For any fan selection or pipe/duct sizing on a life-cycle cost-effective basis, it is essential that calculated predictions of pressure loss should be reasonably accurate. For this, more comprehensive data is essential.

A fresh start was made. One policy decision was to include data on all standard fittings as recommended by HVCA, even in cases where no official data exists. In these cases provisional advice is given so as to spare the practising designer the hours of fruitless searching and indeed soul-searching expended by the author.

Sources
It is over 30 years since any revision of this section of the Guide was carried out. Since then one would hope that fresh research results would be available. Some were found which were useful; some were found which were not useful; and some were known about but not found. This latter case is almost scandalous: in the early 1990s a considerable research project was carried out in Frankfurt with European funding, but the report still languishes inaccessible, somewhere in 'European corridors'. Thus we go to print without access to this useful report.

A survey of foreign guides revealed much contradictory data, generally un-referenced and therefore impossible to verify.

The ASHRAE guide is more professional in this regard, but has large gaps. For instance although there is data for converging flow in tees of ductwork, there is almost no data for diverging flow: how then to design an extract system?

Several foreign Guides, including that of ASHRAE, frequently cite Idelchik as the primary reference. Idelchik, a Belorussian, has spent a life-time collating pressure drop data, and though he does appear to have ignored some of the American data used by ASHRAE, nevertheless his book provides a wealth of data, data however, which is far too comprehensive for everyday use. Nevertheless it is a useful source which was drawn on considerably.

How nice it would be, if a solitary value of pressure loss coefficient z , could be quoted for something as simple as an elbow of a particular shape. No. The friction loss of fluid around an elbow or bend, is found to depend very much on relative surface roughness.

Thus for a particular material, it depends upon diameter, or size. Since modern British and other European metric pipe sizes differ from one another, and from the nominal pipe sizes used in earlier research, accuracy of quoted values cannot be great. Furthermore the slightest rounding of the internal corner can make a large difference. As if that were not enough, the type of upstream joint has an effect too.

No data are available which take into account all the variables of material, size, upstream joint and internal form. Fortunately the variation with diameter becomes slight for diameters greater than about 75 mm. Some selection of available data had nevertheless to be made, even though there appear to be some contradictions yet to be resolved.

Fortunately the matter is simpler for tees. Here the friction pressure drop for flows both to and from the branch, and along the straight, is due to the internal fluid friction of mixing and changing direction; pipe surface effects play an insignificant part. So there is no variation with pipe size or material. However there is one piece of evidence that the type of joint still plays a part.

Contrary to all other guides, ASHRAE gives data for tees of pipework which does depend on size, data which has been in circulation since the early 1930s. Reference to the prime source revealed an interesting explanation. The original researcher only tested a tee of one inch. nominal diameter. But rather than quote his results in terms of friction loss factor z, he quoted them in terms of "elbow equivalents", and of course with elbows there is a size effect. This was just one instance where data from what appears at first sight to be a reliable source, had to be rejected.

What will be a revelation to users, is that for both pipework and ductwork, the values of the loss coefficient z, vary enormously with the proportion of the combined flow which enters or leaves the branch (the relative branch flow) and also varies enormously with the ratio of the branch diameter to the main pipe diameter (the relative branch diameter).

New too will be the revelation that it is sometimes possible to have negative values of the pressure loss coefficient for the branch flow. See Tables 1 and 2. This implies a pressure gain. This phenomenon only occurs for combining flow, for the branch loss factor, and for low values of relative branch flow. This apparent anomaly is explained by the branch flow being accelerated by mixing with the straight flow such that its total pressure increases.

With ductwork components we might hope to have a simpler life. Alas only as recently as 1990 does anyone appear to have tested for size effects when the French tested segmented bends of 250 mm and 400 mm diameter.

The larger diameter always gave appreciably smaller values of z , for 60° , 45° and 30° bends but not for 90° bends. (This anomaly has gone unanswered by the authors of the research paper.) If size has an effect for segmented bends, might it not have an effect for smooth bends, mitred bends and rectangular bends too, we ask ourselves.

It may seem obvious to say that a gentle bend will give less pressure drop than a sharper elbow, but there is corroborative evidence that an optimum relative radius of curvature (r/d) of around 2·5 gives a minimum pressure drop (see Figure 1).

The author has detailed many more of the other anomalous data and contentious judgements and decisions made in the selection of data for this new section of the Guide C in references 1 and 2.

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