Introduction

If we can consider the Air Handling Unit as the heart of the pharmaceutical HVAC system, ductwork is the arteries and the veins. As everything with working in the pharmaceutical industry, we have to carefully design and construct.

The importance of well-constructed ductwork relies on the relatively high pressure that there is inside when terminal HEPA filters exist.

Admissible leakages

EN 12237, “Ventilation for buildings. Ductwork. Strength and leakage of circular sheet metal ducts”, and EN 1507 “Ventilation for buildings – Sheet metal air ducts with rectangular section – Requirements for strength and leakage” establish the ductwork tightness in 4 classes, from A to D (being A the less tight and D the tighter class).

ASHRAE 90.1-2010 defines tightness in classes CL3, 12, 24 and CL48. You can see the equivalence between norms in the graph below:

The ductwork system has to be tested by applying a static pressure and by measuring the leakage.
In the pharmaceutical industry, class B-C or CL6-CL3 are commonly accepted.

For other applications, like VHP ductwork decontamination system, or high containment requirements, class D shall be achieved.

Construction

There are many types of materials of construction. Galvanized steel sheet ductwork is the pharmaceutical industry standard. Avoid another type of material.

For high tightness applications, welded stainless steel is the most appropriate solution, but this is out of the scope of this post.

Ducts are two metal sheets joined. There are many different joints, but they are mostly the source of leakages problems, so we should carefully select the correct type.

1. Longitudinal seam

Ductwork manufacturing normally starts with the fold of two galvanized steel sheets. These two L folded sheets are joined. This is the called longitudinal joint and it is very important to ensure minimum air leakage.

The most convenient joint system for these two L pieces longitudinal seam is the Pittsburgh type:

2. Transverse Joints

The joined ductwork normally has a length that may vary up to 3-5 meters. The joint between them to build long way ductwork is by means of a transverse joint.

German manufacturer METU patented in 1968 the current industry standard with more or less variation. The system is explained here

https://metu.de/en/home-page/products/rectangular-duct-connections

A similar approach is done by Ductmate:

https://www.youtube.com/watch?time_continue=672&v=dCNRl3rm4sA&feature=emb_logo

In any case, slip and drive connections are not recommended.

When joining the transverse connection, the use of neoprene or PE foam gaskets is highly recommended. It is a quick and easy way to achieve an airtight seal at ductwork connections.