The ducts used in heating and ventilation systems can differ in a variety of ways. For example, they can be metal, plastic or composite and square, oval or circular in cross-section. There are pros and cons for each, and many systems use a combination in different locations. However, one of the main ways in which they are classified is according to their flexibility.
The chief advantage of flexible ducting is self evident. It is easy to connect from A to B, routing it around obstructions with minimal constraints. The paper coils attached to vented driers is a familiar example, but for HVAC systems, more durable alternatives include PVC tubing and insulated composites of aluminium.
Flexible ducts and spiral duct should be pulled straight to maximise their internal diameter and the smoothness of the sidewalls; otherwise, there can be significant resistance to the airflow. As with any kind of duct, the same goes for turns. Although it can flex, it should still be kept as straight and direct as possible.
Flexible PVC is one of the cheapest types available and a good choice for things like kitchen extractors. When metal ducts and ductwork parts are located in unheated spaces like lofts, they need about 25mm of insulation to eliminate condensation.
Because of poor energy efficiency, hygiene and fire hazards, the length of flexible duct you are allowed to use may be limited by building codes. This is usually between 1.5 and 6 metres depending on the power of the airflow.
Some rigid ducts are made of plastics, but metal ones (like these https://www.dustspares.co.uk/ductwork-parts/galvanised-ducting/galvanised-steel-spiral-duct.html are preferable for their strength and longevity. Expanded polypropylene (EPP) is also common. Rigid ducts offer the smoothest airflow and least dust accumulation. They are the easiest to cut accurately and can be coated with antibacterial surfaces and sheathed in insulation.
Rigid tubular ducting is the ideal standard material to use in most HVAC and MVHR installations.
In many scenarios, semi-rigid ductwork parts are an ideal compromise between the convenience of a flexible material and the strength of rigid ones. Their strength reduces the bracketing required on longer runs, while the elasticity keeps connections simpler, especially in awkward spaces.
The materials used are resilient to impact, and like rigid ducts, some varieties are given antibacterial and anti-static surfaces.