Plastic and GRP ducts are used to transport air and gases from one process to another. Despite their rudimentary nature (ultimately, ducting is nothing more than a series of ducts in which airflows one way or another) the design of ducting is professional work. A lot of engineering time goes into the design of ductwork.
Unfortunately, there’s also a lot of misinformation about plastic and GRP ducting which makes choosing the right material confusing.
Are these materials substitutable? Do they offer similar performance? When would you use one or the other? What fabrications are available?
In this guide, we’ll discuss the materials, fabrications and use cases of these different types of ducting to help you choose the right material.
Ducting materials
The correct material for ducting depends on the following factors:
- Local environment
- Safety requirements
- Types of gases
- Temperature of the gases
- Positive/negative pressure
- High, medium and low pressure
There are 5 main materials used for ducting:
Polypropylene
Features:
- Extremely durable
- Long service life
- Excellent chemical resistance
- Low mechanical wear
- High heat deflection temperature
Polypropylene (PP) is a plastic and a polymer of propylene. It is tough, rigid, and inert to most common chemicals, making it suitable for many industrial applications.
Polypropylene ducting is mostly used for the ventilation of corrosive fumes, including exhaust gases and fumes from industrial process tanks. PP can withstand chemical gases and solvents with no wear and little maintenance.
UPVC
Features:
- Cheap
- Low weight
- General-purpose
- High chemical resistance
UPVC (Unplasticized Polyvinyl Chloride) is a plastic best-known as a construction material. It is rigid and tough with no plasticisers, making it extremely rigid.
UPVC ducting is chemically resistant to corrosive fumes, gases, and fluids. It is mostly used as gas and airflow ducting for low-pressure solid wall pipes. It is considered a general-purpose ducting, which is in keeping with its use in the building trade.
GRP
Features:
- High mechanical strength
- Low weight
- Exceptional chemical resistance
- Excellent resistance to ageing and atmospheric environments
- Dimensional stability
- Very high thermal insulation
- Excellent resilience to temperature
- Low thermal expansion coefficient
GRP (Glass-Reinforced Plastic) is the highest-strength ducting, suitable for applications that require some extra strength and protection.
GRP is the material of choice for extreme industrial processes, gas transportation, chemical transportation and water transportation. GRP has a higher mechanical strength than plastics with high levels of chemical inertness.
GRP-reinforced PVC
Features:
- Strong
- Low weight
- Resilient
- Dimensional stability
- High thermal insulation
- Resilience to temperature
GRP-reinforced PVC ducting is PVC ducting reinforced with glass fibre. The glass fibre makes this ducting a hybrid material. It sits between PVC and GRP in strength.
This ducting is made by combining the UPVC plastic with resins and glass fibres. It is mostly used as medium-duty ducting for medium pressure applications, like fan connections, risers and main distribution.
Plastic-lined GRP ducting
Features:
- Strong
- Lightweight
- High chemical resistance
Plastic-lined GRP ducting is twin-walled ducting composed of a solid GRP outer pipe with an internal pipe made from PP.
The GRP protects the inner pipe, serving as dual containment. This ducting is widely used to transport gases from one process to another. It is a viable alternative to rubber-lined steel ducting which has a shorter lifespan.
All of these ducting materials are a good choice for a variety of industrial processes, including gas transportation and ventilation. They can be supplied pressure tested to BS, EN and ASTM standards to meet your design specifications.
Ducting fabrications
Plastic and GRP ducting can be fabricated in multiple ways. Today, designs are drawn and refined using computer software. This enables us to produce innovative solutions to problems. Below is a summary of the main types of fabrications executed in ductwork:
- 90° Elbow with internal vanes: This elbow assists air in making a smooth and gradual change in direction.
- Segmented elbow: For large diameter ducts that require a turn. Making elbows in segments allows us to create elbows to any specification.
- Transition pieces: These change the ductwork shape e.g., from rectangular to round.
- Condensate drainage: To allow condensation to escape the ducting safely.
- Protection against electrostatic discharge: This is achieved using anti-static materials such as Anti-Static Polyurethane.
- Manholes: Better known in modern times as access chambers. These are needed when ducting requires servicing and inspection.
Ducting shapes
Duct shape can be round, rectangular or oval. While these shapes can accommodate the same airflow, round is considered the most efficient. Round provides more efficient airflow per square inch, so the ducting can be smaller.
Another reason to choose one shape over another is to accommodate the local environment and maximise airflow since these shapes can be a better fit in different places. For example, a round duct with a 5.1” diameter has the same airflow as a 6” oval. In this case, round is more efficient at the same task.
The surface area of a rectangular duct can have twice the pressure loss of a round duct. This is the key advantage to round ducting – it is more efficient because it loses less pressure. We recommend using round ducting when possible.
Ducting for different pressures
Pressure is a key consideration for plastic and GRP ducts. We need to know how airflows through a duct and at what pressure to design safe, efficient ducting.
Typically, high pressure calls for a robust composite material like GRP. Medium pressure calls for a reinforced material like GRP-reinforced PVC, and low-pressure calls for plastic like PP or UPVC. It is normal to overengineer ductwork to accommodate pressure changes.
We also have to consider positive and negative pressures. These do not affect ductwork in the same way. Positive pressure yields an outward force on the duct. Negative pressure yields a force inwards. Positive pressure occurs when the pressure in the duct is greater than the pressure outside of it while negative pressure occurs when the pressure in the duct is less than the pressure outside of it.
Negative and positive pressure exerts different types of force on the ductwork. It’s important to note that in most cases, positive pressure occurs in ducts and this is the desired pressure. However, sometimes negative pressure is desired. An example is to create suction and exhaust for dangerous fumes.
Chat with our experts
At PDFL, we design, supply, assemble and install ducts, chimney liners, and stacks at plants in all manner of industries. We provide a complete solution, enabling our customers to use one company for their engineering requirements.
Contact our experts about the design, supply and installation of liners and ducts today. Call us on +44 (0)1922 418005 or email sales@pdfl.uk
