Concentric vs Eccentric Reducers

In all systems transporting liquids, gasses, dusts or air from one point to another, there will most likely be a change in diameter in the round duct work. When such a situation occurs it is important to know how to deal with it. There are specific fittings on the market that allow you to connect pipes and ducts with different sizes. If your ducts have the same diameter you will simply use a male connection piece. However, what if you are going from a 150mm diameter duct to a 100mm one? Well, in that case, you will need to use a reducer. These fittings may be used when there is a single diameter change or multiple diameter changes. The same fittings can be used in reverse as increasers.

Types of Reducers

There are two main types of reducers. Concentric reducers and eccentric reducers. Both of them have their pros and cons in different scenarios.

Concentric reducers have the shape of a cone, hence the name. Unlike eccentric reducers, concentric reducers have a common center line. Meaning that when using these fittings you will be able to align the centres of both pipes, the centre of your bigger pipe will be straight towards the centre of the smaller one.

Eccentric reducers are a bit different. One of their sides is straight, with one of the edges running parallel to the connecting pipe and results in the centreline being offset. This is not the reason why eccentric reducers are more commonly used for transporting liquid, but we will get into that in the following paragraph.

When are Eccentric and Concentric Reducers Used

Concentric reducers are used where the pipework is vertically installed and at the discharge side of pumps.

Eccentric reducers are more often used when the pipework lays on a pipe rack. Because of the flat side, aligning and securely mounting the pipes to the rack is easier. Those reducers also come in handy where there is a pump installed in liquid transporting systems. To avoid cavitation, the pipework is connected to the pump via an eccentric reducer that has its flat side facing up. This prevents the forming of bubbles and limits the chance of damaging the propellers inside the pump because of the cavitation phenomenon.

In ducting, we are often transporting heated or cold air and the cavitation phenomenon is not present. Therefore in this industry concentric reducers are used the most, with all of the concentric reducer dimensions.

How are Reducers Made

Concentric reducers used in ducting and ventilation systems are most commonly made from galvanised steel.

Those fittings can also be made out of stainless steel – 304, 316 and 316L. This makes them resistant to corrosion and they can handle weather conditions well. Also, this allows them to be used for transporting corrosive materials like water, steam and air.

The inner and outer surfaces of reducers used in plumbing are well polished with high-grade materials in order to meet the requirements for food, pharmaceutical and other industries.

Installing Eccentric and Concentric Reducers

There are various different ways to install reducers. Which way you choose will vary on the duct work you are changing the size of and what is travelling through the ducting.

• Duct sealant
• Self-drill screws
• Duct tape
• via hose clamps;
• welding;
• threaded.

Much like with the installation of spiral ducting which we covered in one of our earlier blog posts, standard galvanised steel concentric reducers have male spigots which will slip inside spiral duct. A female connection piece will be required if you are joining two male end fittings. Reducers can be fixed using duct sealant, self-drilling screws and/or duct tape.

When using flexible ducting, an often used method of fixing those fittings is using hose clamps, often known as Jubilee clips. These accessories provide a strong grip and firm connection when tightened well around the rigid reducer.

If you are welding reducers in place, you need to make sure you have removed any oils and other contaminants from all surfaces to get a strong weld. Use a cloth with some alcohol to rub all pollutants off the reducer and the pipe.

Threaded reducers, obviously have threaded ends and screw fixed to the pipes. These types of reducers are most commonly used in the plumbing industry and require the pipes to be threaded as well. The perk of this type of connection is that it is flexible, meaning you can shift the angle where the pipe is going when using an eccentric reducer.

As always, if you have trouble choosing the correct reducer for your ducting system our customer support team can help you out. They are professionals in the niche and will gladly assist you if you just give them a call at 01455 616444.