What exactly is the length of the expansion joint used on the fan duct? This question has been asked by almost every purchase. But let's be honest, there's no standard answer. Because the length of fan expansion joint is not determined by patting the head, it depends on the pipe diameter, displacement, working pressure, temperature, and even installation space. For example, for pipes of the same diameter, if the fan outlet temperature is high and the thermal expansion is large, the number of corrugations needed will be more, and the length will naturally be longer. Conversely, if it is only used to dampen vibration, maybe a short section is enough.
Give me a rough range first, so I have the bottom of my mind. For small and medium-sized fans (DN100~ DN500), the length of common metal expansion joints (such as general corrugated expansion joints) is generally between 200mm ~600mm. If it is a non-metal expansion joint (fabric fiber expansion joint), because the material is soft and the compensation angle is large, the length may be shorter, and 150mm ~400mm is more common. However, for large-diameter fans (DN1000 or above), such as induced draft fans in power stations or cement industries, the length of expansion joints may be 800mm or even more than 1m, especially for high-temperature axial expansion joints or structures with guide tubes.
How to set the length? The core is to look at two parameters: axial compensation and fatigue life
Each bellows has a designed compensation amount. For example, a single wave can compensate 10mm, and you need to compensate 40mm, so you have to have 4 waves, plus the pipes and flanges at both ends, and the length will come out. But be aware that the longer the bellows, the better – too long can cause instability, or the space won't fit. Therefore, when selecting the type, the wave number is usually calculated according to the displacement first, and then the pressure level is combined (such as the external pressure single axial expansion joint or the straight pipe pressure balance type), and finally the total length is determined.
Structural form is the premise, length is only the result
Two days ago, I met a customer, and I had to copy the length of others. As a result, the vibration of the fan exceeded the standard after installation. Why? He bought a double hinge transverse expansion joint, but the pipeline on site went in different directions, and the required transverse displacement direction was inconsistent. The length was the same but the structure did not match. So length is only the result, structural form is the premise. For example, the length of double hinge expansion joint for air-cooled island vacuum pipeline is different from that of ordinary axial type. Another example is the rubber compensator, which is shorter in length, but can only be used in low pressure and normal temperature occasions.
Don't forget the deflector
The expansion joint length also includes a flow guide barrel. For example, the corrugated expansion joint used in the power station industry often has a guide tube inside, and the length of the guide tube will occupy a part of the effective length. If the flow rate of the pipeline medium is high, the design of the guide tube is unreasonable, which will generate vortex and noise instead. Therefore, when asking for the length, it is best to report the working conditions (temperature, pressure, medium and displacement direction) to the manufacturer together and let the manufacturer calculate. Don't expect one length to take all fans.