PP Flat to Round

Product Model	PPs round-to-square transition fitting
Category	PP Machined Products
Reference Price	Price on request

Hot Customizable for Working Conditions 11 Technical Parameters

Product Overview

The PP Square-to-Circle Transition Pipe is a connecting fitting used in ventilation ducts to transition between rectangular and circular cross-sections. Due to one end being square and the other circular, it is traditionally referred to as "Sky-Circle-Earth" in the sheet metal industry. The Square-to-Circle Transition Pipe is cut, bent, and welded from PP flame-retardant sheet material based on the unfolded layout. The square end connects to rectangular ducts or square equipment flanges, while the circular end connects to circular ducts, fans, or purification equipment.

In corrosion-resistant ventilation engineering, rectangular square pipes are often used indoors to save space, while outdoor main ducts, fans, and most purification equipment interfaces are circular. A Square-to-Circle Transition Pipe is required to bridge the gap between these two types. A simple connection would result in abrupt cross-sectional changes and gaps. The Square-to-Circle Transition Pipe uses a twisted surface transition section to gradually narrow the square cross-section into a circular one, ensuring smooth airflow conversion and reducing local resistance and air leakage. Plate-welded Square-to-Circle Transition Pipes can be customized to any rectangular size and circular diameter, offering strong adaptability.

The PP Square-to-Circle Transition Pipe processed by Xicheng Environmental Protection uses PP flame-retardant sheet material. The square and circular ends are welded with corresponding flanges, and the twisted surface section is welded from multiple plates and reinforced. It is acid and alkali resistant, self-extinguishing upon ignition, and commonly used at fan inlets/outlets, connecting rectangular ducts to circular vertical pipes, and linking square equipment to circular ducts.

Working Principle

The core function of the Square-to-Circle Transition Pipe is the continuous conversion of cross-sectional shapes. The transition section consists of multiple triangular or trapezoidal twisted surfaces, gradually transitioning from the four straight edges of the square end to the circumference of the circular end. Within this twisted surface channel, airflow gradually shifts from a rectangular to a circular flow state, with the cross-sectional area remaining as constant or changing smoothly to minimize eddies and resistance caused by the shape conversion.

When the Square-to-Circle Transition Pipe also serves as a diameter changer (unequal cross-sectional areas for rectangular and circular sections), it must follow the principle of gradual expansion and contraction for smooth transitions. The length of the conversion section is controlled to avoid sudden velocity changes. The smooth and corrosion-resistant PP sheet material ensures continuous sealing along the weld seams of the twisted surfaces, preventing air leakage and rust in corrosive gases. The flame-retardant components guarantee self-extinguishing performance at the connection point between indoor square pipes and circular equipment.

Structural Composition

The Square-to-Circle Transition Pipe consists of a square flange end, a circular interface end, and an intermediate twisted surface transition section. The square end is a rectangular flange frame that connects to the rectangular duct flange. The circular end is a cylindrical section with a circular flange or socket, connecting to round pipes or fans. The intermediate transition section is formed by bending and welding multiple PP plates cut according to the unfolded layout, with the four corners of the square end gradually converging into the circular circumference.

The twisted surface weld seams are distributed along the length, serving as the primary load-bearing and sealing areas. Reinforcing strips can be added externally. For larger cross-sectional areas or higher negative pressures, additional reinforcing rings are added to the transition section and circular end, while back ribs are added to the square end flange to prevent deformation. The two ends are flanged according to the standards of rectangular ducts and circular equipment, with corresponding hole positions. Square-to-Circle Transition Pipes are often used in combination with flexible connectors and size reducers for fan connections.

Specification Model Table

The table below shows the two-end interfaces and typical connection objects of the Square-to-Circle Transition Pipe, with dimensions matched on-site.

End	Cross-Section	Typical Connection
Square End	Rectangular Flange	Rectangular Duct / Square Damper
Round End	Round Flange / Socket	Round Duct / Fan
Transition Section	Twisted Surface Welded	Cross-Sectional Shape Conversion

Product Features

The PP Square-to-Circle Transition Pipe enables smooth conversion between rectangular and circular cross-sections, serving as a critical transition piece in mixed square-round pipe networks. Its main features include:

PP flame-retardant sheet material twisted surface welded, acid and alkali resistant, and self-extinguishing upon ignition
Connects rectangular pipes to circular equipment from square ends to round ends
Continuous twisted surface transition reduces shape conversion resistance and air leakage
Customizable for any rectangular size and circular diameter
Equipped with rectangular and circular flanges on both ends for easy connection
Reinforced welding in the transition section ensures structural stability under negative pressure
Commonly used with fan inlets/outlets to solve square-round mismatches

Material	PPs Flame Retardant Boards
Processing Technology	%= "Winding face expansion and welding" %
Rectangular Cross Section	Rectangular flange
Round Section	Circular Flange / Spigot
Function	Rectangle and Circular Section Conversion
Flame Retardancy	Self-extinguishing
Corrosion resistance performance	Acid-alkali-resistant
Specifications and Features	Non-standard custom size
Connection Method	Flange Connection
Strengthening Method	Reinforced at the bending point
Installation Orientation	Installed according to airflow direction

Application Industries

Transition from indoor rectangular ducts to outdoor circular main ducts
Connection from laboratory rectangular exhaust stacks to rooftop circular fans
Transition from square dampers or square pipes to circular spray tower interfaces
Connection between rectangular exhaust pipes and circular centrifugal fans at inlet and outlet
Conversion point from square ceiling pipes to circular exhaust stacks
Transition from square equipment outlets to circular ducts
Conversion node for corrosion-resistant exhaust networks with mixed square-round layouts

Typical Process Locations

The square-to-round transition is most commonly used at junctions between indoor rectangular networks and circular equipment or outdoor circular pipes. For example, in laboratories and workshops, rectangular square pipes are used in ceilings to save space. Before the pipes exit the roof or connect to fans, a square-to-round transition converts them to circular, followed by soft connections, reducers, and connections to circular fans. Square-to-round transitions are also used when connecting square interfaces of purification equipment to circular ducts. Installation should be near the section conversion point to minimize the length of irregular pipe segments.

During installation, ensure the square end aligns with the holes and dimensions of the rectangular duct flanges, and the round end aligns with those of the circular equipment flanges. Pay attention to airflow direction and the orientation of the transition section. Both ends of the flanges must be sealed and tightened. The weight of the square-to-round transition and connected components is supported by brackets. When connecting to fans, add vibration isolation with soft connections at the round end. If there is a significant difference in cross-sectional area, the transition section must be long enough to ensure smooth airflow conversion.

They serve different purposes. Reducers only change the cross-sectional size without altering the shape, with both ends being circular or both being rectangular, used for connecting pipes of the same shape but different diameters. Square-to-round transitions change the cross-sectional shape, one end rectangular and the other circular, used for converting between rectangular ducts, circular ducts, circular fans, or circular equipment; when the cross-sectional areas of the rectangular and circular ends are also different, square-to-round transitions also serve as diameter changers. Simple rule: for mixed square-round sections, choose square-to-round transitions; for same-shape diameter changes, choose reducers. In engineering, when connecting a rectangular horizontal pipe to a circular fan, square-to-round transitions, flexible connections, and reducers are often used in combination. When selecting, first draw the cross-sectional shapes and dimensions of both ends, then determine which one or ones of the fitting types to use in series.

The transition section length must ensure a smooth conversion from square to circular twisted surface, while also meeting the requirements for gradual expansion and contraction angles to avoid flow separation at sharp turns. If the transition section is too short, the twisted surface becomes abrupt, leading to increased flow vortices and resistance, and may also cause vibration and noise in the flat section under negative pressure. If it's too long, it results in material waste and occupies more installation space. Generally, the appropriate length is determined based on the rectangle's longer side dimension, the circular end diameter, and the area difference between the two ends to ensure a smooth twisted and conical surface. The square-to-round transition at the fan inlet must particularly guarantee uniform airflow into the impeller. When necessary, refer to the fan manufacturer's connection requirements and recommended length for design. In space-constrained situations, it can be shortened appropriately, but the resistance should be checked and stiffeners added to the flat section to prevent resonance.

When the fan has a circular interface while the connected ductwork is rectangular, a square-to-round shape conversion is required; if both the fan interface and the ductwork are circular, a reducer with a flexible coupling can be used without a square-to-round conversion. In actual engineering projects, when indoor rectangular ducts connect to circular roof or machine room fans, a combination of square-to-round conversion, flexible coupling, and reducers is often employed: the square-to-round conversion handles shape transformation, the flexible coupling isolates fan vibration, and the reducer addresses diameter differences. Whether to use a square-to-round conversion depends on whether the fan interface and duct cross-sectional shape are consistent. During layout, try to place the square-to-round conversion near the fan with smooth transitions to ensure uniform airflow. Before ordering, verify the fan interface dimensions and duct cross-sectional dimensions to ensure this set of transition fittings can be matched in one go, reducing on-site duct modifications.

The square-to-round transition is constructed by welding multiple twisted panels together. The longitudinal seams and the transition ring seams between the square ends and round ends are the primary areas prone to air leakage, with common causes including incomplete welding at seams, lack of penetration, flange deformation, or improper gasket sealing. During processing, ensure accurate unfolding and cutting, continuous full-welding of panel seams after alignment, reinforced flat surfaces on square flanges, and uniform roundness on round ends. During installation, ensure gaskets on both flanges are intact and bolts are tightened diagonally evenly. After installing the negative pressure system, conduct air leakage checks, focusing on twisted panel seams and the square-to-round transition areas. Mark any detected leaks, then perform backwelding or replace gaskets after pressure relief. With accurate cutting, full welding, and flat flanges, the square-to-round transition can achieve sealing performance comparable to standard ductwork.

Ideally, the square-end rectangular cross-sectional area should be close to the round-end circular cross-sectional area to ensure the flow velocity remains nearly constant before and after conversion, avoiding abrupt diameter changes while altering the shape. In design, determine the required cross-sectional area based on the processing air volume and recommended flow velocity first, then specify the rectangular length and width and the round pipe diameter to ensure the areas are comparable and the flow velocity remains stable. If limited by equipment interface constraints, resulting in significant area differences between the two ends, extend the transition section for a smooth conversion, or incorporate reducer sections before and after the square-to-round transition to prevent abrupt velocity changes that cause resistance, noise, and vibration. The fan inlet, in particular, must be avoided from experiencing high-speed deflection. During verification, check the flow velocities of both rectangular and circular cross-sections to ensure they fall within the recommended range, adjusting the rectangular length-width ratio or the round-end diameter if necessary.

For custom square-to-round transitions, provide the length and width of the rectangular end, diameter of the round end, flange standards and hole positions, transition length or available installation space, airflow direction (square-in-round-out or round-in-square-out), and installation orientation (whether eccentric or offset for square and round ends). Also specify plate thickness and fire resistance requirements. When paired with a fan, it's advisable to provide fan interface drawings and on-site pipe connection direction to ensure smooth alignment. Indicate the orientation of the rectangular long side and installation angle to avoid misalignment due to incorrect face direction, preventing on-site connection issues. If the cross-sectional areas at both ends differ, note the control end dimensions or provide a recommended flow rate. Complete parameters and clear drawings enable the manufacturer to accurately perform cutting and one-piece forming, reducing rework caused by dimensional mismatches.

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