Plug Fan vs Housed Fan (Scroll) Comparison

A plug fan and a housed centrifugal fan can use similar wheel principles while asking the surrounding equipment to do different jobs. The housed fan has a scroll that collects radial discharge into an outlet. A plug fan sends air from its wheel into a surrounding plenum; that plenum and its inlet and downstream components become part of the installation to verify. The choice therefore starts with the required airflow, pressure definition and cabinet geometry, not a fixed claim that one construction always saves energy or space.

What changes when the scroll is removed?

In a housed centrifugal fan, the casing guides air leaving the wheel to a defined discharge opening. That opening can simplify a direct duct connection. In a common backward-curved plug-fan arrangement, there is no comparable outer scroll. Air leaves around the wheel into the AHU chamber before passing through coils, filters, dampers or the next opening. A suitable plenum has to provide inlet clearance, room for discharge and a sensible path to the next component.

Removing a scroll changes the physical boundary of the fan and the way performance is tested and installed. It does not by itself prove a particular percentage change in pressure recovery, motor input or sound. Those outcomes depend on the exact wheel, speed, inlet, chamber and system. LONGWELL’s AHU plug-fan family is a relevant route when a plenum is part of the equipment design. A housed industrial centrifugal fan family is a different route when a defined outlet is central to the installation; neither family page approves an exact duty point.

Use the product photograph for construction, not a duty claim

The LONGWELL RLM series catalog shows an open wheel, inlet region, motor and mounting frame. Those visible elements help an AHU designer ask where the assembly can be supported and serviced. They do not show the cabinet around it, a completed inlet transition or the performance of every model suffix. Before selecting any specific unit, obtain the latest assembly drawing and its matching curve or tested configuration.

Ask whether the inlet is centered and clear, how the frame fixes to the cabinet, where the motor and wiring remain accessible, and whether the wheel can be inspected or removed without disturbing other components. A claim that every plug fan is easier to maintain is too broad: access depends on the finished cabinet and service side. The photograph is a family-level construction reference only.

Set the same pressure boundary for both proposals

A useful comparison keeps airflow, air density, speed or control setting and pressure reference explicit. Write down whether each offered value is fan static pressure, fan total pressure or a system pressure across the entire AHU. A housed fan has a defined outlet; a plug fan in a chamber uses a different test and installation boundary. Two headline pressure numbers are not comparable until their definitions and test arrangements are aligned.

The orange path in Figure 2 explains why a downstream plenum, coil face or bend matters to the plug-fan installation. It intentionally carries no percentage or pascal value. Use model-specific test data, then add the real inlet and discharge system effects for the planned arrangement. If one supplier’s data omit the intended plenum or inlet condition, request a corrected selection before ranking the candidates.

Check the cabinet, inlet and downstream space

Draw a section of the equipment, rather than choosing from a thumbnail. Mark inlet clearance, wheel position, service door, motor envelope, coil or filter position, discharge opening and any turn immediately after the fan. A plug fan may allow a different equipment layout, but there is no universal cabinet-length saving. The value depends on what the removed scroll frees and what the plenum must add. A housed fan may remain the more direct choice for a ducted outlet or a cabinet whose geometry is already fixed.

For a retrofit, record the existing unit’s actual dimensions and operating point. Changing to a plug fan may require a new mounting frame, electrical interface, inlet treatment and control setup. It may also alter flow distribution through the next component. A category picture cannot prove replacement equivalence. Obtain the exact candidate drawing and confirm the entire installation boundary with the equipment designer.

Compare power and sound at the expected operating range

Compare electrical input at the same specified airflow and pressure state, not a generic efficiency claim for a wheel type. Include clean and loaded filter conditions, speed range, damper positions and any operating state that changes system resistance. The relevant motor and controller limits must be respected throughout that range. If operating hours are known, apply hours at each state to the corresponding measured or documented input power before estimating annual energy use.

For sound, use comparable sound-power data and octave-band information at the actual duty. The chamber, coil, duct and enclosure can change what is heard outside the AHU. A fan without a scroll is not automatically quieter, and a direct-drive arrangement does not establish a particular vibration result. Ask for current model data and define where sound and vibration will be assessed.

Make a shortlist that can be verified

Start with the system duty and an equipment section drawing. If air must leave through a defined duct connection, evaluate a housed configuration with a suitable outlet. If the AHU has a designed discharge plenum, evaluate a plug-fan configuration, including the LONGWELL EC plug-fan family where its electrical and control features are relevant. In either path, keep the exact model, suffix, curve revision and test boundary together.

A sound choice is the configuration whose documented duty point, installation drawing and full operating range meet the project requirement. Once those inputs are available, price and lifecycle cost can be compared on a common basis.

Need a project-specific selection review? Send the airflow, pressure definition and AHU section drawing to sales@longwellfans.com.