Application of Cross-Flow Fans in Floor Convectors and Trench Heaters

Floor convectors and trench heaters belong to the same type of HVAC terminal units: embedded heating/cooling terminals pre-buried in floor trenches and arranged along curtain walls. They are available in natural convection versions and forced convection versions equipped with cross-flow fans. As the core air supply component for forced models, cross-flow fans fit perfectly with their design requirements of narrow, long trenches and forming uniform air curtains along glass facades.

1. Why Cross-Flow Fans Instead of Axial or Centrifugal Fans

Core features of cross-flow fans: long cylindrical impeller, slim profile, uniform air discharge along the full length, low static pressure and low noise.
✅ Matching the profile of trench housings: Trenches are long and narrow, normally only 100–200 mm in height. The cross-flow impeller has a small diameter and its length can be extended to match the trench. The motor is mounted at the end without occupying vertical space, so it fits inside the trench easily.
✅ Continuous planar airflow: Consistent air output along the full trench length. Air streams rise along the glass to form a continuous hot air curtain, blocking cold downdraft from glass surfaces and preventing condensation — an effect difficult to achieve with discrete air supply from axial fans.
✅ Gentle air supply with low velocity: No draft sensation, suitable for residences, hotels and exhibition halls. EC versions support 0–10 V stepless speed regulation and can be interlocked with thermostats and water valves for variable capacity control.
✅ Noise performance: With proper dynamic balancing and vibration isolation, typical operating noise ranges from 30–40 dB(A), close to indoor background noise. The trench structure further attenuates vibration and noise.

⚠️ Inherent Limitations (critical for selection)

• Very low static pressure: Suitable only for low-resistance air paths. Airflow drops sharply when filters or fin coils are clogged with dust. Service access for cleaning must be reserved; fully sealed construction is not allowed.
• Lower aerodynamic efficiency than centrifugal fans, leading to relatively higher power consumption at full load. EC brushless motors are therefore preferred.
• Large length-to-diameter ratio for long impellers. Intermediate supports and qualified dynamic balancing are mandatory to avoid resonant noise.

2. Assembly Structure & Airflow Path (Cross-Flow Fan Type)

Sequence from bottom to top / air inlet to outlet:

1. Housing (galvanized steel / stainless steel trench) + bottom vibration isolation pads
2. Cross-flow fan assembly (impeller + scroll casing + EC motor)
3. Heat exchanger core: copper tube aluminium fin coil (2-pipe heating / 4-pipe heating & cooling). Electric heating models replace the coil with electric heating modules.
4. Air guide plate and inlet filter
5. Top grille (roll-up grille / linear grille, load-bearing for pedestrian traffic)

Airflow route:
Indoor air enters through the lower part of the grille → drawn in by cross-flow fan → passes transversely through the fin coil for heat exchange → discharged obliquely upward via the air guide plate, climbing along the curtain wall to form an air curtain. In winter, it suppresses cold downdraft from glass; in summer, cooled air rises along glass then settles, achieving precise perimeter temperature control.

Comparison with natural convection units: For the same trench dimension, the heat output of forced cross-flow models can reach 4–5 times that of natural convection types. They are ideal for low-temperature water heat pump systems, large glass areas, high perimeter heat loads and limited trench length.

3. Fan Selection Guidelines for Engineering Implementation

1. Impeller Dimension: Impeller diameter is determined by trench height; common sizes φ40, φ50, φ60 mm. Impeller length roughly equals the effective heat exchange section. Multiple cross-flow fans can be spliced for long trenches with synchronous control.
2. Motor Options
   • EC Brushless DC (mainstream premium): 0–10 V / PWM speed control, low power consumption, adjustable speed, thermostat interlock; preferred for villas, hotels and office buildings.
   • AC Shaded-pole / Capacitor-run motor: Low cost, fixed speed; mostly used for low-end electric trench heaters.
3. Operating Point: Cross-flow fans are low-pressure units, selected at static pressure of 10–30 Pa. Total resistance from grille, filter, fin coil and internal ductwork must be checked and kept below the fan rated static pressure.
4. Temperature Resistance & Material: For hydronic models, discharge air temperature is generally below 50 ℃; ABS impellers are acceptable. For electric heating models with high air temperature, aluminium alloy impellers are recommended to avoid thermal creep and deformation.
5. Noise Control: Rubber vibration isolators between fan and housing. Scroll profile and cutoff position are key for cross-flow noise reduction; the air duct profile shall not be modified arbitrarily. Hard connections that transmit vibration to stone flooring must be avoided.

4. Typical Application Scenarios

• High-end residences with large floor-to-ceiling windows and sunrooms (eliminating cold sensation near windows and anti-condensation)
• Office buildings, exhibition halls, hotel lobbies along glass curtain walls
• Low-temperature water heating heat pump systems (natural convection units deliver insufficient capacity at low supply water temperature)
• Some projects reserve fresh air ports for mixed fresh air supply via the cross-flow fan

5. Common Faults & Maintenance Notes

1. Reduced airflow and rising noise: Filter or fin coil fouling increases flow resistance. Cross-flow performance is highly sensitive to clogging. Annual cleaning (1–2 times) is recommended.
2. Abnormal noise and periodic hum: Failed impeller dynamic balance, bearing wear or resonance. Long impellers without intermediate support discs are prone to shaking.
3. Uneven air distribution along length: Misaligned scroll casing or air guide plate shifts the vortex core, a typical assembly issue for cross-flow fans.
4. Cooling mode: Heating & cooling versions with cross-flow fans must be fitted with drain pans and drain lines to prevent condensate accumulation inside the trench.

6. Comparison: Natural Convection vs Cross-Flow Forced Trench Heaters