A vacuum conveying line for plastic pellets was undersized and the plant did not know it. The line moved 300 kg/h of ABS pellets through a 50 mm line over 20 meters. The blower was a 7.5 kW regenerative unit. The system worked when it was new. After a year, the pellets stopped flowing on humid days. The operator blamed the pellets. The pellets were the same. The air was the difference. The system had been borderline since day one, and humid air pushed it over the edge.

Vacuum conveying moves material by pulling air through a pipeline. The air velocity carries the particles. Below a critical velocity, the pellets drop out of the air stream and the line plugs. The critical velocity depends on particle size, density, and the air density itself. Humid air is less dense. The blower produces the same volume flow but the mass flow of air drops. The pickup velocity drops below the threshold. The pellets plug the line. The system was designed without margin, and the design margin was exactly the difference between dry and humid air.

The pickup velocity calculation

The minimum conveying velocity for plastic pellets in a dilute-phase system is typically 15-20 m/s. The rule of thumb: 1.5 to 2.5 times the particle saltation velocity. For 3 mm ABS pellets at 1.05 g/cm³, the saltation velocity in a 50 mm line is around 8-10 m/s. The conveying velocity should be 16-20 m/s to be safe.

The actual velocity in the line: the blower moves about 170 m³/h at the inlet vacuum. In a 50 mm line, the cross-section is 0.00196 m². The velocity is Q/A = 170/3600 / 0.00196 = 0.0472 / 0.00196 = 24 m/s at the inlet. At the end of the line, the vacuum is lower, the air expands, and the velocity rises. The minimum point is at the pickup point, where the vacuum is highest. At 0.4 bar absolute at the pickup, the 170 m³/h at the blower inlet is only about 68 m³/h at the pickup, giving about 9.6 m/s there. That is below the 16 m/s target. The line was running at half the required pickup velocity from day one.

This is the number that explains everything. The pellets moved when the air was cool and dry (air density higher, more mass flow). On humid days, the air density dropped 3-5%, the mass flow dropped, the pickup velocity fell below the saltation threshold, and the line plugged. The 7.5 kW blower was undersized for the duty. The line needed a 50 mm pickup velocity of 16 m/s minimum at the pickup point, which required roughly double the air volume.

Air consumption is the real design input

Vacuum conveying systems are specified by air volume and vacuum, not by “pellets per hour.” The material flow follows from the air flow. The design process:

  1. Pick the line size (50 mm here).
  2. Pick the conveying velocity at the pickup (16-20 m/s for pellets).
  3. Calculate the air volume at the pickup: Q = v × A × 3600 = 18 × 0.00196 × 3600 = 127 m³/h at pickup conditions.
  4. Convert to blower inlet conditions (vacuum at the blower): at 0.4 bar absolute, the blower inlet volume is 127 × (0.4/1.0)… careful, the blower inlet is at lower pressure, so the volume at the blower is larger. Q_blower = 127 × (1.0/0.4) ≈ 318 m³/h at blower inlet.
  5. That volume requires a 10-11 kW regenerative blower at 0.4 bar vacuum, not the 7.5 kW unit installed.

The installed blower at 170 m³/h delivered only 9.6 m/s at the pickup — about 60% of what the line needed. The system was running on luck.

The humidity effect

Air density at 20°C, 60% RH: 1.2 kg/m³. At 30°C, 90% RH: 1.13 kg/m³. The mass flow drops 6%. The conveying capacity of the air stream scales with density. The 6% density loss pushed the pickup velocity from 9.6 to 9.0 m/s. The saltation velocity for the pellets is around 9-10 m/s. The line sat right at the boundary. Humid days pushed it under. The pellets dropped out and bridged at the first elbow.

The plant could have verified this without any calculation. The telltale sign: the system worked in the morning and plugged in the afternoon (temperature rise), worked in winter and failed in summer, worked with dry pellets and failed with humid ones. Those are all air density stories. A conveying system running on the edge shows the same symptoms a system with a clogged filter shows — reduced flow. The difference: a clogged filter is fixed by cleaning; an undersized blower is fixed by replacing it.

The fixes that work

Three options, cheapest first.

1. Reduce the line diameter at the pickup. A smaller pickup section raises the velocity without more air. Stepping the 50 mm line down to 40 mm at the pickup raises the velocity by (50/40)² = 1.56x — from 9.6 to 15 m/s. The rest of the line stays 50 mm for the expanded air downstream. This is a cheap retrofit and it works when the flow is borderline. The pellets accelerate in the small section, and the velocity rises further as the air expands into the larger line.

2. Install a variable-speed blower. A VFD on the blower lets the plant raise the speed on humid days. The pickup velocity goes up with speed. The blower draws more power (cubic with speed), but the plant only runs it up when needed. The VFD pays for itself in the first summer of operation.

3. Replace the blower with a larger unit. The correct answer for a system that will grow: a 10-11 kW unit. The plant chose the VFD route on the existing blower first (cheap), found the ceiling was still too low on the worst humid days, and replaced the blower the following year. The replacement cost $1,800. The VFD was moved to the new blower.

The line layout that multiplies the problem

The 20-meter line had five 90-degree elbows. Each elbow costs pressure drop, and pressure drop costs air velocity. In dilute-phase conveying, each elbow effectively adds 3-5 meters of equivalent length. The line was really 35-40 meters equivalent. The pressure drop at 318 m³/h through that run at 0.4 bar vacuum was right at the blower’s curve limit. The system had no headroom anywhere: the blower, the line, and the humidity margin were all at their limits simultaneously.

For new designs, the checklist that prevents this class of failure:

  • Calculate the pickup velocity at the pickup point, not at the blower.
  • Add 30% to the air volume for humidity and filter loading.
  • Count every elbow as 4 meters of equivalent length.
  • Size the blower at 0.35-0.4 bar absolute vacuum with 25% margin on the curve.
  • Install a vacuum gauge at the pickup — the first symptom of plugging is the vacuum rising.

The vacuum gauge is the cheapest fix on the list. A plugged line shows a rising vacuum before the pellets stop flowing. An operator who watches the gauge clears the line while it is still a partial plug, not a full one. The plant installed gauges at both pickups after the replacement. The next time the humidity bit, the gauge showed it 10 minutes before the plug, and the operator throttled the air briefly to clear it.

Vacuum conveying is sized by pickup velocity, and the pickup velocity must be calculated at the pickup, where the vacuum is highest. The 300 kg/h line was running at 9.6 m/s against a 16 m/s requirement. Humidity dropped the air density and the line plugged. Size the blower for the pickup velocity plus 30% margin, and put a vacuum gauge at the pickup.