A polyethylene distribution tube that discharges most of its air in the first ten metres is not a ventilation system. It is an expensive draught at one end of the house and a stagnant zone at the other — and it is the single most common way growers end up with uniform equipment and non-uniform crops.
Whether a tube works is decided by pressure distribution along its length, not by the fan on the end of it.
Three Jobs, Frequently Confused
Distribution tubing is used for three functions that have different objectives, different sizing logic and different operating periods. Specifying one and expecting another is a recurring source of disappointment.
Ventilation — exchanging air with outside. The objective is to replace inside air with outdoor air, whether to remove heat, remove moisture, or refresh the atmosphere. Sizing is driven by the required air exchange rate. Whether ventilation can remove moisture at all depends on outdoor conditions, which is set out in ventilation vs. dehumidification.
Circulation — mixing air inside the house. The objective is to eliminate stratification and stagnant zones without exchanging any air with outside. Sizing is driven by the volume that has to be kept in motion and by the velocity needed at canopy level. This function runs continuously in many facilities, including at night when ventilation is closed.
Distribution — delivering conditioned air. The objective is to carry air from a specific source — a heater, or a dehumidifier — to the positions where it is needed. Sizing is driven by the source equipment's airflow and by where the air has to arrive.
A single tube can serve more than one of these, but only if it is designed for the most demanding of them. A tube sized for winter minimum ventilation will not distribute a dehumidifier's full airflow, and a tube designed for heater distribution may deliver air at the wrong height for canopy circulation.
Uniform Discharge Is a Pressure Problem
An inflated tube discharges air through its holes at a rate governed by the pressure difference across each hole. For discharge to be even along the length, the static pressure inside the tube has to be roughly the same at the far end as at the fan end.
That condition is not automatic. Two effects work against it:
Friction along the tube. Air moving along the tube loses pressure to friction. The longer and narrower the tube, the larger this loss, and the less pressure remains at the far end to push air out.
Progressive discharge. Air leaves through holes along the way, so the volume flowing in the tube decreases with distance. This actually helps — velocity falls, so friction loss falls — but it is not enough to compensate on a poorly proportioned tube.
The balance between these determines the outcome, and it is controlled by the relationship between the tube's cross-sectional area and the total open area of its holes.
If total hole area is too large relative to the tube cross-section, air escapes readily near the fan, pressure collapses along the length, and the far end receives little or nothing. The tube may also fail to inflate properly, sagging into the crop.
If total hole area is too small, the tube over-pressurises, the fan is pushed back along its performance curve and moves less air than intended, and the discharge jets may be strong enough to damage foliage close to the tube.
The correct proportion depends on tube diameter, length, hole size and pattern, and on the fan's pressure characteristic. Tubing suppliers publish design data covering these combinations, and the tube should be selected against that data for the specific fan and run length rather than by adopting a rule of thumb. A tube that works well at 30 m may perform poorly at 60 m with the same hole pattern.
Hole Pattern and Where the Air Goes
Commercially supplied convection tubing is punched to defined patterns. Hole diameters commonly range from about 12 mm to 75 mm (0.5 to 3 inches), with typical spacings around 450 mm and 600 mm (18 and 24 inches).
Hole position around the tube's circumference is described by clock positions viewed along the tube — a "4/8" pattern places holes at the 4 o'clock and 8 o'clock positions, discharging downward and outward on both sides. Other common patterns such as 5/7 and 3/9 direct the air more steeply downward or more horizontally.
The pattern choice determines where the air actually arrives:
- Downward-angled patterns drive air toward the floor and into the crop. Useful for canopy circulation and for reaching under benches, but capable of causing plant movement or drying if the tube is close to the crop.
- Horizontal patterns throw air across the house, mixing the upper volume. Useful for destratification, less effective at penetrating a dense canopy.
- Upward patterns are used where the objective is to temper incoming cold air by mixing it with warm air at the ridge before it reaches the crop — a winter ventilation strategy rather than a circulation one.
For humidity control specifically, the relevant question is whether air reaches the canopy, because that is where transpiration occurs and where leaf wetness develops. Air that mixes the ridge volume produces excellent sensor readings and does not address the microclimate that matters.
Jets Move More Air Than They Carry
A useful property of hole discharge: each jet entrains surrounding air as it travels, so the volume of air actually set in motion is considerably greater than the volume passing through the tube.
This is why a relatively modest tube airflow can circulate a large house volume, and it is why jet throw — how far the jet travels before dissipating — matters more than the discharge rate at the hole.
It also means the tube's position relative to the crop is a design variable rather than a convenience. A tube mounted high delivers jets that entrain and mix a large volume but arrive at the canopy weakened. A tube mounted low delivers directly to the canopy but affects a narrower band. Facilities with a wide bay and a single high tube frequently find that the centre of the bay is well mixed and the outer rows are not.
Canopy-level air velocity is the measure that matters, and it is worth spot-measuring rather than inferring. Velocities commonly cited in controlled-environment practice for canopy movement are in the region of 0.2–0.5 m/s, though the requirement varies by crop and canopy structure. Why this velocity matters — for CO₂ delivery as well as for moisture removal at the leaf — is set out in CO₂ uptake in controlled environments.
Coordinating Tubes With Dehumidification Equipment
Where mechanical dehumidification is installed in a house that already has distribution tubing, the two air systems have to be designed as one.
Avoid short-circuiting. If the dehumidifier's return is close to a tube discharge, it will draw in air it has just conditioned rather than the humid air from the canopy. The unit will show excellent performance at its own sensors while the house stays humid.
Decide whether the tube carries the dehumidifier's air. Ducting dehumidifier discharge into a distribution tube gives excellent reach, but only if the tube is sized for that airflow and pressure. A tube designed for a low-pressure circulation fan will not accept a dehumidifier's discharge without redesign, and forcing it produces high face velocities at the near holes and nothing at the far end.
Match the operating periods. Circulation typically runs continuously; dehumidification runs on demand. If they share a tube, the control sequence has to account for what happens when one runs without the other.
Reinforce the existing pattern rather than fighting it. Tubes and horizontal airflow fans establish a circulation pattern. Dehumidification supply and return positions chosen without reference to that pattern can create new dead zones. The layout should be reviewed as a whole.
Four Common Failure Modes
Most air exits near the fan. Total hole area is too large for the tube cross-section and run length, so pressure collapses along the tube. Symptoms: plants near the fan showing wind damage or drying, far end of the house stagnant and humid.
The tube does not stay inflated. Insufficient pressure, usually from the same cause, or a fan that cannot deliver against the system. A sagging tube discharges unpredictably and can rest on the crop.
Air arrives above the canopy. Tube height, hole pattern or jet throw are such that the air mixes the upper volume without penetrating the crop. Symptoms: good sensor readings, persistent leaf wetness and disease at canopy level.
The tube was sized for a different job. A winter minimum-ventilation tube pressed into service as a circulation tube, or a circulation tube asked to distribute dehumidifier discharge. Symptoms vary, but the diagnostic is that the tube performs adequately for its original duty and poorly for the new one.
All four present as "the humidity control is not working." Adding dehumidification capacity to a facility with any of them improves the average and not the uniformity.
Winter and Summer Are Different Duties
Winter. The tube's primary role is usually to temper and mix incoming cold ventilation air so it does not fall directly onto the crop, and to prevent stratification in a closed house. Air volumes are low, and the risk being managed is cold draughts and condensation on cold surfaces.
Summer. Air volumes are high, the objective shifts toward heat removal and canopy air movement, and the tube may be bypassed entirely in favour of open ventilation.
Transitions. These are the difficult periods, when a house may be closed with a high moisture load and outdoor conditions offering little drying potential. This is when circulation and mechanical dehumidification matter most, and when a tube designed only for the winter ventilation duty is least able to help.
A tube specified for one season should be assessed against the others before it is assumed to serve the whole year.
Information to Prepare
- Greenhouse dimensions: bay width, length, gutter height, ridge height, and compartment layout.
- Crop, planting density, canopy height and structure, and bench or floor layout.
- Existing tubing: diameter, length, hole size, spacing and clock position, and the fan connected to it.
- Existing horizontal airflow fans: number, position, capacity and control.
- Ventilation strategy: vent area, forced ventilation capacity, and control sequence by season.
- Heating system and whether it distributes through the same tubing.
- Climate targets by growth stage and by day/night period.
- Measured or estimated air velocity at canopy level, if available.
- Observed problems: location of stagnant zones, condensation, disease patches, and uneven growth.
- Any planned dehumidification equipment, with its airflow and available external static pressure.
Discussing a Project
Yakeclimate designs and manufactures industrial dehumidification equipment for complex climate applications, with a focus on agriculture and energy projects.
We co-develop application-specific dehumidification equipment around the operating conditions, interfaces, and integration requirements of the wider project or system.
Tubing selection, hole pattern and fan sizing remain with the tubing supplier and the greenhouse designer. Our scope is the dehumidification equipment and its airflow and pressure interface with the air-distribution arrangement they establish.
Explore agriculture humidity control, review industrial ceiling dehumidifiers, or contact Yakeclimate to review the operating conditions for your facility.
FAQ
Frequently Asked Questions
Why does my distribution tube only deliver air near the fan?
Because static pressure is collapsing along the tube. This happens when the total open area of the holes is too large relative to the tube's cross-sectional area for that run length, so air escapes readily near the fan and little pressure remains at the far end. The correct proportion depends on tube diameter, length, hole size and pattern, and the fan's pressure characteristic. Tube selection should be made against the tubing supplier's design data for that specific combination.
What do hole patterns like 4/8 and 5/7 mean?
They describe hole positions around the tube's circumference using clock positions viewed along the tube. A 4/8 pattern places holes at the 4 o'clock and 8 o'clock positions, discharging downward and outward on both sides. A 5/7 pattern directs air more steeply downward, and a 3/9 pattern more horizontally. The choice determines whether air reaches the canopy or mixes the upper volume of the house.
Can I use my existing tube to distribute dehumidifier air?
Only if it is sized for that airflow and pressure. A tube designed for a low-pressure circulation fan will not accept a dehumidifier's discharge without redesign; forcing it produces excessive discharge at the near holes and little at the far end. The dehumidifier's airflow and available external static pressure should be checked against the tubing supplier's design data before the layout is committed.
Should air be delivered above the canopy or into it?
For humidity control, into it. Transpiration occurs at the canopy and leaf wetness develops there, so air that mixes the upper volume of the house produces good sensor readings without addressing the microclimate that matters. Canopy-level air velocity is the measure worth checking — commonly cited values in controlled-environment practice are around 0.2–0.5 m/s, varying by crop and canopy structure.
Does the tube replace horizontal airflow fans?
Not usually. They do different things: fans establish a broad circulating pattern in the house volume, while tubes deliver air to defined positions. Many facilities use both, and the two should be designed as one air pattern rather than independently. Introducing either without reference to the other can reinforce dead zones rather than eliminate them.
My humidity control isn't working — is it a capacity problem or a distribution problem?
Check uniformity before adding capacity. If conditions differ significantly between locations in the house — particularly if problems cluster at the far end of a tube, in outer rows, or at low level — the limitation is distribution. Adding capacity to a poorly distributed system improves the average and not the spread, which is usually what the crop is responding to.
References
- Greenhouse convection tubing product specifications — hole sizes, spacings and clock positions
- Polyethylene convection tubing — punched hole configurations
- Stanghellini, C. — Transpiration of Greenhouse Crops: An Aid to Climate Management, Wageningen
