At the same effective knife length, a 1.5 mm slot has 50% more open area than a 1 mm slot. If both must deliver the same average outlet velocity, the 1.5 mm gap will theoretically require 50% more volumetric airflow. In a real system, however, changing the gap also moves the blower operating point and changes pressure, flow distribution, noise and power consumption. The actual result depends on the blower curve, pipe losses, knife design and production requirement.
A 1 mm gap is not automatically more efficient, and a 1.5 mm gap is not automatically better for drying. Select the gap by testing the required process result at the intended line speed, then size the blower for the resulting airflow and total pressure.
The Basic Difference Is Open Area
For a straight, uniform air knife slot, the effective outlet area can be approximated by:
Slot area = effective slot length × slot gap
Consider a knife with an effective open length of 1,000 mm:
- at a 1 mm gap, the nominal opening is 1,000 mm², or 0.001 m²;
- at a 1.5 mm gap, the nominal opening is 1,500 mm², or 0.0015 m².
The second opening is 50% larger. For the same average exit velocity, its theoretical volume flow must also be 50% greater because flow is the product of area and velocity.
This comparison is useful for planning, but it is not a complete design calculation. Air density changes, discharge coefficients, plenum geometry, slot-edge shape and pressure distribution influence actual output. Manufacturing tolerances also become important when the nominal gap is small.
What Happens If the Blower Stays the Same?
Suppose an existing air knife is opened from 1 mm to 1.5 mm without changing blower speed or pipework. The system resistance generally falls because the discharge opening is larger. The blower operating point then moves along its performance curve.
Airflow will usually increase, while blower pressure may decrease. However, the additional airflow may not be enough to preserve the previous outlet velocity across the 50% larger area. The air sheet can become broader but slower. Whether drying improves or worsens depends on the product and how the air interacts with the water.
The opposite change—from 1.5 mm to 1 mm—raises outlet restriction. Total airflow may fall while measured upstream pressure rises. The smaller area can produce a faster, more concentrated air sheet under suitable conditions, but only if the blower can work safely at the new operating point and the gap remains uniform.
This is why a simple statement such as “a smaller gap gives higher velocity” can be misleading. It assumes pressure and flow remain available after the restriction changes. In practice, the blower and the slot respond together.
Comparing the Two Gaps
| Design factor | 1 mm gap | 1.5 mm gap |
|---|---|---|
| Open area at the same knife length | Smaller | 50% larger than 1 mm |
| Theoretical flow for the same outlet velocity | Lower | 50% higher |
| Discharge restriction | Generally higher | Generally lower |
| Sensitivity to gap tolerance | Higher | Lower in relative terms |
| Risk from small debris or contamination | Greater | Lower, depending on design |
| Air sheet | Typically thinner and more concentrated | Typically thicker and potentially higher-volume |
| Blower requirement | Must suit higher restriction | Must supply greater flow for equal velocity |
These are directional comparisons, not guaranteed site results. Knife profile, feed arrangement and operating pressure can outweigh the nominal gap difference.
Drying Performance Is Not Determined by Velocity Alone
Industrial drying is a process outcome. The air must displace water, carry it away and avoid simply moving it to another part of the product. Several factors influence that result.
Product shape and water location
A narrow, high-velocity sheet may work well on a flat surface. Containers, recessed parts and overlapping materials may need more air volume or multiple angles to clear trapped water. The best gap for sheet glass may therefore be unsuitable for bottles or machined components.
Knife angle and distance
Air loses coherence as it travels. Increasing the distance between knife and product can reduce effective force and allow surrounding air to disturb the sheet. Angle also determines whether water is pushed forward, driven sideways or trapped against an edge.
Before increasing blower output, adjust the knife position within safe process limits. A better angle or shorter working distance may improve water removal without added power.
Line speed
At a higher conveyor speed, each area of the product spends less time under the air knife. The system may need more effective air action, another knife or a revised angle. Opening the gap alone does not guarantee a solution, because velocity could fall.
Air distribution
Average velocity can hide weak regions. A poorly fed knife may discharge strongly near its inlet and weakly at the far end. A 1 mm gap also demands good manufacturing consistency: a variation of 0.1 mm represents 10% of the nominal opening, while the same variation represents about 6.7% at 1.5 mm.
Pressure, Airflow and Power Must Be Measured Together
When comparing gaps, record more than pressure. A smaller slot can raise pressure upstream even while total flow falls. A larger slot can increase flow while reducing the pressure reading. Neither result proves better drying by itself.
A useful test record includes:
- effective knife length and measured gap;
- blower speed or command percentage;
- static pressure at a defined location;
- total airflow, when it can be measured reliably;
- outlet velocity at several points along the slot;
- voltage, phase current and true input power if available;
- product type, line speed and drying result;
- ambient conditions and filter status.
Keep the measuring method and locations unchanged between tests. Otherwise, an apparent difference may come from test setup rather than the slot.
How the Gap Affects Blower Selection
The blower must meet the required flow at the total pressure of the installed system. Begin with the effective opening, target process performance and the number of knives operating simultaneously. Then account for losses through the filter, pipes, bends, hoses, valves, manifold and knife.
Do not add a catalogue’s maximum airflow to its maximum pressure as one selection point. Those ratings normally occur at opposite ends of the performance curve. The proposed duty must be plotted on a model-specific P–Q curve at the intended speed.
Also check motor input, current, cooling and continuous-duty limits at that point. Selecting only by motor kilowatts does not confirm that the blower can provide the correct air duty.
Example: A Two-Knife Drying Station
Imagine two identical 1,000 mm air knives operating together. Their combined nominal area is:
- 0.002 m² with two 1 mm gaps;
- 0.003 m² with two 1.5 mm gaps.
If the target average velocity were held constant, the 1.5 mm arrangement would theoretically need 1.5 times the airflow of the 1 mm arrangement. The required blower pressure would not be obtained from this area relationship; it must be determined from the discharge behavior and total system loss.
If only one knife runs during some recipes, the blower will see a different resistance curve. Speed control or a properly engineered bypass strategy may be needed to keep both one-knife and two-knife conditions within the safe operating range.
Applying Towwind Energy-Saving Blowers
A 1 mm versus 1.5 mm gap comparison changes slot area and resistance, but it does not determine the blower model by itself. Confirm the required combined airflow-pressure point against the selected model’s released curve; separate catalogue endpoints are not one duty.
If the delivered blower has adjustable output, confirm its usable range and tune it for verified product widths, line speeds and knife combinations. Check electrical fit, cooling clearance and final connection details before installation.
A useful request for selection should include knife length, slot gap, number of knives, feed-port design, pipe layout, required process result, line speed, duty cycle, site voltage and ambient conditions. For a retrofit, measured pressure, airflow, power and current-system performance are particularly valuable.
Avoiding Unnecessary Energy Use
An oversized slot can waste airflow outside the useful product width. An unnecessarily narrow slot can impose avoidable restriction and may encourage operators to increase speed. In both cases, energy is consumed without certainty of a better process result.
Tune the mechanical arrangement first:
- limit active knife length to the useful working width where practical;
- keep the gap straight, clean and uniform;
- position the knife close enough for effective air action;
- use an angle that moves water away from the product;
- remove avoidable pipe restrictions;
- set blower speed to the lowest value that consistently meets the acceptance standard.
Annual energy should be calculated from measured input power and real operating hours, including separate load states where applicable. Nameplate power is not actual consumption.
A narrower or wider gap may change the energy needed to meet the same drying result. Compare systems at the same production speed and accepted drying quality, using measured input and representative hours rather than a general saving percentage.
How to Run a Useful 1 mm vs 1.5 mm Trial
Use a representative product and stable incoming water condition. Verify that the filter, pipework and knife are clean. Test one gap at a time while keeping knife position, product speed and measurement methods constant.
Allow the blower and process to stabilize, then record the full set of operating data. Inspect the entire product, not only the easiest central region. Repeat the test if water loading varies significantly.
If the 1.5 mm gap improves water movement but leaves lower outlet velocity, a speed adjustment may be evaluated within the blower’s permitted range. If the 1 mm gap produces strong central air but poor end distribution, correct the knife feed or gap uniformity before changing blower size.
The best setting is the one that consistently passes the process requirement with acceptable power, noise and reliability—not the setting with the highest isolated pressure or velocity reading.
Final Recommendation
The choice between a 1 mm and 1.5 mm air knife gap is an engineering tradeoff. At equal length, the larger gap provides 50% more nominal open area and requires 50% more theoretical airflow to preserve the same velocity. Yet the installed result depends on the full blower curve and system resistance.
Treat slot gap as part of one connected system. Specify the process target, calculate the opening, evaluate total pressure loss, select the blower at the required operating point and confirm performance on the actual line. This approach gives buyers a defensible specification and prevents commissioning teams from solving every drying problem by increasing blower power.
Frequently Asked Questions
Does a 1.5 mm air knife gap always use 50% more air than a 1 mm gap?
It requires 50% more theoretical flow only when knife length and average outlet velocity are held constant. With the same blower, changing the gap moves the operating point, so actual flow must be measured or calculated from the system and blower curves.
Does a smaller air knife gap always produce higher velocity?
No. A smaller gap increases restriction and may reduce total blower flow. Higher velocity is possible only when sufficient pressure and flow remain available and the gap is uniform.
Which gap is better for industrial drying?
Neither is universally better. Product geometry, water load, line speed, knife distance, angle, distribution and blower performance determine the better choice.
Can I change the air knife gap without changing the blower?
Possibly, if the blower’s curve, power, current, temperature and operating limits accommodate the new point. Test the complete system before adopting the change for production.
What should be measured during a slot-gap comparison?
Measure pressure, airflow or documented outlet velocity, blower speed, current, true input power if available, gap uniformity and drying results at the same line conditions.