To size one blower for multiple air knives, calculate the airflow required by each knife in every operating mode, add only the demands that occur at the same time, and determine the pressure required along the most demanding active path. The resulting airflow-and-pressure pair is the blower duty to check against a model-specific performance curve. The installed knife count alone cannot determine that duty.
This guide covers the design calculation before model selection. It does not provide a post-installation acceptance test or a procedure for adjusting branch airflow.
Define the Design Boundary and Required Result
Start with the process outcome each knife must achieve at the intended line speed. A knife used for bulk water removal may not have the same air requirement as one used for final edge drying. Record the product, coverage width, operating speed and acceptance basis before converting a knife layout into an air-supply duty.
Identify every component between the blower inlet and the air-knife outlet: inlet equipment, shared pipe, manifold, individual branches, connections and knife plenums. The design calculation must use the intended configuration, not the blower’s maximum airflow or maximum pressure as if both occurred at once.
Record the Demand of Each Air Knife
Give each knife a unique identifier. For each one, record its effective slot length and gap, knife design, required plenum condition, supplier flow data or validated test basis, and the process area it serves. Include whether its gap is adjustable and which production modes use it.
If supplier data specify airflow at a particular gap and plenum pressure, keep those conditions attached to the figure. A slot-area estimate can help screen a concept, but actual discharge also depends on lip geometry, internal distribution, air conditions and manufacturing tolerance. Do not treat a theoretical area-times-velocity estimate as a verified final flow requirement.
Where a knife’s required flow is not known, mark the input as unresolved and obtain manufacturer data or a controlled test. A precise-looking total made from unsupported per-knife figures is not a reliable basis for blower selection.
Determine Which Knives Operate Simultaneously
List the knives installed on the machine, then create a separate operating-mode matrix showing which are active together. A product-width change, cleaning cycle or process transition may change the active set. Include a credible overlap during transitions if the control sequence allows it; exclude combinations that the validated control design prevents.
Do not add every installed outlet automatically, and do not use an average mode when a normal production mode requires more air. The calculated duty must cover each relevant simultaneous condition. Record the mode assumptions so that the selected blower and later control sequence can be checked against the same design basis.
Calculate Combined Airflow for Each Mode
For a given mode, add the required flow of its active knives:
Required mode airflow = sum of the required airflow of every knife active in that mode.
Calculate this separately for normal, peak and any other credible configuration. Different knife lengths, gaps or process duties can produce different individual demands, so multiplying one representative knife by the total count may be wrong. Record the source and conditions of each input alongside the total.
Include only justified allowances, such as a defined range of filter loading or a documented measurement tolerance. Repair a known leak rather than treating it as a permanent process-air requirement. An allowance cannot replace missing knife data.
Calculate the Shared Air Path at Combined Flow
The blower inlet and common header carry the combined flow until the air splits into branches. Estimate losses through the inlet filter or silencer, shared pipe, bends, reducers, flexible connector and manifold entry at the flow that actually passes through each component.
Use the intended internal diameters, developed lengths and fitting arrangement. A header selected around a single knife may become restrictive when more outlets operate together. State the assumptions for filter condition and any control or valve position used in the design case.
Calculate Each Active Branch at Its Own Flow
After the manifold, a branch carries its assigned knife flow rather than the full header flow. For each active branch, estimate the loss through its pipe, fittings, hose, valve, connection and air knife at that branch’s required flow. Include the knife’s required plenum condition where that is part of the supplier data.
Compare the pressure needed along each complete inlet-to-knife route. The route demanding the greatest pressure at its assigned flow governs that mode’s preliminary blower pressure requirement. Do not add pressure losses from parallel branches as if the branches were in series.
Branch flows and pressures interact in a real manifold. If the branches differ substantially, the intended flow split may require a network calculation or supplier verification. The governing-path estimate is a selection input, not proof that each knife will receive its assigned flow without appropriate manifold design.
Establish the Required Blower Duty Point
For each operating mode, pair its combined airflow with the pressure needed through the shared path and governing active branch. This paired requirement is the duty point. If another mode has lower total airflow but a more restrictive path, it may also be important; do not assume the highest-flow mode is the only one to check.
Summarize the calculation in a sheet with these fields:
| Design field | What to record |
|---|---|
| Operating mode | Active knife IDs and the control assumption that makes the combination possible |
| Individual demand | Flow basis, gap and required condition for each active knife |
| Combined airflow | Sum of simultaneous knife demands for that mode |
| Shared-path loss | Inlet, header and manifold assumptions at their relevant flows |
| Governing branch | Branch and knife needing the highest route pressure |
| Required duty | Paired blower airflow and pressure, with documented allowance |
Leave unsupported inputs open for measurement or supplier confirmation. Do not fill the sheet with a generic knife count or a nameplate motor rating.
Check Every Duty Point Against the Blower Curve
Plot the required flow and pressure for each important mode on the exact model and speed curve. Confirm the permitted continuous operating region, electrical supply and any manufacturer-stated limits. A model’s maximum pressure and maximum airflow are separate endpoints; meeting each number independently does not prove that it can deliver the required pair.
Compare the mode points with the intended control range and documented system-resistance assumptions. If the curve or per-knife demand data are unavailable, request them before final selection. Avoid choosing a larger motor solely to compensate for a restrictive header or an uncertain input.
What the Calculation Should Send to the Supplier
Provide Towwind Technology or the selected supplier with the knife schedule, simultaneous-mode matrix, per-knife flow basis, piping and manifold dimensions, shared and branch loss assumptions, required duty points, process target and electrical/control requirements. Ask the supplier to mark every governing duty on the proposed blower curve and state the conditions used for the recommendation.
The sizing calculation ends at a defensible design duty and model-review request. After installation, the actual system still needs separate mode-by-mode commissioning and acceptance checks; those measurements should not be confused with the pre-selection calculation.
Calculation Questions
Should all installed knives be added to the airflow total?
Only the knives that can run together in each credible mode are added for that mode. Check normal production and possible transitions rather than using the installed count alone.
Is required pressure multiplied by the number of knives?
No. Parallel knife flows are combined; pressure is evaluated through the shared path and each active branch. The most demanding complete route informs the mode’s pressure requirement.
What if a knife’s flow requirement is unavailable?
Keep that input unresolved. Obtain supplier data at the intended gap and pressure or validate it by a controlled test before making a final blower selection.