Drain & Vent Hole Calculator

Drain and fill time both use the same falling/rising-head Torricelli relationship as before — flow through an orifice under a changing head, integrated over the whole cavity — but the rack doesn't hold one fixed angle through the dip, so the head at any instant depends on where the crane is in its travel. The tool numerically integrates t = (A_cavity / (C_d × A_drain)) × √(2·head / g) in small steps rather than solving it in closed form, recalculating the head at every step as the angle changes. A_cavity is the cavity's internal cross-sectional area, A_drain is the total open area of the drain hole(s), and C_d is the orifice discharge coefficient — fixed at 0.6, standard for a sharp-edged circular orifice, for every calculation on this page.

Rack angle changes with depth in the tank. Parts go in at 30° from horizontal near the surface, and starting around 4 ft of depth the angle transitions — linearly, reaching 5° by the 6 ft mark — so a long part can lie down and fit inside a tank that isn't as deep as the part is long. Parts retrace the same angle-vs-depth curve coming back out: 5° at full depth, back to 30° within the last few feet before clearing the surface. The effective head driving flow at any instant is the still-unfilled (or still-full) cavity length times sin(that instant's angle) — smaller near full depth than near the surface, which is why most of the fill and drain happens early in each stroke.

Crane speed is now part of the model rather than an unmodeled caveat: the tool evaluates fill and drain against the fastest possible crane stroke — 20 seconds for the full 6 ft of travel, the floor speed for this line — since that's the most demanding case for hole sizing. Running the crane slower than this only helps: it gives the cavity more time at each angle along the way, so real fill/drain time at a slower stroke will be shorter than, or equal to, what's shown here. If the cavity hasn't finished filling or draining by the time the crane completes its stroke, the tool continues the calculation at the angle the part is left at (5° at full depth for fill, 30° at the surface for drain) for as long as it takes.

Fill time is modeled as the mirror image of drain time through the same lower (drain-side) opening: liquid enters low under the same head geometry that governs drainage. This tool does not separately solve compressible air flow out the vent as its own equation — instead, it applies a practical shop rule: the vent's open area should be equal to or greater than the drain's. If the vent is undersized, it becomes the limiting orifice for both directions and actual fill/drain times will run longer than the calculated values — the tool flags this rather than guessing a coupled-flow correction factor.

Buoyancy Check is informational: for round and rectangular tube, it reports the net force between the empty cavity's buoyancy (based on its outer volume, at roughly water's density) and the dry weight of the aluminum wall — the load the rack and crane hardware have to hold against while the part is empty and submerged. It does not change the fill or drain time calculation: the crane holds the rack's position and angle mechanically through the stroke regardless of buoyant force, so it's the angle-vs-depth profile above, not buoyancy directly, that governs flow through the holes. Not shown for box sections/enclosed panels, since that shape has no wall-thickness input to compute a weight from.

Known accuracy gap: this model is a real improvement over a fixed-angle calculation, but it is not field-validated to close precision. Against the one measured data point available to us — a part with a measured drain time of 240 seconds — this model predicts about 167 seconds at the fastest crane stroke: roughly 30% faster than what was actually observed. Treat the times on this page as a floor, not a guarantee — actual fill and drain will likely run longer, and by how much may vary with geometry. Use the margin under a dwell target as a starting point, not a pass/fail guarantee, until more field data narrows this down.

Other assumptions: constant cross-section along the cavity (no internal baffles or closed-off sections), a fixed 6 ft (72") full-submersion tank travel depth for every stage, and the drain and vent holes are at the true ends of the cavity as racked.

Minimum hole diameter: 3/8" (9.5mm) is the smallest drain or vent hole this tool will size or accept, regardless of what the math allows, since smaller holes clog easily with racking hardware, masking, or debris.