Batch vs Continuous Tank Sizing
Batch tanks size from batch size, batch count, fill fraction, and heel; continuous tanks size from flow and residence time or surge. Learn why the design basis must be clear before sizing a tank.
Definition
Batch and continuous tanks are sized from different starting points. A batch tank is sized from the batch size, the number of batches it must hold at once, the working fill fraction, and any heel (dead) volume left between batches — the volume comes from a count of discrete charges, not a flow. A continuous tank is sized from the through-flow and either the residence time the process needs (τ = V/Q) or the surge/buffer capacity it must provide. Both end in a geometric tank volume, but the design basis — discrete batches versus steady flow — is fundamentally different and must be chosen before any sizing begins.
Why it matters
If the design basis is unclear, the tank is sized for the wrong thing. Sizing a batch duty as if it were continuous (flow × residence time) ignores that the tank must physically hold whole batches plus a heel at a working fill level; sizing a continuous duty as if it were batch ignores the flow and the hold time the process actually needs. The two methods can give very different volumes for the same nominal throughput. Stating the basis up front — batch size and count and fill and heel, or flow and residence/surge — is what keeps the sizing defensible and prevents a tank that is too small to hold a batch or too large to turn over usefully.
Formula
Units involved
- •V_batch, V_working, V_geom, V_surge — volume in m³, litres, or gallons
- •n_batches — count, dimensionless
- •fill — working fill fraction, dimensionless (e.g. 0.8)
- •heel — dead/heel volume in m³ or litres
- •Q, Q_in, Q_out — flow in m³/h, L/s, or gpm; τ, t — time in h or min
Concept diagram
Worked example
A duty must hold 2 batches of 10 m³ at 80% working fill with a 1 m³ heel. Compare the batch basis to a continuous basis at 36 m³/h with a 30-minute residence time.
- 01Batch working volume = V_batch × n = 10 × 2 = 20 m³
- 02Batch liquid incl. heel = 20 + 1 = 21 m³
- 03Batch geometric volume = 21 / 0.80 = 26.25 m³
- 04Continuous basis: V = Q × τ = 36 × 0.5 = 18 m³ (working)
- 05Continuous geometric at 0.80 fill = 18 / 0.80 = 22.5 m³
Same ~20 m³ of liquid, but the batch basis needs ~26.25 m³ geometric (whole batches + heel) versus ~22.5 m³ on the continuous basis — the basis changes the answer.
Common mistakes
- •Not stating the design basis before sizing — batch versus continuous gives different volumes for the same throughput.
- •Sizing a batch tank on flow × residence time, ignoring that it must hold whole batches plus a heel.
- •Forgetting the heel/dead volume that stays in the tank between batches.
- •Applying a fill fraction in one method but not the other, so geometric volumes are not compared like-for-like.
- •Assuming residence time is satisfied by a batch tank — reaction or contact time is a separate check from volume.
When to use the calculator
Use the batch-tank-sizing calculator for discrete-batch duty (batch size, count, fill, heel, allowance), and the residence-time and surge-volume calculators for continuous duty. Size both ways when the duty could be run either mode, and compare the geometric volumes.