Section 19 · Design & Build
Design tools
Four small tools for the questions the sizing calculator deliberately leaves alone: how much head the pipework will eat, how big the tank needs to be, how large the air chamber should be, and what to measure before any of it.
Head loss & friction
Every pipe wastes some of the head you have, and the waste is not linear — it grows with the square of velocity and falls steeply as the bore increases. That is why a distribution main is usually cheaper to fix with a bigger pipe than with a higher tank.
The tool uses the standard Darcy–Weisbach relation for the head loss, with the friction factor from the Swamee–Jain approximation to the Colebrook equation — the same approach worked through on the physics page.
with v = Q / A, Reynolds number Re = vD/ν (ν = 1.004 × 10⁻⁶ m²/s at 20 °C), and f from Swamee–Jain. Laminar flow (Re < 2300) falls back to f = 64/Re.
Storage & demand
The ram's output is constant, so the tank only has to absorb the difference between a steady trickle and a bursty demand. Enter the demand you expect and the flow the ram delivers — from the sizing calculator — and this checks the two are compatible before you build a tank around an assumption.
Air-chamber sizing
The chamber's job is to absorb each beat's pulse without letting the pressure swing far. The trapped air follows a polytropic relation between pressure and volume, P · Vn = constant, which for a small change in volume gives a pressure change of
n = 1 for slow, isothermal compression and n = 1.4 for fast, adiabatic compression. A ram's beat lasts around a second, so the air behaves far closer to the adiabatic case than to isothermal.
That gives a defensible starting rule rather than a magic coefficient. Because the adiabatic case produces the larger swing for a given volume, taking n = 1.4 gives the conservative answer — the vessel you need for the swing to be genuinely bounded. Decide how much swing you will accept, and the minimum air volume follows:
with n = 1.4, and ΔVper beat = delivered flow ÷ beat rate. Real behaviour sits between the isothermal and adiabatic bounds, so this is deliberately safe — but it is still an estimate, so confirm against manufacturer data.
The calculator below works out both bounds side by side, because the gap between them is the part worth seeing before you commit to a vessel. Sizing on the isothermal figure is the classic way to end up with a chamber that hammers.
The rule above is derived from the polytropic relation, but it is not the only rule in circulation — published guidance ranges from rules as crude as "make the chamber about the volume of the delivery pipe" to full numerical simulation, and researchers are still working on the effects of vessel volume, orifice size and position. Treat any single rule, this one included, as a starting point. If you must err, err large: excess air volume only costs money, while too little puts the hammer straight into your delivery pipe.
Site survey checklist
Everything on this list is cheap to measure and expensive to get wrong. Print this section and take it to the site — the source assessment page explains the methods.
Before you commit to a site
If you have not yet worked out how much water a ram can deliver from your fall, lift and flow, do that first on the sizing calculator. The tools here refine a design; they do not replace it.