Sizing a Water Retting Tank for Flax: Litres, Days and Water Changes per Kilogram of Fibre

Water retting is the step that decides whether flax straw becomes spinnable fibre or compost, and it is usually sized by whatever tank a household happens to own. The mismatch runs both ways: an undersized tank crowds the stems, restricts water movement, and rets unevenly, while the water demand of the traditional process is large enough that an oversized batch can strain a household supply. This paper sizes the operation properly. From a stated water ratio, a stated fibre yield, and a stated temperature relation for retting time, it computes tank volume per kilogram of finished fibre, the number of water changes a ret requires at several temperatures, and the saving available from recirculation. In the computed results, the tank litres per kg fibre is 179.7; the days at 22c is 7.8; the changes at 22c is 3; the water at 22c l per kg is 75; the days at 30c is 4.5. Size the tank by fibre, not by stems: multiply the fibre wanted per batch by the computed litres per kilogram of fibre, and check the water budget at the warmest expected temperature separately, because warm water shortens the ret but the changes, not the days, set the water bill.. The study is simulated, not measured; all inputs are stated assumptions, printed in the accompanying computation, which readers may replace with their own values and re-run.