The biggest fuel saving in salt making is to remove water before it reaches the fire: running seawater from 2.74% to 10% sodium chloride in the wind cuts the water to boil per pound of salt from 35.5 pounds to 9, a saving of about 75%. Pre-heating the feed with flue heat can save at most about 13.5%, and even with both a community that boils seawater may still burn 9.6 to 34.6 tonnes of wood a year, so large batches need a staged plant, not a bigger pan.
Part of Salt Production & Sourcing
Fuel efficiency in salt making is the reduction of wood burned per pound of salt, by removing water cheaply before boiling, recovering heat, and organising the work in stages.
Where The Fuel Goes
For 100 lbs of sodium chloride from seawater, the energy is about 4,166 MJ, of which 3,605 MJ is latent heat for boiling the water and 561 MJ is warming the feed to 100°C (Yield calculations). The fuel is then that energy divided by the wood’s energy (15 MJ/kg) times the pan’s efficiency. Three things can be improved: the water you have to boil, the heat you recover, and the share of the fire’s heat that reaches the pan.
1. Remove Water Without Fire
The parent’s tip is to run seawater over salt-grass or straw bundles in the wind. This is the principle of the graduation works used historically on brine, where brine trickles over brushwood to expose a large surface to the wind. The effect is large:
- seawater, 2.74% sodium chloride: water per pound of salt = (100 − 2.74) ÷ 2.74 = 35.5
- concentrated to 10%: (100 − 10) ÷ 10 = 9
- saving: 1 − 9 ÷ 35.5 = 0.75, or 75% of the water to boil
That figure counts the boiling load only; the work of lifting and circulating the brine is extra. Wind and sun do this work at no fuel cost, so any ponds or racks that bring brine up to strength before the pan are worth building first (Solar evaporation ponds).
2. Recover Heat
- Pre-heat the feed. Warming feed from 15°C to 100°C is 561 MJ of the 4,166 MJ, which is 561 ÷ 4,166 = 13.5% of the total. That is the ceiling on the saving from pre-heating, and real flue-heat recovery will not reach it all.
- Series pans. Let the hot flue gases pass under a second pan of weaker brine on the way out, as the parent suggests; the first pan finishes, the second pre-concentrates.
- Insulate the firebox and shape it so flames spread under the base rather than past it.
3. Burn Dry Wood
Wet wood spends energy driving off its own water. Season it to 20% moisture or below (Seasoning & Drying) and store it under cover.
Large Batches: Stage The Work
A single large pan struggles because scale builds, the fire swings, and the crystals have to be raked from a boiling pan. A staged plant divides the job:
- Settling tank: the brine rests so sand and sediment settle.
- Pre-concentration: wind or solar to raise strength.
- Main boiling pan: brine is boiled to near saturation, scale forms here and is removed.
- Finishing pan: a smaller pan where crystals form and are raked at a lower heat.
- Drain and dry: bitterns drain; salt dries.
A steady feed keeps the pans full without cooling them. The parent’s tip to boil on windy days applies to the open-air stages.
A Year’s Wood
Using the salt article’s table total of 1,040-1,870 lbs of salt a year and 20.4-40.8 lbs of wood per pound of salt (30% down to 15% efficiency), seawater boiling needs 9.6 to 34.6 tonnes of dry wood a year. Wood is also the community’s cooking and heating fuel, and a coppice of limited area has to supply both (Coppicing & Fuel Management). The same evaporation arithmetic applies to sugar (Boiling and Grading for maple syrup); in each case the fuel can be as large as the food it makes, so decide where the wood comes from before building the plant.
What You Need
- Pre-concentration racks or ponds
- A settling tank
- A main and finishing pan, and a flue that runs under a second pan
- A wood store for seasoned wood
- A log: wood weight per batch, salt weight per batch
Method: Improve Step By Step
Step 1: Measure the baseline: pounds of wood per pound of salt on your present plant.
Step 2: Add pre-concentration first, since it gives the biggest saving.
Step 3: Season the wood.
Step 4: Add series pans and insulation.
Step 5: Re-measure after each change.
Common Mistakes
- Optimising the fire before reducing the water.
- Stacking savings. The percentages above apply to different parts of the load, so they do not simply add.
- Counting wood as free. It has a land cost.
Safety
Fire, steam and salt-laden air
A salt works is a long-burning fire beside large pans of boiling brine. Keep the wood store away from the firebox, keep chimneys clear of soot, and keep a means to smother a fire. Steam from the open pans corrodes metal fittings and wet floors become slippery; ventilate the building, and keep children away from pans and pits.
What’s Next
- Learn next: Purification & Grading — turning the crude salt into a graded product
- Combine with: Evaporation pan design — the pans that carry the load