Industrial drying: moisture balance and thermal cost

It runs the mass and energy balance of a dryer: how much water leaves per hour, how much dried product is left, how much energy that takes and how much fuel and money the dryer uses per hour, per month and per ton. Use it to budget drying a new material, compare fuels, track dryer efficiency and show what wetter raw material costs.

Dry matter goes in and out unchanged. 1,000 lb at 30% moisture (wet basis) holds 700 lb of dry matter; at 10%, those 700 lb become 700 ÷ 0.90 = 777.8 lb of product. The difference, 222.2 lb, is the water evaporated. Simply taking 20% of 1,000 lb (200 lb) is wrong: the correct math goes through dry matter.

Wet basis divides water by total mass and is what moisture balances read; dry basis divides by dry mass and is common for lumber and grain. To convert, db = wb ÷ (1 − wb): 30% wb equals 42.9% db and 10% wb equals 11.1% db. Pick the basis in the first field; when you switch, the calculator converts the values entered.

Useful heat adds heating of the solid and the water plus the latent heat to evaporate (about 1,024 Btu/lb at 122 °F); in the example, 0.51 MMBtu/h, 89% of it for evaporation. Divided by dryer efficiency it gives fuel energy: a direct rotary dryer at 55% needs 0.93 MMBtu/h, or about 1,005 scf/h of natural gas. Typical ranges by type (tray 35–40%, rotary 40–70%, fluid bed 40–80%) come from the literature; measure yours from a day of real fuel use.

With natural gas at $6 per MMBtu, the dryer spends $6.22 an hour, $8.00 per short ton of dried product and about $2,490 a month over 400 hours. Specific consumption is 2,095 Btu per pound of water evaporated, inside the typical 1,646 to 2,881 Btu/lb for direct rotary dryers; above the range, look for air leaks, poor insulation or excess exhaust.

A lot, because water to evaporate grows faster than moisture. In the example, material arriving at 35% instead of 30% raises cost from $8.00 to $10.56 per short ton of dried product, 32% more. Covering the yard, letting material drain before drying and negotiating moisture discounts usually pay back quickly.

This is an average steady-state balance for water only; it does not compute drying time, kinetics or dryer size, and the typical efficiency is only a starting point. Wood, grain and food dusts can form explosive clouds and catch fire in the dryer: housekeeping, temperature control and explosion protection should follow your hazard analysis under NFPA 652 and NFPA 61, and fired dryers fall under NFPA 86.