A Paddy Dryer Facing 24% Moisture After Unseasonal Rain
A cooperative's paddy dryer is rated at 20 t per batch, drying from 20% to 14% (wet basis) in 8 hours using air heated to 60°C. Unseasonal rain brought the harvest in at 24%, and 90 t is waiting under tarpaulins. The miller pays a premium for head rice yield above 60%; trials show recovery falls from 62% to 54% when grain temperature exceeds about 55°C or when one pass removes more than 6 moisture points without tempering. Ambient air is 28°C at 60% RH. The burner delivers 1.2 GJ/h at 85% efficiency, the dryer's overall specific energy is 5.5 MJ per kg of water removed, and the plant can run 20 h/day. Wet grain held above 20% moisture for more than 24 hours at 30°C develops mould and off-odours.
1. Compute the water removal and the revised batch time at 24% feed. What does this do to throughput, and why is the queue a problem?
Read the worked answer
For 20 t at 24%: dry matter = 20 × 0.76 = 15.2 t. Final mass at 14% = 15.2/0.86 = 17.67 t, so water removed = 2.47 t per batch (2,470 kg).
Compare the rated case (20 → 14%): dry matter 16 t, final mass 18.60 t, water removed 1,395 kg.
The water load is 1.77 times the rated value, and since the dryer is heat-limited, time scales the same way: 8 × 1.77 ≈ 14.2 h per batch.
Output per batch falls to 17.67 t in 14.2 h, i.e. about 1.25 t/h against 2.33 t/h at rating - roughly half capacity. Nine hundred tonnes... 90 t wet needs about 5.1 batches, i.e. some 72 hours of drying.
That is the crunch: the queue cannot legally wait 72 hours. Grain at 24% will heat and mould within a day, so the constraint is not dryer capacity but holding time; the operation must buy time before the first batch finishes.
2. The manager wants a single pass from 24% to 14% to clear the backlog. Evaluate this against the milling premium.
Read the worked answer
A single pass removes 10 points, well beyond the 6-point threshold, and the grain spends the end of the batch in air at 60°C. Both triggers in the trials are hit at once, so the expected head rice recovery is the 54% figure rather than 62%.
At 90 t wet feed the saleable milled output difference is large: 8 points of head rice on roughly 75 t of dry paddy is about 6 t of head rice, and at typical premium prices that normally exceeds the cost of a day's extra drying.
Better route: two passes of about 5–6 points each (24 → 18.5%, temper 4–6 h in a bin, then 18.5 → 14%). Tempering lets internal moisture equalise and relieves the stress gradients that cause cracking. Also drop the plenum temperature to about 45–50°C for the last third of each pass, because although 60°C air is tolerable while the grain surface is wet, once the drying front passes, the same air heats already-dry kernels above 55°C.
The pass count, not the tonnage, is the decision variable here.
3. Verify the energy balance and propose a way to hold the 90 t safely while the dryer catches up.
Read the worked answer
Energy per batch = 2,470 kg × 5.5 MJ/kg ≈ 13,600 MJ. Useful burner output = 1.2 GJ/h × 0.85 = 1.02 GJ/h, so time = 13.6/1.02 ≈ 13.3 h, consistent with the 14.2 h estimate - confirming the dryer is heat-limited and cannot be sped up by running the fan harder.
To hold the queue: ambient air at 28°C and 60% RH corresponds to an equilibrium moisture content for paddy near 12–13%, so unheated aeration in a flat-bed or silo will pull the top 1–2 points off at almost no energy cost, and holds the rest cool and safe for several days. That converts a 90 t emergency into a manageable schedule.
Judgement: aerate first, dry in two tempered passes, and use the rated 20 t only as the batch size at 20% feed. Chasing tonnage through the dryer is what destroys the milling premium.