Process & Thermodynamics
Closed-loop heat-pump dehumidification · psychrometric state points · three-stage profile
Read this first — how a heat-pump dryer works (in plain language)
Think of a heat-pump dryer as a dehumidifier running inside a warm, sealed room. Instead of venting moist air to atmosphere (like a conventional dryer, wasting all the heat used to evaporate that moisture), a heat pump captures the moisture as liquid water and reuses the heat to warm the air again. The same air circulates endlessly — dry and hot going in, cool and wet coming out, water squeezed out at the coil, reheated, and sent back. The only energy you pay for is the compressor pumping heat around the loop, plus small losses through the walls.
62°C / 20% RH
on trays
42°C / 71% RH
condenses water
drained (862 L)
62°C / 20% RH
The loop never opens — heat and air stay inside. Only water leaves (as liquid) and electricity enters (to drive the compressor).
Operating principle (technical)
The dryer is a closed-circuit heat-pump dehumidifier. Hot dry supply air from the condenser passes through the coconut trays, picks up moisture (and cools evaporatively), and returns to the heat pump unit. There, a controlled fraction of the return stream (40% bypass) flows through the evaporator coil, where moisture condenses and is drained as liquid water. The cold, dehumidified stream remixes with the bypass and is reheated by the condenser, closing the loop. Latent heat removed at the evaporator is recycled to the condenser — only the compressor work plus envelope losses must be made up from the electrical supply, which is what gives heat-pump dryers their 3–4× energy advantage over vented hot-air dryers.
State points (design point, stage 2)
| State | Description | T (°C) | RH (%) | W (kg/kg) | h (kJ/kgda) |
|---|---|---|---|---|---|
| 3 | Supply air after condenser | 62.0 | 20 | 0.0282 | 136.2 |
| 1 | Return air (wettest trolley) | 36.6 | 98 | 0.0380 | 137.8 |
| 2 | Off evaporator (saturated) | 16.2 | 100 | — | 40.7 |
Two-stage drying profile (anti-case-hardening)
Shelled coconut kernel is prone to surface sealing if dried too aggressively at low humidity. Stage 1 runs at 55°C for the wet phase (0.9 h, ~30% of water removed); stage 2 ramps to 62°C for 5.2 h to finish. Total: 19.7 h.
Energy balance & SMER
| Water removed per batch | 862 kg |
| Peak moisture load (×1.3 stage factor) | 37.6 kW latent |
| Evaporator coil duty (design point) | 94.7 kW |
| Condenser coil duty | 88.1 kW |
| Structure heat loss (PUF 50 mm) | 1.7 kW |
| Electric energy per batch | 214 kWh |
| SMER (kg water per kWh) | 4.03 kg/kWh |
Validation
The thermodynamic model is cross-checked against an independent Python reference implementation. All 20 computed values match to 3 decimals (relative error < 1%). Sanity ranges pass: drying time in 10–20 h, SMER in 2.5–4.5 kg/kWh, coil outlet above 5°C (no frost risk), return RH < 90%.