Fan-and-Pad House That Cools at the Pad but Overheats at the Far End
A 1,000 m² fan-and-pad polyhouse in Jalgaon is growing capsicum in April–May. Outside conditions at noon are 42°C and 25% relative humidity. Inside, the pad end reads about 28°C and 80% RH; the far end reads 38°C and 45% RH. The crop wilts at midday, flowers drop, and some fruit shows sunscald. Algae are growing on the floor near the pads. The grower has been running the foggers almost continuously from 10 a.m. to 6 p.m. to keep the temperature down, but the far end remains hot. The pad is 12 m², the fans are two 1.5 kW units, and the house has a 4 m gutter height with roll-up side vents that are kept closed during the day. The grower wants to know whether to add more foggers, enlarge the pad, or change the ventilation strategy before the next crop.
1. Using the outside and inside readings, calculate the wet-bulb depression and the expected pad outlet temperature. Then explain what the far-end readings tell you about the system.
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Use the wet-bulb depression, not the dry-bulb alone. At 42°C the saturation vapour pressure is about 8.2 kPa; at 25% RH the actual vapour pressure is about 2.05 kPa. From a psychrometric chart the wet-bulb is about 25°C, so the depression is 42 − 25 = 17°C. A well-maintained pad with 80% saturation efficiency should give an outlet dry-bulb of about 42 − 0.8 × 17 = 28.4°C. Near the pad, 80% RH at about 28°C is consistent with that.
At the far end, 38°C and 45% RH contains almost the same absolute moisture (about 3 kPa vapour pressure) as 28°C and 80%; the air has not dried out, it has gained sensible heat. So the pad is cooling at the inlet, but the house is picking up too much heat and moving too little air. That is a system and air-distribution problem, not a pad-efficiency problem alone.
2. Diagnose why the far end is hot and propose the priority corrections to pad area, airflow, shade and ventilation.
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Check four things in order. (1) Pad area and water flow: for 1,000 m², a common design target is about 1 m² of pad per 20–25 m² of floor, i.e. 40–50 m², with face velocity about 1.5–2 m/s and the pad fully wetted. The installed 12 m² is far too small, so the air cannot be cooled enough. (2) Fan capacity and seals: a target of roughly 30–40 air changes per hour for a 4 m high house is 120,000–160,000 m³/h; check fan curves at the actual static pressure, clean blades, and seal gaps and doors. (3) Heat load: whitewash or 50% shade on the roof, check for bare soil and exposed paths, and ensure the crop canopy is not overloaded. (4) Air distribution: use horizontal airflow fans or open internal partitions so cool air reaches the far end; the 45% RH far-end reading shows the air is heating before it returns.
Priority fix: enlarge the pad area, clean or replace the pad, service fans and belts, seal leaks, add roof shade, and run the system earlier in the morning so the structure is cooled before the peak. Do not simply run foggers harder.
3. What is your advice on the continuous fogging and on night-time humidity management to avoid disease?
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Continuous fogging in a fan-and-pad house adds water vapour faster than ventilation removes it. At night, when vents are closed and the roof cools, that moisture condenses on leaves and on the structure, creating long leaf-wetness periods that favour Botrytis, downy mildew and bacterial soft rot. It also wets flowers and reduces fruit set.
Use fogging as a pulse-controlled supplement: operate only when inside RH is below about 65–70%, in short bursts, with the fans running; stop before RH reaches 80%. Avoid fogging after late afternoon. At night, if inside RH is above 85%, ventilate to exchange humid inside air with drier outside air, checking absolute humidity rather than RH alone. Keep the canopy pruned and the floor free of standing water. Fix the algae near the pads because it is a symptom of continuous wetting and a fungus-gnat breeding site. If the pad system is correctly sized, fogging should be needed only during the driest pre-monsoon afternoons, not continuously.