Lettuce Doesn't Wait
Lettuce harvested in summer comes out of the ground at 28–30°C. At this temperature, every extra hour it sits there burns more sugar and water through respiration. By the time the truck reaches the packing house, a chunk of the shelf life that could have been preserved is already gone.
The solution isn't to build more cold rooms, but to pull the field heat out before the product even hits the road. Vacuum precooling does it in 20–30 minutes; forced-air cooling takes 12–15 hours, and cold-room cooling takes 20–24 hours.
The question buyers ask most often is: how big do I actually need? This article answers with real delivery data, from 300 kg farm units to 22-ton export lines.
The Principle in One Sentence
A vacuum precooler lowers the chamber pressure to ≤600 Pa. At this pressure, water on the leaf surface boils at roughly 0°C, and evaporation carries the latent heat away directly (about 2,500 kJ per kg of water), with no need for cold air to penetrate the entire pallet. That is why 20–30 minutes is enough, while forced-air cooling takes 12 hours.
Equipment size is determined by three things: how many kilograms you want to process per batch, how large a chamber that volume requires, and how quickly the vacuum pump can pull the chamber down to 600 Pa.
Real Configurations: From Small to Large
The following data comes from the delivery proposals we analyzed in July 2026:
| Configuration | Batch Capacity | Chamber Volume | Refrigeration Capacity | Compressor | Vacuum Pump | Time to 600 Pa | Actual Cycle |
|---|---|---|---|---|---|---|---|
| CVF-300 Farm Unit | 300 kg | 2.39 m³ | 37.4 kW | Bitzer 12.9 kW | Leybold ~90 m³/h | 10.6 min | 20–30 min |
| CVF-4000-8P Dual Chamber | 4,000 kg | 27 m³ (13.5 per chamber, dual chamber) | 336 kW | Bitzer 124 kW | Leybold ×2 + Feilu ×2 | 7–13 min per chamber | 30–45 min |
| CVF-5000-10P (Shanghai) | 5,000 kg | 45.5 m³ | 364 kW | Bitzer 124.4 kW | Water-ring 55 kW + rotary-vane 2×7.5 kW + Roots ZJ600 | Three-stage, approx. 26 min total | 40–50 min |
| CVF-6000-12P | 5,500–6,000 kg | 45.6 m³ | 498 kW | Bitzer 165 kW | Water-ring 75 kW + Leybold ×2 + Roots | 20–24 min total | 35–45 min |
| CVF-8000-16P US Export | 22,400 kg (16 pallets × 1,400 kg) | 57.24 m³ | 1,544 kW (2×772) | 2× Bitzer 242 kW | 4× Leybold SV0630B (2,520 m³/h) | 9.1 min | 25–30 min |
A few points in this table are worth noting.
Chamber volume is not proportional to kilograms. The 300 kg unit uses 2.39 m³ (about 8 liters per kg), while the 22-ton line uses 57 m³ (2.6 liters per kg). Large machines achieve higher loading density because pallets are standardized.
Pump-down time is where design mistakes happen most. Of the CVF-300's 30-minute cycle, 10.6 minutes is spent pulling vacuum, or 35%. The Shanghai 5-ton machine's three-stage vacuum takes about 26 minutes in total, so the actual cycle is 40–50 minutes, not the nominal 30 minutes. The US 16-pallet line is the exact opposite: 4 SV0630B units pull 57 m³ down to 600 Pa in 9.1 minutes, leaving most of the cycle for actual cooling. When a supplier quotes a 20-minute cycle, calculate the pump-down time first.
Leafy greens tolerate water loss, but not temperature. Lettuce is 94–96% water, spinach 91–93%. A properly controlled vacuum cycle loses 1.5–2.5% water — and that evaporated water is precisely what does the cooling. Above 3% water loss, the leaves begin to wilt and you lose weight on the scale.
Case Study: US 22-Ton Export Line
The largest leafy-green configuration we have analyzed is the 16-pallet machine for a US customer: 22,400 kg per batch, a 57.24 m³ chamber, 304 stainless steel chamber and hydraulic door, 480V/60Hz power, and 1,544 kW of refrigeration capacity supplied by two Bitzer screw compressors. Four Leybold SV0630B units provide a combined pumping speed of 2,520 m³/h, reaching 600 Pa in about 9 minutes.
The analysis showed that the nominal 20-minute cycle is optimistic — the peak load of 1,750 kW exceeds the refrigeration system's capacity, so the actual cycle is 25–30 minutes. For a packing plant with continuous in-and-out flow, that is still fast enough. The engineering lesson: size refrigeration capacity against peak load, not average load.
Case Study: Shanghai 5-Ton Line
The Shanghai customer's 10-pallet machine uses three-stage vacuum: a 55 kW water-ring pump for roughing, two 7.5 kW rotary-vane pumps, and a Roots pump for fine pumping. The nominal 30-minute cycle does not hold up — three-stage pump-down alone takes about 26 minutes, and the actual cycle is 40–50 minutes. It is usable in high-volume scenarios, but if you are sensitive to turnaround time, either reduce the chamber size or increase the pumping speed; do not accept the nominal figure.
What This Means for Buyers
- Work backward from throughput: kg per shift ÷ kg per batch = cycles per shift. Each cycle runs 30–50 minutes including loading and unloading.
- Calculate pump-down: chamber volume ÷ pumping speed × ln(101325/600) × 1.3. If the quoted cycle is shorter than pump-down plus 15–20 minutes of cooling, the numbers have a problem.
- Size condensers to the climate. Evaporative cooling keeps condensing temperature at wet-bulb +5–10°C; in tropical regions air cooling runs 8–12°C higher, and compressor output is derated.
- For export equipment, a stainless steel chamber, 60Hz components, and local voltage (480V in the US, 600V in Canada) are hard requirements. For domestic use, a carbon steel chamber with a good coating is a reasonable cost reduction.
FAQ
Q1: How long does it take to cool lettuce from 30°C to 2°C? 20–30 minutes at ≤600 Pa, depending on the batch and packaging method. The same lettuce takes 12–15 hours with forced-air cooling and 20–24 hours in a cold room.
Q2: Will vacuum precooling dry out leafy greens? A properly controlled cycle loses 1.5–2.5% water, and the evaporated water is exactly the working medium that provides the cooling. Water loss above 3% indicates that the pump-down was too aggressive or that there is a problem with the loading method.
Q3: What is the smallest size a farm cooperative should buy? 300 kg per batch is our smallest standard fruit-and-vegetable unit (CVF-300 class, about 2.4 m³), suitable for a cooperative producing several tons per day. Above that, 1,000–2,000 kg is a common step up.
Q4: Why do quoted cycles often not match reality? Most of the discrepancy comes from pump-down time. A large chamber paired with a small pump spends a third of the cycle pulling vacuum. Before comparing prices, check the pumping speed against the chamber volume.
Q5: Do I still need a cold room if I buy a precooler? Yes. The vacuum precooler handles rapid removal of field heat, and the cold room handles subsequent holding. Working together, the precooler takes most of the heat load off the cold room.
Q6: Can the same machine cool mushrooms and berries? Yes, but not with the same set of parameters. Soft fruit requires staged, gentle pumping to keep water loss at 1–1.5%. A machine with a programmable vacuum curve can handle both.
Summary
Leafy-green precooling has a real size gradient: from 300 kg farm units to 22-ton export lines. The numbers that actually matter are chamber volume, pumping speed, and peak refrigeration load — not the nominal cycle on the datasheet. When selecting equipment, send us your batch size and target temperature, and we will calculate the pump-down time for your chamber.
Contact us at yuanxianmachinery.com for a precooling solution evaluation.
Author: Kenny Pan, Dongguan Yuanxian Food Machinery Co., Ltd. | August 6, 2026