Industry Study Notes | 2026-08-10 | Yuanxian Machinery Internal Technical Learning Topic: Vacuum System Technology (Vacuum Pump Principles / Pumping Speed Calculation / Water Vapor Trap Design / Vacuum Level Control)
I. How Many Pumps Are Actually in Vacuum Cooling Equipment
First, correct a common misconception: a vacuum precooler/rapid cooler is not "a single vacuum pump pumping all the way down". By process stage, a complete vacuum system usually consists of two types of pumps working together:
| Pump Type | Applicable Stage | Features | Typical Configuration |
|---|---|---|---|
| Water Ring Pump | High-Temperature Initial Stage (90→50°C) | High-temperature resistant, water vapor resistant, tolerant of water carryover | Nash 7.5kW |
| Rotary Vane Pump | Medium-to-Low Vacuum Stage | High ultimate vacuum, stable pumping speed | Leybold SV300 (7.5kW) |
| Roots Pump | High Pumping Speed Boosting | Connected in series with the rotary vane pump to increase middle-stage pumping speed | ZJ600 (5.5kW, with water ring pump) |
| Screw Pump | Oil-Free Applications | No oil contamination, low maintenance cost | Dry Screw (Export Model) |
The typical process for a food vacuum cooler is two-stage: in the first stage, the water ring pump operates alone, lowering 90°C product to 50°C (the water ring pump withstands high temperature and water vapor, making it suitable for the high-humidity, high-temperature initial stage); in the second stage, the refrigeration system (compressor + moisture trap) + rotary vane pump operate in tandem, the moisture trap condenses water vapor to reduce the load, and the rotary vane pump continuously evacuates, lowering the product to 0–10°C.
II. How to Calculate Pumping Speed: Empirical Formulas Work Better Than Theoretical Formulas
The theoretical formula S = V × ln(P0/P1) / t is not sufficient for sizing because it does not account for leakage, product outgassing, or piping losses. For actual sizing, Yuanxian uses the empirical formula:
Pumping speed S = V_chamber × k / t_evac
V_chamber = vacuum chamber volume (m³)
k = empirical coefficient (accounting for leakage and product outgassing), taken as 5–8
t_evac = target evacuation time (min)
Example (CVF-1000-2P): chamber 8m³, k is taken as 6, evacuate to 600Pa in 5 minutes, required S = 8×6/5 = 9.6 m³/min ≈ 576 m³/h. With 1 SV300 (~300m³/h), actual evacuation to 600Pa takes about 8–10 minutes.
Pumping Speed-Throughput Quick Reference Table (Knowledge Base Vacuum Pump Selection Reference Curve):
| Throughput | Recommended Pump Configuration |
|---|---|
| 500kg | 1×SV200 |
| 1000kg | 1×SV300 or 1×RD0360A |
| 2000kg | 2×RD0360A or 2×SV300 |
| 3000kg | 3×SV300 |
III. Common Pitfalls: Pump and Chamber Mismatch
This is a high-incidence design flaw in the solution library, in two forms:
Case A: Insufficient pump capacity for medium-to-large machines. An 8m³ chamber equipped with a single 5.5kW pump (e.g., SV100B, 100m³/h) actually takes ~32 minutes to pre-pump to 600Pa (including pipeline losses), while the entire cycle is only 15–20 minutes, which means no cooling. An 8m³ chamber should be equipped with at least an SV200B (5.5kW) or Daluotong DLT-200.
Case B: Insufficient pump capacity for small machines. A 2m³ chamber equipped with a 2.4kW pump (SV25, ~25m³/h) requires ~24.6 minutes for pre-pumping. Upgrading to an SV100 (5.5kW) costs only ~2000 yuan more, reducing pre-pumping time from 25 minutes to 6 minutes.
Assessment formula: actual pre-pumping time = (V/S) × ln(101325/660) × 1.3; if the calculated value exceeds 70% of the claimed cycle, it is the bottleneck.
IV. Water Vapor Condenser: The Vacuum System's "Heart Partner"
The water vapor condenser (evaporator) is responsible for condensing water vapor into frost and protecting the vacuum pump from emulsification. Two key points:
- Evaporating temperature: standardized to -10°C ~ -5°C (not -10°C ~ -15°C); all copy and proposals shall be based on this.
- Prerequisite: Vacuum pumping with a rotary vane pump must be paired with a water vapor condenser; the condenser first condenses water vapor to reduce the load, so the pump can operate stably.
V. Vacuum Level Control: Pressure Reduction Rate Determines Quality
Rapid food cooling cannot be pumped all the way down in one go; the rate must be limited according to temperature range to prevent surface boiling and freezer burn:
| Temperature range (°C) | Maximum pressure reduction rate (mbar/min) | Risk control |
|---|---|---|
| 60–50 | ≤100 | Prevent excessively rapid surface temperature drop |
| 50–40 | ≤30 | Prevent internal water boiling |
| 40–30 | ≤10 | Allows expansion of aerated foods |
| 30–20 | ≤5 | Prevents freeze-drying effect |
| 20–10 | ≤2 | Stable final temperature |
6. One-sentence summary
Vacuum system design comes down to three things:Pump type selected according to process stage(water-ring pump for high-temperature stage, rotary vane pump for low-vacuum stage, Roots pump added for high pumping speed), Pumping speed checked against empirical formula(k is 5–8, reserve margin for air leakage), Water vapor trap is a prerequisite for rotary vane pumps(evaporation temperature -10°C ~ -5°C). During design review, first check whether the pump is sufficient, then check whether the water vapor trap is provided, and finally check whether the pressure reduction rate is reasonable.