How to Read Vacuum Degree and When Pumps Engage: Vacuum Measurement and Control Logic for Vacuum Cooling Equipment

The vacuum curve on the panel is not decoration for the customer to look at—it is the baton for the entire vacuum cooling unit: when the pump engages, when it backs off, when the valves open, when they close—all depend on it. The conclusion first: pressure first determines how low the product temperature can be pulled, then determines how the pump set is sequenced; and the low-pressure reading during the evacuation stage and the absolute pressure during normal circulation inside the chamber differ by two orders of magnitude—one set of vacuum gauges cannot measure both accurately.

First, distinguish the equipment types. Fruit and vegetable vacuum precoolers handle room-temperature produce (25–30°C infeed), while food vacuum rapid coolers handle hot food (80–90°C infeed). Both rely on reducing the pressure inside the chamber so that water boils at low temperature, and evaporation carries away latent heat. Below, this is explained in three layers: “what pressure corresponds to, who reads it, who controls it.”

1. Pressure determines the final temperature: pressure actually corresponds to the boiling point of water

The pressure inside the chamber determines how low the water temperature can be pulled—the lower the pressure, the lower the boiling point. This is standard physical property data, not an empirical estimate:

Chamber pressure	Corresponding boiling point of water
4,250 Pa	30°C
2,340 Pa	20°C
1,230 Pa	10°C
870 Pa	5°C
610 Pa	0°C

To pull the product down to around 0°C, the chamber pressure must be brought to around 600 Pa; if the customer only needs 10°C, 1.2 kPa is enough, and the pump does not need to stay at full pull. The “ultimate vacuum” on the equipment nameplate is exactly this lower limit. Conversely, when a customer specifies a final temperature requirement, the corresponding pressure point is already fixed.

2. The target vacuum levels of the three product lines differ by an order of magnitude

Product line	Target vacuum	Description
Fruit and vegetable vacuum precooling	≤600 Pa (some models ≤400 Pa)	From 8 m³ small and medium chambers to 45 m³ and above large chambers, all stay in this range
Food vacuum rapid cooling	≤600 Pa	The entire line uses the same range
Vacuum freeze-drying	<10 Pa (some ≤5 / ≤7 Pa, a few models ≤20 Pa)	More than 60 times lower than precooling/rapid cooling

Precooling and rapid cooling stay in the 600 Pa range, while freeze-drying must enter the 10 Pa range. The pump sets on the two sides are not the same thing, which is also why it is difficult for one unit to handle both process types at the same time.

3. One set of vacuum gauges cannot measure accurately across two orders of magnitude

There are at least two widely separated pressure ranges on vacuum cooling equipment:

• Evacuation stage and holding stage: from atmospheric pressure down to several hundred pascals; this stage requires readings accurate to the pascal level;
• Normal circulation condition inside the chamber: precooling/rapid cooling operates near vacuum, but for materials prone to freeze damage such as cut flowers and berries, the lower limit of absolute pressure inside the chamber must be kept at around 800 Pa; below this value there is a risk of freeze damage, and this stage also requires pascal-level readings;
• Atmospheric pressure section in the piping: some valves and control circuits operate in the 50–60 kPa range.

Between 600 Pa and 50–60 kPa there is a difference of two orders of magnitude. Using a wide-range compound gauge to read 600 Pa, the pointer is squeezed into a tiny division at the far left of the dial, and the error is enough to cover the entire control window; using a 0–20 kPa range gauge to read atmospheric pressure conditions, the gauge is directly driven beyond its range.

The approach is to select gauges by range segment according to order of magnitude: the low-pressure section (evacuation, holding, freeze-damage lower limit) uses a 0–22.2 kPa range, and the atmospheric pressure section uses a 0–105 kPa range. Choose the same manufacturer and same model series so that the reading basis is consistent and easy to compare. When troubleshooting on site why the vacuum level keeps fluctuating, “the measurement point falls in an airflow turbulence zone” is also one of the causes—the gauge should be installed on a straight pipe section with stable airflow.

4. Who reads, who issues commands

The vacuum control system itself has accuracy specifications: high-vacuum measurement error ±0.1%, long-term stability ±0.2% full scale/year, with matching vacuum gauges and vacuum modules. Above that is the control layer:

• The freeze-dryer approach is very direct—the vacuum gauge uses an electric contact vacuum gauge, automatically controlling the start/stop of the Roots pump according to a preset vacuum value. When the pressure reaches the set point, the gauge itself sends a switching signal out, with no need for a person to watch it.
• A more complete control scheme is PLC: small models use an economical PLC with a 7-inch touch screen, while medium production models upgrade to the S7-1200 level with a 10-inch screen, with safety protection covering three paths: vacuum, temperature, and refrigeration system. The pressure signal enters the PLC, and the PLC then determines pump start/stop and valve actions.

In other words, the pressure gauge serves as both eyes and hands: it is simultaneously the sensor and the trigger source for the execution logic.

5. Three hard logics of pump set sequencing

1. A Roots pump cannot evacuate on its own. It is a booster pump and relies on the pressure difference across it to push gas to the next stage. The correct sequence is to first use a rotary vane pump or water ring pump to evacuate the system below 1 kPa, then automatically start the Roots pump. If the backing pressure has not been reduced, the Roots pump turns but does no work—this 1 kPa threshold is the dividing line between freeze-dryers and precooling/rapid cooling machines in terms of pump sets.

2. Rapid coolers are divided into two stages. The first stage uses a water ring pump, which can withstand high-temperature, high-humidity water vapor; after the chamber temperature has dropped, a rotary vane pump (or dry screw pump) is then used in coordination with the refrigeration system. Switching between the two stages is done by a pneumatic ball valve.

3. Vent before stopping the pump. An electromagnetic vent shut-off valve is installed on the vacuum pump piping: when the pump stops working, it automatically closes the vacuum line and at the same time lets air into the pump, preventing vacuum pump oil from being sucked back into the vacuum line under negative pressure. If this step is omitted, pump oil will enter the process piping.

Accessories are likewise determined according to pressure logic: a vacuum butterfly valve is installed on the vacuum line between the drying chamber and the condenser for easy opening and closing at any time; corrugated flexible hoses are used between the pump and piping to isolate vibration; the pump inlet diameter follows the pump, DN32 for small models, DN50–DN65 and upward for medium models.

6. How to troubleshoot abnormal vacuum

Symptom	What to check first
Vacuum cannot be pulled down	Door gasket or flange leakage, emulsified pump oil, valves on the piping not fully open
Pulling down too slowly	Pump pumping speed does not match chamber volume (occurs more frequently than other causes), piping too narrow, water accumulation or frost in the moisture trap
Roots pump does not start	Backing vacuum has not reached 1 kPa, electric contact gauge set value drift, Roots pump cooling water interruption
Vacuum fluctuates back and forth	Internal leakage of vent valve or butterfly valve, measurement point falls in an airflow turbulence zone

7. Remember three sentences

Pressure is not just a number—it is the upper limit of how low the product temperature can be pulled. Freeze-drying uses the 1 kPa threshold to bring the Roots pump in, while precooling and rapid cooling only need to stay in the 600 Pa range. Before stopping the pump, first let the vent valve let air into the pump.

FAQ

Is lower vacuum always better for a precooler? No. The pressure only needs to be low enough to reach the boiling point corresponding to the final temperature; going below the value required by the process is equivalent to needlessly increasing the pump load. For materials prone to freeze damage such as cut flowers and berries, the lower limit of around 800 Pa must also be maintained.

Why should the vacuum gauge be provided in two units? The evacuation/holding stage is at the pascal level, while the atmospheric pressure section of the piping is in the 50–60 kPa range, a difference of two orders of magnitude. A single gauge will either be inaccurate in the low-pressure stage or be driven beyond range by atmospheric pressure conditions.

Why does the Roots pump always start late? It works by pressure difference and must wait until the backing pump has evacuated the system below 1 kPa. Starting late is normal logic, not a fault; if it never starts, first check the backing vacuum, the electric contact gauge set value, and the Roots pump cooling water.

To see the standard configurations of specific models under each product line, refer to the three product lines: fruit and vegetable vacuum precooler, food vacuum rapid cooler, and vacuum freeze-dryer; more articles on equipment operation and maintenance parameters are collected in the Technical Library. If you need the vacuum level and pump configuration calculated according to your chamber volume and target final temperature, you can contact us directly.