The Use of Vacuum Pumps and Systems in Wood Drying

The vacuum system plays a dual role as both the "engine" and the "exhaust fan" during the drying process:

Establishing and Maintaining the Vacuum Environment: This is the basic function, creating the conditions for low-temperature boiling and the pressure differential driving force.

Removing Non-Condensable Gases: Primarily removes air from the wood cell cavities and any air that might infiltrate from seals. These gases do not condense, occupy space, affect the vacuum level, and must be continuously evacuated.

Removing Condensable Water Vapor: This is the main load. The vacuum pump needs to promptly remove the large amount of water vapor evaporating from the wood out of the drying chamber to maintain a stable low-pressure environment.

In wood drying, depending on the drying process and equipment, vacuum pump configurations mainly follow two patterns:

1. Intermittent (Cyclic) Vacuum Drying

This is the most common mode. The drying process cycles between "Heating" and "Vacuum" phases.

Workflow:

Heating Phase: The vacuum pump is off. Hot water or steam is circulated through the heating platens, conducting heat into the wood. The chamber pressure rises (may even be slightly above atmospheric pressure), allowing heat to penetrate the wood and heat the moisture.

Vacuum Phase: Heating stops. The vacuum pump starts, rapidly reducing the chamber pressure to the target vacuum level. Under low pressure, the superheated moisture inside the wood instantly vaporizes and is rapidly driven out by the pressure differential.

Cycle Repeats: The "Heat - Vacuum" cycle repeats multiple times until the wood reaches the target moisture content.

Requirements for the Vacuum Pump:

High Pumping Speed: Needed to quickly reduce chamber pressure from atmospheric to the target vacuum in a short time, shortening each cycle's duration.

Ability to Handle Large Amounts of Water Vapor: Each vacuum phase releases a surge of water vapor.

Common Pump Types:

Working Principle: The Roots pump has a very high pumping speed but cannot compress gases to atmospheric pressure alone; it requires a liquid ring or screw pump as a "backing pump" to provide the necessary backing pressure.

Advantages: Within the required vacuum range, the system's pumping speed is much greater than a single pump, significantly reducing evacuation time and improving drying efficiency. Relatively lower energy consumption.

Disadvantages: Higher initial investment, more complex system.

Advantages: Simple structure, robust and durable, can directly handle media containing large amounts of water vapor, tolerant of moisture, and less prone to damage even if small amounts of sawdust are ingested. Relatively low cost.

Disadvantages: Ultimate vacuum is limited by water temperature (typically around -94 kPa), higher energy consumption, requires a continuous supply of service water and produces warm water.

Liquid Ring Vacuum Pump: This is a very classic and commonly used choice.

"Roots Blower + Backing Pump" System: For large, efficient industrial drying equipment, this is a superior option.

2. Continuous Vacuum Drying

In this mode, heating and vacuum application occur simultaneously and continuously. It usually requires special heating systems (e.g., high-frequency/microwave heating or heat sources effective in a vacuum environment).

Requirements for the Vacuum Pump:

Need to remove water vapor and non-condensable gases continuously and steadily.

Due to continuous operation, pump reliability and durability are critical.

Common Pump Types:

Advantages: No working fluid in the pump chamber, completely tolerant of water vapor, avoids issues like oil emulsification or water contamination. Long maintenance intervals, reliable operation. Lower energy consumption than liquid ring pumps.

Disadvantages: Highest initial investment cost.

Dry Screw Vacuum Pump: Offers significant advantages in this continuous mode.

Large Liquid Ring Pump: Can also handle the task, but long-term water and electricity consumption and warm water discharge need consideration.