Essential for Vacuum Drying: Complete Analysis of Four High-Efficiency Vacuum Pumps with Pros and Cons Comparison!

Vacuum obtaining equipment, also known as vacuum pumps, are devices used to produce, improve, and maintain vacuum conditions. According to their working principles, they can be divided into two types: gas transfer pumps and gas trapping pumps. Gas transfer pumps work by continuously sucking in and discharging gas to achieve the purpose of evacuation. Gas trapping pumps achieve evacuation by adsorbing or condensing the pumped gas inside the pump. These two types of pumps are further divided into various forms based on specific structures. This article only introduces several vacuum pumps commonly used in vacuum drying.

Main parameters of vacuum pumps:

(1) Pumping speed: When the pump is equipped with a standard test dome and operates under specified conditions, it is the ratio of the gas flow rate passing through the test dome to the equilibrium pressure measured at a specified position on the test dome. Abbreviated as pumping speed, in m³/s or L/s.

(2) Pumping capacity: The gas flow rate passing through the pump inlet, in Pa·m³/s or Pa·L/s.

(3) Ultimate pressure: When the pump is equipped with a standard test dome and operates under specified conditions without introducing gas during normal operation, it is the lowest pressure that tends to stabilize, in Pa.

(4) Starting pressure: The pressure at which the pump can start without damage and has pumping action, in Pa.

(5) Foreline pressure: The outlet pressure of a vacuum pump whose exhaust pressure is below one atmospheric pressure, in Pa.

(6) Maximum foreline pressure: The pressure beyond which the pump cannot operate normally, known as the maximum foreline pressure, in Pa.

(7) Maximum working pressure: The inlet pressure corresponding to the maximum pumping capacity, in Pa. At this pressure, the pump can operate continuously without deterioration or damage.

(8) Compression ratio: The ratio of the outlet pressure to the inlet pressure for a given gas in the pump.

Vacuum Pumps Commonly Used in Vacuum Drying

1. Water Ring Pump

Main advantages of water ring pumps:

· Simple structure with low manufacturing requirements, no need for intake or exhaust valves;

· Gas compression is nearly isothermal, with no mutual friction between metal contact surfaces inside the pump, reducing the risk of combustion or explosion accidents;

· Suitable for pumping gases containing steam, moisture, or solid particles;

· The rotating parts in the pump chamber are sealed by liquid from the fixed parts, requiring no lubrication.

Main disadvantages of water ring pumps:

· Low efficiency, generally 30%–50%, with high energy consumption;

· High noise, mostly above 95 dB, some up to 105 dB, causing severe noise pollution.

2. Steam Jet Pump

A steam jet pump is a low-vacuum pump that utilizes high-speed jet flow formed by water steam passing through a nozzle to entrain the pumped gas, thereby achieving a certain degree of vacuum in the pumped container. Steam jet pumps feature simple structure, stable and reliable operation, and can pump gases containing water vapor, dust, flammable/explosive substances, or corrosive gases. They have a large pumping capacity and are widely used in metallurgy, chemical industry, food, pharmaceutical, and other fields. Their disadvantages are high energy loss and low pumping efficiency. Water steam jet pumps use water steam as the working medium, ejecting supersonic steam jet from a Laval nozzle to entrain the pumped gas, thereby achieving gas extraction. The compression ratio of a single-stage pump is generally 8–10. To obtain lower working pressures, multiple jet pumps need to be connected in series.

3. Roots Vacuum Pump

Roots vacuum pump (abbreviated as Roots pump) is a rotary positive displacement vacuum pump without internal compression. It evolved from the Roots blower. Depending on the operating pressure range of the Roots pump, it can be divided into dry Roots pumps that discharge directly to atmosphere and wet Roots pumps, which belong to low-vacuum Roots pumps; there are also medium-vacuum Roots pumps (mechanical booster pumps) and high-vacuum multi-stage Roots pumps. In recent years, Roots pumps have been widely applied.

Generally, Roots pumps have the following characteristics:

(1) Large pumping speed over a wide pressure range;

(2) Equipped with a bypass overflow valve, allowing startup under atmospheric pressure and shortening pumping time;

(3) Clearance between rotors and between rotors and pump chamber walls; no friction among moving parts in the pump, no lubrication required, and oil-free pump chamber;

(4)  Symmetrical rotor shape with good dynamic balance, smooth operation; high-precision gear transmission selected for low noise during operation;

(5) Compact structure with small footprint; usually horizontal structure, with vertical gas flow in the pump chamber, facilitating the removal of pumped dust or condensates;

(6)  Suitable rotor profile and precise grinding processing for high volumetric efficiency;

(7)  Low operation and maintenance costs.

In vacuum engineering applications, Roots pumps are generally connected in series with fore pumps (such as rotary vane pumps, slide valve pumps, and water ring pumps) to form units, used as mechanical booster pumps in the medium-vacuum range; double-stage or multi-stage Roots pump units can achieve high vacuum; for dry, clean, oil-free pumping systems, air-cooled Roots pump units are commonly used; for systems with water vapor, wet Roots pumps are preferred.

From the structural principle of Roots pumps, the rotors can operate at high speeds, resulting in very high pumping speeds (up to over 100,000 m³/h) and simple structure with economical operation. Therefore, Roots pumps are widely used in metallurgy, petrochemical, light industry papermaking, electrical and electronic, food, and other industrial sectors. To avoid misoperation of Roots pumps, a bypass overflow valve is generally installed. The bypass overflow valve is designed based on the pump's permissible pressure difference ΔP, allowing startup under atmospheric pressure and enabling continuous operation of the Roots pump and fore pump simultaneously, thereby significantly reducing container pumping time (by 30%–50%).

4. Screw Vacuum Pump

Screw vacuum pumps belong to the dry pump series. Depending on the screw profile, they can be divided into screw type and spiral type. The main characteristics of screw vacuum pumps are: reliable operation and long service life; good dynamic balance; strong adaptability, capable of pumping dust, condensable vapors, and various other gases; convenient operation and maintenance. These pumps are widely used in electronics, nuclear energy, chemical, pharmaceutical, food, and other industrial fields.

Screw vacuum pumps come in vertical and horizontal layouts. Vertical layouts are suitable for vacuum pumps with small pumping speeds, compact size, and tight structure; horizontal layouts are suitable for vacuum pumps with large pumping speeds. The advantages of horizontal pumps are stable structure, low center of gravity, smooth rotation, low vibration, and low noise. Vertical pumps use spiral single-thread screw rotor profiles, which are simple to process, low cost, and easy to maintain.