How Does a Liquid Ring Vacuum Pump Work?
A technical guide to liquid ring vacuum pump working principles — how the liquid ring forms, how vacuum is created, and how service-liquid temperature affects attainable vacuum.
A liquid ring vacuum pump creates vacuum by using a rotating liquid ring — typically water — to compress and discharge gas. The liquid ring acts simultaneously as a sealant, a coolant, and the compression medium.
This guide explains the working principle step by step, covers the role of the service liquid, and describes how single-stage and two-stage designs differ.
What Is a Liquid Ring Vacuum Pump?
A liquid ring vacuum pump is a rotating positive displacement pump. An eccentrically mounted multi-blade impeller rotates inside a cylindrical casing. Service liquid — usually water — is fed into the casing and thrown outward by centrifugal force to form a rotating ring against the casing wall.
Because the impeller is offset from the casing centre, the distance between the impeller hub and the liquid ring varies continuously around the circumference. This varying distance creates the compression cells that do the pumping work.
Main Components
| Component | Function |
|---|---|
| Cylindrical casing | Houses the impeller and liquid ring |
| Multi-blade impeller | Rotates eccentrically; creates compression cells |
| Inlet port | Admits gas to expanding cells |
| Discharge port | Expels compressed gas from contracting cells |
| Service liquid (sealing liquid) | Forms the liquid ring; seals, cools, and partially condenses vapors |
| Shaft seal | Prevents leakage along the shaft |
| Service-liquid separator | Separates discharged liquid from the gas stream |
Step-by-Step Working Principle
1. Formation of the Liquid Ring
When the impeller rotates, centrifugal force throws the service liquid outward against the casing wall. Because the impeller is mounted eccentrically, the liquid ring is thicker on one side of the casing and thinner on the opposite side.
2. Inlet — Expanding Cells Draw in Gas
On the side where the liquid ring is thickest, the space between the impeller hub and the liquid ring surface is smallest. As the impeller rotates toward the thinner side, each inter-blade cell expands. This expansion lowers the pressure in the cell and draws gas in through the inlet port.
3. Compression — Contracting Cells Compress Gas
As the impeller continues rotating toward the side where the liquid ring is thinnest, the cells contract. The liquid ring moves inward, reducing cell volume and compressing the trapped gas.
4. Discharge — Compressed Gas Is Expelled
When cell pressure reaches the discharge pressure, the compressed gas is expelled through the discharge port. The cycle — intake, compression, discharge — repeats continuously with every rotation.
Role of the Service Liquid
The service liquid performs three functions simultaneously:
Sealing: The liquid ring fills the gaps between impeller blades and casing wall, preventing gas from bypassing the compression stage.
Cooling: Because the process gas remains in direct contact with the service liquid throughout compression, heat generated during compression is absorbed by the liquid. This keeps gas temperatures low compared with dry-running compressors.
Partial condensation: The service liquid can cool the gas and condense or absorb part of the condensable vapor during compression. Actual performance depends on vapor load, service-liquid temperature and operating pressure. High-vapor processes may benefit from upstream condensation or a liquid separator.
The service liquid must be continuously supplied, cooled, and either recirculated (closed-loop system) or replaced (once-through system). See Closed-Loop Vacuum Systems for recirculation design guidance.
How Vacuum Is Created
Vacuum is created by the pressure differential between the expanding inlet cells and the system being evacuated. As the cells expand, gas flows from the process vessel — at lower pressure — into the pump inlet. The pump continuously removes gas from the system, lowering the system pressure until equilibrium is reached between the gas removal rate and any gas ingress.
The lowest pressure the pump can sustain — the ultimate vacuum — is limited primarily by the vapor pressure of the service liquid at its operating temperature.
Effect of Service-Liquid Temperature
Service-liquid temperature is one of the most important operating parameters for a liquid ring pump.
The minimum achievable absolute pressure is closely related to the vapor pressure of the service liquid. As service-liquid temperature rises, its vapor pressure rises, and the attainable vacuum becomes shallower. Conversely, lower service-liquid temperature generally supports deeper attainable vacuum.
Practical implication: Liquid ring pump performance is strongly affected by service-liquid temperature. Lower service-liquid vapor pressure generally supports deeper attainable vacuum, while actual performance depends on operating conditions and the selected pump series. Consult the performance curves for the specific series and service-liquid temperature when specifying a pump.
Single-Stage vs. Two-Stage Designs
Single-stage liquid ring pumps pass gas through one compression stage. Examples from the ZT Pumps range include the 2BVA series, LD series, and SZ series.
Two-stage liquid ring pumps pass gas through two compression stages in series within a single casing. The 2S two-stage series is an example. Two-stage designs can maintain stronger pumping performance at lower inlet pressures, but ultimate pressure depends on the specific series, service-liquid temperature and operating conditions.
For vacuum levels that require performance beyond what a liquid ring pump alone can provide, liquid ring pumps are commonly used as backing pumps in combination with a Roots vacuum booster. See Roots Vacuum Booster Systems for system design guidance.
Note on the 2BE series: The 2BE is a single-stage, double-acting design — not a two-stage pump. "Double-acting" refers to the dual inlet/discharge port arrangement that improves flow balance and capacity, not to a second compression stage.
Advantages and Limitations
Advantages
Handles wet and condensable-vapor gas streams: The service liquid can cool the gas and condense or absorb part of the condensable vapor during compression. This makes liquid ring pumps well suited to vacuum drying, distillation, and solvent recovery where dry-running pumps would require upstream condensation.
Low gas temperature rise: Because compression occurs in contact with the service liquid, gas temperature rise is minimal compared with dry-running designs. This is relevant when pumping thermally sensitive gases.
Simple, robust construction: With only one rotating part — the impeller — liquid ring pumps have very few wear components. There are no valves, pistons, or complex sealing arrangements. This simplicity contributes to long service life and low maintenance requirements.
Tolerant of liquid slugs and particulates: Small amounts of liquid carryover or particulates in the gas stream are generally tolerated without damage to the impeller, provided the service liquid flow is maintained.
Material flexibility: Pump casings and impellers can be supplied in cast iron, stainless steel 304/316, or other materials to suit the process fluid and gas stream. Material compatibility must be confirmed for the specific application.
Limitations
Ultimate vacuum limited by service-liquid vapor pressure: A liquid ring pump cannot achieve vacuum below the vapor pressure of its service liquid at operating temperature. For deeper vacuum, a Roots booster or alternative technology is required.
Service-liquid consumption and treatment: The service liquid must be supplied, cooled, and either recirculated or disposed of. Closed-loop systems reduce consumption but add equipment.
Lower efficiency at deep vacuum: Pumping capacity falls as inlet pressure approaches the service-liquid vapor pressure. Performance curves should be checked at the actual operating point, not only at rated conditions.
Not suitable for gases incompatible with the service liquid: If the process gas reacts with water or requires a specific service liquid, the system must be designed accordingly.
Typical Applications
Chemical Processing
Liquid ring pumps are widely used in distillation, evaporation, crystallization, drying, and solvent recovery. Their tolerance for condensable vapors and corrosive gas streams — with appropriate material selection — makes them a common choice in chemical vacuum applications.
Pharmaceutical Manufacturing
Vacuum filtration and drying of active pharmaceutical ingredients (APIs) often use liquid ring pumps. For applications requiring deeper vacuum, liquid ring pumps may serve as backing pumps in a multi-stage vacuum system. Final vacuum level and vapor load requirements should be confirmed for the specific process.
Food and Beverage
Vacuum evaporation for juice and dairy concentration and vacuum packaging use liquid ring pumps. Stainless-steel configurations may be selected where process compatibility and cleanliness requirements call for them. Final material and hygienic requirements should be confirmed for the specific process.
Power Generation
Steam condenser air-removal systems in power plants use liquid ring pumps to handle wet steam and non-condensable gases. The ability to handle wet gas streams is essential in this application.
Pulp and Paper
Paper machine vacuum systems — forming, pressing, and drying sections — rely on liquid ring pumps for their ability to handle large volumes of moist air and fiber-laden gas.
Oil and Gas
Vapor recovery units, crude oil degassing, and vacuum distillation in refineries use liquid ring technology. Liquid ring compression generally limits gas temperature rise because the process gas remains in direct contact with the service liquid. Suitability for flammable or hazardous gases depends on the complete pump configuration, materials, motor, instrumentation and applicable hazardous-area requirements.
Corrosive Gas Streams
With appropriate material selection, liquid ring pumps can be used for many corrosive process-gas streams. Material compatibility must be confirmed for the specific gas and service liquid.
How to Select a Liquid Ring Pump
When specifying a liquid ring vacuum pump, define the following parameters before selecting a model:
- Required vacuum level — absolute pressure at the pump inlet under operating conditions
- Gas flow rate — m³/h at inlet conditions (not atmospheric conditions)
- Gas composition — condensable vapors, corrosive components, particulates, flammable content
- Service liquid type and temperature — affects attainable vacuum and material selection
- Material requirements — based on gas and liquid corrosivity
- Seal type — mechanical seal or packing, depending on process requirements
Use the ZT Pumps Pump Selector to filter by vacuum level, flow rate, and application, or contact our engineering team for application-specific guidance.
ZT Pumps Liquid Ring Product Range
ZT Pumps manufactures a range of liquid ring vacuum pumps covering single-stage and two-stage configurations. The full range — including specifications, performance curves, and available materials — is listed on the Liquid Ring Vacuum Pumps category page.
Key series include:
- 2BVA series — direct-drive and coupled single-stage designs; compact footprint
- 2BE series — single-stage double-acting; high-capacity applications
- 2S two-stage series — two-stage design for stronger performance at lower inlet pressures
For system-level applications, liquid ring pumps are also available as part of Roots vacuum booster systems and closed-loop vacuum systems.
FAQ
What liquid is used in a liquid ring vacuum pump? Water is the most common service liquid. Other liquids — such as glycol solutions, solvents, or process-compatible fluids — can be used when water is incompatible with the process gas or when a lower freezing point is required. The service liquid must be compatible with the pump materials and the process gas.
Can a liquid ring vacuum pump handle condensable vapors? Yes. The service liquid can cool the gas and condense or absorb part of the condensable vapor during compression. Actual performance depends on vapor load, service-liquid temperature and operating pressure. High-vapor processes may benefit from upstream condensation.
What is the difference between single-stage and two-stage liquid ring pumps? A single-stage pump compresses gas in one stage. A two-stage pump passes gas through two compression stages in series, which can maintain stronger pumping performance at lower inlet pressures. Ultimate pressure depends on the specific series, service-liquid temperature and operating conditions.
How does service-liquid temperature affect performance? Higher service-liquid temperature raises the vapor pressure of the liquid, which limits the minimum achievable vacuum. Lower service-liquid temperature generally supports deeper attainable vacuum. Performance curves should always be checked at the actual service-liquid temperature for the installation.
When should I use a Roots booster with a liquid ring pump? When the required vacuum level is below what the liquid ring pump can achieve alone, a Roots vacuum booster can be added upstream to extend the vacuum range while the liquid ring pump continues to handle the wet gas stream. See Roots Vacuum Booster Systems for guidance.
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Written by
Zhongtian Pumps Engineering Team
The Zhongtian Pumps Engineering Team specializes in industrial vacuum pump manufacturing, application engineering and vacuum system design. Nantong Zhongtian Machinery Manufacturing Co., Ltd. has manufactured industrial vacuum equipment since 2002.
