Compressor Discharge Pressure: How Liquid Ring Compressors Reach Higher Pressures

Discharge pressure is the figure every compressor duty is built around. It decides how much work the machine must do, how hot the gas becomes, and, beyond a certain point, whether one compression stage is enough. For wet, corrosive, or flammable process gas, it also determines which type of compressor can do the job safely.

What Compressor Discharge Pressure Means

Compressor discharge pressure is the pressure of the gas at the compressor outlet, where it leaves the machine and enters the downstream piping. In refrigeration and air conditioning, the same term describes the high-side pressure of a refrigerant circuit. In process plants, it describes the pressure a process gas compressor must deliver to a header, reactor, condenser, or recovery unit.

Discharge pressure is quoted in one of two ways:

• Gauge pressure is measured relative to the surrounding atmosphere and is marked with a G, as in MPa(G) or bar(g). Product specifications usually use gauge pressure, so 0.7 MPa(G) means 0.7 MPa above atmospheric.
• Absolute pressure is measured from a perfect vacuum. At sea level, absolute pressure equals gauge pressure plus about 0.101 MPa (1.013 bar).

The difference matters as soon as compression ratio comes into play, because compression ratio is always worked out in absolute terms.

Compression Ratio and Discharge Temperature

Compression ratio is the absolute discharge pressure divided by the absolute suction pressure. A compressor that draws gas at atmospheric pressure and delivers it at 0.7 MPa(G) works at a ratio of about 7.9, not 7.

Compression ratio drives discharge temperature. When gas is compressed without heat removal, its temperature climbs as the ratio rises. A published example for a dry compressor shows the effect: gas entering at about 16 °C and compressed at a ratio of about 3.5 leaves at a theoretical 112 °C. As the ratio rises further, the temperature keeps climbing until it reaches the limits of materials, seals, or the gas itself. This is why dry compressors split high overall ratios across several stages with cooling in between.

A liquid ring compressor behaves differently. The gas is compressed in direct contact with the service liquid, which absorbs most of the heat of compression. The liquid typically warms by only about 6–8 °C as it passes through the machine, and compression stays close to isothermal. Discharge temperature therefore stays low even as the compression ratio rises, a major advantage for flammable, heat-sensitive, or polymerising gases.

What Sets Discharge Pressure in a Process Plant

In a process plant, required discharge pressure depends on downstream requirements. Typical drivers include:

• The pressure of the header or pipeline the gas is fed into.
• The pressure needed to condense a vapour, as in solvent recovery or chlorine liquefaction.
• The inlet pressure of a downstream unit, such as a reactor, an absorber or a flare gas recovery system.
• Pressure losses through coolers, separators and piping between the compressor and its destination.

A liquid ring compressor is a positive displacement machine, so it delivers a near-constant volume of gas across a range of pressures. Its ports, however, give it a built-in pressure ratio set by their geometry. Running far from that design ratio costs extra power. Matching the machine’s design to the duty's actual discharge pressure is therefore what keeps a liquid ring compressor efficient.

How Liquid Ring Compressors Reach Higher Discharge Pressure

Every single stage has a practical limit on the pressure ratio it can handle efficiently. As the ratio climbs, internal losses grow, and stage efficiency falls. A liquid ring compressor reaches higher discharge pressures in two ways.

Balanced, rigid stage design. A high pressure rise loads the impeller and shaft. Compressor designs counter this with symmetrical or double-acting construction that balances the forces on the rotor, and with shaft support that limits deflection.

Two or more stages in series. Splitting the overall ratio across stages lets each one work closer to its best efficiency. A study of a two-stage liquid ring compressor found that the first stage, running at a high compression ratio, lost efficiency as the ratio rose, while the second stage operated around its efficiency peak. With suitable impeller design, the two stages shared the overall ratio almost equally, at about 1.1:1.

Kenflo’s liquid ring compressor range covers three discharge-pressure bands. The compression ratios below assume suction at atmospheric pressure; a different suction pressure gives a different ratio.

Kenflo Designs for High Discharge Pressure

The Kenflo CBK is built for high discharge pressure at medium and low capacity, and its design answers each mechanical demand that pressure brings:

• Zero theoretical axial force. A symmetrical impeller with double suction and double discharge balances the pressure forces on the rotor.
• Support at both shaft ends. Two symmetrically mounted cylindrical roller bearings carry the radial load, and paired angular contact ball bearings locate the rotor axially. All bearings are SKF.
• A high-strength shaft. The shaft is made of duplex stainless steel 1.4462, and the impellers are dynamically balanced.
• Pressure-tight sealing. Rubber O-rings seal the casing and withstand high pressure, and the shaft can carry a single or double mechanical seal or a dry gas seal.

For discharge pressures beyond a single CBK, the Kenflo 2BWX tandem compressor package connects liquid ring compressors in series. It reaches a limit discharge pressure of 1.3 MPa(G) across 1 to 1,400 m³/min, while the working process stays close to isothermal. The package includes a heat exchanger and a gas-liquid separator, so the working liquid is recycled, and it can be fitted with monitoring sensors and control elements. Typical services include toxic, flammable, and explosive gases in chemical processes, as well as the recovery of industrial exhaust gas.

Conclusion

Compressor discharge pressure sets the compression ratio, and compression ratio sets the work and heat of every compression duty. Liquid ring compressors keep that heat in check through near-isothermal compression and reach higher pressures through balanced stage design and series staging. From the high-capacity CBA2 to the high-pressure CBK and the 2BWX tandem package, Kenflo covers discharge pressures from near atmospheric up to 1.3 MPa(G).

Guangdong Kenflo Pump Co., Ltd. has built pumps since 1954, and the Kenflo engineering team works with plants to match each liquid ring compressor to the process discharge pressure.