Water-Flooded Oil-Free Screw Compressor GP Series


IHI has developed GP55 (output 55 kW) and GP75 (output

75 kW) of water-flooded oil-free screw compressor GP series and started their sales.

These are the first large twin screw-type compressors of 55 kW or larger lubricated with water without using lubricating oil in Japan.

The GP series expanded the lineup from 15 kW to 75 kW by starting the sales of “GP55/75” (Fig. 1).

1. Outline of GP series

1.1 Water-flooded screw compressor

The GP series compressors are classified as water-flooded twin screw compressors. The twin screw type compressor is classified into an oil-flooded compressor that injects oil into the compression chamber (hereinafter called air end) and an oil-free screw compressor that does not use oil. The oil-free screw compressor is further classified into a water-flooded screw compressor that injects water into the air end and a dry screw compressor that does not inject water or oil into the air end.

The dry screw compressor and oil-flooded screw compressor have the following disadvantages.

The dry screw compressor cannot seal rotor gaps with water or oil because it does not inject water or oil into the air end. To minimize the leak of compressed air, therefore, it is necessary to rotate the rotor at a high

speed, thus increasing the noise value. Since there is nothing to cool the compressed air in the compression stroke, the discharge temperature of a single-stage compression type reaches 300?C or higher, and the power required for compressing increases. A 2-stage compression type capable of lowering the discharge temperature to about 200?C improves the efficiency, but its structure result in high cost.

With the oil-flooded screw compressor, low-speed rotation becomes possible because the rotor gaps can be sealed with oil injected into the air end. Since the compressed air is cooled by oil in the compression stroke, sufficient efficiency can be obtained even through the single-stage compression and it is simple in mechanism. But since it uses oil, it is troublesome and costly to maintain and requires environmental measures.

Since the GP series (water-flooded oil-free screw compressor) injects water into the air end in place of oil, it removes such disadvantages of the dry screw type and oil-flooded screw type. 1.2 Features of GP series

The features of the GP series are cited below.(1) Environment-friendly

The GP series uses water for cooling and lubricating the air end and does not use oil at all. For this reason, it is free from oil mist, oil odor,

Water-Flooded Oil-Free Screw Compressor GP Series

HACHIYA Kazun ori : Compressor Engineering Department, Compressor & Machinery Division, Industrial Machinery






Fig. 1 Water-flooded oil-free screw compressor GP series


: Air flow

: Air + water flow : Water flow : Drain

Minimum pressure valve

Air end

(compression chamber)


Solenoid valve

Solenoid valve

Solenoid valve

Inlet valve

Solenoid valve

Water cooler




Water filter

Oil-free compressed air

Automatic drain To water filter

Water cooler

Suction filter

Air suction

Drain trap



Inlet of cooling water

Outlet of cooling water

Automatic water supply

Manual drain To manual drain port

(Note) Blue frame: Circuit when air-cooled type water-cooled type changing is done.

(b) Water- cooled type

Water-Flooded Oil-Free Screw Compressor GP Series


3. Co

n clusio


The water-f looded oil-free screw compressor GP series saves energy and reduces maintenance and environmental load, removing various disadvantages found with the conventional compressors. The GP55/75 now being marketed can better meet customers’ needs. IHI, a general manufacturer of oil-free compressors, will further expand its compressor business by means of the GP series.