Using a gas to entrain & compress gases
Jet Compressors are used in the process, pulp and paper, petroleum, power, gas and many other industries to circulate steam or gas, boost or recover low-pressure steam, and to mix, transfer, and compress gases. Jet compressors use a jet of high pressure gas to entrain a low-pressure gas, mix the two thoroughly, and discharge the mixture at an intermediate pressure. Gases can be steam, air, propane, or other mixture - basically any gas. When both motive and suction gases are steam, the compressor is generally referred to as a Thermocompressor.
Compared with other types of compressing equipment, Jet Compressors offer definite advantages:
- They are simple in construction and have no rapidly rotating parts to wear, adjust, or replace.
- Jet Compressors can be made from practically any machinable material including carbon steel, bronze, stainless steels, titanium, hastelloy, etc.
- Require little or no maintenance and only require periodical inspection, therefore they can be installed in remote locations.
- Steam Jackets can be fitted to either part or the whole of the compressor when the duty requires it in order to prevent freezing, etc.
- Can be used in potentially explosive atmospheres without additional safeguards as they have no electrical components.
- The Jet Compressor not only performs the primary function of compressing and mixing gases but, in addition it takes the place of a reducing valve and salvages much of the energy lost in the reduction of the gases pressure.
- Compared with other types of equipment Jet Compressors are low in original cost and in upkeep.
Two different types of jet compressor are available:
Variable Area Nozzle Compressor
This type of Jet Compressor is used when motive, suction, or discharge conditions vary and it is necessary to control the discharge pressure or flow. Control of this compressor is accomplished by a spindle which regulates the motive gas flowing through the nozzle, however unlike a control valve where energy is lost, the spindle can reduce flow without reducing the available energy. Control of the spindle can activated by temperature, pressure, flow or suction to motive ratio and variation of the spindle travel can be achieved with any suitable actuator.
Fixed Nozzle Compressor
This type of Jet Compressor has no regulating spindle, and are generally used where operating conditions are stable. These compressors will accept some level of variation however the operating envelop is much smaller than that of the Variable Area Nozzle Compressor, however these compressors are much more stable when higher compression ratios are necessary.
Typical Compressor Applications
A few of the primary applications where eductors are used can be seen below, this list is by no means exhaustive as their uses are numerous.
Fixed Nozzle Jet Compressors make it possible to mix natural gas or L-P gases in desired proportions without the need for any complicated apparatus. In many installations jet compressors are used when the necessity arises for producing a gas of a specific heating value and density to substitute for another commonly used gas. The compressors will operate at peak load conditions or to provide emergency supplies in case of emergency breakdowns. Using jet compressors propane, butane or natural gas can be mixed with air in a specific proportion to obtain the desired heating value. In gas mixing applications automatic spindle control is not important as these units are designed for a constant mixing ratio. Therefore once correctly adjusted the fixed nozzle compressor will perform consistently time after time. To obtain a system with enough flexibility to match load variations, multiple units are usually installed and used in parallel operation.
The compressors in this application do not have regulating spindles, but the set of units is sized so that 15 variations of capacity are available and each unit is designed to handle double the capacity of the preceding unit. Each gas compressor is operated either wide open or is shut off by means of a valve. A check valve should be installed on the suction side of each compressor to close automatically in order to prevent back flow of gas in to the intake manifold when a compressor is shut off.