Using a steam to entrain, recycle & compress gasses
Thermocompressors are used in the process, pulp and paper, petroleum, power, gas and many other industries to circulate steam and boost or recover low-pressure steam. Thermocompressors use a jet of high pressure steam to entrain a low-pressure steam or 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.
- Thermocompressors 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 where necessary in order to prevent freezing, etc.
- Can be used in potentially explosive atmospheres when supplied with either hand wheels or ATEX certified actuators.
- Thermocompressors 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 Thermocompressors are low in original cost and in upkeep.
Thermocompressors are 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.
Typical Thermocompressor 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.
In a recirculating operation, the jet compressor entrains steam which would normally be wasted, it does this by using higher pressure steam as the motive force to entrain the low-pressure steam, and then discharge at an intermediate pressure to the system pipe work. Typical systems could be a tire vulcaniser, a paper mill Yankee Dryer (both illustrated below), drying roll, or any type vessel where it is difficult to remove condensate such as for instance, a heat exchanger. Process plants where heating is accomplished by condensing steam may find condensate removal to be a problem, to help over come this many plants use jet compressors instead of reducing valves as the jet compressor increases the velocity of steam flow through the system and carries condensate with it. In this application, without a compressor condensate must be removed by an often impractical method of blowing down the system.
The thermocompressor recirculates with out loss of heat or energy, by using exhaust steam which might otherwise be wasted the compressor improves heat transfer rates and increases the heating or drying capacity of the system. Energy normally lost in a reducing valve is used to circulate the steam through the system. The thermocompressor increases flow velocity and sweeps condensate along with the steam.
Thermocompressors used to recirculate steam are usually automatically controlled to allow for accurate control of the downstream pressure. Control is no more complex than that required for a reducing valve, namely control by pressure and temperature. Essentially, controls should provide for a variable throttling range and a reset feature. The jet compressor will desuperheat to some extent. However, where steam pressures and temperatures are higher than is desired, additional desuperheating equipment should be used. For information on our range of desuperheaters please see here .
Shown above, Watchcase vulcanisers are being used to making rubber tires and tubes, this offers an excellent example of Type 426 Steam Jet Compressors engaged in a recirculation duty. Here a constant circulation of steam is necessary to avoid air and condensate pockets which would result in under cured spots in the product. The compressor discharges at a velocity sufficient to maintain proper recirculation of the steam and to accelerate removal of condensate that would otherwise lower the efficiency of the operation. Only enough live steam is required to make up for the lost steam that has been condensed through the process.
In this application, Type 426 Steam Jet Compressors are used for recirculating in connection with a Yankee Dryer for paper manufacture in Paper mills. Regulation here must be precise to maintain proper steam temperature on the drying rolls used for tissue paper. Pressure difference between the suction and discharge must be maintained at a level high enough to overcome the combination of the pressure drop within the roll, the centrifugal force of the condensate to be removed, and the pressure losses in the piping and condensate flash tank.
Reducing and Compressing
Jet compressors can be used to advantage in plants by compressing steam or reducing pressure. For instance, a plant may have been built originally to provide steam at certain pressures. Over a time conditions might change to an extent where the available pressures no longer provide a sufficiently wide range, a Jet Compressor can be used to correct the condition.
In cases where steam pressure is too high, the jet compressor can mix the high-pressure steam with exhaust steam and produce the required discharge pressure. In such operations, the cost of a jet compressor plus steam saving should be compared with the cost of a pressure reducing valve. Where steam pressure is too low, and high-pressure steam is available for operating the unit, the compressor can mix the two and boost the low-pressure steam. Low-pressure steam can be flashed-off condensate, steam from a low-pressure main, exhaust from a turbine, or from any other source.
Here, two Type 426 Variable Area Nozzle Jet Compressors are used to boost the pressure of flash steam from a condensate receiver which normally would be at atmospheric pressure. The two compressors use high-pressure steam to entrain flash steam and discharge at an intermediate pressure into the distribution pipe line for supply throughout the plant. A pressure controller measures the pressure in the pipe line which in turn controls the positioner fitted to the actuator on the thermocompressor. A pressure relief valve acts as a guard against the building up of pressure in the flash vessel. The control system operates the two units in sequence and allows operation at a satisfactory entrainment ratio with varying capacities. The first one operates up to its full capacity before the second begins to operate. When the load starts to decrease the second unit shuts down completely before the first unit begins to reduce.