Thermocompressors Operation

Steam jet thermocompressors or steam boosters are used to boost or raise the pressure of low pressure steam to a pressure intermediate bettveen this and the pressure of the motive high pressure steam. These are useful and economical when the steam balance allows the use of the necessary pressure levels. The reuse of exhaust steam from turbines is frequently encountered. The principle of operation is the same as for other ejectors. The position of the nozzle with respect to the diffuser is critical, and care must be used to properly posidon all gaskets, etc. The thermal efficiency is high as the only heat loss is due to radiation [5]. 

Do the steam systems operate at the needed pressure rather than at the available boiler pressure Too large a pressure leads to a higher condensation temperature for steam. This causes an unnecessarily large temperature differential for heat transfer. Fouling and other problems arise as a consequence, reducing the energy efficiency. In other words, use utilities at the lowest practical temperature. A simple solution to reduce the steam temperature is the use of a thermocompressor. This device uses high-pressure steam to increase the pressure of low-pressure steam to form steam at a desired intermediate pressure. 

The Standard Messo multistage vacuum crystallizer Figure 8.52) provides a number of cooling stages in one vessel. The horizontal cylinder is divided into several compartments by vertical baffles that permit underflow of magma from one section to another but isolate the vapour spaces. Each vapour space is kept at its operating pressure by a thermocompressor, which discharges to a barometric condenser. 

Thermocompressors can be of two basic types fixed nozzle types or units with a reguiating spindle. Fixed-nozzle thermocompressors are more efficieni and are used when the thermocompressor can be operated under steady-loac conditions. Thermocompressors with regulating spindles are designed for use when performance approaches critical conditions and changes in suction loac and suction and discharge pressures are expected. 

For spindle operated thermocompressors, shown in Figure 18-2(b), the nozzle design is quite different from that of fixed nozzle units. The nozzle and spindle assembly combines a rounded entrance orifice with a straight section into which a tapered spindle is guided. Operation is much like that of a needle valve. The length of the spindle travel is based upon the design requirements. 

Spindle operated units can be provided with either manually or automatically operated spindles. Thermocompressors with manually-operated spindles are designed for applications where loads will remain steady but where changes in operating conditions may occur and when operating conditions are not sufficiently established and some flexibility in nozzle orifice size is preferred.Thermocompressors with automatically-controlled spindles are used where pressure, suction, or discharge conditions vary and it is necessary to control discharge pressure or flow. 

A characteristic performance curve for a spindle-operated thermocompressor is shown in Figure 18-4. 

Figure 18-4 Characteristic performance curve of spindle operated thermocompressor.

Because of the unpredictable performance of thermocompressors, control of evaporators using them is more difficult than for a conventional system where it is necessary to set only steam and feed rates to maintain a constant evaporation rate. One way to provide flexibility with better operating stability is to use two or more thermocompressors in parallel. This permits capacity control without loss In energy economy. 

Spindle regulated jets are usually not acceptable except for thermocompressors. A discussion of their application is given in Chapter 18. For critical vacuum pumps, regulating spindles are an ineffective means of control and can lead to unstable jet operation. 

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