Thermocompressors Control

The prime advantage of a single fixed-nozzle thermocompressor is its compactness. Single nozzle units are preferred when large compression ratios are required. Single nozzle units are used for small capacity application and when several parallel units are needed to provide some degree of control. 

Unlike a control valve where energy is lost as flow is throttled, a spindle reduces the flow rate without reducing the energy available per unit flow. This permits the thermocompressor to use the maximum energy available at all ranges of flow and extends the capacity variation of a single unit. 

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. 

Figure 18-5 and Figure 18-6 show typical capacity rates and sizing curves for fixed multiple nozzle and regulating spindle thermocompressors, with steam as both motive and suction fluids. These curves should be used to approximate thermocompressor size and performance. The ectual efficiency can vary with the size of the thermocompressor, the control variability, the system, and the manufacturer. 

Figure 18-7 can be used to estimate the purchase cost for fixed-multiple-nozzle and automatically-controlled-spindle thermocompressors. 

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. 

A steam-jet thermocompressor is employed when low-pressure steam is used for the evaporation and the available steam pressure is high. The Jet compressor essentially replaces the existing pressure regulator or reducing valve that controls the steam pressure (and temperature) for evaporation and can be applied easily to single-effect and multi-effect units under the right conditions. 

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