Working principle


Due to cost reasons, single-stage centrifugal compressors and high-pressure fans are commonly used in mechanical steam recompression systems. Therefore, the following explanation is for this type of design.

A centrifugal compressor is a volume control machine, which maintains a volume flow rate almost constant regardless of the suction pressure. The change in mass flow rate is proportional to the absolute suction pressure. The compression cycle of a single-stage centrifugal compressor is depicted in an enthalpy entropy diagram.

Power required for a single-stage centrifugal compressor:

For example, compress saturated water vapor from the evaporator from the suction state p1=1.9 bar, t1=119 ℃ to p2=2.7 bar, t2=161 ℃ (compression ratio Π= 1.4). The compression cycle follows a polytropic curve 1-2, increasing the specific enthalpy of steam Δ HP.

For the specific enthalpy h2 of steam, the equation for the internal efficiency (isentropic efficiency) of the compressor is used:


The obtained value is h2=2785 kJ/kg( η 0.8 is suitable for single-stage centrifugal compressors in water vapor media. T2=161 ℃ relative to h2 and p2. Now this steam can be used to heat the first effect evaporator. Firstly, it loses its overheating and cools down to saturation temperature t3 (130 ℃) and pressure p2 (2.7 bar). At this temperature, it enters the heater of the evaporator. base on



 

Among other factors, the unit variable compression work hp depends on the multi-party index κ And the molar mass M of the inhaled gas, as well as the inhalation temperature and required pressure rise. For the actual coupling power of the prime mover (electric motor, gas engine, turbine, etc.), a larger mechanical loss margin is considered. A single-stage centrifugal compressor with a impeller made of standard materials can achieve a water vapor pressure rise with a compression factor of 1.8. If higher quality materials such as titanium are used, the compression factor can reach up to 2.5. In this way, the final pressure p2 is 1.8 times the suction pressure p1, or a maximum of 2.5 times, which corresponds to an increase in saturated steam temperature of about 12-18K, with a maximum temperature rise of up to 30K, depending on the suction pressure. As for evaporation technology, the usual practice is to represent its pressure based on the corresponding boiling point temperature of water. In this way, the effective temperature difference is directly represented.

For example, the suction pressure p1=1 bar corresponds to 100 ℃, the final pressure p2=1.7 bar corresponds to 115.2 ℃, and the pressure ratio Π= P2/p1=1.7, saturated steam temperature rise is 15.2 K.

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