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Working principle of the compressor

For cost considerations, single-stage centrifugal compressors and high-pressure blowers are the most widely used in mechanical vapor recompression (MVR) systems; the following discussion will focus on these configurations.

For cost considerations, single-stage centrifugal compressors and high-pressure blowers are the most widely used in mechanical vapor recompression (MVR) systems; the following discussion will focus on these configurations.

Centrifugal compressors are volumetric‑control machines, meaning that, regardless of the suction pressure, the volumetric flow rate remains nearly constant. By contrast, the mass flow rate varies in proportion to the absolute suction pressure. The compression cycle of a single‑stage centrifugal compressor is depicted on an enthalpy–entropy diagram.

Power required by a single-stage centrifugal compressor:

For example: Saturated water vapor from the evaporator is compressed from an inlet state of p1 = 1.9 bar, t1 = 119 °C to p2 = 2.7 bar, t2 = 161 °C (compression ratio Π = 1.4). The compression process follows the polytropic curve 1–2, and the specific enthalpy increase of the vapor is Δhp.

For the specific enthalpy h2 of the steam, using the equation for compressor isentropic efficiency:

The resulting value is h2 = 2785 kJ/kg (ηs = 0.8 applies to a single‑stage centrifugal compressor operating with steam). t2 = 161°C corresponds to h2 and p2. At this point, the steam can be used to heat the first‑effect evaporator. First, it loses superheat and cools to the saturation temperature t3 (130°C) at pressure p2 (2.7 bar). At this temperature, it enters the evaporator’s heater. Based on…

 

 

Among other factors, the specific polytropic compression power hp depends on the polytropic exponent κ, the molar mass M of the intake gas, as well as the inlet temperature and the desired pressure rise. For the actual coupled power of the prime mover (electric motor, gas engine, turbine, etc.), a larger margin for mechanical losses is factored in. A single-stage centrifugal compressor made from standard materials can achieve a steam pressure ratio of 1.8; with higher‑quality materials such as titanium, this ratio can reach up to 2.5. As a result, the final pressure p2 is 1.8 times the intake pressure p1, or up to a maximum of 2.5 times. This corresponds to an increase in the saturated steam temperature of approximately 12-18K, with the maximum temperature increase reaching up to 30K, which depends on the intake pressure. In terms of evaporation technology, the common practice is to express the pressure based on the corresponding boiling point temperature of water. Thus, the effective temperature difference can be directly indicated.

For example: an inlet pressure p1 = 1 bar corresponds to 100 °C, while the final pressure p2 = 1.7 bar corresponds to 115.2 °C; the pressure ratio Π = p2/p1 = 1.7, and the saturation‑steam temperature rise is 15.2 K.