Vapor-compression Evaporation - Comparison

Comparison

These two compression-type evaporators have different fields of application, although they do sometimes overlap.

• An MVR unit will be preferable for a large unit, thanks to the reduced energy consumption. The largest single body MVR evaporator built (1968, by Whiting Co., later Swenson Evaporator Co., Harvey, Ill. in Cirò Marina, Italy) was a salt crystallizer, evaporating approximately 400 metric tons per hour of water, featuring an axial-flow compressor (Brown Boveri, later ABB). This unit was transformed around 1990 to become the first effect of a multiple effect evaporator. MVR evaporators with 10 tons or more evaporating capacity are common.
• The compression ratio in a MVR unit does not usually exceed 1.8. This means that, if evaporating at atmospheric pressure (0.101 MPa and 100 °C), the condensation pressure at the heat exchanger will be 116.9 °C. Deducting the boiling point elevation boiling point rise (8 K for a saturated salt solution), this leaves less than 8 K delta T at the heat exchanger, resulting in a very large heating surface. Axial-flow and Roots compressor may reach slightly higher compression ratios.
• Thermocompression evaporators may reach higher compression ratios - at a cost. A compression ratio of 2 is possible (and something more) but unless the motive steam is at a reasonably high pressure (say, 16 bar g - 250 psig - or more), the motive steam consumption will be in the range of 2 kg per kg of suction vapors. A higher compression ratio means a smaller heat exchanger, and a reduced investment cost. Moreover, a compressor is an expensive machine, while an ejector is much simpler and cheap.

As a conclusion, MVR machines are used in large, energy-efficient units, while thermocompression units tend to limit their use to small units, where energy consumption is not a big issue.

The efficiency and feasibility of this process depends on the efficiency of the compressing device (e.g., blower, compressor or steam ejector) and the heat transfer coefficient attained in the heat exchanger contacting the condensing vapor and the boiling "mother" solution/liquid. Theoretically, if the resulting condensate is subcooled, this process could allow full recovery of the latent heat of vaporization that would otherwise be lost if the vapor, rather than the condensate, was the final product; therefore, this method of evaporation is very energy efficient. The evaporation process may be solely driven by the mechanical work provided by the compressing device.