Regardless of evaporator type evaporators may also be grouped based on energy source used and connection as follows:
Some Basic Equations
1. Normal steam driven
Single effect (fig. 1)
Multiple effect (ME) (fig. 2)
2. Thermal Vapour Recompression (TVR)
Single effect
Multiple effect (ME)
3. Mechanical Vapour Recompression (MVR)
Single effect (fig. 3)
Multiple effect
A normal steam driven single effect evaporator needs roughly the same amount of primary steam as the amount of water evaporated. Due to reuse of the vapour, the steam demand for a four effect evaporator is only about 0,28 tons of primary steam per ton of water evaporated.
The greater the number of effects, the lower is the steam demand, but on the other hand, the investment goes up, and so does also the cost for pumping, maintenance etc. Additionally, a very long multiple effect evaporator is complex to control and operate.
As a result there exists a financial optimum, which for large evaporators lies at 6 - 10 effects. The smaller the flow, the lower is the optimum number.
In a TVR evaporator a low cost steam ejector is used for recycling part of the vapour to the heating side of the first effect. The primary steam pressure is beneficially 3 - 10 bars. Thus a four-effect TVR evaporator may have the same steam demand as a normal five or six effect evaporator. Because the ejector is sensitive to changes in capacity, the turn down capability of such an evaporator is limited. In cases where the steam is produced in a power plant, the use of higher pressure steam for the evaporator reduces the electrical production of the power plant, and may thus prove to be uneconomical
In an MVR evaporator an electrically powered compressor is used to increase the pressure (and corresponding condensing temperature) of all vapour for reuse as heating medium in the heating side of the first effect (heat pump principle). The MVR evaporator thus needs no primary steam at all (except for start up purposes, where also electrical heating alternatively can be used), as well as no external condenser nor cooling water for condensing the vapour. MVR evaporators are normally single effect and very easy to control. The best conventional MVR evaporators operate at a power demand that corresponds to 40 stage evaporation (electrical power input for MVR compared to energy content of primary steam for a steam driven single effect evaporator), while the new Windsor Evaporator typically has a power and that corresponds up to 80 stage evaporation.
MVR evaporators are very competitive in cases where no power plant or boiler is available for steam production.
