Mechanical Vapour Recompression MVR

EVAPORATORS IN THE DAIRY INDUSTRY
As an alternative to the thermo-compressor, the mechanical vapour compressor has experienced extensive use in evaporators within the dairy industry during the past fifteen years. Electricity is often the selected source of energy for the compressor, but also diesel motors are used. Other processes may require steam at low pressure, and the compressor can be driven by a steam turbine acting as a reducing valve. All procedures are determined by local price policy for energy. However as a rule of thumb, an MVR solution is profitable, if the price/kW ≤price/kg steam x 3. The decision as to which type of compressor to use, is traditionally influenced by the desired quality of the end product - the milk powder - and in the MVR evaporator there is a very short residence time, resulting in low viscosity of the concentrate.

MECHANICAL VAPOUR COMPRESSOR FEATURES
The mechanical vapour compressor is a fast revolving high pressure fan (≈3000 rpm) capable of operating under vacuum. At low boiling temperatures the volume of the vapours is enormous (see page 21). Consequently, there is a limit as to the lowest temperature levels used in practice. As the energy applied to the compressor is utilized most efficiently by low compression ratios, the obtained temperature/pressure in-crease is limited. Therefore, a large heat transfer surface is required, which tends to increase the capital costs of the equipment.

As it is essential to operate an MVR unit at a low overall temperature difference between the vapour evolved from the product and the heating medium as a result of the compression, it is essential that the boiling point elevation of the product is kept at a minimum. Otherwise this could further minimize temperature differences available for the evaporation. This, too, limits the maximum concentrations aimed at in evaporators of this kind. Fig. 11 illustrates a one-effect MVR evaporator, and Fig. 12 the corresponding heat flow diagram.

Fig. 12  Heat flow diagram MVR evaporator

THE PROCESS
To begin with the incoming cold milk is preheated by concentrate, then by condensate from the heating section of the calandria, followed by a final pasteurization by live steam. The vapour is compressed in the MVR unit and used as heating medium, as it releases the latent heat by condensation. A vacuum pump, together with a small amount of cooling water, maintains the desired vacuum in the system.

As it can be seen no energy leaves the plant as warm condensate, and only a minor part via the cooling water (depending on the pasteurization temperature desired). The MVR evaporator is in this context very often used as precondenser of milk products for transport purposes, where the required solids content is in the range of 30-35% and thus the boiling point elevation is limited. With the concentrate leaving the plant at low temperature, this kind of installation is a strong competitor to hyper-filtration.

The working cycle of a mechanical compressor is shown in Fig. 13. The vapour is sucked from the separator represented as point A at a given temperature/pressure level ta/Pa and compressing it to point B':t'r/Pr. The compressed vapour is desuperheated to B:tr by spraying water into the outlet of the compressor. The compressed vapour is condensed on the heat exchanger surface in the calandria from point B to C, where it is discharged as condensate. Simultaneously, water is evaporated from the milk and separated in the separator from where it leaves at point A.

The MVR evaporator offers much better capacity flexibility / turn-down cabability, as only the RPM on the fan needs to be adjusted.

Usually, the MVR evaporator is combined with a TVR unit, if solids contents suited for a spray drying plant are desired.,(see Fig. 14) The steam consumption per kg evaporated water is of course less than in a multi-effect evaporator, but if the MVR unit is driven by an electric motor, the electrical energy consumption will increase. Since only limited amounts of cooling water is required, this combination offers a very attractive solution, however, a higher investment should be anticipated. Under special energy price conditions it is advantageous to replace the TVR unit with an additional MVR unit to compress the vapour over the last effect. (see Fig. 15) It is therefore recommended that each case is studied carefully taking local conditions such as steam, electricity and cooling water prices into consideration.

Fig. 14  Combined MVR/TVR evaporator

Fig. 15 Combined MVR/MVR evaporator