Vapour Recompression TVR

ENERGY SAVING THERMO-COMPRESSOR
Another way of saving energy is by using a thermo-compressor which will increase the temperature/pressure level of the vapour, i.e. compress the vapour from a lower pressure to a higher pressure by using steam of a higher pressure than that of the vapour. Thermo-compressors operate at very high steam flow velocities and have no moving parts. The construction is simple, the dimensions small, and the costs low.

In Fig. 6 the principle of the thermo-compressor is shown.

Fig. 6  Thermo-Compressor

In the live steam nozzle (1) the pressure of the inflowing steam is converted into velocity. A jet is therefore created which draws in part of the vapour from the separator of the evaporator. In the diffuser (2) a fast flowing mixture of live steam and vapours is formed, the speed of which is converted into pressure (temperature increase) by de-celeration. This mixture can now be used as heating steam for the evaporator. In Fig. 7 a flow sheet of a two-effect evaporator with thermo-compressor is shown, and in Fig. 8 the corresponding heat flow diagram is shown.

Fig. 7  Two-stage evaporator with thermo-compressor

Fig.  8  Heat-flow diagram. Two-stage evaporator with thermo-compressor

EFFICIENCY IN THERMO-COMPRESSOR
The best efficiency in the thermo-compressor, i.e. the best suction rate, and thereby a good economy, is obtained when the temperature difference (pressure difference) between the boiling section and the heating section is low.
Thermo-compressors must be adapted to the operating conditions. But these conditions can vary, be it, for example, that the heat resistance of the heating surfaces increases during operation due to deposits on the heating tubes. The suction rate will then decrease considerably. In evaporators that have to serve various capacities a number of compressors with different characteristics is used. Further, a thermo-compressor, which has been designed for a higher live steam pressure, can draw a larger amount of vapour from the separator than one built for a lower pressure. For simplification we will in the following use an efficiency of 1:2, but new designed thermo-compressors will under certain conditions operate with an efficiency of 1:3.

By adding a thermo-compressor we have then in a two-effect evaporator by means of 1 kg live steam evaporated 4 kg of water, i.e. the saving of steam is as great as that obtained by addition of two effects in multi-effect evaporation. Dividing a given total t between the first and last effect in multi-effect evaporators requires an enormous heating surface and consequently an expensive installation.

The total heating surface can be reduced by increasing the ∆t, which is done by increasing the temperature of the first effect heating section, resulting in a higher boil-ing temperature. This will, however, result in fouling (microthin deposits of mainly milk proteins on the tubes resulting in a decrease of the K factor), especially, if the milk has a high acidity. Also crystallization of Caphosphate will at high temperatures result in deposits

Boiling temperatures higher than 66-68ºC (depending upon product and milk quality) in the first effect should generally not be used, if a 20 hours' continuous production time is aimed at.
The surface of the single effect of the evaporator is calculated from the following formula:

S= B×h " / K×∆t    (9)

Where	S:	heating surface m2
	B:	water vapour kg/h
	h":	specific heat Kcal/kg (condensing enthalpy)
	K:	heat transfer coefficient Kcal x m-2 x h-1 x ºC-1
	∆t:	temperature difference or the driving force ºC (between heating media and boiling liquid)

The most critical factor during the design of an evaporator is K, as it is a function of product properties and temperature level used. It is influenced by:

Evaporator temperature
Specific heat
Density
Boiling pressure
Boiling point elevation
Heat conductivity
Viscosity
Surface tension

Other factors influencing the design:
Temperature sensitivity of the product
Chemical behaviour

The K factor therefore differs from product to product, especially, due to the boiling point elevation which is a function of concentration of molecules and thus influenced by the composition of the product and the solids content. At 9% solids, as in skim milk, the boiling point elevation is less than 1ºC, whereas it is several degrees at 48-50% solids. Only extensive laboratory work makes it possible to conclude this factor.

The capacity (C) of an evaporator is

C = K x S x ∆t   (9.1)

Thus the capacity of an evaporator can be increased by more surface or higher boiling temperature in the first effect. It is not recommended to use higher temperature than 66-68ºC, as discussed above.

The total ∆t (usually from 66ºC to 45ºC = 21ºC) is divided between each effect. This means that in a three-effect evaporator each effect will have a big ∆t corresponding to a relatively small surface and low investment costs, and by an increased number of effects, whereby the steam consumption goes down, the available ∆t becomes smaller in each effect. This requires larger heating surface and the investment costs go up.

The thermo-compressor is incorporated between the first and second effect (mono-thermal compression), the first and the third (bi-thermal compression), or between the first and fourth effect (tri-thermal compression). The influence on the steam economy and the investment costs is significant. If we look at a 7-effect evaporator with mono-thermal compression (see Fig. 9), we can evaporate 9 kg of water using only 1 kg of steam. For tri-thermal compression we place the thermo-compressor between the first and fourth effect (see Fig. 10). We can now evaporate 13 kg of water using only 1 kg of steam, as we utilize all the vapour from the first effect to heat the second effect, the vapour from this is then used in the third effect, from where part of it is compressed in the thermo-compressor.

Fig. 9  Mono-thermal compression.

Fig. 10 Poly-thermal compression.

If the thermo-compressor increases the temperature of the vapour by 9ºC we have a ∆t of 3ºC for each effect in a tri-thermal evaporator. In order to maintain the 9 kg evapo-ration in the first three effects a bigger surface is needed compared to the example with mono-thermal compression.
With a given length and diameter of the tubes the number of tubes can therefore be calculated according to the previously mentioned formula (9).
However, one major drawback in multi-effect evaporators is the long residence time, where the product is exposed to heat. Although it is at low temperature, it will have a negative effect on the viscosity of the concentrate. See page 97.