Module 1. Evaporation
Lesson 7

7.1 Introduction

The economy of the evaporation system increases with number of effects, simultaneously the capital investment is also increase with number of effects. Hence overall economy of the operation is based on the cost benefit ratio. Further the improvement in economy Is possible by the use of vapour recompression system.

7.2 Cost Factors in Evaporator Selection

If the cost of 1 m2 of heating surface is constant, regardless of the number of effects, the investment required for an N-effect evaporator will be N times that of a single effect evaporator of the same capacity. The choice of the proper number of effects will be dictated by an economic balance between the saving in steam obtained by multiple effect operation and the added investment costs brought about by the added effect.


Fig. 7.1 Cost factors in evaporator selection

The relations are shown in the graph. The annual fixed charges may be taken as a percentage of the first cost of the evaporator. Since the cost per sq.mt. of heating surface increases somewhat in small sizes, the curve for the first cost is not a straight line except in the upper part of its range. The cost of steam and water fall off rapidly at first but soon show the effect of the law of diminishing returns. Labour costs may be considered constant, since only one operator is needed except with a very large number of effects. The total cost of operating the evaporator is the sum of all these curves and usually shows a marked minimum for the optimum number of effects.

7.3 Vapour Recompression in Milk Condensing Plant

Vapour recompression is a process by which the low pressure vapour produced from the boiling milk in the calandria is recompressed to a higher pressure. This recompressed vapour is used for heating the milk again in either same effect or in previous effect. Because of this, the steam consumption per kilogram of evaporated water is reduced considerably, lowering the processing cost for the condensed milk.

There are two ways for recompressing the low pressure steam i.e. Thermal Vapour Recompression (TVR) by steam-jet vapour compressor or thermo compressor and Mechanical Vapour Recompression (MVR). At present in India TVR is common, while MVR is getting importance recently due to its extremely favourable characteristics for conservation of energy.

7.3.1 Thermal vapour recompression (TVR)

In thermo compressor, the kinetic energy of a jet of steam is used to compress the vapour. It consists of a steam nozzle, suction chamber with inlet for sucking in the vapour, mixing chamber and recompression chamber as shown in Figure. (Fig. 7.2)

The process of thermo compression on enthalpy-entropy (h-s) diagram is depicted in Figure. Here, live steam at pressure P1(state-1) is almost isentropically expanded in the nozzle to suction pressure P2 (state-2). Steam pressure usually employed in the condensing plant is about 8-12 bar and suction pressure about 0.2-0.3 bar depending on the effect from which the vapour is drawn. The expanded steam emerges from the nozzle as a jet of steam. The velocity of the steam is about 1000 m/sec.

In the mixing chamber the sucked-in vapour is entrained and carried away by the expanded steam. The vapour is accelerated as the steam transfers its kinetic energy to it. The mixing occurs at constant pressure, the enthalpy is increased and state point-3 is reached. From this point onwards, the cross-section of the thermo compressor increases, and so the kinetic energy of mixture is converted into potential energy. The pressure of the mixture is increased almost isentropically from state-3 to state-4. In this way the low pressure steam taken from a lower effect is compressed to a higher pressure corresponding to the inlet pressure of previous stage operating at higher pressure and temperature.


Fig. 7.3 Enthalpy - entropy diagram of thermal vapour recompression (TVR)

The amount of vapour, MV and amount of live steam, Ms are related as follows (Kessler, 1981).

MV/Ms= [0.8 (h1-h2) / (h4-h3)] – 1

Where h1, h2, h3, h4 is enthalpies of steam at various state points.

It can be seen from the above equation, that if the vapour to live steam ratio is higher, the factor h4-h3 decreases. This means that rise in temperature of compressed vapour is smaller. A thermo compressor with a vapour to live steam ratio of 50 : 50, gives a temperature rise in the compressed vapour of about 15oC. But, if the proportion is 60 : 40, the temperature rise is only 11oC.

Generally, the vapour drawn from the first effect is recompressed and used for the same effect again. However, more recently, the trend is to draw vapour from the second or third effect and use the recompressed vapour in the first effect. This is due to the fact that the evaporating capacity of earlier effects is increased.

With thermo compressor drawing vapour from the second effect, one must choose the right thermo compressor to achieve a temperature which lies at least 5oC above the boiling temperature of the first effect. The performance of thermo compressor is influenced by the heat transfer rate in calandria, suction pressure, discharge pressure and the motive steam pressure.

7.3.2 Mechanical vapour recompression (MVR)

Here, the low pressure vapour is compressed mechanically i.e. employing single or multiple stage radial flow compressors or by axial flow compressors. These compressors may be driven by electric motors, I.C. engines or steam turbines. (Fig.7.4)

Layout for single effect evaporation and the process of mechanical compression is shown on enthalpy-entropy diagram. The quantity of vapour MV drawn from the evaporator is at saturation condition with pressure P1, temperature t1 and enthalpy h1. This condition is shown on h-s diagram as state-1. The mechanical compressor compresses the vapour almost isentropically to a pressure P2, temperature t2 and enthalpy h2. This is superheated steam, which is not suitable for heating the milk as such, because of its bad heat transfer properties. It is cooled down to saturated state-3 i.e. temperature t3 and enthalpy h3 at constant pressure P2.

This is done by diverting a portion of condensate at temperature t4 and injecting it in the superheated steam. The condensate evaporates by consuming superheat from the compressed vapour. The mixture thus achieves final state-3 with temperature t3 enthalpy h3 and pressure P2 . The amount of steam available is thus increased by the amount of condensate mixed.


Fig.7.5 Enthalpy - entropy diagram of mechanical vapour recompression (MVR)

At this stage is should be remembered that the energy required to drive the MVR may be costlier than steam. Thus actual saving will be somewhat less depending on the prices of steam and other forms of energy employed to run MVR.

With increasing energy costs, evaporators with MVR become increasingly competitive with multi effect evaporators with TVR. Apart from being extremely economic MVR has other advantages.

1. The maximum evaporating temperature of first effect can be reduced to such as extent that burning on of product is minimized.

2. The lowest effect of evaporating temperature i.e. of last effect can be high enough which results in lower viscosity of the concentrate facilitating easy handling of concentrate. The pre-heating of concentrate before drying may be avoided or may be reduced to a less drastic treatment.

3. The higher temperature in the final effect results in reduced choking of calandria. Thus the plant can be run for a longer period before cleaning.

4. The need for cooling water is considerably reduced or totally eliminated.

A major disadvantage of MVR is the greater expenditure on equipment, maintenance cost and noise problem, but most of the studies indicate that the payback period for MVR is about 2-2.5 years.

Last modified: Friday, 5 October 2012, 4:25 AM