Most of the evaporators use a low pressure steam for heating purpose. Due to addition of heat to a solution in the evaporator by condensation of steam, the solution in the evaporator will boil. If the vapours leaving the evaporator are fed to some form of a condenser, then the heat associated with the vapours will be lost and the system is said to make poor use of steam.
The vapour coming out of an evaporator can be used as a heating medium for another evaporator operating at a lower pressure and temperature in order to provide a sufficient temperature difference for the heat transfer in that evaporator.
When a single evaporator is put into service and the vapours leaving the evaporator are condensed and discarded, the method is known as single-effect evaporation. The economy of a single effect evaporator is always less than one. Generally, for evaporation of one kg of water from a solution, 1 to 1.3 kg of steam is required.
The method of increasing the evaporation per kilogram of steam by using a series of evaporators between steam supply and condenser is known as multiple-effect evaporation. It is the one way to increase the economy of evaporator systems. The method of re-using the latent heat is called multiple-effect evaporation.
A multiple effect evaporation system is commonly used in large scale operations. In such a system, evaporators are arranged in series so that the vapour from one evaporator is used as a heat medium for the next one that is operating under at a pressure and temperature lower than the previous one. Each unit in such a series is called an effect. In case of a tripple-effect evaporator, if the first effect is operating at atmospheric pressure, then the second and third effect operate under vacuum. Steam is fed to the first effect and the vapour from the third-effect is condensed in a condenser connected to a vacuum pump (which is used to maintain vacuum in the last two effects).
With a multiple-effect evaporation system, it is possible theoretically to evaporate N kg of water from 1 kg of live steam fed, where N is the number of effects.
If the vapour from one evaporator is fed to the steam chest of the second evaporator as steam supply, the system is called as double-effect evaporator system and so on.
The methods used for feeding a multiple evaporation system are :
1. Forward feed, 2. Backward feed, and 3. Mixed feed.
1. Forward feed :
In this arrangement, the thin liquid feed flows in the same direction as the vapour flow. Fresh feed enters the first effect and steam is also fed to a steam chest of the first effect. The vapours produced in the first effect are fed to the steam chest of the second effect as a heating medium and the concentrated liquor from the first effect is fed to the next effect in series, as shown in Fig. The pressure in the second effect is less than in the first effect and so on. Thus, this arrangement does not require pumps to transfer concentrated solution from effect to effect. This is used when the feed is hot and the product may get damaged at high temperatures.
2. Backward feed :
In this arrangement the feed solution and the vapour flow in opposite directions to each other. Fresh feed (thin liquid) is admitted to the last effect and then pumped through the other effects. The steam is admitted to a steam chest of the first effect and the vapours produced in the first effect are fed to the steam chest of the second effect (evaporator) and so on. The pressure in the first evaporator is highest and that in the last effect is lowest. If the solution is very viscous then we have to adopt a backfeed arrangement as the temperature of the first effect is highest and thus the corresponding viscosity of the solution will be less. Fig. shows this type of arrangement. Since the feed flows in the direction of increasing pressure, pumps are used for the transfer of solution from one effect to another effect. It is used when the feed is cold, since it saves a large quantity of steam and increases the economy.
3. Mixed feed :
In this feed arrangement, steam is admitted to a steam chest of the first effect and the vapours leaving the first effect are fed to the steam chest of the second effect and so on. Feed solution is admitted to an intermediate effect and flows to the first effect from where it is fed to the last effect for final concentration. This arrangement is a combination of the forward and backward feed adopted for the best overall performance. Fig. shows this type of arrangement.
Difference between forward feed and backward feed arrangements :
1. In the forward feed, the flow of the solution to be concentrated is parallel to the steam/vapour flow.
1. In the backward feed, the flow of the solution to be concentrated is in opposite direction to that of steam/vapour (counter current).
2. Forward feed arrangement does not need pumps for moving the solution from effect to effect as vacuum is maintained in the last effect.
2. Backward feed arrangement needs pumps for moving the solution from effect to effect as transfer of the solution is to be done from the evaporator operating at a low pressure to that operating at a higher pressure.
3. In the forward feed as heating of the cold feed solution is done in the first effect, less vapour is produced per kilograms of steam fed, resulting into a lower economy.
3. In the backward feed, the solution is heated in each effect which usually results in a better economy than that with a forward feed.
4. With a forward feed, the most concentrated solution is in the last effect wherein the temperature is the lowest and the viscosity is the highest. These conditions lead to reduction in the capacity of the system as a whole due to low overall coefficient in the last effect in case of thick liquors which are very viscous.
4. In the case of backward feed, the concentrated solution is in the first effect wherein the temperature is the highest as steam is admitted to that effect and the viscosity is the lowest, thus the overall coefficient can be moderately high inspite of high viscosity.
5. In the case of forward feed, the maintenance charges and power cost are less.
5. In the case of backward feed, the maintenance charges and power cost are more for the same duty.
6. The forward feed arrangement is less effective thermally.
6. The backward feed arrangement is more effective thermally. (At high feed temperatures).
7. Forward feed is more economical in steam.
7. At low values of feed temperature, backward feed arrangement gives higher economy.
8. Forward feed is very common (largely used) as it is simple to operate.
8. Backward feed is not common as it necessitates the use of pump between effects.
The choice of optimum number of effects will be determined by an economic balance between the savings in steam obtained by using a multiple-effect evaporation system and the additional investment cost resulting from the added heat transfer area.