Evaporation steam consumption

Crude solution throughput Amount of crystals Water evaporated Steam consumption Temperature levels... 

Monitor performance of multiple effect evaporators steam consumption, concentration of incoming dilute feed and outgoing concentrated solutions... 

Determine total evaporation required, and estimate steam consumption for the number of effects chosen. 

Superheated steam may be needed where steam distribution pipework in a plant is over extended distances, resulting in a loss of heat and increase in wetness of the steam. Another case may be where a process requires a temperature above the working pressure of the plant. The third case is where steam is used for turbines. Here it improves the performance of the turbine, where for every 6°C increase in steam temperature it can produce a saving of about 1 per cent reduction in steam consumption. Superheaters may also be supplied as independently fired units. These may be used when either the amount of superheated steam required is much less than the boiler evaporation or is only needed on an intermittent basis. 

It is decided to bleed off 0.25 kg/s of vapour from the vapour line to the second effect for use in another process. If the feed is still heated to the boiling-point of the first effect by external means, what will be the change in the steam consumption of the evaporator unit ... 

The single effect evaporator uses rather more than 1 kg of steam to evaporate 1 kg of water. Three methods have been introduced which enable the performance to be improved, either by direct reduction in the steam consumption, or by improved energy efficiency of the whole unit. These are ... 

Exit air usually is maintained far from saturated with moisture and at a high temperature in order to prevent recondensation of moisture in parallel current operation, with a consequent lowering of thermal efficiency. With steam heating of air the overall efficiency is about 40%. Direct fired dryers may have efficiencies of 80-85% with inlet temperatures of 500-550°C and outlet of 65-70°C. Steam consumption of spray dryers may be 1.2-1.81bsteam/lb evaporated, but the small unit of Table 9.19(b) is naturally less efficient. A 10% heat loss through the walls of the dryer often is taken for design purposes. Pressure drop in a dryer is 15-50 in. of water, depending on duct sizes and the kind of separation equipment used. 

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