Performance of evaporator-capacity-steam economy-methods to increase steam economy-heat transfer in evaporators-boiling point rise BPR-boiling point elevation-duhring’s rule

 

Performance of evaporator

Two basic criteria for performance of evaporator.

•  Capacity
• Steam economy

Capacity of evaporator

“Number of kilogram of water evaporated per hour.”

Or;

“ Amount of water evaporated per hour.”

Steam economy

“Number of kilogram of water evaporated per kilogram of steam fed to evaporator.”

Or;

“Amount of water which is converted into vapors through steam.”

• Steam economy is the ratio of capacity and consumption.
• Steam economy is unitless quantity.
• Steam economy for single effect evaporator is 0.8. 

That means 80 percent of that steam is used to generate water.

Methods to increase steam economy:

The operation cost of evaporator can be reduced by increasing steam economy.

There are two methods to increase steam economy.

1.    By use of multiple effect evaportaors. 

2.    By vapor recompression.

In Multiple effect evaporators the vapors which is generated in first effect can be used as heating media for second effect. Similarly, the vapors generated from second effect can use as the heating media in third effect. But these vapors can not be use as heating media for previous effects. For this purpose compression of vapors take place. Compression increase the pressure and temperature of vapors. So these vapors can be used in previous effects in comparison to subsequent effects to increase the steam economy.

Steam economy for single effect evaporator<1

Steam economy for multiple effect evaporator>1 ,but lower capacity than single effect.

Because; in single effect evaporator the vapors generated through steam did not further use. But in multiple effect evaporator vapors generated through steam are futher use for next effect as heating media.

Heat transfer in evaporators:

Pumps, valves and interconnecting pipes are available for transfer of liquid from one effect to another that increases both equipment and operating cost.

The rate equation for heat transfer is;

Q=UAdT

Where,

Q is heat transfer per unit time,

U is Overall heat transfer coefficient,

A is heat transfer surface, and

dT is temperature difference between two streams.

But applying this equation have some difficulty in deciding correct value for temperature difference which is known as Boiling point rise (BPR) or boiling point elevation (BPE).

At same pressure , a solution has a boiling point greater than that of water and the difference between its boiling point and that of water is BPR or BPE.

To concentrate the salt solutions usually more heat is required to vaporize unit mass of water, so that the reduction in capacity of a unit may occur.

Duhring’s rule:

The value of BPR or BPE cannot be calculated from physical data of the liquor. For this purpose during’s rule is used. Duhring’s rule is used to find the change in BPR with pressure.

“ It states that the ratio of the temperature at which two solutions (one of which can be pure water) exert the same vapor pressure.”

Duhring's rule to find the change in BPR with pressure of solution of NaCl in water.

If the boiling point of the solution is plotted against that of water at the same pressure, then the straight line is obtained.

Thus if pressure is fixed, the boiling point of water is found from steam table and the boiling point of solution from figure.

The BPR is much greater with strong electrolyte such as; salt and caustic soda.