MEASURES OF EVAPORATOR PERFORMANCE

There are three main measures of evaporator performance:

• Capacity (kg vaporized / time)
• Economy (kg vaporized / kg steam input)
• Steam Consumption (kg / hr)

The performance of a evaporator is evaluated by the capacity and the economy.

Economy

• Economy is the number of kg of water vaporized per kg of steam fed to the unit.
• The rate of heat transfer q through the heating surface of an evaporator, by the definition of overall heat transfer coefficient, is product of three factors

1. The area of heat transfer surface A
2. The overall heat transfer coefficient U
3. The overall temperature drop ΔT

Q = U * A * ΔT

• Economy calculations are determined using enthalpy balances.
• The key factor in determining the economy of an evaporator is the number of effects.
• The economy of a single effect evaporator is always less than 1.0. 
• Multiple effect evaporators have higher economy but lower capacity than single effect.
• The thermal condition of the evaporator feed has an important impact on economy and performance. 
• If the feed is not already at its boiling point, heat effects must be considered. 
• If the feed is cold (below boiling) some of the heat going into the evaporator must be used to raise the feed to boiling before evaporation can begin; this reduces the capacity.
• If the feed is above the boiling point, some flash evaporation occurs on entry.

Capacity

• Capacity is defined as the no of kilograms of water vaporized per hour.
• If the feed to the evaporator is at the boiling temperature corresponding to the absolute pressure in the vapor space, all the heat transferred through the heating surface is available for evaporation and the capacity is proportional to q.
• If the feed is cold, the heat required to heat it to its boiling point may be quite large and the capacity for a given value of q is reduced accordingly, as heat used to heat the feed is not available for evaporation.
• if the feed is at a temperature above the boiling point in the vapor space, a portion of the feed evaporates spontaneously by adiabatic equilibration with the vapor-space pressure and the capacity is greater than that corresponding to q. This process is called flash evaporation.

Steam consumption

• Steam consumption is very important to know, and can be estimated by the ratio of capacity divided by the economy. 
• That is the steam consumption (in kg/h) is

                                          Consumption = Capacity/Economy.

Heat transfer in evaporators

• The rate equation for heat transfer takes the form:

Q = U * A * ΔT

where:

1.  Q is the heat transferred per unit time
2. U is the overall coefficient of heat transfer
3. A is the heat transfer surface
4. T is the temperature difference between the two streams. 

• In applying this equation to evaporators, there may be some difficulty in deciding the correct value for the temperature difference because of what is known as the boiling point rise (BPR) or boiling point elevation (BPE)
• If water is boiled in an evaporator under a given pressure, then the temperature of the liquor may be determined from steam tables and the temperature difference is readily calculated. 
• At the 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 the BPR or BPE. 
• For example, at atmospheric pressure (101.3 kN/m2 ), a 25 per cent solution of sodium chloride boils at 381 K and shows a BPR of 8 deg K. If steam at 389 K were used to concentrate the salt solution, the overall temperature difference would not be (389 − 373) = 16 deg K, but (389 − 381) = 8 deg K. Such solutions usually require more heat to vaporise unit mass of water, so that the reduction in capacity of a unit may be considerable. 
• The value of the BPR cannot be calculated from physical data of the liquor, though Duhring’s rule is often used to find the change in BPR with pressure. 
• Duhring’s rule states that the boiling point of given solution is a linear function of the boiling point of pure water at the same pressure.
• Thus, if the boiling point of the solution is plotted against that of water at the same pressure, then a straight line is obtained.
• Thus, if the pressure is fixed, the boiling point of water is found from steam tables, and the boiling point of the solution from Duhring’s plot.
• Different lines are obtained for different concentrations.
• The boiling point rise is much greater with strong electrolytes, such as salt and caustic soda.