MULTIPLE EFFECT DISTILLATION OF SEAWATER WATER USING SOLAR ENERGY - THE CASE OF ABU DHABI SOLAR DESALINATION PLANT - Energy Research Developments: Tidal Energy, Energy Efficiency and Solar Energy - page 46

where T 2d is the design brine maximum temperature in the first effect (top effect), T 1d is the
design brine temperature in the last effect (bottom effect), BPE av is the average boiling point
elevation in the N effects. For the evaporator in the solar desalination plant the following
values are used: T 2d = 68 o C, T 1d = 40.7 o C, BPE av = 0.71 o C. The average heat transfer
coefficient for the N effects, U (kcal/h m 2o C) was estimated from:

C ( T ) + C ( T )

U = ( 1888 × L + 1313 ) ×

1 2

(53)

2

where C is a correction coefficient which is dependent on the brine temperature, L is the load.
The design value is that when L = 1.0, i.,.

C ( T ) + C ( T )

U d = ( 1888 + 1313 ) ×

1 2

(54)

2

The correction coefficient can be expressed by the equation:

C ( T ) = − 0 4678 + 0 050 × T − 0 0005 × T

2

+ 0 17 × 10

− 5

× T

3

(55)

b b b b

The operating condition of the evaporator depends on the load parameter L = ( M d / M d100% ).
The operating temperatures for each load are evaluated as follows:

• Calculate the brine temperature in the last effect from knowledge of the seawater

temperature and the condenser load:

T = T + 1

2 c 2

Q

c

= T + + 1

(56)

c 1

m × C

sw p

0 Q

h

= T + + 1

c 1

m × C

sw p

In this equation the assumption is made that the condenser load, Qc, is 90% of the heating
load, Q h due to heat loss to the environment. It is also assumed that the last effect brine
temperature is smaller than the condenser outlet temperature by 1.2oC. These assumptions are
verified by actual measurements at the evaporator of the solar plant.
Calculate an approximate value for the first effect temperature, T1,

T = T + ( L × δ T + BPE ) × N

(57)

1 2

Calculate an average overall heat transfer coefficient for the N effects,