Partial Evaporation Example

1/31/2007

Shell and Tube Heat Exchanger

Water 1 1ˆ,m H

4 4ˆ,m HSteam

3 3ˆ,m H

2 2ˆ,m H

5 5ˆ,m H

Vapor

Liquid

Condensate

• Water feed, m1, is at 20 oC & 1 bar, 100 kg/hr

• Steam feed, m4, is saturated vapor at 2 bar

• Steam condensate, m5, is saturated liquid at 2 bar

• If both m2 and m3 exist, they are in equilibrium with each other at 2 bar

Scenarios

• Water stream exits as a liquid below saturation temperature

• Water stream exits as liquid and/or vapor at saturation temperature

• Water stream exits as a vapor above saturation temperature.

Mass/Energy Balances1 2 3

3 2

4 5

100

0 if kinetic/potential energy changes are ignored

m m m

m m

m m

H

2 2 3 3 5 5 1 1 4 4

1

4

5

ˆ ˆ ˆ ˆ ˆ 0

ˆ 83.9 /

ˆ 2706.3 /

ˆ 504.7 /

m H m H m H m H m H

H kJ kg

H kJ kg

H kJ kg

Scenario 2:Water out is at saturation conditions

Vary m2 and calculate m4

2 2 3 3 5 5 1 1 4 4

2

3

ˆ ˆ ˆ ˆ ˆ 0

ˆ 417.5 /

ˆ 2675.4 /

m H m H m H m H m H

H kJ kg

H kJ kg

15.15

35.66

56.18

76.69

97.20

117.71

0

20

40

60

80

100

120

140

0 20 40 60 80 100

Amount of Vapor Produced, m 3 (kg/hr)

Ste

am

Ne

ed

ed

, m4 (

kg

/hr)

Scenario 1:Subcooled liquid out of exchanger

Vary m4 and calculate m2 temperature

1 1 4 4 5 52

2

ˆ ˆ ˆˆ m H m H m HH

m

0

50

100

150

200

250

300

350

400

450

0 5 10 15

Steam Flow Rate, m 4 (kg/hr)

Sp

ec

ific

En

tha

lpy

, H2 (

kJ

/kg

)

20

40

60

80

100

120

140

Tem

per

atu

re, (

oC

)

Scenario 3:Superheated vapor out of exchanger

Vary m4 and calculate m3 temperature

1 1 4 4 5 53

3

ˆ ˆ ˆˆ m H m H m HH

m

2650

2660

2670

2680

2690

2700

2710

2720

2730

2740

2750

115 120 125 130 135 140 145 150 155

Steam Flow Rate, m 4 (kg/hr)

Sp

ec

ific

En

tha

lpy

, H3 (

kJ

/kg

)

20

40

60

80

100

120

140

Te

mp

era

ture

, (oC

)

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140 160

Steam Flow Rate, m 4 (kg/hr)

Ou

tle

t T

em

pe

ratu

re (

oC

)