| 
|
| Selecting
appropriate enhancement is a key to an optimized heat exchanger.
The final goal must be to achieve the ratio of outside heat
transfer coefficient to inside heat transfer coefficient as
one (ho/hi = 1). Table 1 shows the design parameters and output
data. The final selection for this application was based on
the basis of ho/hi close to one. Therefore, case C, with inside
enhancement was selected as an optimized design. Enhancement
was achieved via twisted tape turbulators with H/D = 4.1,
tape width 0.5” (12.7 mm), and thickness 0.025”
(0.635 mm). Selection D would had been satisfactory too but
it would have resulted in 20% extra cost and 50% extra pump
power requirement. The unit was installed and has been operational
for several years with no thermal or mechanical problems.
Table 1 Comparative case study of ammonia flooded evaporator
|
Parameter |
A |
B |
C |
D |
Shell OD, in (mm) |
16 (406.4) |
16 (406.4) |
16 (406.4) |
16 (406.4) |
Tube OD, in (mm) |
0.75 (19) |
0.75 (19) |
0.75 (19) |
0.75 (19) |
Tube wall, in (mm) |
0.065 (1.65) |
0.049 (1.245) |
0.065 (1.65) |
0.049 (1.245) |
Passes |
2 |
2 |
2 |
2 |
Under/over capacity |
-78% |
-75% |
0% |
+6% |
Pressure drop, psi (bar) |
20.0 (1.38) |
35.0 (2.41) |
6.8 (0.47) |
10.8 (0.74) |
ho/hi |
2.90 |
4.10 |
1.20 |
1.87 |
|
Design
capacity: 487,200 Btu/hr (143 kW) |
Refrigerant:
Ammonia @ -40°F (-40°C) saturated temperature |
Process
fluid: 29% wt/wt CaCl2 brine |
Process
flow: 450 gpm (28.4 l/s) |
Process
inlet: -29.4°F (-34.1°C) |
Process
outlet: -32°F (-35.6°C) |
|
A:
No enhancement |
B:
Outside enhancement only |
|
C: Inside enhancement only |
|
D: Both sides enhanced
|
