1. Michael Owen Univ. of Stellenbosch/Dept. M&M Eng./STERG
A hybrid dephlegmator for incorporation into an air-cooled steam condenser
Michael Owen
Univ. of Stellenbosch/Dept. M&M Eng./STERG
Energy Postgraduate Conference 2013

2. Overview Air-cooled condensers (ACCs): what, where, why?
Hybrid (dry/wet) dephlegmator concept
HDWD performance evaluation
Concluding remarks

3. 3
ACCs: what, where, why?

4. ACCs: what, where, why? (cont.)4
ACCs: what, where, why? (cont.)
Wet cooling using evaporative condensers
Approximately ½ of all power plants in the U.S. use evaporative cooling¹
36 of the 37 major operating solar thermal power plants are wet cooled²
Water intensive: > litres per day per MW installed³
Typically accounts for > 80 % of the water consumption at a plant4
The use of alternative means of cooling therefore holds the greatest potential for decreasing water consumption related to electricity production

5. ACCs: what, where, why? (cont.)5
ACCs: what, where, why? (cont.)

6. ACCs: what, where, why? (cont.)6
ACCs: what, where, why? (cont.)
ACCs use ambient air to cool and condense the process fluid
No water is directly consumed in the cooling process
ACCs are therefore an attractive option considering current global water security concerns
ACCs allow for flexibility in plant location
Location not constrained by proximity to water resources
Plants can be built nearer fuel/energy supplies or load centers → increased reliability and reduced transmission costs
ACCs experience a reduction in cooling effectiveness at high ambient temperatures
Q α Tv - Ta
A dynamic relationship exists between condenser and turbine performance
Reduced cooling effectiveness → decreased plant output on hot days

7. Hybrid (dry/wet) dephlegmator7
Hybrid (dry/wet) dephlegmator
Conventional
Replace conventional dephlegmator with a HDWD
Originally proposed by Heyns and Kröger5
2 stages connected in series:
1st stage – ACC (inclined finned tubes)
2nd stage – hybrid plain tube bundle
2nd stage operation:
Low ambient temperatures → DRY
High ambient temperatures → WET
Wet mode
Both latent and sensible heat transfer
Measurably enhanced air-side heat transfer coefficient
Hybrid

8. HDWD performance evaluation8
HDWD performance evaluation
3 street ACC with a HDWD (operating wet) provides the same turbine output benefits as:
A 33% over-sized ACC (4 streets vs 3) - at a much reduced cost
A conventional ACC with spray cooling enhancement – while consuming 20 % - 30 % less water and avoiding fouling and corrosion issues

9. HDWD performance evaluation (cont.)9
HDWD performance evaluation (cont.)
One-dimensional performance analysis to identify the best configuration for the 2nd stage HDWD bundle
More rows of smaller diameter tubes in a multipass arrangement with a single tube row in the final pass
Highest heat transfer rates (up to 35% increase for an ACC)
Lowest ejector loading
Slightly higher water consumption (although still negligible compared to an evaporative cooler)
Higher steam-side pressure drops A B C D E F G H I J K
Tube dia., mm
38.1
19.0
No. of tube rows 16 25
No. of tubes/row
156
314
No. of passes 3
12/3/1 2
13/2
14/1 1 15
20/5/1
20/5
38.1
19.0 16 16 25

10. 10
Concluding remarks
While ACCs offer water consumption advantages over evaporative cooling towers they experience decreased performance during hot periods
Alternative and enhancement strategies have thus far proven problematic
A novel hybrid dephlegmator concept is proposed that:
Offers enhanced cooling performance
While consuming only a small amount of water
Is able to reduce the load on the ejector
Simple and cost effective
Experimental data is required to fully quantify the exact performance characteristics of the HDWD

11. 11
References
Carney B, Feeley T, McNemar A. NETL Power Plant Water Research Program. EPRI Workshop on Advanced Thermoelectric Cooling Technologies 2008; Charlotte NC.
NREL 2010:
Gadhamshetty V, Nirmalakhandan N, Myint M, Ricketts C. Improving Air-cooled Condenser Performance in Combined Cycle Power Plants. J Energy Eng 2006; 132/2:81-88.
DiFilippo M., Reclaiming Water for Cooling at SCE’s Mountainview Power Plant, Presentation: EPRI workshop on Advanced Thermoelectric Cooling Technologies, Charlotte, 2008.
Heyns, J.A., Krӧger, D.G., Performance characteristics of an air-cooled steam condenser with a hybrid dephlegmator, R&D Journal of the South African Institute of Mechanical Engineers, Vol. 28, pp , 2012.