Dry Cooling Tower Analysis
With a Wet Cooling tower the "cold water" temperature approaches the "wet bulb" temperature. Consequently the current "cold water" temperatures during the peak temperatures in the summer are 86-88 deg F (the wet bulb temperatures are seldom above 70 deg F). The "wet bulb" temperature is the coldest temperature that can be achieved by evaporation. A Dry Cooling tower can only approach the dry bulb temperature (ambient). Since the ambient temperatures during the peak load season at the Navajo Plant exceed 105 deg F and realistically the best approach to this temperature is 5 deg F (Marley Cooling Tower Co.), a "cold water" temperature of 115 deg F was used (to account for some fouling of the exterior of the tubes).
The maximum backpressure for the Navajo turbines is 5" Hg (2.46 psi-absolute).
Use existing Condensers with Dry Cooling Towers.
Using Heat Exchanger Institute calculations for surface condensers and the thermodynamic computer model PEPSE (NUS Corp.) full load could not be achieved without exceeding the maximum recommended fluid velocities in the condenser tubes. Current tube velocities are 6.5 ft/sec. The maximum velocity recommended by the Heat Exchanger Institute is 12 ft/sec. Current flows per each Unit at Navajo are 260,000 gpm. To maintain full load with 115 deg F "cold water" temperatures the flows would have to be 718,000 gpm. The tube velocities for this flow would be 17.5 ft/sec.
Enlarge Condenser to keep tube velocities at 12 ft/sec limit.
Using the same tools as above the flow would be 648,000 with acceptable tube velocities if the condenser were increased in size by 59%. This is physically impossible considering the Main Turbine pedestals surround the existing condenser.
Derate Unit load using existing condenser with 12 ft/sec limit.
The flow would be 494,000 gpm (per Unit). The Unit load would have to be reduced to 515 Net MW during the peak load season (69% of existing capacity or a 235 MW per Unit derating - 705 MW derating for the plant).
Air-Cooled Condenser Analysis
Conversations with Balcke-Durr Consolidated (formerly Zurn) personnel specializing in "Air-Cooled" condenser applications led to the following:
There is no Air-Cooled Condenser in the world as large as would be required by the Navajo Plant with it's ambient conditions and 5" Hg turbine exhaust limitations. The closest in size is in South Africa with a design ambient temperature of 64 deg F (versus over 105 deg F for the Navajo Plant) at an exhaust pressure of 5.3" Hg. Though the plant output is larger than the Navajo Plant the air cooled condenser sizing for the Navajo Plant would be much larger (57% larger).
A combined cycle plant is being built near Boulder City, Nevada. The maximum ambient temperature used for the maximum off-design specification was 108 deg F (actual ambient conditions will be somewhat higher). The corresponding backpressure at this temperature is 7.8 " Hg (where the Navajo turbine maximum is 5.0" Hg).
The Balcke-Durr engineer made adjustments for the differences in steam condensation heat load and for the difference in backpressure requirements. This would result in a structure taking up 18 acres (nearly 800,000 sq ft) of land with a height of 160 ft. The cost of the air-cooled condenser would be $240,000,000. Construction costs, turbine exhaust modifications, and moving the transformers, subsynchronous resonance protection equipment, and other auxiliary equipment would easily increase this cost to the half billion dollar mark.
As there is not as large a piece of this type of equipment in existence, there have been no retrofits to any plant near the scale of this plant (one tenth of the heat load with much cooler ambient conditions is the closest I have been able to find). If David Wegner or his contacts know of a comparable retrofit project I'm available for discussions.
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