Direct natural draft, vertical bundles
Condenses turbine exhaust steam in finned-tube bundles standing around the base of a natural-draft tower.
Steam condenses in vertically arranged finned-tube bundles around the base of a hyperbolic tower, and the warm air column in the tower drives the draft. Kröger's natural-draft direct dry cooling system.
Pack: Natural draft. In the plans: Pro (as its one pack), Expert, Enterprise and Academic. Plans
The analyses it offers
- RatingThe plant at one condition, solved once.
- DesignFix a target and solve for one dimension.
- Parametric sweepThe rating repeated along one or more axes.
- Annual runThe rating once per weather point over a year.
- Transient simulationThe plant after an event.
The transient scenarios
- Step in steam admission (cold start)
- Ramp to design admission
- Islanding (settle, then step up)
- Islanding search: the admission a back-pressure limit allows
- Step on one boundary channel
- Step in the ambient temperature
- The ambient temperature following a profile
What it predicts, and what that was compared with
The app's capability statement for this system, as every report prints it.
| Analysis | Predicts | Compared with | Agreement |
|---|---|---|---|
| Steady rating, and the sweep, design and annual runs built on it | Air mass flow rate, heat transfer rate, steam back pressure and every loss of the draft equation | The published design point of the reference tower | Heat transfer rate within 0.5%; air mass flow rate 3.5% above the published value |
| Discretised transient | Steam back pressure, tube wall temperature and air mass flow rate through a start-up, a load change or islanding | The CFD co-simulation of the reference tower (Strydom, Pretorius and Laubscher, 2026), run under its own conditions; no measured start-up | Peak back pressure 146.8 kPa at 891 s against 142.2 kPa at 850 s; wall peak 383.8 K against 382.9 K; air mass flow rate within 1.1% at 1 500 s; islanding peak 34% low |
Its limits
Crosswind on a natural-draft tower is represented by the method of Kröger, Vol. 2, Section 9.2: mean inlet and outlet pressure coefficients in the draft equation and a maldistribution factor on the heat transfer rate, fitted on horizontal bundles filling the inlet. On a vertical ring of deltas the draft-side coefficients carry a multiplier of 0.5, measured on the air mass flow rate of four CFD series. The heat transfer rate lost per unit of air flow is where the CFD literature disagrees, by 0 to 18% from 6 m/s upward, and every report of a windy case states this. Not modelled for this system: fouling that develops in service and degradation (a stated fouling resistance is carried as given), freezing and freeze protection, hybrid or adiabatic inlet assistance, and the plume. Every correlation is a fit over a declared range; the report lists any excursion at the solved state, and a result outside a declared range is an extrapolation and not a prediction.
The validation cases of this system
- Another modelKong's annular (180°) NDDCS against its own CFD
- Another modelKong's apex sweep: the apex trend is not a settled reference, and the model sits between two of them
- Another modelKröger's crosswind method on Kong's two delta rings, 3 to 15 m/s
- Another modelKong's triangular (60°) NDDCS against its own CFD
- Another modelLiu's one-tower 2 x 660 MW plant under crosswind: the largest ring, a held load, and a sixth air-flow multiplier
- Worked exampleNDDCS design pointDirect natural-draft tower, vertical bundles
- Another modelStrydom et al. (2026) co-simulationIslanding scenario
- Another modelStrydom et al. (2026) co-simulationStep scenario