Rating Mode¶
Purpose¶
In Rating mode, the cooling tower predicts the water outlet temperature for a tower with known characteristics (KaV/L). This mode is used to evaluate the performance of an existing tower under different operating conditions.
Required Inputs¶
| Parameter | Description | Typical Range |
|---|---|---|
| Inlet Water Temperature | From the connected water stream | 30 - 60 °C |
| KaV/L (Merkel Number) | Known tower characteristic | 0.5 - 5.0 |
| Air Dry-Bulb Temperature | Ambient air temperature | 15 - 45 °C |
| Air Wet-Bulb Temperature | Ambient wet-bulb (or RH) | 10 - 35 °C |
| L/G Ratio | Water-to-air mass flow ratio | 0.5 - 2.5 |
| Atmospheric Pressure | Local barometric pressure | 90,000 - 105,000 Pa |
| Calculation Method | Merkel or Poppe | — |
KaV/L from Design Mode
You can obtain the KaV/L from a previous Design mode calculation, from manufacturer data, or from fill correlation:
where \( C_1 \) and \( C_2 \) are fill-specific constants (configurable in the editor).
Step-by-Step Procedure¶
-
Set Calculation Mode to Rating.
-
Enter the known KaV/L value (e.g., 0.8144).
-
Configure air conditions (Tdb, Twb) and L/G ratio.
-
Run the calculation.
The solver uses bisection to find the outlet temperature that produces the specified KaV/L. The convergence tolerance is 0.0001 K.
Outputs¶
| Output | Description |
|---|---|
| Water Outlet Temperature | Predicted cold water temperature |
| Cooling Range | Tw,in - Tw,out |
| Approach | Tw,out - Twb |
| Tower Efficiency | Range / (Range + Approach) |
| Heat Rejected | Q = mw * Cpw * Range |
| Air Outlet Temperature | Temperature of exhaust air |
| Air Outlet Humidity | Humidity ratio of exhaust air |
| Evaporation Rate | Water lost by evaporation |
Design-Rating Round-Trip Consistency¶
One important validation is the round-trip test: a Design calculation produces a KaV/L, which is then used as input for a Rating calculation. The Rating result should reproduce the original outlet temperature.
Example:
| Step | Mode | Input | Output |
|---|---|---|---|
| 1 | Design | Tw,out = 30.00 °C | KaV/L = 0.8144 |
| 2 | Rating | KaV/L = 0.8144 | Tw,out = 30.00 °C |
Verified
The DWSIM implementation achieves 0.00 K deviation in the Design-Rating round-trip for both Merkel and Poppe methods (see Internal Validation).
Performance Curves¶
Rating mode is particularly useful for generating performance curves — how the outlet temperature varies with changing conditions:
- Variable wet-bulb: How does tower performance change seasonally?
- Variable L/G: What happens if the water flow rate changes?
- Variable Tw,in: How does the tower respond to process upsets?
These analyses can be automated using the Automation API.