Design Mode¶
Purpose¶
In Design mode, the cooling tower calculates the required Merkel number (KaV/L) to achieve a specified water outlet temperature. This is the mode used when sizing a new tower or evaluating whether a given set of conditions is feasible.
Required Inputs¶
| Parameter | Description | Typical Range |
|---|---|---|
| Inlet Water Temperature | From the connected water stream | 30 - 60 °C |
| Desired Outlet Temperature | Target cold water temperature | 20 - 40 °C |
| 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 | — |
Step-by-Step Procedure¶
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Create and connect streams: Place the Cooling Tower on the flowsheet. Connect a material stream as the water inlet (hot water). Connect another material stream as the water outlet.
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Configure the inlet stream: Set the water inlet temperature, mass flow rate, and pressure. Assign a property package (e.g., Steam Tables or NRTL).
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Open the Cooling Tower editor by double-clicking the unit operation.
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Set Calculation Mode to Design.
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Set Calculation Method to Merkel or Poppe.
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Enter air conditions:
- Set the Air Dry-Bulb Temperature (e.g., 25 °C)
- Set the Air Wet-Bulb Temperature (e.g., 20 °C) — or switch to Relative Humidity input
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Set L/G Ratio (e.g., 1.0).
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Set Desired Outlet Temperature (e.g., 30 °C).
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Run the calculation (Ctrl+F5 or the Solve button).
Outputs¶
After a successful calculation, Design mode produces:
| Output | Description |
|---|---|
| Required KaV/L | The Merkel number needed to achieve the target outlet temperature |
| Cooling Range | Tw,in - Tw,out |
| Approach | Tw,out - Twb |
| Tower Efficiency | Range / (Range + Approach) |
| Heat Rejected | Q = mw * Cpw * Range |
| Evaporation Rate | Water lost by evaporation |
| Makeup Water Rate | Total water replacement needed |
| Fan Power | Based on air pressure drop and efficiency |
Design Guidelines¶
Practical L/G Selection
- L/G = 0.75 - 1.5: Typical for induced-draft towers
- L/G < 0.8: Large air flow, lower KaV/L but higher fan power
- L/G > 2.0: Small air flow, higher KaV/L and more difficult to achieve
Feasibility Check
If the calculated KaV/L exceeds approximately 3.0 - 4.0, the design may be impractical. Consider:
- Increasing the L/G ratio (reducing air flow)
- Relaxing the approach (higher outlet temperature)
- Using a more efficient fill type
Example¶
Given: Tw,in = 40 °C, Tw,out = 30 °C, Tdb = 25 °C, Twb = 20 °C, L/G = 1.0, mw = 1 kg/s
Results (Merkel method):
| Parameter | Value |
|---|---|
| Required KaV/L | 0.803 |
| Range | 10.0 K |
| Approach | 10.0 K |
| Efficiency | 50.0% |
| Heat Rejected | 41.8 kW |