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Indirect-Contact or Closed-Circuit Evaporative Cooling Tower

An Indirect-Contact or Closed-Circuit Cooling Tower is a hybrid system that combines the efficiency of evaporative cooling with the cleanliness of a closed-loop process. It physically separates the process fluid from the atmospheric air and spray water, while still using evaporation for heat rejection

Core Principle: Two Separate Loops

The “closed-circuit” name comes from the primary process loop being completely sealed (closed). Heat is transferred from this closed loop to an open evaporative loop through a heat exchanger.

Two Independent Fluid Circuits:

1. Primary (Process) Loop – CLOSED:
   1. Contains clean process fluid (water, glycol mix, or other coolant).
   1. Circulates inside tubes or coils.
   1. Never exposed to atmosphere – no evaporation, contamination, or concentration.
   1. Goes directly to sensitive equipment (lasers, servers, PLCs, medical devices).
2. Secondary (Evaporative) Loop – OPEN:
   1. Contains spray/makeup water (often called “sump water”).
   1. Circulates outside the coils and is exposed to air.
   1. Operates like a small open cooling tower, with evaporation, Blow down, and drift.

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How It Works: The Indirect Contact Process

1. Hot Process Fluid Entry: Hot fluid from the process enters the coil bundle at the top.
2. Internal Flow: It travels through the sealed coils, releasing heat through the tube walls.
3. External Evaporative Cooling:
   1. Recirculated spray water is pumped from the basin and sprayed/n distributed over the exterior of the coil bundle.
   1. Simultaneously, air is drawn vertically through the coil and spray water by a fan.
   1. A small portion of the spray water evaporates on the coil surface, absorbing latent heat and cooling the coil.
4. Indirect Heat Transfer: Heat moves from the hot internal fluid, through the coil wall, to the evaporating external water film.
5. Cooled Fluid Return: The now-cooled process fluid exits the coil bundle and returns to the process.
6. Air/Water Exhaust: Warm, humid air is discharged, and excess spray water falls back to the basin.

Key Advantages

• Process Fluid Protection: The primary loop stays clean and uncontaminated—no exposure to air, dust, minerals, or biological growth. Ideal for critical processes.
• Freeze Protection: Easy to add glycol to the sealed primary loop for winter operation without affecting evaporative performance.
• Reduced Water Treatment: Only the secondary open loop (spray water) requires treatment, which is typically less critical.
• Lower Pumping Power: The primary loop often operates at lower pressure drops than open tower systems with heat exchangers.
• Year-Round Operation: Can operate in dry mode (as an air-cooled coil) in freezing conditions by shutting off the spray pump.
• No Cross-Contamination Risk: Eliminates risk of process fluid contaminating the tower water or vice-versa.