Types of Closed-Circuit cooling Tower
Closed Circuit Cooling Towers (CCTs) can be categorized in several ways, primarily based on their airflow generation method and their physical design and orientation. Understanding these types is key to selecting the right tower for a specific application.
Here is a breakdown of the main types:
1. Classification by Airflow Generation Method
This is the most fundamental way to categorize CCTs.
A. Mechanical Draft Towers
These use mechanical fans to force or draw air through the tower. They are the most common type.
- Forced Draft:
- Design: The fan is located at the base of the tower and forces ambient air into the tower. Air is discharged vertically out the top.
- Characteristics:
- The fan and motor are located in the cold, incoming air stream, which can lead to potential icing issues in winter and puts the motor under less thermal stress.
- The fan is typically a centrifugal type, which can develop higher static pressure, making it good for systems with higher air resistance.
- The discharge plenum is under positive pressure, which can lead to more water droplet escape (“drift”) if not properly designed.
- Best for: Indoor installations or where the fan noise needs to be isolated (the fan can be inside with ducting).
- Induced Draft:
- Design: The fan is located at the top of the tower and induces or draws air upward through the tower. Air is discharged vertically out the top.
- Characteristics:
- This is the most common design for modern CCTs.
- The entire tower is under negative pressure, which promotes a more even distribution of air across the coil and fill.
- The fan is in the hot, saturated exhaust air stream, which prevents recirculation and places the motor in a harsher environment (requires a well-rated motor).
- The fan is typically an axial type (like a large propeller), which is more energy-efficient for moving large volumes of air at low pressure.
- Best for: The vast majority of outdoor industrial and HVAC applications.
B. Natural Draft Towers
- Design: These do not use fans. Instead, they rely on the natural buoyancy of warm, moist air. The exhaust air is warmer and less dense than the outside air, causing it to rise through a large hyperbolic chimney, which in turn draws cooler air in at the bottom.
- Characteristics:
- Extremely large and expensive to construct.
- Have very high initial capital costs but extremely low operating costs (no fan energy).
- Used almost exclusively for very large heat loads, typically in power plants (500+ MW).
- Rare for standard industrial or commercial CCT applications.