In the recent past, changes in the climatic temperature and a decrease in the power reserves have necessitated the optimization of the existing power plants and any upcoming ones. Plant efficiency and output control the cost of power thus the need for improved plant performances. In gas turbines, inlet air cooling or fogging is used to increase its output and improve efficiency (Jones & Jacobs 2000).
High pressurized fogging system
This is a system used in gas turbines to enhance their performance resulting in the improvement of the turbine’s output and efficiency.
Components of the fogging system
The fogging system consists of various components including:
- High-pressure pump skid. Water going through the fogging nozzles must be at high pressure for atomization to take place. The high-pressure pumps are used for pressurizing the water and forcing it through an orifice. Other pumps cause the water to swirl (for swirl jet nozzles) or to impact on a pin (for impaction pin nozzles) making it split into droplets (Jones & Jacobs 2000).
- Fogging nozzles in the inlet air ducts. The fogging nozzles are the “heart” of the fogging system and are of two types: the swirl jet and the impaction pin nozzles. Their principle of operation is based on the conversion of water pressure into velocity when the water is strained through an orifice. The high-velocity water is turned into a conical sheet of water which splits after expansion and eventually breaks into filaments of water then droplets. The very small droplets of water produced are evenly distributed in the inlet duct. This ensures faster evaporation leading to less droplet fallout and obstructions that may cause damage to the compressor. To make certain that the fog droplets are small, the nozzles must be large in number.
- The type of nozzle, the operating pressure and the nozzle capacity influence the size of the fog droplets.
- Duct drain. The duct drain is located at the inlet or downstream near the nozzle manifolds. Its function is to remove any accumulated water in the inlet duct. This is due to the water condensing or coalescing as it moves down the duct.
- Control system. The control system is used to control the operation of the pumps and fogging nozzles. The water flow is controlled depending on the ambient conditions and the number of pump stages. The pumps are each connected to a certain number of nozzles hence can be turned on sequentially in case of a temperature rise or a change in the humidity (Mee Industries Inc. Technical Division 2002).
Operation of the fogging system in gas turbines
The fogging system in gas turbines works by forming a big evaporative surface area through atomization of the water supply into billions of very small spherical droplets. When the water travels down the inlet duct, it is pressurized by the high pressure pumps to ensure atomization. Depending on the type of nozzle, the operating pressure and the nozzle capacity, the water is converted into very small droplets to ensure their evaporation. For any accumulated water which has not evaporated, it is collected by the drain duct to prevent compressor damage. When the ambient conditions change, the system detects this through sensors (for temperature and humidity) and adjusts accordingly the amount of fog required (Mee Industries Inc. Technical Division 2002).
The fogging system is a very effective method for enhancing the performance of the gas turbine by improving its efficiency and output. It consists of high pressurized pumps, nozzles, duct drains and a control system for adjusting the ambient conditions to cool the compressor for the turbine and prevent its damage.
Jones, C &Jacobs, J., 2000. Economic and technical considerations for combined cycle performance and enhancement options. GER 4200. Web.
Mee Industries Inc. Technical Division, 2002. Understanding and evaluating high pressure fogging systems for gas turbine inlet cooling. AN-GT-240. Web.