Inlet fogging systems for combustion turbines achieve their effect at the molecular level: The cooling effect occurs by converting thousands of gallons of water into single evaporated molecules suspended in the air. The right fog pattern comes down to a matter of selecting the best nozzle design and proper placement in the inlet air stream.
Two Types of Nozzles
Two types of nozzles are commonly used for inlet fogging: impaction-pin nozzles and swirl-jet nozzles (Figure 3). Both designs operate on the principle of converting high water pressure into increased velocity as water passes through a tiny nozzle orifice. As water exits the nozzle orifice, it forms a conical sheet of water that gets ever thinner the farther it moves from the orifice. Surface tension causes the cone to first break down into fingers of water, and eventually, air instability breaks these fingers down into minute droplets for fast and efficient evaporation.
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| 3. Two fog nozzle types. Two types of nozzles are in general use for gas turbine fogging applications: impaction-pin nozzles and swirl-jet nozzles; impaction-pin designs predominate. Courtesy: Mee Industries Inc. |
Although they operate on the same principles, the designs differ in how they create that cone of water, and this affects the size of droplets produced. Impaction-pin nozzles consist of a smooth, short, straight-through orifice with a specially engineered impaction pin in front of the orifice. A fine jet of water is emitted, which immediately strikes the pin, causing the water to form a cone of micro-fine particles. Swirl-jet nozzles, on the other hand, rely on the nozzle’s internal geometry to create the cone. An internal swirl chamber forces the water to exit the nozzle tangentially to the axis of the orifice, forming a hollow cone.
Both nozzle designs produce droplets in the sub-50 micron range, and either one will work for particular, noncritical applications. For example, when fogging nozzles are placed near the ceiling to cool or humidify a factory, greenhouse, warehouse, or restaurant, there is plenty of time for even the larger droplets to fully evaporate before falling to the floor. However, in higher precision applications, such as inside HVAC ducting or for turbine inlet cooling, droplet size is critical.
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