Polar Works-Inlet Cooling Technology Overview-Evaporative Cooling

Evaporative Cooling

While Polar Works isn't involved with evaporative cooling, it's worth understanding how this technology works. The main disadvantages that others identify with evaporative cooling are the limits on year-round effectiveness and the potential compressor damage that can be caused by impingement, when un-evaporated water droplets enter the gas turbine compressor (usually from over-spray fog systems, or in over-spray water systems). Many gas turbine manufacturers have expressed a concern over the use of this technology, saying that it might cause erosion or corrosion to the IGV's or to the first stages of compressor blades. While we at Polar Works don't believe that impingement is something about which you should be concerned, we think we can do better by focusing on dry inlet cooling instead.

Evaporative Cooling is comprised of three major categories:

Evaporative Media

First popularized in arid climates, evaporative media gained growing acceptance in the South and East in the late 1980's. The evaporative media systems are reasonably priced, particularly if they are designed into the inlet filter house right from the beginning; back-fitting this technology is not as easy. Effectiveness of this technology is typically around 85%. One of the drawbacks of evaporative media is that the number of hours that the system is effective is rather low for most climates. Moreover, the small parasitic pressure drop associated with air-flow across the media is a year-round performance penalty. Some operators remove their media in the fall, and replace it in the spring. The media itself is a consumable item, requiring replacement every other season or so, depending on local weather, and on water quality.

Fogging

Around 1990, many operators tried to overcome the pressure drop and fouling problems of evaporative media systems by adding foggers to their inlet filter houses. Because the foggers were placed upstream of the filters, the fog droplet size needed to be very small, so as not to wet the filters. High pressure mechanical fog nozzles could achieve droplet sizes less than 50 microns, while air-atomized foggers could achieve fog droplets of about 10 microns. The mechanical systems were originally prone to plugging and maintenance, although their performance has improved through the years, largely due to feeding those systems water of much higher quality. Mechanical systems also have been achieving much smaller droplet sizes. The effectiveness of fogging systems was slightly higher than typically found in evaporative media systems, often achieving a measure in the low 90 % range.

Over-Spray

Mechanical foggers have evolved into over-spray systems, where upwards of 150% of the water required to be 100% effective is used. This additional water enters the compressor as fine drops. In fact, some systems now forgo fogging as the water delivery method, and have started using fine droplets in spray patterns. Some operators and gas turbine manufacturers are concerned about compressor blade erosion due to droplet impingement. The droplets are carried back several stages into the compressor, where they eventually evaporate, providing a delayed cooling of the air, working essentially as a cheap compressor intercooler. Many operators of existing gas turbines have been using their on-line water wash nozzles for the same effect for years, mostly without any harm being discovered.