| Frequently Asked Questions |
| If more plant operators, A/E's and generation planners knew the answers to the following questions, then EVERY gas turbine would have an inlet cooling system! If you have a question that we can answer for you, e-mail us at info@polarworks.com. |
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| What are the key financial considerations in evaluating inlet air cooling technologies? back to top |
| Inlet air cooling projects are usually conceived of as capital improvements to existing plants. As a result, financing rules require shorter payback periods (usually within one year) than current inlet air cooling technologies can deliver. Even for plants and technologies that show favorable payback, many projects are financially constrained by restrictive covenants in their debt agreements; they are not allowed to undertake capital improvements or additional debt. This sometimes results in sub-optimal outcomes, whereby projects that could be beneficial are not built. |
| At Polar Works, we believe that an inlet cooling system, when properly designed, more nearly approximates a new capacity addition rather than a capital improvement project. This means that a large inlet cooling project should be treated as if you were building a new power plant, and should be financed in the same way. |
| How important is "pressure drop" in an inlet cooling system? back to top |
| "Pressure drop" is usually measured in "inches, water gauge" (in. w.g.), in the U.S. For most gas turbines, a good rule of thumb is that for every 4 inches pressure drop, the gas turbine output will drop by 1%. We think that this is important; but like all measures, there is more to the story than just the rule of thumb. For example, if a system is consistently providing several percent additional gas turbine output, over many operating hours per year, then the inlet pressure drop is not as significant. The opposite example is where a system only provides a few percent additional power output, and over relatively few hours per year, then the performance impact is significant. |
| Standard specifications for inlet filter systems used to be 4 in. w.g., even though most of them were capable of pressure drops of around 1". The pressure drop associated with the inlet cooling system is additional. We typically see that cooling systems can be designed for about 1" pressure drop across most DX refrigeration coils. Pressure drops across water coils might be a little higher, and across evaporative media film, even a little higher. Pressure drop across fog nozzle manifolds is usually insignificant. If you have a coil system, remember to measure pressure drop on a humid day, where there is significant water condensation occurring; pressure drop might be higher than was originally specified. |
| Why is humid air lighter than dry air? back to top |
| Air is mostly nitrogen and oxygen, which have molecular weights (stay with me here folks) of 28 and 32 respectively. Water has a molecular weight of 18. Every water molecule in the air is displacing a heavier molecule of nitrogen or oxygen. Remove the water, and you have more room for nitrogen and oxygen. |
| If dry air is denser than humid air, why does evaporative cooling work to make denser air? back to top |
| Because evaporative cooling cools the air, making it denser. There are two counteracting forces here: cooling makes the air denser, and humidification makes the air lighter. In the case of evaporative cooling, the force of cooling is much greater than the force of humidification. The net effect is more density. |
| If I have a choice of putting more water through the compressor, or more air through the compressor, what is the problem with putting more water through? back to top |
| If you read some reports on the joys of putting through more water in a gas turbine, you'd expect that they have devised a flammable version of water. As far as we know, water still doesn't burn! But, natural gas does burn, and it does so more readily in the presence of increased oxygen. More Gas = More Power. |
| With regards to evaporative cooling, what does the term "effectiveness" mean? back to top |
| The Doctor of Psychrometry answers: "effectiveness" is a measure of approach to Wet Bulb temperature. The amount of cooling available to an evaporative cooling system is measured by the difference between the ambient Dry Bulb Temperature (DBT) and the ambient Wet Bulb Temperature (WBT). If you have ever used a sling psychrometer, these were the two measurements that you took. If the temperature difference is 10 degrees, and if the effectiveness of a system is 85%, then the maximum amount of cooling available will be 0.85 X 10 degrees = 8.5 degrees. This can also be described as "approach" to Wet Bulb. When the difference between DBT and WBT is less than 5 degrees, effectiveness drops off fairly rapidly. So, remember that effectiveness is usually a best-case scenario, and can't be expected over a wide range of weather conditions. |
| What are the issues associated with "impingement" in evaporative cooling systems? back to top |
| Impingement refers to un-evaporated water droplets entering the gas turbine compressor. These are typically found in 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. |
| At Polar Works, we are of the opinion that most over-spray systems are not going to pose a threat to the compressor. Most operators know that operating an on-line water wash systems dumps a lot more water into the compressor, at much larger droplet sizes, than any over-spray system will do. This practice (operating on-line wash during peak periods) is both widespread and as old as the advent of cleaning systems. |
| Nonetheless, at Polar Works we are believers in DRY inlet cooling. The jury is still out as to whether long-term corrosion to the inlet duct will cause rust to get downstream to the compressor as foreign objects. We have heard of inlet ducts that were reduced by evaporative cooling to rotting shards. We're not the alarmist types, and we're not saying over-spray is bad, but we think that we can do better. |
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| Tel: 518.583.7733 E-mail:info@polarworks.com © 2000 Polar Works, LLC |