Most of you are familiar with one of the main parameters which affects most on the output and efficiency of a gas turbine, yes that is the ambient temperature. As you know, the gas turbine output depends on the mass flow of air sucked into the compressor, and mass flow is inversely proportional to the ambient air temperature, because density of air reduces as the ambient temperature increases or vice versa, thus the higher the ambient temperature the lower the density of air causing the reduction in mass flow.
To improve gas turbine performance during hot summer, there are various techniques used in the modern gas turbine power plants, however there are two common types of gas turbine inlet air cooling; chilling and evaporative. Where, evaporative cooling can reduce temperatures up to couple of degrees to the wet bulb temperature while with chilling gas turbine air inlet temperature of about 6 degrees of Celsius can be achieved.
From above it is noticed that in case of evaporative cooling temperature reduction is limited close to the dew point while from OEM limitation point of view, plant operation below 6 degrees is not allowed to avoid icing at the compressor inlet which may act as FOD (foreign object damage) in compressor.
Using chilling to reduce inlet air temperature below dew point temperature, mechanical and absorption chillers are used. Mechanical chillers are most suitable for this purpose while in case of absorption, hot water or steam is used which will consequently increase the heat rate therefore this method can be used where there is waste steam of hot water available. Mechanical chillers are used to provide relatively cool air below ambient air dew point with consistency at the gas turbine inlet. It utilises electrical power which may add in station auxiliary load noticeably subsequently may impact on the heat rate.
There are two methods of evaporative cooling of gas turbine inlet air; the evaporative coolers and the fogging. In fogging special designed fog nozzles in combination of very high pressure supply pumps, are used. Fog nozzles are designed very carefully to atomise water drop to sub-micron to achieve very fast rate of evaporation. Following very important factors are considered during designing and installation of a fog system at turbine inlet:
1. Special nozzles which reduce droplet size to sub-micron,
2. Selection of very high pressure pumps,
3. Pure demineralized water to be used
4. Selection of very fine (sub-micron) LP and HP filters,
5. Installation of nozzles array well before the compressor inlet to provide enough time for evaporation, to avoid water carryover into compressor
6. Installation of special catcher channel to collect agglomeration and condensation on the duct walls and silencers etc,
In some cases the wet compression is also used to increase gas turbine performance in conjunction of evaporative coolers and fogger. In which, over fogging is used to carry over the water droplets to compressor where it absorbs by air due rising temperature of air at each compressor stage thus reducing work on compressor so increase in output to turbine. Also due to this water absorption reduces the compressor outlet temperature which allows to burn more fuel and consequently increase of output without increasing of flame temperature.
In evaporative coolers which consists of a media followed by drift eliminator, where water is sprayed over the special media which become wet and airflows through honeycomb passages in the media evaporates the water with in and reduces temperature. Then cooled air passes through the drift eliminators to separate suspended water droplets from air to avoid carryover to the compressor. The evaporation rate depends on the wet bulb temperature. The evaporative coolers become more effective in the regions where there is less humidity in atmosphere. Following very important factors must considered before installing evaporative coolers:
1. Chose this type of cooling in regions where atmosphere is less humid,
2. Selection of effective media,
3. Installation of FOD screen between drift eliminators and gas turbine inlet silencer,
4. Protective paint for the inlet duct and internals
5. Water feed and blowdowns,
6. Quality of water supply,
Finally I personally would prefer to use evaporative coolers in the region with dry atmosphere, because of high reliability and effectiveness.
Nawaz Ali Lakho
Plant Manager, Prodiesel (Pvt) Ltd. Alshuaiba 150 MW Power Station,
Basra, Republic of Iraq.