By M. Ameri, H. R. Shahbazian, H. Hosseinzadeh and M. Nabizadeh
Media evaporative cooling is suited to regions where the air temperature is high and the humidity is low. This article looks at inlet air cooling technology and an installation at the Fars combined cycle plant in Iran.
Ambient conditions, especially temperature, have great influence on gas turbine performance. Since the density of air decreases during warm days, the mass flow rate through the gas turbine decreases and causes a drop in the output power.
Typically, during the hot summer days there can be a power output reduction of between 0.5 per cent and 0.9 per cent of the ISO output power for every 1°C rise in the ambient temperature.
Solving this problem is very important because peak demand also occurs in the summer. Gas turbine inlet air-cooling is one way of enhancing output power in the summer.
One common method of inlet air-cooling is evaporative cooling, which is appropriate for warm and dry ambient. Since most gas turbines in Iran are installed in such areas, this method can be used to enhance the performance of gas turbines. A media evaporative cooling system was designed, manufactured and installed at the Fars combined cycle power plant.
GT inlet air cooling methods
There are two types of evaporative cooling:
1) Inlet fogging: In this method the high pressure water is sprayed in the inlet air by nozzles. It is very important that all the water droplets should evaporate before reaching the compressor in order to prevent the erosion of its blades. These nozzles produce water droplets of less than 20 micrometre diameter. Since the ratio of area to the volume of these small droplets is large, they evaporate quickly. Therefore, the humidity of inlet air is increased and its temperature is reduced. In this method, the inlet air can become saturated or oversaturated. Also the pressure drop in this system is negligible.
The advantages of this method are: the inlet air dry bulb temperature approaches the wet bulb temperature; low pressure drop; and less initial cost compared to the compression and absorption refrigeration systems.
To prevent the corrosion of nozzles and deposit on the compressor blades, the water should be very pure. The power consumption of this method is high due to having to pump water at high pressure. This method is appropriate to cool the compressor inlet air. However, if the water spray system is not designed well, the first rows of compressor blades will be damaged.
2) Media evaporative cooling: Media surfaces are made of flexible cellulose fiber and consist of honeycomb-shaped cells. Inlet air is cooled by spraying water into these cells and the subsequent evaporation of water. Increasing the contact area between water and air will enhance the evaporation. The flexible honeycomb-shaped cells increase the contact area between water and air.
The following important points should be considered for the selection of a media system:
- The pressure drop in this system is more than in other evaporative cooling systems. However, this pressure drop has negligible effect on the gas turbine output.
- Power consumption in this system is less than in other systems.
- This system does not need demineralized water and can use well water.
- This system needs periodical replacement of media (every three or four years).
- The installation costs are higher than other systems.
- If this system is installed in the filter room, the shutdown period of the gas turbine for installation of the system is considerable.
The media evaporating cooler system is selected based on the technical and economical evaluation of these methods.
Media evaporative cooling
This system uses a media to evaporate water. It is used widely for gas turbines especially in dry and hot areas. The performance of this system is based on water evaporation, which consumes latent heat of vaporization and reduces the air temperature.
Figure 1 shows the schematic of the media system. In this system the water is pumped from the tank below the cooler to the distribution header above the cooler. This water is distributed over the surfaces. The outlet water is gathered below the cooler and drained to the water tank.
Figure 1. Media system schematic
The media surfaces have a thickness of about 20 cm or more and cover the entire cross section of the inlet air duct. The compressor inlet air passes through the media surfaces and the water evaporates to the saturation limit. The remaining water is used for continuous purging and discharging the deposited materials from the media surfaces. The amount of circulating water should be at least two to three times that of evaporated water. A PLC control system is used to control the stages of gas turbine inlet air cooling, performance of pumps, and protect the equipment. A local meteorology station is used in this system to measure the air temperature and the ambient humidity and is interlocked with the turbine control system.
Figure 2. The Fars media evaporative cooler system air duct and its diffuser
To prevent the carry-over of water droplets from media surfaces and damaging the compressor blades due to the high velocity of air, the air velocity should be limited. The cooler air is passed through a mist eliminator after the evaporative cooler and the water droplets are removed. The efficiency of the evaporative cooler is reduced at high velocities for Celdek media surfaces with various thicknesses.
On the other hand as the inlet air velocity increases the pressure drop in the system also increases. Based on these facts, it is obvious that if the inlet air velocity is high, the evaporating cooler efficiency is reduced and the pressure drop is increased. Therefore, a diffuser is used at the inlet air duct to reduce the inlet air velocity. However, it should be noted that the pressure drop is very small.
Figure 3. The installed media evaporative coolers at the Fars combined cycle power plant
The operating point of the evaporating cooler system is set on the cooler controller with respect to the ambient temperature. This point should not be set at low temperature, otherwise ice may be formed in the inlet duct and can cause the media surfaces to crack. If the ambient temperature falls below zero degrees, the system is drained completely to prevent freezing.
The Fars combined cycle power plant has a total of six GE Frame-9 gas turbines with a rated power output of 123.4 MW (at ISO conditions). Due to increased power demand in recent years, the optimization of these units was considered to compensate for the power lost in warm seasons. The installation and operation of a media evaporative cooling system to cool the inlet air was carried out for units 1 and 2.
The design dry bulb temperature and relative humidity was selected as 35°C and 20 per cent, respectively based on the site ambient conditions studies.
Table 1 shows the results of performance tests for unit 1 in August 2004. It shows that the output power is enhanced by 11.11 MW. The steam turbine output is increased by 1.6 MW.
The results show that the difference between performance test data and theoretical values is small. This difference can be due to a variation in turbine performance or the efficiency of the cooling system. The air inlet temperature drop increases if the ambient relative humidity decreases and the ambient temperature increases. The output power enhancement will increase as well.
Using media evaporative cooling in units 1 and 2 of the Fars combined cycle power plant has increased the mean output power for each gas turbine by 11 MW, or 14 per cent. The total power increase for one block of combined cycle power plant (two gas turbines and one steam cycle) is 23.52 MW.
The results of technical and economical evaluations show that using this system is more cost effective than using new gas turbines for generating the power. Moreover, it is very suitable for central regions of Iran where the air temperature is high and the humidity is low.
- Cyrus B. Meher-Homji & Thomas R. Mee III, 1999, “Gas Turbine Power Augmentation by Fogging of Inlet Air”, proceeding of the 28th Turbomachinary symposium, PP. 93-113
- CELdek 7060-15 Evaporative Cooling Pad. https://munters.com