1 CTG Inlet cooling General
On hot days, the gas turbine output can reduce 10-20%. Typically, every 1 oF rise of ambient air temperature reduces the gas turbine output by approximately 0.3 to 0.5% . Stand-by units cost $300-450/kW plus maintenance fees. There is a clear business case for inlet cooling technologies as power augmentation options.
Fogging is of various types. Natural fog with droplet sizes of 1 to 10 mm. In saturated fog cooling, the air is saturated, which means 100% relative humidity. In overspray or high-fogging, more water is supplied than needed for saturation. In spray cooling, droplet inertia dominates, leading to a relatively large slip velocity. Droplets are greater than 20mm. In mist cooling, slip velocity is zero and droplet size is between 5~20mm.
In inlet cooling, an active refrigeration system achieves temperatures below wet bulb temperatures. Compressor-based refrigeration chillers or heat-driven absorption refrigeration based chillers are used. Systems can combine electric, absorption chilling with partial thermal storage.
In indirect chilling systems, the evaporative cooling is limited by wet bulb temperature. In direct wet systems, spray water is introduced into the air stream or onto a medium in the main inlet air stream.
Indirect “dry” systems are similar to the indirect chilling system except that the separated primary source is provided by evaporative cooling.
Excess water vapor causes increased specific heat, which causes less cooling during expansion, so exhaust temperature goes up. Higher exhaust temperature causes the GT controller to back off on fuel flow, which reduces the power boost and increases CO emissions. Can be corrected by modifying GT controls.
Gas Turbine inlet cooling can significantly augment the output power during hot or dry days, but not necessarily the thermal efficiency. Among various inlet cooling schemes, fogging is the most economic approach, costing $35-60 per kW. Fogging droplet size should be smaller than 20 microns to minimize erosion and maximize evaporation in the compressor. Wet compression increases axial velocity, blade inlet velocity, incidence angle and tangential component of velocity. Most erosion occurs at leading edge and one spot of trailing 10-9´edge of the rotor suction side.
Winter time site temperature
Summer time site temperature
2 SEC inlet cooling system
MHI 501G inlet cooling system is use the deminlerzation water through a calcium filter to remove the calcium and make up to the inlet cooling basin, the logical set up if the temperature below above 17 degree C, then the pump will start up, for the intial time, the pump will start 5 minutes then stop 5 mintues 3 times then keep continuously operation,
3 Inlet cooling challenge:
3.1 startup reliability, several factors impact the start up reliability, the pump prime, the pump is located on the platform which is 3 meters is high, so any leakage to the air will lost the pump prime, so install a prime line to help start up
3.2 inlet cooling operation time
The initial set up inlet cooling start up at 17 degree C, then engineering dig out the MHI logical found MHI permit the inlet air can cooling tower to 8 degree, then change the set point to 12 degree to start the inlet cooling system
3.3 inlet cooling water PH and conductivity
Due to the PH requirements so the plant operation manual test the PH and conductivity per each shift
3.4 inlet cooling vs CTG plant output
1 degree C inlet air decrease the plant output can increases 3 MW.
4 Solution
Inlet cooling
With MHI permission to run CT inlet evaporative down to 75% load on CT the logic needs to be modified to reflect that. Current logic won’t allow inlet cooling to occur below 90% load.
Switches are mounted very close to the front of the evap media. When the evaps are put in service they are effecting the temperature readings and not giving a accurate ambient temperature resulting in loss of run permit.
ex: CT2 switch reads +- 3 degrees higher than ambient and needs to be calibrated but can’t get at it while running.
5 performance evaluation
The humidity
Wet bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water into the air at a constant pressure
6 Conclusion