We usually calculate compressor efficiency by this method for the past 10 years and based on that we planned for a compressor off-line wash.

ηIsen =
T1 = Actual compressor inlet temperature (ºR)
T2 = Actual compressor outlet temperature (ºR)
P1 = Compressor inlet pressure (Psia)
P2 = Compressor outlet pressure (Psia)
k = Ratio of specific heats = 1.4

<b>Moderator's note:</b> I think ηIsen and (ºR) are special characters that did not show up correctly.

Based on that formula our compressor efficiency was calculated.
Values were P2=203 Psi, P1=14.14 Psi, T1=79.8 F, T2=799 F and efficiency showed 86% Approx.We do have Evap system ON (Method of turbine inlet cooling) during this period.

Now the scenario has changed and we stopped evap system and installed a new system which is called Wet Compression system (Method of turbine inlet cooling). This system reduced 4~5 degrees more temperature drop of compressor inlet temperature (T1) than evap system and also about 50degree temperature drop in Compressor discharge temperature (T2).

When we calculate the compressor efficiency it crossed 95%. We were like wohhhh what we will do now? How will we establish our offline wash and another question came into mind that whether this efficiency formula is correct? Because in winter season fully cleaned compressor shows efficiency to reach about 87%, while in summer fully cleaned compressor efficiency shows 86%?. Please guide.

(Our turbine is Siemens Westinghouse W251 B11, 48.8MW ISO,Operating in combined cycle mode)

 

Hello Khattak.

Where are you measuring T2? is it the compressor discharge itself? in our units (5 x 251B11) it is measured in the combustors chamber space so it is influenced in some measure by the firing temp. Once you clarify this, I suggest to refer your data to a reference temp and pressure so you can compare your calculations regardless it is Winter or summer.

Regarding off-line washing, may be wet compression will help you to keep the unit cleaner...

Rgds.

 

Hello and thank you for your valuable time.

First to answer your question to measure the T2.In our case, it is also measured in the combustors chamber space.

Secondly sorry for posting the incomplete formula for compressor efficiency. Here it is

ηIsen = ((p2/p1)^k-1/k)) / (T2/T1)-1

T1 = Actual compressor inlet temperature (R)
T2 = Actual compressor outlet temperature (R)
P1 = Compressor inlet pressure (Psia)
P2 = Compressor outlet pressure (Psia)
k = Ratio of specific heats = 1.4

Please can you suggest regarding this new wet compression effect on efficiency changes?

 

The wet compression is adding water into the compression process, and that water is 'soaking up' the heat added by the process - the heat of compression is being used to vaporize the water. Water vapor also has a higher specific heat capacity, so it takes more heat to raise the temperature of the wet air than it did with dry air... thus cooler temperatures at the compressor inlet.

Since the wet compression process adds more water per pound of air than the evaporative cooling did, you'll see a change in the calculated compressor efficiency due to the lower CIT.

The compressor efficiency calculation is based on isentropic & adiabatic conditions (no heat added or subtracted) - by adding water, you've gone outside the assumptions of the calculation.

I would recommend you have two water wash calculations to track RELATIVE changes in compressor efficiency, one with wet compression ON and one with it OFF (i.e. summer and winter conditions). That way you can track how much efficiency has changed, and use that as the trigger for when to run a water wash.