Calculating Steam Turbine Condenser Sub cooling 2

[thermalarchitect]

At most of the power plants within my organization, there is only a indication of back pressure at the exhaust of the steam turbines. There usually is not a second variable needed to find the exact conditions of the steam leaving the turbine.

I am very interested in quantifying condenser sub cooling and making some adjustments to operations, but how can I do this with the lack of data needed to properly find exhaust conditions?

Is it reasonable to assume that the quality stays the same through a range of up to 4inHG back pressure? If so, how far could the range of inHG go while still maintaining the same assumption of nearly constant quality?

I need a thermo lesson, it's been too long since college.

 

[rmw]

The quality does not stay the same. Do you remember the Mollier diagram from your long ago thermo days? You need to get one and plot your turbine expansion line by taking known points along the turbine such as extraction points or crossover points, feedwater heater inlet conditions, whatever. Each of these defines a point on the expansion line of the turbine.

Plot this line down to the range of back pressures that you experience and you will be able to note the differences in quality.

With the back pressure that low, I can guarantee you that your are in the moisture region. But the slope of the expansion line, where it intersects the back pressure line will tell you the quality. (A true turbine expansion line will have a recovery point that moves back up the pressure line due to pressure recovery effects as the high velocity exhaust in the turbine hoods expands into the cavernous expanse of the condenser, but we'll ignore that for now.)

You use the word subcooling, which is proper terminology for a condenser parameter, but not the one you are drivng at. The term subcooling is usually atributed to a hotwell condition where the condensate in the hotwell is colder than the saturation temperature corresponding to the actual back pressure of the exhaust.

What you are talking about is quality, or moisture, which is not subcooling, rather a specific condition of steam for that given temperature and pressure at the ELEP or expansion line end point.

If you measure hotwell temperature, it should correlate to your back pressure, and if it is colder than the corresponding saturation pressure, then that is a problem. True subcooling is not a desirable thing.

Another thing, be careful where you take your condenser back pressure readings. While it is not large, there is actually pressure drop between the turbine exhaust hood and the condenser tube bundle. Your instrumentation will probably not be sophisticated enough to detect it, but it exists. Further, velocity effects near the turbine hoods can cause errors in the back pressure reading. The pressure should only be taken from a still area, or from a device called a basket, which provides a still area for the pressure pick up.

I hope this helps.

rmw

 

[thermalarchitect]

Thank you for your reply, I will have to dig out the Mollier diagram.

I am merely trying to quantify the amount of unnecessary heat rejection that may be occuring in our condenser systems (both ACC's and water tube bundle types).

It sounds like I could just create an equation based on the Mollier diagram and have the back pressure be the input and the resultant temperature be the output of the computation. I have a server based historian system to look at these plants real time data and I could input that equation and the hotwell temp into the system to create a real time indication of sub cooling.

That is where I was going with this. I was actually just interested in how other folks are calculating unwanted condenser heat rejection based on the instrumentation that I have (which is industry standard)

 

[davefitz]

If you only wish to determine the heat load to the condenser , there are several simple methods and also some complex methods( as used in performance tests).

For the water cooled condenser, you could measure the cooling water inlet and outlet temps and estimate the water flow based on the circulating pump performance curves and the monitored pressure increase across the pumps.

A similar estimate can be had with the ACC but you would use the fan perfromance curves, inlet dry bulb gtemp and air exhaust temp.


Yet another estimate can be had from a 1st law energy balance; the energy supplied to the steam turbine cycle from teh boiler steam flows must equal the turbine output plus condenser heat load plus feedwater heater duties plus standard losses. The boiler steam flows are calcualted from the indication of condensate feed from the condensate flow nozzle, etc. With today's digital control systems it should beeasy to program it to output these energy flows.

 

[rmw]

HEI has a condenser sizing/rating software that is easy to use, and you can fill in most of the parameters from plant instrumentation data. It is very inexpensive.

http://www.heatexchange.org/pub/default.htm

This should allow you to determine the heat load on your condenser and get a steam flow, and see if it matches your turbine heat balance.

I have used it a lot knowing no more than inlet and outlet temp, back pressure and CW flow.

You should be able to get your CW flow from reading your pump curves (gauge readings corrected for height with respect to pump center line or discharge.) If you know the power factor at your plant, or have a good approximation, you should be able to back check your pump curve readings.

Further, if you have good instrumentation on the condenser CW inlet/outlet pressure, head loss across the condenser is a good way to verify your pump curve readings.

If you have everything right, the pump curve, power calculations and head loss across the condenser should be consistent indicators of your CW flow.

Nicer if you have flow measurement, or a weir.

rmw

 

[mauner]

I don't know that determining the total heat rejected answers the original question.

I think rmw hit the nail on the head. The other parameter needed is condensate inlet temperature.

Although subcooling is not desirable, it is unavoidable and sometimes built into the steam cycle design. Subcooling occurs when condensate droplets form and remain in contact with the condenser tubes. The number I remember is that we didn't want more than 2F of subcooling.

You need to be careful not to cause condensate pump cavitation by reducing the amount of subcooling below a design value. Condenser hotwell level may not provide sufficient NPSH to prevent condensate pump damage.

Plant designs that use feedwater heaters do much more to improve cycle efficiency than saving a couple of btu/lbm of sensible heat loss.