Condenser Heat Load 7

[dkm0038]

I am designing an air cooled heat exchanger for a steam condenser. The specs that i have recieved for steam is a flow rate of 99,800 lb/hr, 969.89 BTU/lb, and at 4.6 inHg. The heat load given is 86,958,734.

The heat load was calculated by the flow rate times the difference between the enthalpy of vaporization (969.89)and the enthalpy of the condensate (98.56).

My question is why was the heat load calculated as shown above. It seems that the necessary heat load would be the flow rate times the enthalpy of vaporization, which i thought was the energy transfer needed to condense the steam.

Thank You for any help anyone may have.

 

[davefitz]

The condenser load will be determined by the value of the enthalpy * mass flow of all streams entering minus the enthalpy * mass flow of the subcooled condensate leaving.

The LP turbine L-0 blade useed energy end point UEEP may be 10% liquid by weight in a modern steam turbine, and thus this enthalpy is less than sat steram . Also, there are other streams mixing with this , such as cascading drains from the LP heaters, and turbine leakage, etc.

Finally , there may be considerable subcooling occurring in the condenser, depending on its detailed design. Normally, one tries to avoid subcooling as it increases the amount of oxygen that is saturated in the condensate, but it happens in some designs anyway.

 

[sheiko]

dkm0038,

One issue in the way the latent heat was calculated is taking the difference between vapor and liquid phases enthalpies at 1% flash. This is NOT correct (only valid for pure component flow). For a mixture, take the heat flow difference for the stream at 0% flash and at 1% flash. Then devide by the total mass flow.

Moreover, if you have water or non-condensibles in your stream, the latent heat for the initial flahses will be huge. Since the presence of water and non-condensibles is hard to predict accurately in normal refinery operation, it is safer to exclude these components from the latent heat calculations.

"We don't believe things because they are true, things are true because we believe them."

 

[rmw]

Huh????


rmw

 

[sheiko]

rmw,
Please elaborate

"We don't believe things because they are true, things are true because we believe them."

 

[GuyFromDenver]

If the exhaust from a turbine is not superheated it will cavitate in the blades and beat the snot out of them. it is for that reason that most commercial power generating stations back pressure the turbine exhaust so that it is not in the quallity region on the way out. You take an efficiency hit by doing so, but the trade off of replacing multi-million dollar hardware a couple time a year offsets the cost of back pressure.

Smaller scale Rankine cycle engines use organic working fluids such as toluene, isopropane, etc (Google organic Rankine cycle engine for more info) as they leave the turbine with more superheat then they went into it with as the entropy at lower pressures is greater then at higher pressures at most of the T-S plot.

BHut back to the rest of the story. When I size heat exchangers I go mostly off of experience from exchanger manufacturers that I have grown to trust. Always size for your maximum operating conditions then consider less then optimal ambient temperatures, exchange media and then still go a little larger.

 

[stgrme]

The exhaust end blades in a steam turbine, operating in the wet region, do not "cavitate"; they "erode" due to the impact of moisture droplets. Virtually all condensing steam turbines have exhaust end blades that operate in the wet region.

Contrary to GuyFromDenver's claim, real commercial plants do not back pressure their turbines to avoid the wet region. The losses in output would be substantial.

Designs for blading operating in the wet region incorporate measures to mitigate erosion, such as erosion-resistant materials or flame-hardening of inlet edges, interstage moisture removal and increased axial spacing between stationary and rotating blades. There is also empirical evidence that the rate of erosion slows from an initially high rate to a more moderate rate due to roughening of the blade surface. There may be exceptions, but exhaust end blades in many turbines, operating in the wet region, last 15 to 20 years or longer.

 

[rmw]

Or even 30 years with replacements of the wear strips on the last stage buckets (blades.) Again, quite contrary to GuyFromDenver's claim, power plants do all they can to get the deepest vacuum they can possibly get with the cooling water that they have to work with. When you burn as much fuel as a large power station can burn, even small percentage increases mean big bucks savings.

And... what is going on is not cavitation, it is condensation. The steam begins to condense into small water droplets as it gets into the wet region below the saturation line on the Mollier diagram.

rmw

 

[FredRosse]

Moisture in turbine exhaust has been a technical issue since the beginnings of steam turbines, well over 100 years ago. As a design issue, it is generally considered good practice to keep exhaust steam moisture below 15%, although I have seen turbine vendors offering exhaust moisture as high as 17%. Nuclear plants, which operate with wet steam throughout most of the turbine expansion line process, have separation devices (internal to the turbine and external to the turbine) to keep moisture in the blading reasonably low.

Once the machine is installed and running, the operators always try to get the lowest practical exhaust pressure. With over 40 years in the power industry, I have never heard of commercial plants adding back pressure to protect turbine blades.

Having said that, there can be situations at large utility plants where lower turbine exhaust pressures actually reduce output. All turbines have a limit, where lower exhaust pressure gives diminishing returns and further reduction in exhaust pressure gives no increase in power. The lower exhaust pressure also implies lower condensate temperatures, and this then requires more extraction steam for feedwater heating. The net result is that the cycle efficiency is actually reduced somewhat with the lowest available exhaust pressure during winter operation. Some plants account for this and limit the circulating water flow to stay above the exhaust pressures that hurt cycle efficiency.

 

[brlewis]

I have attached a gatecycle analysis for your project that you may be interested in.
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http://files.engineering.com/getfil...7cbf-432d-866a-c97cbbd90839&file=G-AIRCC.pdf>

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http://files.engineering.com/getfil...2-49ce-a108-23d0a8754ae4&file=AirCCModel.pdf>

There are a lot of parameters needed to refine the model for the air cooled Heat exchanger and I guessed on your turbine parameters. The heat balance should be close.
If you post a list of changes you want to make I will rerun it for you.

 

[brlewis]

One more thing. All of my experience has been theoretical. I have been using this program for the last 5 yrs. and am curious how it relates to actual real processes. Please provide some feedback on how this compares with your steam generator and condenser. I am a firm believer in modeling when tempered with live field examples. It would be extremely difficult to do all the calculations that modeling provides.
All of my modeling has been internal and not for profit. My efforts have been modeling complete MSW gasification gas turbine combined cycle power plants. I have done 100's of senarios. I am willing to share some of my experience on a limited basis. I was interested in your project.

 

[dkm0038]

thanks brlewis, I will go through the info and come back to you

 

[tgmcg]

Condensed water droplets upon impact do in fact cause damage via cavitation.

Erosion tends to produce relatively smooth wear surfaces. Cavitation produces rough, pitted surfaces. Run your hand along the leading edge of an airplane propeller sometime. It's as rough as a wood rasp. Cavitation.

 

[EdStainless]

Good description FredR.
In cases that I have checked it is common to see about 5% moisture in turbine exhaust. Hardfaced last stage blades will handle this.
Plants do have a minimum pressure that they will run. If for some strange reason they can actually reach it they either throttle cooling water flow, idle a pump (both bad practices, or they recirculate some cooling water.
Turbines reach a point where either lower pressure does not result in more power, or where the risk of damage becomes excessive.
The impact of droplets has nothing to do with cavitation, it is mechanical impact damage. If you had steel balls that were 0.002" diameter moving at >500 ft/sec you would get the same results.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[muscovy]

Typical exhaust of a turbine is about 10% moisture.

 

[rmw]

Depends... on the starting point and the slope of the turbine expansion line (as viewed on a Mollier diagram).

I've seen turbines with every bit of the 10% that muscovy[b/] mentions in their exhaust while the BFP turbine(s) for that very same unit was struggling to get into the same condenser with any moisture.

rmw