Condenser Heat Load
Condenser Heat Load
(OP)
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.
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.





RE: Condenser Heat Load
The heat load is calculated using 969.89 Btu/lb as the latent heat of condensation --- and that is the correct figure to use. Actually, I get 969.74 Btu/lb using the online NIST database - but that nit-picking.
You obviously have your enthalpies mixed up. The enthalpy of the saturated condensate at 4.6 in Hg = 181.96 Btu/lb
The enthalpy of the saturated steam at 4.6 in Hg = 1,151.70 Btu/lb. The difference is the latent heat of vaporization (or the latent heat of condensation, however you want to describe it). Check it out.
RE: Condenser Heat Load
So the latent heat of vaporization equals the enthalpy of vaporization minus the enthalpy of saturated liquid?
and also is it correct to get these values at the operating pressure (4.6 inHg), or would there ever be a reason to get it at atmospheric pressure? Thats what I'm thinking he got the enthalpy of saturated liquid from.
Thanks Again
RE: Condenser Heat Load
RE: Condenser Heat Load
dkm0038:
No, the latent heat of vaporization DOES NOT equal the enthalpy of vaporization minus the enthalpy of saturated liquid. Your terminology is wrong and that's probably what's giving you the wrong idea and/or concept of what takes place inside the condenser and on a Mollier (P-H) diagram. There is no such thing as "enthalpy of vaporization". Enthalpy is the property of a substance and not of an action or process. Allow me to explain in detail in order to make sure we both agree on what is happening.
I assume that the steam condenses in the condenser at a constant pressure of 4.6 inches of Hg (gauge). Actually, this is not really true because there has to be a pressure drop within the condenser (otherwise, there would be no flow), but it is what is done in practice and this assumption yields a conservative answer. I also assume that the inlet steam is SATURATED (as opposed to superheated). I'm also assuming that the formed condensate is also saturated (as opposed to supercooled). This means that you must evacuate the condensate as fast as it is formed. Under these conditions, the thermodynamic process is a horizontal line on the Mollier diagram that starts at the saturated vapor curve line on the right hand side of the diagram and extends horizontally to the left portion of the curve that represents the saturated liquid line. This horizontal line should be directly on top of the pressure value of 4.6 inches of Hg (gauge pressure – don't forget to add atmospheric to convert it to absolute pressure) which can be read on the Ordinate axis of the Diagram.
Note that the horizontal line defines what is happening in the condenser: you are taking saturated steam and condensing it at constant pressure. The point at the saturated vapor curve defines the condenser inlet and its enthalpy can be read directly below, on the Abscissa axis. The point on the saturated liquid curve defines the product condensate and its enthalpy can also be read below, on the abscissa. The definition of the load on the condenser is the heat removed from the steam in order to convert it to condensate and this equates to the enthalpy of the vapor minus the enthalpy of the condensate – as represented by the length of the horizontal line. If you use the Mollier Diagram, you have the enthalpies of both streams. You can also use the NIST free database which you can find at: http://webbook.nist.gov/chemistry/fluid/. Either way, you should find that the difference between the enthalpies is the 969.74 Btu/lb.
Additionally, I believe you either have a typo or someone made an error in the calculations' results you were given. If you multiply the steam mass flow rate by the latent heat of vaporization you were given (& which I confirmed as correct), then you obtain 96,795,022 Btu/hr and NOT 86,958,734 as the heat load you report. Somewhere, something is amiss and I recommend you check it out.
I hope this helps to bolster your confidence in the correct calculated load.
RE: Condenser Heat Load
BTW, as an aside, the accuracy of the estimated heat duty doesn't increase by expressing it in more significant figures than those used for the enthalpies or the flow rates.
RE: Condenser Heat Load
If this is a real-world application, you need to consider two additional factors. First, there is a pressure drop through the condenser which will lower the total pressure at the outlet. Secondly, there is always a certain amount of air leakage into the system. At the outlet end, this affects the partial pressure of the steam. Usually this is taken as about 80% (per HEI standards) of the total pressure at that point, further reducing the condensing temperature at the outlet. The remaining mixture of steam and air is usually taken to a jet-ejector sytem to maintain vacuum in the condenser and reduce back pressure in the steam turbine. I'm assuming this is a steam turbine condenser based on the conditions you are starting with.
Regards,
Speco
RE: Condenser Heat Load
Based on the original values you gave us, your answer is correct. The change in enthalpy between the inlet and outlet flow times the mass flow rate is your original answer. That's how much heat (btu/hr) was lost by the steam. Any thermo, heat transfer and engineering handbook will bear that answer.
RE: Condenser Heat Load
To dkm0038, if this is exhaust steam from a turbine, what was the quality assumed? The heat duty given to you may have been the result of assuming a certain steam quality.
See please, thread666-143727: Turbine exhaust steam quality.
RE: Condenser Heat Load
Regards
RE: Condenser Heat Load
Let me see if I can summerize all of this i think my trouble is in the different names the same physical quantity.
This is for a 9.50 MW steam turbine generator operating at4.6 inHg(A) with a mass flow rate of 99,800 lbm. I was always under the impression that for pure condensing (no desuper heating or subcooling)the latent heat load is just the mass flow rate times the latent heat of vaporization.
is it true to say that the latent heat of vaporization is the enthalpy of vaporization (hfg) which is further the difference between enthalpy of saturated vapor (hg) and the enthalpy of saturated liquid (hf)?
RE: Condenser Heat Load
To sailoday28. It seems that the kinetic energy of steam entering a condenser at 200-250 f/s should be a very small amount of the steam enthalpy at 4.6 in Hg (A). Would you confirm ?
To dkm0038. This site may give you some ideas about air-cooled turbines exhaust steam condensers:
ww
RE: Condenser Heat Load
RE: Condenser Heat Load
Therefore, for the conditions you state above, I would consider the UEEP equal to 969.89 Btu/lb at an absolute pressure of 4.6 inch Hg. The corresponding liquid enthalpy is 98.56 Btu/lb and the heat load is the mass flow times the difference in these two enthalpies.
Best of luck!
RE: Condenser Heat Load
thanks for the responce but,
if we do what you say and the heat load would be (99800lb/hr)*(969.89Btu/lb - 98.56Btu/lb) = 86,958,734 Btu/hr
yet the enthalpy of vaporization (or condensation in our case)for steam at 4.6 inHg is aprx 1019.5 Btu/lb. and if this is the energy per pound to condense steam then the heat load to condense would be (99800lb/hr)*(1019.5Btu/lb) = 101,746,100 Btu/hr?
Is this correct. I understand that what stgrme was saying may be standard practice but does this mean that there isn't total condensing going on?
thank you
RE: Condenser Heat Load
To assist everyone on this thread, I'm attaching the calculations for the condenser load at two pressure: 4.6 inches of mercury absolute and gauge.
Note the simple answer.
The original cited load is still wrong - if the steam is saturated. That may be part of the basic data that hasn't been given.
RE: Condenser Heat Load
If the steam were saturated, the exhaust enthalpy would be 1118.05 Btu/lb. The heat of vaporization is the difference between the enthalpies of saturated steam and saturated water at the same pressure. For the present case, that difference is approximately 1019.5 Btu/lb.
Since the exhaust enthalpy from the turbine (969.89 Btu/lb) is less than the saturation enthalpy (11180.5 Btu/lb), less heat needs to be removed in order to condense the exhaust steam.
I hope this helps!
RE: Condenser Heat Load
RE: Condenser Heat Load
RE: Condenser Heat Load
rmw
RE: Condenser Heat Load
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.
RE: Condenser Heat Load
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."
RE: Condenser Heat Load
rmw
RE: Condenser Heat Load
Please elaborate
"We don't believe things because they are true, things are true because we believe them."
RE: Condenser Heat Load
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.
RE: Condenser Heat Load
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.
RE: Condenser Heat Load
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
RE: Condenser Heat Load
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.
RE: Condenser Heat Load
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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.
RE: Condenser Heat Load
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.
RE: Condenser Heat Load
RE: Condenser Heat Load
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.
RE: Condenser Heat Load
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.
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Plymouth Tube
RE: Condenser Heat Load
RE: Condenser Heat Load
I've seen turbines with every bit of the 10% that [b]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