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danieldecko (Mechanical) (OP)
29 Oct 04 16:57
I have a question regarding to Heat recovery steam generators. For ex. a HRSG with a two pressure level steam generator has some design parameters; such as pinch point and evaporator approach temperature for both sections. I would like to know if gas to steam temperature difference at superheater exit is also a design parameter? For ex. let's suppose that in a combined cycle, a gas turbine exhaust combustion products at 510 C. If we say, gas to steam temp. diff. at superheater exit is 32 C; steam turbine inlet temp. becomes 478 C. For ex., then we wanted to change the gas turbine and we want to put a gas turbine with 540 C exhaust temperature. So, if we fix the gas to steam temp. diff. at superheater exit, a desuperheater should be used after 1st stage of high pressure superheater to maintain the same steam turbine inlet temperature. However, if it is not a fixed parameter at the design point of HRSG, only the amount of steam produced in high pressure drum will be changed.

In conclusion, I would like to ask whether gas to steam temp. diff. at superheater exit is a fixed parameter at design stage or not? Or, If any source could be supplied related to HRSG design, I would be grateful. Thanks...
Helpful Member!  quark (Mechanical)
30 Oct 04 1:58
I don't have knowledge on these issues but there is an excellant website which helped me much with some other things.

http://www.hrsgdesign.com/design0.htm

The owner of the above website is a member here.

Regards,

davefitz (Mechanical)
31 Oct 04 10:49
there are different ways of arrivign at the same design. One method is to avoid designing a superheater that requires an effectiveness greater than 0.9, using the e-NTU method for determineing heat exchanger effectiveness.

In most cases, the fluid stream with the minimum value of Cp*W is the superheater steam , so  using that stream as governing , we have :

e,max = 0.9 = (Tsho-Tshi)/ (Tgi-Tshi)
where Tshi= superheater inlet stema temp = sat steam temp
Tsho= temp leaving superheater on the design day
Tgi= gas temp entering HRSG on the design day.

The value of e is also correlated to the design of superheater, viz, surface area of fins, heat trasnfer coeeficient, number of passes, etc. Usually a value of 0.9 is feasible and economical.
timbones (Mechanical)
31 Oct 04 12:34
When you first size the HRSG, the gas to steam temperature difference is an important consideration because it determines the size of superheater. However it is not a fixed variable during operation. The steam temperature should not be desuperheated at the first stage desuperheater unless you are going to exceed the operating temperature of the piping or the steam turbine. Desuperheating reduces the efficiency of the cycle.

That is not to say desuperheating is all bad. If the plant requires significant amounts of duct burning capacity, desuperheaters can be useful for ensuring steam temperature doesn't get to high. In such a scenario, the desuperheater may be operating a significant amount of the time. You only get to choose the superheater once and that means its impossible to have a superheater that is sized right for all conditions. If you skimp on the size of the superheater, it can hurt your efficieny and you may find operating scenarios where the steam is too cold for the turbine (this problem is usually the worst on cold days where the gas-turbine is at part load).
scsekhar (Mechanical)
3 Nov 04 6:34
It may not be a good idea to look at superheater in isolation while designing HRSG. One has to consider the heat balance of the entire two pressure system, fixing the pinch and approach points and including for any blowdown or steam extraction.

S.C.Sekhar
JKEngineer (Chemical)
5 Nov 04 21:06
I used to do a lot of steam system analysis incorporating HRSG systems.  We used rules of thumb for the approach or pinch temperatures at several points along the boiler.  They basically were driven by economics.  Too tight a pinch drove the capital cost up too high, too loose a pinch caused a loss of energy recovered.  As scsehkar says, you have to consider the balance of the entire system.  

Before getting into all those analyses, I did a Master's thesis on the Use of Post-firing in a 2 Pressure Waste Heat Boiler in a Coal Gasification Plant.  It also used multiple pinch point considerations, but the study looked at the relative steam generations at each pressure and the impact on the steam plant.  An estimate of the cost, based on the relative sizes of the various sections was also included.  

Bottom line, you can't approach the GT exhaust gas temperature too closely with the superheated steam temperature; and you can't ignore the interactions through the entire HRSG train.  

HTH

Jack M. Kleinfeld, P.E.  Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering
www.KleinfeldTechnical.com

JKEngineer (Chemical)
5 Dec 04 9:45
As a follow-up to my previous post:
The approach and pinch temperatures will set the performance and sizing of the HRSG.  In general, unless you do something special, one of them will be the controlling limit and the others will not be met.  That is, if you can tolerate a 50F superheater approach, a 25F boiling bank approach, and a 50F economizer approach in terms of econimic sizing of the boiler components, only one of them is likely to control the system sizing.  At the point that you have met the first or most limiting of them, the others locations will have larger than economically limited approaches.  
HTH
Jack

Jack M. Kleinfeld, P.E.  Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering
www.KleinfeldTechnical.com

athomas236 (Mechanical)
6 Dec 04 19:11
The minimum pinch point I have seen is 7 degrees C

athomas236
timbones (Mechanical)
7 Dec 04 12:10
JKEngineer, I disagree with you that one of the criteria will limit the others. I can easily design a boiler where all three prescribed criteria of superheater approach, boiler pinch and economiser approach are satisfied.

As an example, lets say I had a superheater approach to exhaust gas temperature of 50°F, I could improve this by adding surface to the superheater until I had an approach of say 20°F. Since I am doing more heat transfer in the superheater than before, adding the surface would change the boiler pinch, which I could counteract by adding more surface to the evaporator and economiser thus satisfying all three criteria.
JKEngineer (Chemical)
13 Dec 04 23:51
Timbones -
Your example violates the concept of a pinch limit.  The number is approximate, certainly, but is a guide to what is economic.  If you take the pinch limit as meaning: "the approach between the steam and the gas at this point can be as close as, but no closer than the number" (because you can't afford it) which is what is usually meant by a pinch limit spec, then your adding more area to get 20F having specified 50F is circular.  

In addition: the rate of change of temperature with heat transfer from the gas is essentially a straight line.  The same plot, temperature vs heat transfered for the water/steam, is not.  There is a large flat in the curve for the evaporation.  Therefore, you are not at liberty to set all three approach temperatures.  You might get lucky, but generally one will limit.  

To take your specific example, by increasing the area in the superheater to get a 20F approach, you are allowing more heat to be removed from the gas in the superheater.  That heat goes to the steam.  Since the conditions of the steam are set, P,T,h for the boiling bank and the superheater outlet, you are superheating more steam.  That additional steam has to be heated and evaporated - needs more gas than the first case.  You need to plot the two energy balance curves, for gas and steam, as a function of heat transfered or "position" in the boiler.  HRSGs are generally linear.  


Jack

Jack M. Kleinfeld, P.E.  Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering
www.KleinfeldTechnical.com

timbones (Mechanical)
14 Dec 04 11:51
JKEngineer, I think you are assuming that I have fixed the amount of steam that the boiler is going to make which is not the case. If I change my superheater exit temperature to be 30°F hotter but still maintain the boiler pinch that I had before, my boiler will definately make less steam.

To put it another way, assume I have determined a fixed exhaust gas temperature, exhaust gas flow and a boiler pressure (so evaporation temperature), if I assume a specific pinch, this will give me a set exhaust gas temperature at the beginning of the evaporator section and will hence give me a set amount of heat transfer that can be accomplished with the combination of superheater and evaporator. I can bias this heat transfer between making more steam or hotter steam. If I choose to make hotter steam by adding more superheater area, my steam exit enthalpy is higher and therefore steam massflow must be reduced to maintain the energy balance (but evaporator pinch is still held constant).

Tim
JKEngineer (Chemical)
14 Dec 04 12:48
Timbones -
I am assuming a design calculation for a given amount of gas at a given source temperature and specified steam conditions, T and P delivered by the HRSG and BFW T.  The calculation yields the amount of steam that can be raised for the specified gas flow and T and the steam conditions.  Once they are set, there are three temperature approaches or pinches to deal with, the superheater, the boiling bank, and the economizer.  Typically, one will control or be reached before the others, becoming the limiting case.  Your comment in the post before my last of increasing area to decrease the approach ignores the purpose of setting the approaches, economics, and the impact on the inter-related energy balances of the gas side and steam side of the boiler.  

If you are going to allow the steam conditions to vary, the game changes.
Jack

Jack M. Kleinfeld, P.E.  Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering
www.KleinfeldTechnical.com

timbones (Mechanical)
15 Dec 04 15:42
Now you have me confused. If you already know what the steam outlet temperature and flow are, then you have already fully constrained the design in which case all three approach criteria will be limiting at the same time and they become a product of the design rather than a criteria for it.

When I size a HRSG, I use the pinches and approach temperatures as a guide to how much surface is economical. I then calculate, using these pinch criteria, how much steam the boiler can make (not the other way around which seems to me what you are suggesting). Steam outlet temperature is dictated by my superheater approach temperature (in essence the two temperatures tell me the same thing for a given exhaust temperature). Then, knowing how much heat transfer needs to happen in each section of the HRSG, I work out how much surface I need and whether that amount of surface is economical or not. With enough experience, I know what pinches will give me economical HRSG sizes, which is the whole point in using pinches as a design criteria in the first place.

Within limits, I can arbitrarily set and meet all three pinch/approach criteria at once provided I want to spend the bucks on the appropriate boiler surface in the economiser, evaporator or superheater. Of course each change alters the steam outlet flow and temperature. If they didn't, they wouldn't be design criteria.

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