How to raise condenser vacuum? 4

[semarang]

Hi all.
I've got an issue about how to raise the condenser vacuum by reducing the exhaust steam temperatures, who come as the Temperature hot inlet of condenser. Is it really true, that if we can reduce the temperature of steam that will condense in condenser, so the vacuum of condenser will raise?
In my case, I've got turbin with its exhaust pressure design 64 mmHga. Exhaust Steam that coming to condenser is 47oC, and the condensor pressure is 85 mmHga. Can we decrease the pressure of condenser by reducing the temperature of the steam (maybe by using the hood spray system)?


Dwi Handoyo S

 

[25362]

There are a variety of possible reasons for getting 4-5 deg C higher than design conditions. Have you identified the cause ? After uncovering the real factor you'd be in a better position to decide on the desirable procedure to reduce temperatures and increase vacuum at the final surface condenser, if at all feasible.

 

[davefitz]

I think you've got it backwards.

The exhaust pressure is usually equal to the saturation pressure , based on a saturation temperature equal to the the cooling water temperaure plus 12 C. If the cooling water temperature is too high ( as on a hot sumer day), then the condenser pressure will be high. You can prove this by plotting the exhasut saturation temperature vs the cooling water temperature , and the 2 lines should be parallel over the year, separated by about 12 C. The temperature difference will increase over 12C if you have fouling of the condenser tubes ( on the water side) or if the circulating water pumps are not providing the full design circ water flowrate.

The steam turbine exuast temperature is almost always equal to the saturation temperature, as most modern turines exaust in the 2 -phase flow range of 7-11% liquid by weight .

 

[25362]

In addition to what davefitz indicated, the presence of noncondensables (CO₂) and air inward leakages could play an important role in the performance of the condensers, the condensate removal pumps, and the vacuum-producing equipment.

Differing loads and the degree of superheat of the turbine entering steam may also be factors in fixing the presure (and temperature) at the surface condenser.

Water-side fouling may also be a cause in below-design performance of surface condensers as well as of the intercoolers of a steam-jet set up.

Lower-than-design ccw linear velocities in the tubulars would result in lower HTCs and a poorer condenser performance, hurting the vacuum.

Condensate back-up (partly plugged lines, boot's LLC failure, pumps' in-leaking of air) can also interfere by reducing the available surface of the condenser. This could be detected by sensing the temperature of the condensate. If it is markedly "sub-cooled", it may indicate a back-up.

Additional condensed water, if any, would oblige to check the condensate-removal pumps for probable cavitation effects.

Jet problems such as caused by xs wear of steam nozzles could also be suspected if the vacuum is not to expectation.

Again, it appears logical that if you can identify the causes of below-design performance, you'll be in a better position to suggest ways to bring the condenser back to design conditions. Good luck.

 

[semarang]

Thanks for ur reply, Mr. 25362 and Mr. davefitz.

I've already checked the air extraction equipment of condenser. All is on a good operating condition.
To davefitz, what u said is true, I've been plotting the vacuum with the temperature of cooling water. More bigger the temperature, so the vacuum is decreasing. But there is something interesting to catch. In my case, when the turbine is on full load, 200 MW, the vacuum is only 85 mmHga with its 47 C of exhaust steam (differential temperature of cooling water is 9 C). In the low load, 100 MW, the vacuum reach 60 mmHga with its 39 C of exhaust steam (differential temperature of cooling water is 7 C). The design exhaust pressure of the turbine is 64 mmHga. With that condition, can we increase the vacuum on 200 MW by reducing the exhaust temperature (using hood spray with its suplly < 40 C)? If so, is it stii safe for turbine's work?

Thanks for your reply.

Dwi Handoyo S

 

[25362]

A priori, one reaches the conclusion that because of the small temperature (and latent heat) difference &quot;spray quenching&quot; wouldn't be so effective.

Spray quenching would be indeed more effective in steam desuperheating by exploiting the vaporization of the sprayed condensate droplets, a larger temperature difference, and a larger thermal conductivity of the hotter steam.

It appears that condensing some of the steam by adding sensible heat to the sprayed condensate water would need a great (impractical ?) amount of water. Whether the surface condenser would be more effective (with a lower LMTD and a larger condensate flow rate) at full load is a moot question.

Anyway, the cool condensate should be air-free, not to add noncondensables to the fully loaded system.

The vendor should be asked about the desirability of a continuous commissioning of the hood spray concerning probable harm to the turbine blades. The vendor could also advise on the heat transfer (i.e., cooling) contribution attainable from the spray.

 

[engy74]

dear Semarang,

from the indication given above, one may suggest that your condenser is under sized with respects to its turbine, i.e. it can not extract all the heat by the condensation of the steam: but this seems very strange; another thing I suggest YOu to verify consist of the temperature and pressure of the steam entering in your turbine: first, to understand if they are different with reference to the design data, second to understand how this variation affects your condenser performances ; could You indicate, if it's possible, how the steam parameters (pressure and temperature) change with the load of the plant?
and, which kind of plant are we discussing about? a new plant, a refurbished plant?
all the information to undestand - before modifing the condensing system - if really the condenser is undersized (if it was designed for another application, for istance) , or others are the problems involved in your plan...
Best regards,
Engy 74

 

[25362]

One &quot;normal&quot; reason for the lack of correspondence between the total pressure in the surface condenser and the condensate's temperature and vapor pressure is the unavoidable presence of noncondensables (air, CO₂, etc.).

As an example, at 100 MW, 39^oC correspond to a VP of about 53 mm Hg vs the reported 60 mm Hg. Meaning the gases being removed to the vacuum-producing equipment (ejectors ?) contain 12% mol noncondensables and 88% water.

Am I right ?

 

[rmw]

Another reason not often considered in condensers for causing a lack of correspondence between total pressure, and saturation pressure is superheat. Yes, superheat coming from an inefficient or overloaded Boiler Feedpump Turbine (BFT) exhaust, or a high energy in-leakage coming from a defective trap or the like (valves left open, drain and dump valve seats defective) on a source capable of supplying high energy, like a stop valve drain, a feedwater heater extraction line drain, a reheater piping drain, etc. etc., and there are many possibilities.

Condensers, unlike feedwater heaters are ill-equipped to handle other than saturated or low quality steam. Superheated steam vapor will blanket the condenser tubes and act like non-condensible fouling. U-values for this sensible heat transfer are pathetically low because the renyolds numbers (velocities) are virtually laminar in this region.

So, check all drips and drains that dump to the condenser for abnormal conditions.

Good luck,

RMW

 

[engy74]

Hi everyone;
I submit You other doubts reguarding this condenser
1) temperature of the water at the inlet of the condenser: are we sure that the design temperature corresponds to the real inlet temperature?
2) pressure and temperature of the steam before the turbine: are these parameters the design parameters for the turbine?
To rmw: interesting observations: but now I'm curious to have, at minimum, a P&I of the plant...
Bye,Engy74

 

[semarang]

Thanks to all the respond.
It seems very impractical for lowering the condenser pressure by hood spray in this 20 years old condenser. Isn't it?
But if we try to see in its heat transfer point of view, is't still impractical to use hood spray's supply (who reach 32 C) for lowering the 46 C steam into 42 C?

Dwi Handoyo S

 

[25362]

Dear Samarang, speaking theoretically from the heat transfer point of view, in the limit, so to say, one may spray enough cold condensate water so as to have a direct contact condenser, aka barometric condenser, altogether dispensing with the surface condenser.

But then the amount of water needed for a water heat up of, say, 10^oC (10 kcal/kg) to condense the steam (more than 500 kcal/kg) would easily be in the 50:1 mass range.

If one assumes only partial condensation by the use of the cold spray, the liquid-vapour contact efficiency, being what it is, may result in spray cold water covering tubes of the surface condenser, reducing the available LMTD and the HTC, as well as available surface of heat exchange. It may act as a surface-cancelling condensate backup.

Theoretically your idea of helping the surface condenser with spray cooling could be effected, but the system geometry and size should, then, be designed differently.

 

[rmw]

Dear Samarang,

While agreeing with 25362 that the amount of spray water to accomplish what you wish to do would be massive, as it is in any direct contact condenser, I can tell you that I have heard of exactly what you are suggesting being done with a spray header system in the turbine exhaust ducts above the condenser neck, not the same as the hood sprays used for back end cooling during start up conditions.

This was done in a unit about your size, but about twice the age of your unit.

However, I would point out to 25362 that aside from the tubes in the rows of the outer edges of a condenser tube bundle, the tubes in a condenser, by definition are already blanketed in moisture and droplets from the dropwise condensation action, and the addition of the spray water component would be negligible.

Also, some condensers, in order to deaerate and heat the make up condensate, spread this condensate directly on top of the condenser tube bundles, in a much higher mass per unit of foot print area than the spray water would ever do, without detrimental effects.

If anything, this cooler water would continue to act as a condensing medium as it distributed itself down through the tube bundle, absorbing the heat of the exhaust steam, allowing the condenser tubes to remain cooler, longer, and enhancing their overall LMTD.

As to the use of the hood sprays that do exist, although I deem the concept as impractical as you say, with the concurrance of the turbine manufacturer, I would give it a try once, on a limited basis, and if you see any noticable change, I would then design a proper spray system in, or right below the condenser neck, away from the high velocity area right in the hoods.

So, I guess my last paragraph said the same thing as 25362's last paragraph, only differently.

RMW

 

[25362]

RMW comments are instructive. We are not speaking of evaporative cooling. Mass transfer as well as heat transfer move, in our case, in the same direction. Sprayed water is supposed to gain sensible heat.

One has always to pay attention to temperature differences as the driving force for heat transfer. Samarang wishes to reduce the temperature of the condensing steam by 4 ^oC at full load. This would affect the LMTD in the already overloaded surface condenser.

Would the effect of hood spraying compensate for this loss of LMTD ? And if the water covering the tubes is colder than the condensing vapour, would the heat transfer suffer ? It is a moot question.

Anyway, I think, as RMW does, that a well-designed direct-contact (spray) system could help a surface condenser but the set should be designed a priori for the purpose.

 

[25362]

At the risk of repeating myself: from the data supplied at 100 MW and at 200 MW, the loss of vacuum is due, at least in part, to vacuum equipment deficiencies in removing noncondensables (partly generated by deteriorated turbine shaft seals), and part due to poor heat transfer in the surface condenser by insufficient surface and other possible reasons.

For those designing vacuum column overhead condensers for vapours containing steam, it is generally accepted as a fact that it is the film of noncondensables which presents an additional resistance to HT over the layer of condensate forming on the tubes' surface.

 

[semarang]

Thanks for the valuable respond to Mr. 25362 and Mr. RMW.
To RMW, your statement about spray header system interesting me. Maybe you can send me (dwihandoyo03@yahoo.com) more information about it especially on its technical aspect?


Dwi Handoyo S