Calculating the Energy savings of reducing steam boiler pressure

[BronYrAur]

I am trying to quantify the energy savings of reducing a boiler pressure from 140 psig to 125 psig. It is a 700 BHP Cleaver Brooks model 4WI-200-700-200 with a natural gas input of 28,576,000 BTUH. There is a feedwater economizer on the stack.

How can I quantify the savings?

 

[IRstuff]

But, what would that cost you? Presumably, someone came up with that pressure to achieve some sort of design requirement. Blithely assuming that saving energy is the only result can lead to DISASTER.

to wit, we had a process that ran at 1100°C, and worked perfectly fine. Some young whippersnapper process engineer decided that 1050°C would substantially prolong the life of the chamber and made the change without consulting anyone in the original process development lab. The end result is we had miserable yields for years, and no one even figured out what was happening until we tried to produce a part that depended on the process step so heavily that it became obvious that the process was broken. After spending a hundred thousand dollars of tiger team experiments, it was discovered that the IBM technical journal from 12 years prior had published exactly what happens when the temperature is dropped. A quick revision to the process schedule for the next lot bumped up the yield on that part by a factor 6 times.

TTFN (ta ta for now)
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[MintJulep]

Savings should primarily be from reduction of system losses.

Calculate the piping and boiler shell losses at both temperatures.

 

[georgeverghese]

The Cleaver Brooks 4WI boiler book ( which is on the net ) has a worked example which should help you with this estimate - see pages 11-13. Working backwards, I get

Eff at 125 psig for this model = 83% ( from table 13)
Stack temp drop on going from 140 psig to 125 psig = 9degF ( from fig 3)
Estimated eff increase = 9/40 = 0.225% ( given that CB claim a 1% increase in eff for every 40degF drop in stack temp -see narrative on page 12 - I estimate this to be approx 36degF increment in stack temp per 1% decrement in net thermal eff )
So current eff at 140psig = 82.78%

Hardly anything to shout about..

 

[BronYrAur]

Thank you. I will check out that reference. Regarding what it will cost, I've looked into that I ready. This is a hospital that was served from another steam source at 140 PSI. When recent additions were constructed, a new central plant was also constructed to handle just those additions. I think 140 PSI was selected just to match what was there. I have double-checked pipe sizes, steam traps, and pressure reducing valves. There are a couple of pressure reducing valves that will need to be increased in size but that's about it. All of the steam that is consumed is step down to either 85 PSI or 15 psi. So there is no direct need for 140 PSI. 125 was more or less an arbitrary starting point to reduce pressure and hopefully save energy. And now I'm trying to quantify the savings to see if it's a worthwhile endeavor.

 

[racookpe1978]

Reducing the exit gas temperature by 9 degrees will "probably" not be enough to cause condensation of acidic gases in the exhaust, but in other cases it might. Be cautious.

 

[Latexman]

Why not run the boiler at 85 psig, maximize cost savings, and eliminate one level of pressure reduction controls? Or, is pressure control deemed too critical for this?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[BronYrAur]

It's a hospital that needs 85 going to the sterilizers.. the powers-that-be are nervous enough about the possibility of reducing to 125. No logical reason behind their concerns other than it has worked fine all these years at 140 why Rock the Boat?

It's funny how clients are sometimes Their Own Worst Enemy. They get on a kick to save energy but then they are reluctant to do any of the things that might actually save them.

the purpose of my analysis is to make sure there will be no detrimental effects to this pressure reduction. But it would be nice to offer that against an energy savings because there will be some cost to change out a couple of prvs. So I'm trying to quantify that Energy savings. I know there will be some savings just in a reduced temperature and reduce radiation losses. But I was hoping the majority of the savings would be in reduced natural gas consumption at the burner. I still need to look into that Cleaver Brooks reference mentioned earlier.

 

[georgeverghese]

There may be more fertile ground for energy savings with the low grade heat from the stack exhaust than with steam pressure reduction. Examples would be to generate a chilled water stream at say 10degC (to supplement existing cooling requirements at this hospital or some neighbouring facility) with a lithium bromide based chilling unit or extract shaft power for some mechanical driver using an organic Rankine cycle(ORC). There are many vendors who are in this business. Government subsidies for such projects are usually available, given that these result in CO2 emissions reduction. But beware that at times when the boiler is running partload, boiler exhaust temp would be lower and supplementary gas firing may be required at the stack exhaust to keep up chiller production / maintain adequate shaft power.

 

[georgeverghese]

Given that this boiler operates at 83% eff at design load, suspect you'd be struggling to get a decent power extraction or chilling duty stream from the exhaust.

 

[MortenA]

Energy savings would correspond to energy losses, since these are directly proportional to dT, you can just take the dT at on pressure and divide it with dT at another temperature (to outside, soil or waterever is most correct - and you will get an estimation of the savings thats fairly correct.

Best regards,

 

[BronYrAur]

What DT are you referring to MortonA? I did some radiation calcs based on the different surface temps of 140 psig vs 125 psig. That is of course a T^4 relationship in absolute temps. Not sure what DT you mean.

Whether the surfaces were bare or insulated, the difference in the radiative losses seems to be about 3%. The problem is that I have no way to quantify the amount of radiative loss I have in the first place. I would have to quantify the surface area of everything in the system, which would be a significant undertaking. All I know is that it will be 3% less (assuming I did the calcs correctly).

I know there are other savings, such as flash loss, blowdown, failed trap loss, etc that all will be helped by lowering the pressure. I'm probably safe to say that the savings in natural gas will at least be 1%, but I'm half-ass guessing at a lot of it. I need something more concrete.

 

[MortenA]

well energy losses to surroundings? If its just less energy loss in dP then look that up in a steam table?

So at 140 psig the steam temperature is 361 F
At 125 spig the steam temperature is 353 F

So if the surroundings have a temperature of say 80F then dT(140)=281 and dT(125)= 273, and the expected energy savings should be around 273/281=0.97 or 3%

This is based on common heat transfer equation: Q=h*A*dT - you dont know h*A - but you can assume they remain constant!

Best regards, Morten

PS: It seems like we agree in methodology

 

[MortenA]

Note: You could be off on your ambient temperature - but since the steam is so hot, and not so different for the two pressure it wont matter much

 

[IRstuff]

While T^4 is potentially an issue, it's not as much as one might think, given the steam temperatures. That's because the radiation of interest is from the outermost surface of the insulated pipe to the ambient, and since there's an inherent design requirement to minimize that surface temperature, it'll be pretty close to ambient to begin with, and certainly it wouldn't be higher than about 50ºC, simply for safety.

TTFN (ta ta for now)
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[davefitz]

Theoretically the main energy savings ( in steady state) could occur by eliminating the frictional losses assocated with pressure drop thru the valves, ie, operate with all valves wide open. This mode of operation would require replacing safety valves to lift at the drum pressure that occurs with valves wide open , and require that the boiler + steam distribution system can safely operate with no reserve pressure . As a practical matter none of those options are permitted. To quickly respond to changes in steam demand it is neccesary to have reserve pressure in the steam drum therefore some throttling at the steam outlet valve is needed. The boiler feed pump must be capable of feeding the water when the drum is at the safety valve lifting pressure, thus some throttling at the feedwater valve is required durign normal operations. If the throttling at the main steam outlet throttle valve is excessive, energy gains could be had by adding a steam turbine generator in parallel with the throttling valve.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[gruntguru]

So if the surroundings have a temperature of say 80F then dT(140)=281 and dT(125)= 273, and the expected energy savings should be around 273/281=0.97 or 3%
Don't forget this would be a 3% reduction in HEAT LOSS not a 3% reduction in ENERGY INPUT.

je suis charlie

 

[Rspain]

Hi There,

It is not an easy question, and it involves many potential risks.

Reducing pressure of a boiler, for small differences in pressure, ceteris paribus, leads to little savings.

Reducing boiler pressure will result in lower temperature of the flue exhaust gas (burning fuel), hence there are some savings there (sensible heat lost in flue gas). As a thumb approx rule, you can say 1% overall energy efficiency in the boiler for every 22ºC reduced in the flue gas. Thus expect no more than a 0.2-0.3% boiler efficiency improvement from this effect (183ºC vs 178ºC - difference in steam, expect no big difference in flue gas temp).

Reducing pressure in the boiler will have as a direct consequence a reduction in the pressure of the steam in the pipelines all the way between the boiler itself and the PRVs. This might also mean slightly lower heat losses to the air from the pipes, again small difference (you can check the delta[deltaT]).

You must be very carefull no to reduce too much the pressure of the boiler, since the lower you run a boiler (pressure) the smaller the real capacity of the boiler, since being lower density the steam (higher speed through outlet of boiler); the chances of having extremly undesirable and damaging water carryover increase.

NOTE: from energy point of view, the enthalpy of 140psi steam and 125psi steam is essentially the same (aprox. 0.2% difference) ...

Hope it helps,
BR;
RTO

 

[MortenA]

@guntguru, just out of curiosity: How how would the two differ in this particular case?

 

[Rspain]

Hi MortenA,

In a steam system, heat loss is just a fraction of Energy Input in the boiler. Typically, boiler energy input has as its biggest fraction the actual process energy consumption.

So saving 3% in heat loss does not mean overall energy savings of 3%.

BR,
Rto