## Calculating the Energy savings of reducing steam boiler pressure

## Calculating the Energy savings of reducing steam boiler pressure

(OP)

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?

How can I quantify the savings?

## RE: Calculating the Energy savings of reducing steam boiler pressure

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)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

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## RE: Calculating the Energy savings of reducing steam boiler pressure

Calculate the piping and boiler shell losses at both temperatures.

## RE: Calculating the Energy savings of reducing steam boiler pressure

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..

## RE: Calculating the Energy savings of reducing steam boiler pressure

## RE: Calculating the Energy savings of reducing steam boiler pressure

## RE: Calculating the Energy savings of reducing steam boiler pressure

Good luck,

Latexman

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

## RE: Calculating the Energy savings of reducing steam boiler pressure

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.

## RE: Calculating the Energy savings of reducing steam boiler pressure

## RE: Calculating the Energy savings of reducing steam boiler pressure

## RE: Calculating the Energy savings of reducing steam boiler pressure

Best regards,

## RE: Calculating the Energy savings of reducing steam boiler pressure

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.

## RE: Calculating the Energy savings of reducing steam boiler pressure

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

## RE: Calculating the Energy savings of reducing steam boiler pressure

## RE: Calculating the Energy savings of reducing steam boiler pressure

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

## RE: Calculating the Energy savings of reducing steam boiler pressure

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

## RE: Calculating the Energy savings of reducing steam boiler pressure

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

## RE: Calculating the Energy savings of reducing steam boiler pressure

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

## RE: Calculating the Energy savings of reducing steam boiler pressure

## RE: Calculating the Energy savings of reducing steam boiler pressure

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

## RE: Calculating the Energy savings of reducing steam boiler pressure