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# over pressure

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Member Posts: 4
can someone tell me how many btu it takes to make six pounds of steam after you reach boiling point??

i know it takes 970 to make steam

• Member Posts: 23,438
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Not sure exactly what you are asking...

It takes about 970 BTU to evaporate a pound of water to steam at atmospheric pressure.  So six pounds would about 5800.  Now if you continue to heat the steam produced, so as to raise the temperature, that takes more BTUs -- and if you don't change the volume, you'll increase the pressure at the same time.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 4
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over pressure

jamie  i think i know what you are saying but here is the reason i am asking.

at my place of work we run six pounds of steam and from my past experience i would like to lower it to 1 to 1.5 pounds to save energy and my boss says no it has run that way for years there must be a reason. we are in another year of budget cuts which will ultamately mean a reduction in my hours.  so i would like to show him how much we are waistingon a simple spreadsheet.  so once again i ask for clarification, to make six pounds of steam pressure will 5800 btu/pound of water do it??
• Member Posts: 16,894
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Is this system

used just for heating the building, or for manufacturing processes too?
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting
• Member Posts: 2,398
edited April 2011
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Pressure vs. Production

OK, I see it now. What Jamie said is correct regarding "heat per pound" of steam.

The difference in fuel used can be demonstrated by clocking the burner and meter (an abbreviated simple form).

Basically, time how long it takes to make 1.5 psig from "lift-off" (the steaming point), then how long it takes to make 6 psig from that point. There is your essential difference.

The tough part is how the steam is used, what causes the steam pressure to degrade and how fast. This goes to the "process" question that Steamhead raised.  If strictly heating, your "cruise time" to the higher pressure will degrade at a slower rate than if used for steam kettles as an example.

I do not consider 6 psig to be a "process pressure" though. Process pressures tend to be at least 35 psig up to whatever is needed and for various reasons.

I suspect the application, being only 6 psig, may be "Heating and Ventilating", meaning there may be air heating coils in air handling units which require 3 to 5 psig in the coil, after a control valve, to allow residual pressure for the steam trap.

When set up for this, the pressure is maintained nearly constantly regardless of need, just to be "ready". Thus there are fewer "down cycles" compared to more passive radiation. (Ventilation loads being more constant, you see.)

If the system is just radiators, with or without traps, I would think 1.5 psig would be plenty and it would probably work on less. Any higher pressure is wasted fuel and can affect system operation and noise.

As for a quantified fuel savings in heating only, that is hard to say. Higher pressure, if it retards the return of condensate due to a malfunctioning trap, would hold off temperature satisfaction and potentially over-heat other spaces. See how hard to quantify that would be.

But from a time standpoint, if it takes 2 minutes to make 1.5 psig from lift-off, it may well take another 2-3 minutes to make 6 psig. Once the burner shuts off, if it takes two minutes to degrade back to cut-in (where the burner starts again), the losses are minimal, you are firing to maintain 6 psig from a higher baseline. Maybe 10%? A guess.
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

• Member Posts: 4
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over pressure

ok guys sorry my lack of information is making it to tough.

the system is for heating only it is supplying steam for fin tube radiation and tube bundle heat exchangers to convert to water heat through air handlers with coils.

the system also has vacume return for the condensate.

originaly the building was total steam heat through univents and radiation fin tube.

after a rebuild for air quality the univents were removed and replaced with water heat rooftop units and vav boxes with reheats.

i have rebuilt the steam traps and the system is working well other than gliches in the energy management program.

i see no reason to run six pounds of steam to heat with radiation or water coils and am looking to save some energy by turning the pressure back to 1 to 1.5 pounds at most. still looking for the answer -does the required 970 btu raise as the pressure raises-- or does it stay the same clear through six or more psig.

if it stays constant then i am looking at 5820 btu / pound of water to create 6 pounds of steam   right???
• Posts: 2,067
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Steam Tables

Hi - The attached is a handy reference that might be of help to you

- Rod
• Member Posts: 2,398
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The heat exchanger

with its presumed control valve, has the same effect as an air handling unit coil control valve does on the steam system. You need a certain higher pressure to impose a certain pressure drop across that control valve to maintain what is known as "valve authority".  This is why I suspect that 6 psig is necessary for that device alone.

There is probably not a lot you can do about that.
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"