Heat Content and Steam PSI

DrPepper

if they have been distributing steam at 50 psi and drop it to 15 this is what happens...
2000 #/hr in a 3" pipe at 50 psi moves at ~4500 ft/min
that same 2000 #/hr in a 3" pipe at 15 psi has to go over ~9000 ft/min.
If the system was originally sized "small" because "industrial doesn't care about noise". Then you may be at 8000 - 10000 ft/min now. Next step is low coil output and erroded pipe.
That 50 psi steam has a specific volume of 1.36 Cu.ft/lb
At 15 psi the same lb. has is 1.83 cu.ft/lb. The coil doesn't care where it gets get's it's steam, it just goes into a vacuum if the piping can't keep up. It's a common mistake and must be looked at.
Go to the Spirax/Sarco web site, they still have literature and charts that would help you.
Good luck.

Ken Sappio

Heat Content vs Steam PSI

I am involved with the installation of a steam boiler for industrial application. The boiler will supply steam to heating coils for heating air. The steam will flow through 5/8" copper tubes and the air will flow across aluminum fins. The boiler was fitted to operate up to 50 PSI but new controls have been bought to make it a low pressure boiler, max 15 psi. Does reducing the pressure decrease the heating capacity of the boiler? If so, by how much? Thanks in advance.

Rich L.

Pressure temp relationship

Reducing the pressure of the boiler will reduce the max temp output of the boiler.

Steam at 15 PSIG = 250 degrees F. Steam at 50 PSIG = 298 degrees F.

See: http://www.broadleyjames.com/PDFs/102.pdf

This will send "cooler" steam to your heating coils and slightly derate them. However the largest output of heat is in the change of state from steam to water. you get 970 BTU's of heat energy (per pound of water) from that change with no drop in temperature. The rest of the heat comes at 1 BTU per degree temp change per pound of water.

Hope this helps.

Brad White_203

Other thoughts

If the coil is to handle outside air, those in my experience have one-inch tubes, often with internal distributor tubes to get steam all the way across the coil for every other tube. This is also a form of freeze protection but is not absolute protection.

Also the fins are, for longevity, often tinned (solder coated).

Rich L is correct on the principles of pressure and temperature. You can expect a perceptible drop-off in coil capacity if the coil was originally sized for medium pressure and now will be on low.

As for the boiler capacity, BTUs are BTUs for all practical purposes. Your useful heat (heat of evaporation) will be more per pound though, at the lower pressure. Less fuel will be spent getting up to pressure and you will not be superheating.

If you have boiler operator laws, your actual pressure may even be less than 15 PSI as a practical matter, often 12 PSI with a 3 PSI margin to relief valve lift pressure.

bob_46

Latent Heat

at 15 PSIG about 946 BTU/LB at 50 PSIG 912 BTU/LB

Charlie from wmass

You nay also find the tappings for the supply off the boiler are far to small to allow the steam to leave the boiler as it wants to. It is not alway a good thing to try and use a high pressure boiler at low pressure. You may find it cheaper to buy the boiler you need and sell this one to someone who needs it than to use this one for low pressure.

Ken Sappio

More Info/questions

The boiler has a max working pressure rating of 150 Lbs. and the boiler company says it can be fitted to operate at any max pressure up to 150. We wanted 15 psi but it was ordered with a 50 psi saftey valve and pressure controls with 0 to 50 psi operating range. We asked that they send a 15 psi safety valve and controls that will allow only max of 15 psi operating pressure. The boiler has both a hourse power rating and Lbs/Hr rating - 30 HP, 1035 Lbs/Hr. I asked the boiler company if going from operating at 50 psi down to 15 psi will derate the boilers output capacity, they said that the hourse power of the boiler is rated at zero (0) psi so going from 50 psi to 15 won't cause me any problems, but I take everything with a grain-of-salt so that is why I am asking. So is a 1035 Lbs/Hr boiler at 50 psi still a 1035 Lbs/Hr boiler at 15 psi. Also, I am wondering if the energy (BTUs ?) that are required to cause water at 15 psi to turn to steam is the same amount of engery to cause water at 50 psi to go to steam? Oh, by the way, the coil builder sent me data sheets indicating that the coils were built using 13 psi as the design steam pressure, so I should be ok there. Thanks again.

grumpyplumber_2

Is the boiler rated for low pressure? mine here (ok the co. bought them but they ARE mine) are designed to run at 30-125 psi- less than 30psi and blow wet steam. (CB 100hp fire tubes). Check your design ratings. Pressure to the coils can be reduced with 1/3-2/3 regulators. And yes, I have saved $$ (20%) by reducing my boiler pressure to 50psi from 80psi

scrook_2

firing rate & steam properties

A 30 HP boiler is a 30*33,475 = 1,004,000 BTU/hr boiler or a 30*34.5 = 1035 Lb/hr or 30*9.8095 = 294.3 kW These assume 14.7 psia or 0 psig pressure and density for Lb/hr.

Its rated firing rate doesn't change w/ steam or water pressure, however the steam conditions do change:

From steam tables:
At 14.7 psia (0 psig) it'll be 212° take 970 btu/lb to boil and yield 26.3 cu ft/lb steam

At 25 psia (10.3 psig) 240° take 28 btu/lb to heat to 240° from 212° then 952 btu/lb more to boil to 16.3 cu ft/lb steam

30 psia (15.3 psig) 250°, 945 btu/lb to boil, 13.8 cu ft/lb

65 psia (50.3 psig) 298°, 912 btu/lb to boil, 6.65 cu ft/lb
etc...

Notes:
1 BTU/lb to raise liquid temperature 1°F, 900+ BTU/lb to boil, as listed above, decreases as pressure rises, but extra heat's added to heat liquid hotter first.

for steam densities (lb/cu ft) invert the steam specific volumes (cu ft/lb) e.g. 13.8 cu ft/lb = 0.0725 lb/cu ft

Atmospheric pressure (0 psi Gauge) = 14.7 psi Absolute

If you have 13 psi design coils you should be good w/ a 15 psi relief and what, about 9-12 psi operation.

More questions?

DrPepper

Relief valve not control...

Boiler working pressure is like flange or fitting "working pressure" it is a rating. Nothing more.

If you run the boiler at 15psi, that is determined by the boiler controls (pressuretrol or elecrtonic). The relief valve is there to protect the devices downstream of the boiler. So, it MUST be set at a higher pressure than the boiler set point but, lower than the point that would cause damage to equipment. For instance a 15psi boiler may use a 30psi relief.
1035 lbs/hr is the same at 50 or 15 psi. But it will act a whole lot different in the piping.

A 1,350,000 btuh boiler is not hooked up with copper, and you really better know what you are doing to get it right.

Not to be a smart guy, but you sound like you need some education on steam systems. Read the directions on this boiler, diagrams on near boiler piping, and if the rep is any good, listen to him. You may even want to call in an advisor to make sure before you lay your money down.

Best of luck to you,

Ken Sappio

Scrook - no more at this time

Thank you. You answered all my questions I have at this time, and throughly too. I appriciate it.