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what pressure level should be used for a 15 section cast iron steam boiler?

elfie
elfie Member Posts: 264
have a steam boiler with substantial excess capacity



it is a 3 million btuh boiler that is connected to about 600 btuh of radiation capacity



it is a 2 story facility (about 12 radiators and 2 hot air blowers with steam coils)



what ever PSI level we use there is short cycling



seems the lower pressure the better and may be more efficient.  currently using 7.0 cutoff and 5.0 cutin.  also used 3.0 and 1.5 cutin and that seemed to work



seems a large system with such a low level of demand could run on a psi level of 2-3 psi



thanks for feedback

Comments

  • talk about over-sizing!!!

    i am sure that such a large boiler will have a 2-stage gas valve, wich would be either flowing at 100%, or 75%. if this valve was controlled with a vaporstat, then the burner would come on high fire, and then as the pressure rose to a few ounces, the valve would switch to lower flow.--nbc
  • elfie
    elfie Member Posts: 264
    no vaporstat

    no vaporstat  - use pressure trol



    not sure that i could get this beast of a boiler to maintain pressure below 1 oz  so vaporstat may not make sense



    burner has modulating capability (ie. burner rate reduces when it reaches a certain PSI level), but because of oversizing, the modulating capability is useless; the manufacturer advises to not use a burner rate below 2.4m btus for this size boiler (which is about 50% of the total burner capability - a burner rate below this level may create condensation due to uneven heating and thats not good.



    i am just trying to get a sense as to what the pressure level should be set at..
  • Dave in QCA
    Dave in QCA Member Posts: 1,777
    Need more information

    *  A complete and accurate survey of the radiation organized by  by type.

    ie, cast iron radiators

    convectors

    fan coils

    ducted air handlers with steam coils

    blast heaters

    What other unusual conditions exist, i.e., are there places where condensate is lifted to return lines by steam pressure?  What controls, separate from the boiler being on or off control the temperature and the operation of any devices connected to the boiler.  For examplte is there a pneumatic control system, selfcontained thermostatic radiator valves, etc.

    Large steam systems can be very complicated and its impossible to give an accurate answer with only partial information.  There has to be more than 600 BTU of radiation connected.  That would be one very small radiator.
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
    http://grandviewdavenport.com
  • elfie
    elfie Member Posts: 264
    edited November 2012
    more infor

    sorry its 600 thousand btus of radiation capacity (not 600)



    there are 12 cast iron rads and two 200 thousand btu blowers.(each blower coil has a 2 inch connecting steam pipe).; each rad has a thermostatic trap (all recently replaced as well as well as other traps in system so this reduces steam pressure loss to condensate return pipes.



    thats all the radiation being supported by the steam boiler (and most of the time the rads are not needed so that reduces radiation capacity)



    there is one location that requires a condensation lift to reach condensate return pipe but its only about 10 inches (ie. an end of main pipe is about 10 inches below the condensate return line) and I believe 2 psi will lift condensate about 4 ft so thats not an issue.  there is a condensate return tank/pump to get condensate back to the feedwater tank
  • Dave in QCA
    Dave in QCA Member Posts: 1,777
    edited November 2012
    conflicting issues

    Wow, So you actually have about a million BTU of load connected.  If you factor for piping losses and pickup, and the efficiency of the boiler (80%), you would need to fire at 1,675,000 BTU to support everything running at once.  Still, you have a LOT of excess capacity.



    Common advice for firing boilers is not to fire the boiler at a rate less than 50% of its rated capacity, not the capacity of the burner.  But, I have not been involved in the conversations with your manufacturer and would have to defer to what they said, and/or what the meant.



    Given the fact that this is a maintained steam system, if I were setting this boiler up, I would set the operating cutout for 5 psi and cutin at 2.5 - 3 psi.  I would set the modutrol to be in high fire at 2.5or3 psi and low fire at 4 psi.  In actual operation in moderate weather, I would probably leave the Hi/Lo/Auto switch set in the Lo position.



    While on one hand, cycling causes ineffiencies, on the other hand running an oversized boiler when it is not needed causes even more ineffiencies.  Remember too, when your boiler shuts off at 5 psi, the water temp in the boiler is 228 F.  When the burner goes off, the boiler water will continue to boil as steam is consumed by the system.  As this occurs, the temperature of the boiler water will decrease in conjuction with the falling pressure, all the while the boiler is still boiling and producing steam.  The heat content of the boiler is being transferred to the coil or radiator that is consuming the steam, even when the burner is off.  When the boiler has cooled to 219 F, the pressure will be 2 psi.  The 10 degree drop in water temperature generated steam of equal heat content to what was lost from the boiler as its temperature declined.



    Also, I think if this building were mine, I would start looking for efficiencies that could be gained by improving insulation and tightening the building envelope.
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
    http://grandviewdavenport.com
  • elfie
    elfie Member Posts: 264
    thanks

    thanks for a great reply



    does increasing the pressure improve heat up speed?



    i have heard that lower pressures can be of more benefit to heat up speed.



    thanks
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