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how much does pressure cost?

adayton_2 Member Posts: 130
Some of it in fact does just vanish in the air (basement) where the BTUs are usually not required/needed. also it COSTS a much larger load of BTUs to jack up steam to 2 or 4 POUNDS Per Square than it does to reach steam at 5 oz. and once the radiation is already flaming steam HOT and on their way to satisfying the thermostat, why should the boiler be STILL chugging maddly away stuffing tons more BTUs into steam JUST in order to maintain 2 POUNDS of "pressure"?(when the radiation is ALREADY HOT). Excess heat rolls back with condensate into the boiler pot where the boiler happily EXUDES heat into the basement from it's jacket. Now also add to all this the fact that laws of physics state it is HARDER (read more expensive) to heat water that is already HOT (215* PLUS @ 2PSI) to get it to keep making steam. Others brighter minds here can certainly confirm these concepts in more certain scientific terms. Bottom line its easier(thus cheaper) to get heat into cooler substances than it is to get heat into HOT substances.....



  • joe_66
    joe_66 Member Posts: 30
    What does pressure cost?

    One pipe steam that works perfect at 10o.z..Vaporstats get thrown out and pressurtrols get installed and set at 2psi.What does this pressure cost?
  • Brad White_35
    Brad White_35 Member Posts: 11
    The cost of

    additional firing time to maintain more pressure than is necessary. An obvious answer but one that probably bears some monitoring with data loggers to take note of burner run times in comparable systems. But figure if a boiler has to fire for 12 minutes to make 2 psi and only 3 minutes to make 12 ounces, I would say 9 minutes of combustion time has been wasted at least for that cycle.

    Your actual time may vary. Void where prohibited :)
  • Plumbob
    Plumbob Member Posts: 183

    You mean the extra heat just vanishes into thin air?
  • Brad White_35
    Brad White_35 Member Posts: 11
    Exactly, Alfred.

    Well said.

    The first and second laws of thermodynamics rule.


  • ttekushan_2
    ttekushan_2 Member Posts: 57
    Please explain...

    I'm missing something here. If this is the case, are all those exhaust heat economizers out there that are used to preheat the boiler feed water a fraud?

    My understanding is that boiler horsepower ratings are only valid without feedwater, i.e. that cool return water lowers the rated output of the boiler. This is confirmed in my experience in two pipe systems with somewhat undersized boilers. Is this wrong also?

    Additionally I have found that condensate return temps are not necessarily proportional to pressure, but saturation of the radiation.

    For example, a project a few weeks ago involved a boiler running on a vaporstat and ounces of pressure. Consensate would return warm (traps all new, btw) after extensive run after deep setback. This week a higher (but still low) pressure commercial system (9000 edr) running 5-7 lbs due to presence of unit heaters in addition to radiators. Condensate still returning to boiler room tepid after 1 hour of operation. One hour was sufficient to bring up temp from 45 degrees to 60 degrees (building's being rehab'ed so its unoccupied). Of course, on shut down the boiler continues to produce steam until the pressure reaches atmospheric.

    Under these conditions, which is therefore more efficient?
  • Brad White_36
    Brad White_36 Member Posts: 30

    No, exhaust heat economizers are not a fraud, boiler horesepower is a steady-state output rating with feed water flow rate equal to evaporation rate and injected at 210F (I believe that is the temperature). Condensate temperature is a function of first giving up its latent heat (condensing) then whatever losses are given up on the sensible side to the point of measurement. The two different systems you describe have differerent pressure needs and different piping configurations I am sure.

    None of the points raised has anything to do with the question that I can see. The answer still remains that the most efficient system is the one that heats the building using the least fuel and at the lowest pressure to do the job. There is no point in superheating steam or running a higher pressure than you need to. In fact higher pressure steam delivers less heat before condensing. Low is the way to go.
  • Tom R.
    Tom R. Member Posts: 139
    Boiler HP

    Actually, the wording goes "from and at 212 deg. F. " which doesn't mention feedwater being added or not. The reclaiming of heat from the exhaust is a legitimate way to save fuel, but for longer unnecessary run times the loss is in the fuel spent heating the combustion air which does nothing to heat the space but is expelled up the chimney. The higher the steam pressure, the higher the exhaust gas temperature and the more heat gets wasted. The only reason for needing more pressure would be to move the steam longer distances.
  • adayton_2
    adayton_2 Member Posts: 130

    perform two "economies". 1) reclaim heat from exhaust stack, 2) utilize that heat to "PREheat" makeup water that would otherwise be injected into the boiler as a COLD Shock which is not to healthy for your boiler. OR to PREheat COLD condensate that is returning from a COLD system start in the morning after deep set-back at night. This also so the boiler does not get COLD Shocked..

  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Yes Tom

    you are right. I should have remembered "from and at 212F". And thanks for reinforcing my points.

    While feedwater is not specifically mentioned, you have to admit, it will be :)

    My steam mentor, the late Ray Stevens, always stressed heating the feed water to near the boiling point (sparger tube deaerator) so as to maintain status quo in the boiler. As a practical matter, 210 was as high as you could practically go even with high pressure steam and not cavitate the boiler feed pumps. And that is what stuck.

    Thanks Ray, wherever you are.
  • Christian Egli_2
    Christian Egli_2 Member Posts: 812
    Just to add my two cents

    I love steam.

    I value economisers. Like the scrupulous accountant they ferret out all the loose pennies and recapture them into a usable form. Sometimes, some pennies are not worth running after, it depends on the cost of the economiser and the scale of your boiler room.

    I admire heated feed water tanks. Like the front line soldiers they take all the beating that comes with fresh water and feed water assaults. A feed tank heated to 210F or so, sees most of the heat shock and most of the sludge fall out the boiler would otherwise see. Instead of replacing a fouled up boiler you replace a sacrificial feed tank. Depending on the scale of your boiler room, if the heated feed tank is just as precious as the boiler, then there is no cash benefit.

    Here is a link to a thread that dealt with higher pressure cost. Oooh, it's precious

  • Tom R.
    Tom R. Member Posts: 139
    Especially a deaerating feed water tank

    which drives out the oxygen from make-up water.
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