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Steam, pressure, water, and power consumption of an idle machine

Hi all,

This discussion started as a sidebar to another: "Leaving your machine on overnight?" I'm splitting it out, since it is about to wander fairly far from the original topic. Please forgive the somewhat kludgy attempt at preserving the original diversions...

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Scott said:

Theoretically water consumes the most energy at physical change state, i.e. when it's turned into steam.  Though not a feature I am aware of on any current machine, it would be ideal to turn the steam boiler temp down to just below boiling overnight.  No need for long heat soak in the morning (like when starting a cold machine) and probably far less cycling through the night.  You could also insulate the boilers, like old Lineas were, and it would improve shot-to-shot consistency.  Even more so if you have a copper boiler, they loose the most heat.  

I wouldn't recommend shutting a machine off for the same reasons mentioned above.  Though I would add if your dissolved hardness is that high, you really need to do something about it regardless.  Aside from the wear on the machine, hard water takes more energy to heat. 

 

Jeremiah said:

Technically, if the steam is not released then the heat of transition is not lost. Because of the pressure, the boiling point is raised and the water boils only when pressure is released. The reduction of energy used if turned down would be related to a lower heat gradient only. Good theory though.

I'm going to disagree with you there, I think.

In pressurestat-controlled systems, the heater will cycle on and off as needed. It does this even at idle to maintain system pressure. Since the steam boiler's heating element is submerged in water, it heats the water directly and the steam only indirectly. Despite this, you'll notice that the system pressure responds rapidly to the cycling of the heater - beginning to increase within seconds of cycling on and stopping its increase shortly after the power is interrupted. This suggests to me that the  pressure increase is being driven by an increase in the amount of steam in the system - aka phase change as opposed to a steam temperature driven pressure increase.

So the water in the tank is boiling, even when the system remains closed.

Right?

(Still wrapping my head around where the heater's energy goes and how the heat loss affects the mixture. Those brain cells are a bit dusty and bored these days, and appreciate the opportunity to come out and play, so thanks!)

Jeremiah said:

Technically, if the steam is not released then the heat of transition is not lost. Because of the pressure, the boiling point is raised and the water boils only when pressure is released. The reduction of energy used if turned down would be related to a lower heat gradient only. Good theory though.

Hey Brady, the pressure in the boiler (as read on a steam pressure gauge) is only absolute above the water line.  There is a pressure differential in the boiler between the steam and the water.  Steam, like air, is compressible and as its compressed it draws energy.  

Jeremiah - The pressure in the boiler is maintained not by the seals, valves, o-rings, etc; but by the heat transferred from elements to the water.  No heat = no pressure.  The boiling point of water is relative to it's pressure this is true; however the fact that there is steam in the boiler, water would have to pass the change state and therefore it has been boiled.  No boiling = no steam.  

I'm tired to, so this is only so so coherent.  Fun stuff.  I've done boiler inspections for large building heating systems; radiant and steam.  And I usually get to talk to the city boiler inspector about this stuff.  I think I'm one of the few people he meets that geek out on stuff like this.  And he keeps asking me to come down and get my licence.  

Oh I tried to edit my poorly worded and less than thought out answer.  So if you find error, I don't doubt it. I'm tired.

It's basic physics, the increased pressure and temperature due to application of heat in a sealed container is what allows for flashing and rapid steam production when the valve is opened. Until this happens, the water is under pressure and heat increases proportionally to the energy input, with the phase transition heat (the extra heat required for water to turn to steam - which was what started the discussion) not being a factor until a valve is opened despite a bit of steam trapped in the headspace (at equilibrium).

Therefor, a reduced setting on the boiler would result in a savings related to the amount of heat required to maintain the relative temperatures only, and the boiling point of water at atmospheric pressure would be just an arbitrary point along the spectrum of potential heat input to energy usage or savings, with the heat of transition coming into play when pressure is released allowing for flashing to steam and necessitating additional heat input to maintain boiler temperature and pressure.

It's just a small technicality, not a big deal. Very on point of you to be considering phase transition and stuff.

Let's think this through.

When a machine is fired up for the first time, the boiler contains about 50% volume of air, 50% volume of water. As the element heats the water to the boiling point, both the water (to a relatively small degree) and the air in the boiler expand. As long as the expansion happens relatively slowly, the anti-vacuum valve will allow this air to escape - this is the hissing you hear when you fire the machine up from cold. Its only after the water begins to boil that the flow of gas out the AV valve reaches a rate that will seal it. At this point, the gas in the boiler is a mixture of the original air and newly-made steam.

As long as the machine is not turned off, there is no way for more air to enter the system, yet we're taking gas out of the boiler all day via the steamwand. That means that at some point, the gas in the tank will be entirely steam (with perhaps some portion of gas that came in dissolved in the water), right? That means that phase change is relevant, even at idle.

Scott, could you elaborate on this pressure differential? It was my understanding that the pressure was equal throughout the boiler. Just because water is treated as incompressible doesn't mean that is can't be pressurized, right?

Good thread.

For the record, Jeremiah, I agree with your eventual conclusion: the phase-change energy that went in to the steam is not lost unless the steam leaves the system. The only route for energy out of this sealed system is heat loss (conductive, convective, radiant) from the boiler. I just disagree about how energy goes back in to the system and what it does once it is there.

You're really overthinking this thing, dude. The air is expressed as a matter of course. We're talking about a working shop environment I assume, and you really should get the air out before beginning. The eventual conclusion was the whole point, that the need to make up for phase change heat comes only with the release of pressure. 212 degrees is only the boiling point at atmospheric pressure and not relevant.

I don't know how you disagree about energy going "back" into the system. We input energy from the heating elements, heat is lost as you stated, the extra amount of heat required to change liquid to gas comes into play when pressure is released. The steam in the head space is of minimal concern to this discussion.

I'm not trying to debate, just pointed out a slight inaccuracy. Any debate should cite the gas laws and energy of phase transition. Sorry to question anyone's authori-tie. (Cartman accent)

 

Jeremiah, does this sound like what you are getting at - 

So, assuming that once a steam boiler has pressurized, the entire machine is heat soaked and no steam is release - the affect of change state of water to gas - as in my previous argument as it relates to energy consumption - is not relevant as the boiler is pressurized to 1.5 bar (or a manifold absolute pressure of 2.5 ATM) and a temperature of around 245 degrees - well below the boiling point of water (262 degrees @ 2.5 ATM).  

 

This assumes one thing of course.  That the heating elements, when fired, are only heating the surrounding water to a temperature below 262 degrees it maintain said pressure.  I wonder if that is the case. 

 

What we need to do is meter the power consumption of a machine at various states. 

 

And dude, you should be debating.  You're good at it.  And it's the internet.

Correct, and any energy saved would be related to the lesser heat gradient. Anybody insulate their boilers? Seems more likely route for better efficiency.

I have seen old Lineas that were insulated.  

 

The T-Plan machines at big green have insulation, but it's pretty minimal.  

 

I can't think of one machine were the boiler is physically isolated from the body/frame.  It's always a metal to metal connection.  Hence the whole machine, with all that metal surface area, acts like a heat sink - transferring energy away from the boiler.   It's why machines placed next to the window at drive-thru bars have a harder time with shot consistency.

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