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Thread: Boiler pressure as water temperature

  1. #1
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    Boiler pressure as water temperature

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    It occurs to me that most coffee people dont have a heap of thermodynamics training, what a surprise that is.
    So some of the properties of water and steam are a black art to most people, so I thought I would try and shine some light on the subject.

    Saturated steam pressure is the pressure of the steam and water in a simple boiler, this pressure determins the temperature of the water and steam, and it cant rise above this temperature (become superheated) until all the water has changed to steam, or the steam has been removed from close contact with the water that is boiling.
    This pressure can be directly related to its temperature, and we have steam tables that give the information, as follows.


    Now how to read this information.
    The pressure, in Bar, is absolute, that is, it is measured from zero pressure, so the one bar line is just below one atmosphere in pressure, one atmosphere (normal pressure) is 1.01325 bar (14.7 PSI), which is added to the gauge pressure to get absolute pressure, clear as mud?

    One bar is 100 KPa, or 14.5 PSI, if you look at the one bar line, the temperature of the water will be just under what is considered boiling point at one standard atmosphere, 99.6 degrees C.
    So another way of looking at this it to see it as the temperature it will boil at for the pressure, hence the "Denver affect" of not getting a good coffee in Denver as it is too high, and the pressure is .835 bar or 12.1 PSI, and the boiling point of water at this pressure is roughly 94 degrees C.

    For most Heat exchanger machines, like my Giotto, the boiler pressure is displayed.
    This has a relationship to the temperature of the water in the thermo siphon itself.
    In the case of my Giotto, when new, was a displayed pressure of 1.1 bar, which is a temperature of 122 degrees C, roughly (how much do you trust the gauge?).
    Over time (years) this crept up to 1.3 bar (125 degrees C), until I extracted my digit and adjusted the pressure stat that turns the element on and off, down to 1.1 bar again.



    This process can be reversed, and if you read the temperature accurately, the pressure can be inferred from this.



    Note the temperature at 1.01325 Bar

    BTW, a pascal is a very small pressure, one kilogram weight on a piston one square meter in area produces a pressure of one pascal.
    A Bar is 100 KPa, or 100.000 Pa.
    So to reverse this, one Bar pressure (Gauge), or two bar pressure (absolute) will lift 100 tonne on a one meter square piston.

    All the other stuff to the right of the pressure and temperature is thermodynamic stuff to calculate how much energy is required to boil the water and other stuff that you dont need to know about.
    But as I am a marine engineer who works on a steam turbine ship, and who has just finished a year back at college, needs to know.

    I hope this helps, and doesnt confuse too much, but knowledge is power, and no knowledge is worse, IMHO.

  2. #2
    A_M
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    Re: Boiler pressure as water temperature

    Quote Originally Posted by 7758524F1B715752515342360 link=1261297726/0#0 date=1261297726
    It occurs to me that most coffee people dont have a heap of thermodynamics training, what a surprise that is.
    So some of the properties of water and steam are a black art to most people, so I thought I would try and shine some light on the subject.

    Saturated steam pressure is the pressure of the steam and water in a simple boiler, and cant rise above this (become superheated) until all the water has changed to steam, or the steam has been removed from close contact with the water that is boiling.
    This pressure can be directly related to its temperature, and we have steam tables that give the information, as follows.


    Now how to read this information.
    The pressure, in Bar, is absolute, that is, it is measured from zero pressure, so the one bar line is just below one atmosphere in pressure, one atmosphere (normal pressure) is 1.01325 bar (14.7 PSI), which is added to the gauge pressure to get absolute pressure, clear as mud?

    One bar is 100 KPa, or 14.5 PSI, if you look at the one bar line, the temperature of the water will be just under what is considered boiling point at one standard atmosphere, 99.6 degrees C.
    So another way of looking at this it to see it as the temperature it will boil at for the pressure, hence the "Denver affect" of not getting a good coffee in Denver as it is too high, and the pressure is .835 bar or 12.1 PSI, and the boiling point of water at this pressure is roughly 94 degrees C.

    For most Heat exchanger machines, like my Giotto, the boiler pressure is displayed.
    This has a relationship to the temperature of the water in the thermo siphon itself.
    In the case of my Giotto, when new, was a displayed pressure of 1.1 bar, which is a temperature of 122 degrees C, roughly (how much do you trust the gauge?).
    Over time (years) this crept up to 1.3 bar (125 degrees C), until I extracted my digit and adjusted the pressure stat that turns the element on and off, down to 1.1 bar again.



    This process can be reversed, and if you read the temperature accurately, the pressure can be inferred from this.



    Note the temperature at 1.01325 Bar

    BTW, a pascal is a very small pressure, one kilogram weight on a piston one square meter in area produces a pressure of one pascal.
    A Bar is 100 KPa, or 100.000 Pa.
    So to reverse this, one Bar pressure (Gauge), or two bar pressure (absolute) will lift 100 tonne on a one meter square piston.

    All the other stuff to the right of the pressure and temperature is thermodynamic stuff to calculate how much energy is required to boil the water and other stuff that you dont need to know about.
    But as I am a marine engineer who works on a steam turbine ship, and who has just finished a year back at college, needs to know.

    I hope this helps, and doesnt confuse too much, but knowledge is power, and no knowledge is worse, IMHO.
    Well presented and makes perfect sense to me..... Actualy it takes me back to my collage days and steam systems.... But then, it was all non metric and what a mix of values.... The maths would always tip me over the edge.......

    This explination and graphs will assist me in getting right, when next I am at base camp... ;D :D ;)

    Actually... Many often forget the importance of altitude... However it only comes into play at the extreams or if you have very tight valuse and or accuracy... Thus for coffee, most of us should be fine...

    Look forward to more reminders / technical information as it comes to hand...

    Knowladge is power, but a little and or if out of context is outright dangerious ;D :D

    I-brick on the river Styx


  3. #3
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    Re: Boiler pressure as water temperature

    oh dear thermodynamic steam tables. A complicated subject for those who have never studied thermodynamics.

    Probably one of the more complicated mechanical engineering subjects in 2nd year with a fairly high fail rate.

    These steam tables are pretty much useless imho. We all know that temperatures in the boiler is not the same as the water that comes out for brewing as most machines use heat exchangers. This is where Heat and Mass transfer comes in handy. And trying to calculate the final temperature of the brew water coming out of a group head would be extremely tough work.

    I wouldnt trust the temperature differential between the boiler and what comes out of the group head would be always the same given a range of boiler pressures; i.e. it may not be a linear relationship.

  4. #4
    Sleep is overrated Thundergod's Avatar
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    Re: Boiler pressure as water temperature

    Quote Originally Posted by 08272D30640E282D2E2C3D490 link=1261297726/0#0 date=1261297726
    knowledge is power, and no knowledge is worse
    Quote Originally Posted by 08272E2C3B042827282E2C242C273D490 link=1261297726/1#1 date=1261299393
    for coffee, most of us should be fine
    I pull the lever and coffee comes out.
    What more do I need to know?
    If I pull the lever and coffee doesnt come out I take it to Renzo at Di Bartoli.

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    Re: Boiler pressure as water temperature

    Quote Originally Posted by 7B475A414B4A5D48404B2F0 link=1261297726/3#3 date=1262957731
    If I pull the lever and coffee doesnt come out I take it to Renzo at Di Bartoli.
    And he tells you to remember to dose the basket!

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    Re: Boiler pressure as water temperature

    I really like this thread.

    It is commonly discussed on here that boiler pressure affects temperature but to be able to explain the why and how much to people succinctly is great.

    As TG said, for most people it probably wont affect how they pull the lever on their machines, but for background theory, its awesome. *

    Good stuff.

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    Re: Boiler pressure as water temperature

    One thing that wasnt mentioned (or I couldnt see it) is that your pressure gauge measures "gauge pressure" - that is, the difference between the actual pressure in the boiler and the atmospheric pressure around the boiler.

    So a typical gauge reading of 1.2 bar is actually an absolute pressure reading of 2.2 bar (1.2 + 1.0 bar atmosphere)

    Hence the saturated water in your boiler is typically at 123C

    (Side note about that Denver thing: If their local air pressure is 0.9 bar, they should tweak the gauge pressure 0.1 bar higher to get the same boiler temp.)

    You can also see that adjustments of 0.1 bar change the saturated steam temp by ~1.4C (approximately linear across a small range).

    Changing the boiler temp will have a similar effect on the brew temperature (not 1:1 but probably close).

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    Re: Boiler pressure as water temperature

    Quote Originally Posted by 7758524F1B715752515342360 link=1261297726/0#0 date=1261297726
    BTW, a pascal is a very small pressure, one kilogram weight on a piston one square meter in area produces a pressure of one pascal.
    Uh, no. its about 10 times smaller than that.
    /nerd

  9. #9
    Mal Dimal's Avatar
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    Re: Boiler pressure as water temperature

    Thinking of a HectoPascal I think Andy.... ;)

    Mal.

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    Re: Boiler pressure as water temperature

    Quote Originally Posted by 6748425F0B614742414352260 link=1261297726/0#0 date=1261297726
    BTW, a pascal is a very small pressure, one kilogram weight on a piston one square meter in area produces a pressure of one pascal.
    1 Pa is indeed a very small pressure, but lets be nit picky. Assume g= 9.81 m/s/s.

    1 kg mass on a 1 square metre area is actually 9.81 Pa. You would need a mass of 0.102 kg acting on 1 square metre to produce 1 Pa.

    I assume you meant to say 1 N acting on a 1 square metre area would produce 1 Pa of pressure. *;)

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    Re: Boiler pressure as water temperature

    Although I am a mathematics guru of sorts. All that was double dutch to me.

    All I wanted to know and you may be able to help is:

    Owning only an EM430 (cant afford 480 yet). I have an EM6910 which unlike the grinder I believe to be good coffee machine.

    Can I slow the pour down with my not fine enough grind using the thermoblock temperature? The only adjustment I have is +/- 2 or 4 degrees. Current factory setting 92.

    I think I still have to convince hubby on the 480.

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    Re: Boiler pressure as water temperature

    Quote Originally Posted by 72574158360 link=1261297726/10#10 date=1268703240
    Although I am a mathematics guru of sorts. All that was double dutch to me.

    All I wanted to know and you may be able to help is:

    Owning only an EM430 (cant afford 480 yet). I have an EM6910 which unlike the grinder I believe to be good coffee machine.

    Can I slow the pour down with my not fine enough grind using the thermoblock temperature? The only adjustment I have is +/- 2 or 4 degrees. Current factory setting 92.

    I think I still have to convince hubby on the 480.
    i am going to say NO.. you need to grind finer or tamp harder or dose more (or a combination of all three) depending on what your doing now

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    Re: Boiler pressure as water temperature

    Quote Originally Posted by 7C594F56380 link=1261297726/10#10 date=1268703240
    Can I slow the pour down with my not fine enough grind using the thermoblock temperature? The only adjustment I have is +/- 2 or 4 degrees. Current factory setting 92.
    I dont think so and if it does at all it is hardly noticeable.

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    Re: Boiler pressure as water temperature

    Thanks I cant grind finer but I was going to next fiddle with quantity and tamping. But I was going to try both ways less and more so thank you for steering me in the right direction first. I just have to wait for my birthday for the 480. I have talked about nothing else since joining coffeesnobs.

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    Re: Boiler pressure as water temperature

    Quote Originally Posted by 7451475E300 link=1261297726/10#10 date=1268703240
    Although I am a mathematics guru of sorts. All that was double dutch to me.
    Im afraid that there are a few engineering types hanging out here in the forum MineAllMine and sometimes the urge to splurge engineering technobabble can be overpowering ::) ;). Much easier just to refer to the Tables if this sort of info is rarely sought and ever needed..... :)

    Mal.

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    Re: Boiler pressure as water temperature

    Quote Originally Posted by 0C293F26480 link=1261297726/13#13 date=1268704724
    Thanks I cant grind finer but I was going to next fiddle with quantity and tamping. But I was going to try both ways less and more so thank you for steering me in the right direction first. I just have to wait for my birthday for the 480. I have talked about nothing else since joining coffeesnobs.
    I know its simple but try it until the new grinder comes home
    Use a mortar and pestle after grinding in the EM430 to make it finer

    Its a thousands of years old method that still works ;)

    KK

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    Re: Boiler pressure as water temperature

    THANK YOU SOOOOOOOOOO MUCH. Am I dumb for a supposedly smart person (in MENSA).

    I have a mortar and pestle. Just never thought of it. Logging off right now to do just that. :D ;D :D ;D

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    Re: Boiler pressure as water temperature

    Andy you are a genius. The pour pressure was perfect according to the gauge on the machine but my taste buds say it was better than perfect. My first non bitter coffee in a very long time. Thank you again.

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    Re: Boiler pressure as water temperature

    Quote Originally Posted by 202C252828214D0 link=1261297726/11#11 date=1268704067
    i am going to say NO.. you need to grind finer or tamp harder or dose more (or a combination of all three) depending on what your doing now
    Tamping harder will not slow down the average extraction rate. All tamping harder serves to do is to create a greater head space between the bottom of the shower screen and the top of the coffee bed, thus increasing the initial wetting period. What this means is that instead of seeing the first drops of espresso at 4 seconds, you may see it at 5 or 6 seconds instead. Once extraction is in full swing, the tamp pressure is fully alleviated against 9 bars of extraction pressure.

    Ill say it once and Ill say it again, these thermodynamic steam tables are useless for espresso machinery employing HX technology unless youre also well versed in heat and mass transfer. What they are useful for are for single boiler dual purpose machines such as the Silvia if it had a boiler manometer on it.

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    A_M
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    Re: Boiler pressure as water temperature

    Quote Originally Posted by 4F797A786F7979655969626565660A0 link=1261297726/18#18 date=1268752965
    Tamping harder will not slow down the average extraction rate. All tamping harder serves to do is to create a greater head space between the bottom of the shower screen and the top of the coffee bed, thus increasing the initial wetting period.
    Err I would suggest there are lots of assumptions here with teh first being that some one is dosing by weight alone....

    If I tamp harder or softer; it is always one of also maintaining the same head space.

    Further more, I can change my tamp and go from a fast pour to a great extraction to a choke using the same grind and the same weight... Harder to do an a single basket... but easy with a double..

    Can be done on a domestic as well as a commercial.

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    Re: Boiler pressure as water temperature

    Firstly, I cant quite follow what youre saying however:

    Of course the assumption is the person pulling shots is dosing exactly the same every single time.

    What needs to be clarified is when tamping, as long as you tamp with sufficient pressure to remove all air pockets/voids within the coffee bed, it doesnt matter what pressure you tamp at. Of course if you tamp at 5 kg vs 20 kg, there is going to be a massive difference as 5 kg isnt nearly heavy enough to remove the voids in the coffee bed.

    Changing tamping styles however will have an effect, for example the nutating tamp - which I do not use for speed and consistency issues.

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    Re: Boiler pressure as water temperature

    I didnt mean to start an argument/debate in all this. Koffee Kosmos has solved my rough grind problem with the simple idea of a mortar and pestle until new grinder can be conned from hubby. Works perfectly. All the other stuff is way beyond my ken.

    Thanks anyway

  23. #23
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    Re: Boiler pressure as water temperature

    Quote Originally Posted by 6144524B250 link=1261297726/21#21 date=1268794646
    I didnt mean to start an argument/debate in all this. Koffee Kosmos has solved my rough grind problem with the simple idea of a mortar and pestle until new grinder can be conned from hubby. Works perfectly. All the other stuff is way beyond my ken.

    Thanks anyway
    Excellent news MaM.... [smiley=thumbsup.gif]

    So long as youre now experiencing decent coffee, then everything is right with the world 8-) ;D

    Mal.

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    Re: Boiler pressure as water temperature

    Behmor Brazen - $249 - Free Freight
    Absolutely. I had been drinking my 6910 coffe for 2-3 years thinking couldnt get much better after my old single thermostat system where I had to wait to cool boiling frothing steam to make next coffee. If I hadnt talked to Garrett at Pony Express after tasting his coffee and he told me what the problem was, I would have been none the wiser. BUT not I have a tasting for what might be and using the mortar and pestle fixes pour time and pressure. Taste is noticebly better.

    You are right though everything is as right as it can be.



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