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Thread: Pros and cons of dual boiler and HX machines

  1. #1
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    Pros and cons of dual boiler and HX machines

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    Now some may see this as a Ford vs Holden question; its not. I understand basically how both these types of machines work, but they obviously use very different processes to achieve a similar end result. There must therefore be a number of differences in the way these machines are used, which would translate into good and bad points.

    What would be a breakdown of the pros and cons of each of these machines? What are things to aim for / avoid with each type? Any other useful bits of info? Not looking to buy one (at this stage), just furthering knowledge.

    Greg

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    Re: Pros and cons of dual boiler and HX machines

    Hi Greg,

    Dont have time to get into the thick and thin of it yet - Ill come back and do that - but the very first thing that I would point out is that not all HX machines are created equal and not all DB machines are created equal. So I would always approach the subject being very wary that generalisations will not always be useful.

    Cheers,

    Luca

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    Re: Pros and cons of dual boiler and HX machines

    Yep figured that, thanks. It appeals to us for the guy at the mower shop to say Victas are universally better than Rovers so we as an ignorant consumer can buy a Victa in full confidence its the best; but when we really know about something such is so rarely the case.

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    Re: Pros and cons of dual boiler and HX machines

    Ill second that. Each and every machine is different. However the design principles are common, even amongst the differences.

    Heat exchanger machines: The brew water is heated by the steam boiler. The temperature of the steam boiler is quite a lot higher than the optimal brew temperature (125oC or so) so these machines are not made to operate in what can be called a steady-state condition. Rather they operate in a dynamic equilibrium condition, where cold feed water is heated and mixed within the heat exchanger volume and then pumped to the group. To get it to operate like this from a standing start requires an initial flush to remove some of the superheated water. With careful flushing, very consistent temperatures are possible from shot to shot.

    These machines were designed to work under nearly constant operating conditions, where a dynamic equilibrium is possible. Making them work in the home isnt too hard either. I suspect the most difficult operating regime would be somewhere between the optimum dynamic equilibrium regime and the one-off regime, where the brew temperature may be harder to control.

    HX machines also use a trradeoff between the steam boiler pressure and brew temperature, as one controls the other. In practice, some very well designed machines have been produced that hold a very stable temperature and steam well.

    HX machines also use less power, fewer parts and are simpler, so are more economical to build.

    Dual boiler machines use the principle that you make no compromises in either steam performance or brew operation, as there is a dedicated boiler for each. Therefore the steam boiler can run at a higher temperature and give more/better steam performance, and the brew boiler can operate close to brew temperature. However, there is much more to it than that, as a boiler simply running at or close to brew temperature doesnt ensure a stable or consistent brew temperature. Design has a lot to say about how well the machine works. For example, the La Marzocco and Synesso saturated brew group is used to ensure that the brew boiler and group are maintained at very close to the same temperature. So in principle it should be possible to walk up to a good dual boiler machine and pull the same shot as you would get from a machine in constant operation. In practice this still isnt possible as no matter how hard you try, things heat up and cool down at different rates, so some dynamic equilibrium must be reached for truly consistent results. This can usually be remedied by a simple flush from go, and the flush in this case is just to heat things up to operating temp. Also and importantly flush volumes arent at all critical, as you cant overheat/overcool the group (within reason). This ma well be one of the main distinguishing factors of this design.

    In fact, these are really only minor differences in operation.

    In short, the generic categories are less important than the actual design. Id pick a Synesso over two Silvias side by side any day, even if the Silvias are fitted with a PID. Of course I have to win the lotto to get the Synesso ;-)

    Cheers and Merry Christmas to all :D :D :D :D

    Mark.

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    Re: Pros and cons of dual boiler and HX machines

    As Sparky said.

    Just let me add in a nutshell:
    If we assume that both machines are well made then the major difference is that with the HX there is only one boiler that has to serve two tasks (coffee and steam) that need two different temperatures while the twin boiler machine has one dedicated boiler for each that has to only do one task well.
    Both make good coffee but the twin boiler makes the task easier and less of a hit and miss especially with less experienced people.

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    Re: Pros and cons of dual boiler and HX machines

    Quote Originally Posted by Monti link=1166750333/0#4 date=1166846982
    As Sparky said.

    Just let me add in a nutshell:
    If we assume that both machines are well made then the major difference is that with the HX *there is only one boiler that has to serve two tasks (coffee and steam) that need two different temperatures while the twin boiler machine has one dedicated boiler for each that has to only do one task well.
    Both make good coffee but the twin boiler makes the task easier and less of a hit and miss especially with less experienced people.
    Yes and no, in my opinion Monti...A well made HX is also temp stable. My machine is capable of knocking out as many shots as I want without temp variation. The routine, whether hx or dual boiler is to flush until the water looks good and then go for it- doesnt matter which style of machine you use.

    What many CSers can forget is that the dual boiler machines- Minore II for example- are chock a block full of components and technology whereas the HX machines are simple- you lift the hood and its like looking under the bonnet of an old car...there is not much in there- therefore not much can go wrong.

    I have little doubt that ownership of a dual boiler machine will be more expensive in the long term because they are so full of boiler, water lines and electronics- which are not that crazy about heat....

    Both types of machine are capable of superb coffee with a modicum of training...just as they are capable of garbage. You need only ready Sparkys Giotto post pre vs post phone support....there was a world of difference in her coffee.

    No amount of technology will substitute for a poorly trained operator ;)

    2mcm

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    Re: Pros and cons of dual boiler and HX machines

    So, are you saying that there is no benefit in having twin boilers for temperature stability and that the need for temp surfing and purging is the same for both?
    I wouldnt think so. Many articles from respectable sources state that those are the benefits that twin boiler machines have over HX machines.
    So, why would you say that?

    Also, it goes without saying, that the better temp stability one will have and the more the machine can control for you the more technical they are going to be on the inside.

    PS: Both machines having the same quality build and same person operating them. We are only discussing twin boiler versus HX and not barista expertise nor machine build quality.

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    Re: Pros and cons of dual boiler and HX machines

    Quote Originally Posted by Monti link=1166750333/0#6 date=1166855307
    So, are you saying that there is no benefit in having twin boilers for temperature stability and that the need for temp surfing and purging is the same for both?
    I wouldnt think so. Many articles from respectable sources state that those are the benefits that twin boiler machines have over HX machines.
    As I alluded to before, there is a lot more to consider than how many boilers something has. It might be simplest to start by talking about dual-boilers. I think that it is not too much of a stretch to say that this class of machines is pretty much epitomised by the La Marzocco commercial machines. Other manufacturers make dual and multiple boiler machines, but it seems to me that most of this is a reaction to the meteoric rise in popularity of LM machines over the last seven years due in large part to them being the machine of choice for the World Barista Championships. Now, Sparky made the crucial point that part of the reason why LMs work as well as they do is that they use a saturated group head. Im going to expand on that a little bit because it is very important. The saturated group is much simpler to understand than a thermosyphon group. In a nutshell, a huge mass of water circulates throughout the group ... it is basically an extension of the boiler. At a guess, Id say that the LM and Synesso saturated groups hold over a litre of water. Thats a pretty significant amount in a Synesso with 1.9L brew boilers. All of this water is heated to brew temperature and that, in combination with the dedicated boiler, is what gets you your temperature stability. Well, that and the temperature controller. On earlier lineas, the temp controller is just a thermostat with a bit of a deadband. Some temperature surf these, some dont bother. On post 2005 lineas in Australia and the GB5s, the temp controller is a PID, which does a much nicer job. Of course, there are more things going on under the hood, but I fear that most people will probably already be nodding off ... ;P

    When one uses one of the PIDded LMs (or a synesso), a brief warming flush is standard practice. If the LM has brew water preheating, you can flush 10mL or 500mL and your espresso should taste more or less the same. This is because the group temp cant be raised above the brew temp and the influx of 500mL of water that is only maybe 10C colder than the other 10L of water sitting in the boiler is going to have very little, if any, impact on everything else thats going on. In practical terms, a flush is always necessary to make sure that the group head is clean. (That is the reason given for flushing the group in barista comps and it can be done at any time after removing the portafilter; you still get the points.)

    It has always been a pet peeve of mine that people will assume that any dual boiler machine is ipso facto going to perform like a LM. (Or even that early LMs will perform like the later ones.) A simple way to illustrate the point is to consider taking two silvias and transplanting the guts into one frame. You would then have a machine with two boilers, but with all of the temperature surfing, brew pressure and steam surfing problems that are endemic to silvias. And some manufacturers are making machines that are basically two single boiler machines in one body. I guess the other machine to consider is the minore. Well, the obvious difference between the expobar and an LM is that it has a different group. One consequence of this has to do with the fact that the thermosyphon group, unlike the saturated group, does not have water circulating around immediately above the portafilter. Im not 100% sure, but it seems to me that this water must be a lot colder than the water in an expobar HX, so I would expect that successive shots will warm up the group, raising the temperature of successive shots. I seem to recall that Abe Carmeli had some figures on home-barista.com to that effect. And there are other differences to be expected as well.

    The flip side of the coin is then HX machines that offer thermal stability similar to LMs. There is a rumor floating around that the NS Aurelia actually out-performed the LM machine in the temperature tests during the WBC machine trials. (Although changing brew temperatures on that machine is rather more involved.) I have used a few HXs that perform fine with just a small flush of a relatively nonspecific amount. There is a surprising number of things that can be tweaked! Thats not to say that there arent really crappy HX machines out there. There are. Heaps of them.

    ... and then theres the whole e61 preinfusion vs LM preinfusion thing and another gagillion factors ... way too much to write, but I hope that I have shown why I dont like generalisations!

    Cheers,

    Luca

    Saturated group from an old LM linea:



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    Re: Pros and cons of dual boiler and HX machines

    Spot-on Luca [smiley=tekst-toppie.gif]

    Mal.

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    Re: Pros and cons of dual boiler and HX machines

    Behmor Brazen - $249 - Free Freight
    Good post Luca.

    But I am failing to understand why people are comparing Silvias with their small (300 ml) single boilers, no E-61 thermosyphon groups, no heat exchangers, wide deadband thermostats.... with machines out of their class.

    A Silvia is a $7-800 machine. Anything else with is above $2000.

    --Robusto



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