# For arguments sake, Rocket Cellini premium plus or Expobar leva dual boiler?



## Scubadoo (Sep 3, 2012)

My friend popped into a London retailer to look at a Rancilio Silvia and Francino machines but came away talking about the Rocket Cellini Premium Plus. idiot!

At that price point, which way would you go? Any other options?

Cheers

Dave


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## jimbow (Oct 13, 2011)

Wow, that's quite a leap! Might also want to consider Izzo machines although that might be pushing the budget further still and also perhaps some single group commercial offerings.


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## Scubadoo (Sep 3, 2012)

I know, I know! Good job it's a business purchase!

If I'm honest, I know it's too much money but they do look gorgeous. The Rocket in particular but the dual boiler of the expo appeals.

Yikes!


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## RoloD (Oct 13, 2010)

I thought I was going to faced with that decision once (Rocket or Expobar Dual Boiler).

The Rocket is undoubtedly more handsome but the Expobar is definitely the better spec. Owners of both machines seem very happy with them.

Rockets do have style - it's much harder to lust after an Expobar.

In the end, I bought a second-hand Olympia Cremina. I'm now converted to the lever cause.

I wouldn't quite say the espresso from the Cremina is better than sex, but I've certainly had worse sex than the coffee it produces.


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## Scubadoo (Sep 3, 2012)

RoloD said:


> I wouldn't quite say the espresso from the Cremina is better than sex, but I've certainly had worse sex than the coffee it produces.


Ha ha, love it!


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## Scubadoo (Sep 3, 2012)

The ECM Barista looks a good machine as well. Any one have any experience of it?

Is there anywhere that stocks these machines anywhere near Bristol? It'd be nice to see them in person.


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## garydyke1 (Mar 9, 2011)

For what its worth the Expobar is a great machine and delivers value for money that is hard to beat. I dont have any upgraditis what-so-ever


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## Scubadoo (Sep 3, 2012)

Thanks Gary.

I think we're heading down the Rocket route. Will let you know what we get.


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## Spazbarista (Dec 6, 2011)

Another Dual owner here. I faced that decision 9 months ago. The only advantage of the Rocket machines is a better finish and some higher spec parts. Disadvantages are temperature stability (highly important) is not as good as Dual.

Also the Dual is made from generic parts that are possibly more easily available, cheaper, and user serviceable. This becomes a real advantage when you realise that you will have to service any espresso machine sooner than you thought. The Dual has a cult following (google 'Brewtus')

If I was faced with the choice again I'd still get the Expobar, but I'd go for the plumbed in model.


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## RoloD (Oct 13, 2010)

Or if you want to join the lever cult, wait for the Londinium 1 to be launched at the end of this month.


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## vintagecigarman (Aug 10, 2009)

RoloD said:


> Or if you want to join the lever cult, wait for the Londinium 1 to be launched at the end of this month.


Quiet! Don't tell everyone! If the Londinium 1 is as good as I expect it to be, I'll be wanting to sell my Rocket on here!


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## Scubadoo (Sep 3, 2012)

Cool, I'll give you £500 for it, just as a favour.


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## Calidore (Jan 22, 2012)

I have the Rocket. It's a great machine. As with all heat exchanger models temperature stability is related to how consistent the user is with cooling flushes. For me it took about two days to get the hang. It makes really great coffee. Brilliant foamer too. And people who come into my kitchen do a little recoil in wonder when they see it. That gives me childish pleasure every time.


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## Scubadoo (Sep 3, 2012)

Thanks Calidore.

So how do you actually judge your flushes and when do you do them?

Is it just about getting rid of the water that's sitting in the pipe running through the boiler in order to draw fresh cold water through the heat exchanger?


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## lookseehear (Jul 16, 2010)

There is an oft-linked article on Home Barista about heat exchanger machines. In general though, your reference point is when the water stops spluttering out of the grouphead. Regardless of how long the machine has been idle, if you count the same number of seconds after the spluttering stops it should get you in the same ballpark place. Most people tend to:

- flush until spluttering stops plus an amount of time dependent on your individual machine

- grind, distribute tamp etc (this time lets your steam boiler temp/pressure recover before brewing) - it should hopefully take a similar amount of time each time you do this

- insert portafilter and brew


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## garydyke1 (Mar 9, 2011)

This cooling flush lark is what made me go for a dual boiler PID in the first place. A lot less flushing and guesswork. Horses for courses though I guess


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## RoloD (Oct 13, 2010)

Calidore said:


> ... As with all heat exchanger models temperature stability is related to how consistent the user is with cooling flushes. For me it took about two days to get the hang.


 Some would argue with this.


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## RobD (May 2, 2012)

RoloD said:


> Some would argue with this.


Im not sure how this conclusion is drawn by londinum Espresso. Because in my years of thermal modelling, if you have a thermal mass at lets say 102.7c at some point all the parts will reach a saturation point especially if they are made from copper whit has a thermal conductivity of around 400 W M2 Kevin Deta T, so unless the thermosyphon is intentionally loosing around 10c in the circuit some portion of the liquid delivered will be above a good brew temp?


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## RoloD (Oct 13, 2010)

Well RobD, can't argue with that because I am not an engineer. But am I right in thinking not all heat exchanger designs incorporate a thermo-syphon - or am I wrong?


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## PolarBearDave (Jul 24, 2012)

I wouldn't say a saturation point, but equilibrium point - where the heat transferring through the metal balances the heat loss at that particular geometry. Once you start to run out from the lump of the boiler into tubes and smaller complex shapes (grouphead), then the exposed area for heat loss jumps up dramatically.


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## Scubadoo (Sep 3, 2012)

RoloD said:


> Well RobD, can't argue with that because I am not an engineer. But am I right in thinking not all heat exchanger designs incorporate a thermo-syphon - or am I wrong?


Well the rocket definitely has one and apparently does need a flush so I'm surprised the Fracino doesn't.


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## jimbow (Oct 13, 2011)

My understanding is that the water in the thermosyphon does lose temperature. This principle is in fact crucial to its operation as a convection current is set up within the thermosyphon. The water in the thermosyphon section that runs through the boiler (the HX) is heated and then rises drawing cooler water into the space it previously occupied (inside the HX). The heated water continues to rise until it reaches the group head where it begins to cool and sink down the cool (return) side of the thermosyphon, and return to the HX. When the pump is activated, fresh water is injected into the centre (or bottom, depending upon design) of the HX and displaces the water inside along the thermosyphon towards the group head, where the thermosyphon is now open to the atmosphere. In some HX designs, water is displaced along both sides (the hot and cool) of the thermosyphon which produces a more balanced brew water temperature profile. I find Fracino machines seem to work this way with little or no cooling flush necessary as a result.


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## jimbow (Oct 13, 2011)

Article on HB here categorizing HX designs and the way they behave:

http://www.home-barista.com/espresso-machines/ideal-brew-temperature-management-by-hx-espresso-machine-type-t20661.html


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## RobD (May 2, 2012)

i have only seen the schematics for and original E61 HX machine, and if some of the machines use mixing system or deliberate heat loss arrangements then yes I can see you would have around a 10c difference.

thermosyphons work by cold water being denser that hot water so as the water cools it sinks, but it does rely upon the pipe work always having clean rise & fall, its a very old water circulation system that has been used in household heating systems and older vehicles for years, but to lose 10c in such a small system although is not good design and realise that a lot of the designs are really E61 hybrids with only the PF holder element looking the same?

Will be interesting to see what the real temps are when I can get round to making a group head thermometer


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## PolarBearDave (Jul 24, 2012)

I know it's not a E61 or even a HX machine but on my Silvia, the temperature of the grouphead head lump that sticks out from the bottom of the boiler is ~85C when the top of the boiler is ~102C. And that's effectively the same lump of brass, which is also insulated in my case....


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## RobD (May 2, 2012)

Hi Dave

the reason your boiler is at 102 but your group is only at 85 maybe because its brass and that only has a thermal conductivity of 102 W/mk where as copper is 400 W/mk so has 4 x the heat transfer capability, if it was made from aluminium it would be 220 W/mk. its all to do with the number of free electrons in the material that are able to transfer the heat energy K from one and other and on to other materials.

but in a boiler that is predominately copper with copper tubes the heat will be transferring much faster than your losses unless you use a lot of un insulated tube, or cold infusion/mixing system.

it is making me wonder what will happen to my flushing temp if i insulate the entire HX system apart from the head.


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## jimbow (Oct 13, 2011)

Fantastic stuff!

Don't the copper tubes go into the brass group head though? Would not all that exposed brass act a bit like a radiator?


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## RobD (May 2, 2012)

Hi Jimbow yes the group head will act like a rad and it will depend on 4 factors. 1 the temp of the water supplied to the head. 2 the thermal transfer rate of the material the head is made from, 3 the surface area of the groupehad that is open to air. 4 the ambient air temperature

The water is the heat exchange medium in the Grouphead apart from where the copper tubes connect, so the surface area the water is in contact and that materials properties of that surface will be the first main thermal junction, then you have the mass of the head to warm because until it gets above the surrounding ambient air temp i will not transfer any energy, as heat is transferred in a pretty liner manner, so if your have a surface area open to air that is 1 meter square 1 deg warmer than the surrounding air you will transfer 0.024 watts of energy. if its 100c warmer you will transfer 2.4 watts. but this is working with steady state models, and probably why its best to give a coffee machine a warm up time to give all the parts enough time to reach a thermal equilibrium.

Sorry if you have lost the will to live







:exit:


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## PolarBearDave (Jul 24, 2012)

Yes I see that brass has a lower thermal conductivity, but there's considerably more cross sectional area (easily x10+ I would guess) to transfer the heat as its more like a lobe connected to the bottom, compared to the thin walls of a couple of copper pipes running from a boiler to a remote grouphead...

I would be surprised if any remote grouphead had a closer equilibrium temperature than one that's integrated into a boiler, no matter what it's made of (except maybe with a very effective thermosyphon flow).

Anyway, back to the flushing. I understood flushing was needed because the boiler in a HX machine is at steam temperature. If the brew water sits in it, rather than just moving through it, it's gets too hot and needs to be flushed out.

Maybe with a good thermosyphon system, the water keeps moving enough when the pump isn't on, leaving the boiler and being cooled at the grouphead, that it doesn't overheat?

Sent from my iPad using Tapatalk HD


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## RobD (May 2, 2012)

Hi Dave

The quick version- it's the water that is doing the heating not the copper tubing, so if there is more surface area for the water it will transfer more heat.

The only reason that copper is used in coffee machine is the same reason its used in household plumbing, not for its thermal properties, but because of its ease of fabrication and relatively neutral chemical properties to water.


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## PolarBearDave (Jul 24, 2012)

Yes indeed I agree, the metal has nothing to do with it, it's all about flow of water.

Reading that HB article on HX machines I think explains it as the boiler is at ~125C and the thermosyphon is designed to keep the grouphead at the actual desired shot temperature e.g. 102C (~20C drop) which stabilises the shot temperature when it finally comes through it.

However, if the thermosyphon is too effective, the grouphead overheats (pulling the temperature too close to 125C) and needs a cooling flush before pulling the shot.

So maybe in machines where less of a flush is required, the thermosyphon system is more tuned to keep the grouphead at just the right temperature? However, maybe if the heat transfer is kept low, it takes a long time to heat up? Always a trade off I suppose

Sent from my iPad using Tapatalk HD


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## jimbow (Oct 13, 2011)

Depending upon the design of the specific HX machine, I do not believe the water inside the thermosyphon actually reaches the same temperature as the steam boiler. The water inside the HX will rise out of the HX (the section of the thermosyphon circuit running through the boiler) as it heats and along the thermosyphon towards the group head where it may transfer a small amount of it's heat energy before returning back down the other side of the thermosyphon circuit and into the HX once more.


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## PolarBearDave (Jul 24, 2012)

Just realised I got my temperatures wrong in the previous post - the HX boiler's at ~125C and the brew temperature should be ~92-95C so in fact it should be about a 30C drop between the boiler and grouphead, via the thermosyphon system.


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## jimbow (Oct 13, 2011)

Depending upon design I am not sure the brew water actually spends enough time within the HX to reach the temperature of the water within the boiler. I think that the convection current in the thermosyphon means the water will exit the HX before it gets that hot.


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## PolarBearDave (Jul 24, 2012)

Ahh I think I see what you're saying. I didn't mean the actual water in the thermosyphon - more the thermosyphon system as a whole maintains the grouphead at 20-30C less than the boiler. This would be through both heat losses and as you say the water circulating at a lower temperature.


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## Scubadoo (Sep 3, 2012)

It almost feels a shame to get back to the original topic, so sorry!

We're getting close to ordering the Rocket Cellini and eureka Mignon. I e-mailed bella barista and claudette rang me the next morning to chat over the options and offer advice. Very impressed with them so far, and the best price that I've seen.

I'm weighing options of other models and still have niggling doubts that I should go for a PID machine like the Izzo Alex PID or maybe even a double boiler. It's difficult to know having not had a machine before. I think the Cellini is probably plenty to spend really and by the looks of it will mAke great coffee.

Need to get some cups, knock box and a milk jug now. I was thinking of some of the ancap ones or the branded ones from Origin look nice. And I thought I'd start with the cheap classic jug from coffehit. What's the best size, 20oz? And a grindenstein knock box.

Oh, and some coffee! I have a local shop in Bristol called Twoday coffee roasters, might try there.

All very exciting!

Thanks for the advice along the way.


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## jimbow (Oct 13, 2011)

PolarBearDave said:


> Ahh I think I see what you're saying. I didn't mean the actual water in the thermosyphon - more the thermosyphon system as a whole maintains the grouphead at 20-30C less than the boiler. This would be through both heat losses and as you say the water circulating at a lower temperature.


Ah okay.

Scubadoo, lots of great places to buy freshly roasted coffee online. HasBean, SquareMile, Origin, Extract and Hands On are all well respected and roast to order online.

Ibwould also make sure you get a set of small jewellers scales (0.1g precision) which you will need for weighing coffee and the espresso.


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## Scubadoo (Sep 3, 2012)

The Rocket lands on Monday!


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## Spazbarista (Dec 6, 2011)

Brilliant! Rocket or Expobar....neither will disappoint. Rocket definitely has the edge in build quality.


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