Superheated

[MrJack]

A while back, someone suggested I reinvent the steam boiler to get around the issue of wet steam (see Izzo Alex Duetto II Wet Steam Problem)

Well, it seems Slayer thought it was a problem worth solving - they have brought out a new line of espresso machines with superheaters. I wonder if they have a successful patent (despite superheaters being used in steam service for over a century)?

They do look really really nice, but sadly the marketing machine is in overdrive already. Just watched a video where they claim that their drier steam has "less water content in it". Now, unless it's hooked up to an N2 bottle, that seems unlikely...

[trentski]

Don't let science get in the way of good marketing

[jbrewster]

Presumably they meant "less *liquid* water content".

[simonko]

I was just contemplating the other day for a home build of passing steam (rather than brew or cold water) through a thermoblock. The TB I looked at has a thermal cutout at 190°C, so waiting on datasheets to see if if that's just a factory TS appropriate for typical use, or I can pump up that puppy to cater for the steam flow.

It's not the dilution that bothers me too much (I'm happy enough with the milk quality from a machine that gives me 10% milk dilution), it's just a extra variable to have in help dissociate HX temp from steam without going dual boiler.

Will be interesting to see what they've done. Thanks for pointing it out.

[MrJack]

Presumably they meant "less *liquid* water content".

Presumably, but it will undoubtedly be an error perpetuated through the specialty coffee media, forums etc...

[MrJack]

I was just contemplating the other day for a home build of passing steam (rather than brew or cold water) through a thermoblock. The TB I looked at has a thermal cutout at 190°C, so waiting on datasheets to see if if that's just a factory TS appropriate for typical use, or I can pump up that puppy to cater for the steam flow.

It's not the dilution that bothers me too much (I'm happy enough with the milk quality from a machine that gives me 10% milk dilution), it's just a extra variable to have in help dissociate HX temp from steam without going dual boiler.

Will be interesting to see what they've done. Thanks for pointing it out.

As I recall, most boilers are at around 125°C. Don't think you'd need >60°C of superheat. The main difficulty would actually be reducing the heat output and making sure it is safely controlled.

[simonko]

I was initially concerned about the small time of contact steam would have, but thinking about it now the steam entering would cool the TB anyway. More up to whether the TB element could keep the temp high enough I'm thinking.

[MrJack]

The heat demand and heat transfer rate would be higher for liquid water.

[trentski]

Won't you just end up heating your milk faster than you can texture it?

[MrJack]

Maybe, but not necessarily.
Dry steam will be lower mean density than wet steam at the same pressure. Therefore, for approximately the same volumetric flowrate (and thus velocity) there could be a lower mass flowrate through the orifice in the steam wand.

You really only need to superheat enough to ensure that even with cooling occurring in the path to steam tip, the steam remains above it's boiling point temperature. So your steam may actually only be a few degrees hotter when reaches the milk (and the extra enthalpy from a few extra degrees is not really significant).

Hard to say without doing some modelling really (or some experimentation).

[simonko]

Yeah, I think the idea is to make it drier steam rather than dry steam - I've read completely dry steam is unsuitable for heating applications.

Here's a nice little vid, although I wonder if it is indeed the steam causing the paper to burn or radiation from the pipe.

[MrJack]

Yeah, I think the idea is to make it drier steam rather than dry steam - I've read completely dry steam is unsuitable for heating applications.

Superheated means the steam is above the boiling point temperature at the current pressure. That can't happen until the steam is dry (because otherwise additional heat just leads to more of the liquid being vapourised (aka increases the steam "quality").

I'm not sure of the basis for what you read, but superheated steam is used in industrial applications. "Unsuitable" is likely to depend on the application (and is perhaps driven by economics or controllability, rather than heat transfer).

For our (relatively unusual) use case, the only potential concerns I can can see are burn risk:
a) to people, because you can't see dry steam.
b) to the milk, if you have localised heating for some reason.

I think a is more relevant than b.

Be very careful with steam, it's potentially very hazardous.