Posted on July 29th, 2008 by Steve Cassidy

I like Miso Soup

No, I haven’t been taken over by a random word generator : I genuinely like the stuff. Not just because it perfectly complements some Sashimi or a Bento Box; but because it helps me think about air-conditioning and heat. Miso soup is a mixture of stock – or dashi – with paste – or Miso. It’s supposed to arrive hot, and if you leave it in a coolish room you can see the little particles of paste circulating in an almost textbook perfect case study of convection: something very few people actually believe is really going on around us, despite being taught about it in school (by a mad Welshman with crinkly wavy hair, a la Dilbert, in my case, but I digress)…

When the weather is hot, and I’m standing in people’s server rooms and they are going nuts with fear and loathing about their precious servers going into meltdown, I like to ask them about Miso soup: and if they get all confused (and then angry and then don’t pay my bill), I ask them how much they think the air inside the typical hot-air balloon actually weighs.

Very few get it right: the answer is, about five tons. Once you get that idea into your head, getting emergency cooling for servers sorted out starts to make a good deal more sense – and those elephant’s-trunk so-called aircon units which harassed managers tend to put in as a reflex action during these periods, start to look more like a way to throw kilowatts into the air for very little benefit, than like a smart way to stop your servers going into meltdown.

Got any good “boy stood on the burning deck” stories from extreme heat or wild weather?

Tags: cooling, Miso soup, server

Posted in: Green, Real World Computing

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5 Responses to “ I like Miso Soup ”

Grimer Says:
July 30th, 2008 at 7:25 pm
I just calculated the weight of the air in a balloon and came up with a rather different answer:

Small one man balloon = 595 cubic metres = 595,000 litres
1 mol of air (at room temp) = 22.4 litres
80% Nitrogen + 20% Oxygen = (0.8×15x2) + (0.2×16x2) = 30.4g per mol

Therefore 595,000 litres = 26,562.5 mol = 807,500 grams = 0.8075 tonnes

However, hot air has less density than cold air (PV/T=K)

0.9486 kg/m³ for dry air heated to 210 °F (99 °C)

This would mean that 595 cubic metres of hot air = 0.564 tonnes

An average balloon has a capacity of 2,832 m³. If we scale up the previous answer:

2832/595 = 4.75
4.75 x 0.564 tonnes = 2.68 tonnes

Anyway, it isn’t convection that keeps a balloon in the air. It is due to the difference in the density of the atmospheric air and the air trapped inside the balloon. This is a difference of over 30%.

Hot air balloons rise for exactly the same reason that ‘blimps’ rise. They are full of a low density gas. It has nothing to do with convection (although you could argue that convection is the phenomenon of a lower density gas rising up through a higher density gas).

It sounds like you’re very lucky to get paid and don’t really know what you’re talking about.

Sorry…
Steve Cassidy Says:
July 30th, 2008 at 11:18 pm
Charmed, I’m sure. Though in order to critique like that you need to show where I associate convection with balloons (I don’t: the convection is in the soup, which you carefully ignore, like all good critics with a chip on their shoulders). As for the five ton figure, it was told to me while I was standing in a balloon with a qualified balloon pilot who, since his job includes not crashing into the ground, I’m prepared to listen to.

It sounds to me like you desperately need to be bad-tempered and misinterpretative for the sake of it, and need to find some way to vent your own hot air. Which is rather were I came in…
Drab Says:
July 31st, 2008 at 8:02 am
Steve

I agree wholeheartedly with your comments on Grimer’s behaviour – he was definitely looking for a fight. However you do not comment on his mathematics, I suppose I could go online and check it out for myself but it would be nice to know whether you accept his basic calculation re weight of air inside a balloon and hence the wieght of air inside an average server room, which would lead us nicely into reasonable recommendations for server room cooling.

Cheers

DRAB
Steve Cassidy Says:
July 31st, 2008 at 12:46 pm
You don’t need to start from a balloon to get figures for server rooms, though it is a forest of incompatible units fed into simple bits of maths: a room of a given volume will contain a fairly well understood quantity (=weight) of air. The gap in appreciation comes in when dealing with the kilowatts of power pulled by a server rack, versus the usual rating of an aircon unit. Aircon people deal in BTU’s – which handily hides how much power they suck in on their own account. My guess (without having to calculate how many moles per kilofortnight are sucked up the elephant’s trunk air exhaust) is that a portable aircon unit sized hopefullly to cook 1.25kW of servers is itself drawing (and dissipating) a further 3-5kW – and a painful amount of that dissipation is back into the room, from the surface of the “trunk” (=air exhaust).

However, I can read the mood of the day and I’ll place a small side bet this turns into a long thread about weighing hot air ballons with a periodic table, a slide rule, and a pyrometer…
server rack cooling Says:
October 13th, 2008 at 2:33 am
server rack cooling…

I never thought I will agree with this opinion, but you know… I agree partially now…