Comments on: I like Miso Soup

By: server rack cooling

server rack cooling — Mon, 13 Oct 2008 02:33:37 +0000

server rack cooling…

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

By: Steve Cassidy

Steve Cassidy — Thu, 31 Jul 2008 12:46:56 +0000

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…

By: Drab

Drab — Thu, 31 Jul 2008 08:02:23 +0000

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

By: Steve Cassidy

Steve Cassidy — Wed, 30 Jul 2008 23:18:35 +0000

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…

By: Grimer

Grimer — Wed, 30 Jul 2008 19:25:06 +0000

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…