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# [Beowulf] Re:running hot?

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Mark Hahn hahn at mcmaster.ca
Thu Mar 19 12:51:04 PDT 2009

```>> are you running your machinerooms warm to save power on cooling?
>
> How much would that really save?  Is there a study somewhere
> demonstrating substantial power savings?

yes, that's unclear to me as well.  I've heard people in the
machineroom infrastructure biz claim that 25% of your power
goes to cooling.  that seems like a lot to me

we have a number of 30T Liebert chillers: 3-phase 575V total
required fuse is 80A.  but we don't run the humidifier (11.6A)
and rarely the reheat (30.1A).  fan is 11A (10 HP) and each
compressor is 20.5A.  these numbers are all taken from the
electrical sticker inside the unit, and must represent peak.
I had an electrician measure currents on the feed, and IIRC,
the number was cycling between 24 and 40A depending on whether
the unit was at 50% or 100% cooling.  (that makes some sense:
8A fan, 16A/compressor.)  I don't know offhand how to convert
40A 3phase 575V into KW power, though - is there a sqrt(3) in there?
30T extracts 105.5KW, though.

> Whatever the steady state temperature in the room the AC still has to
> pump out heat at the same rate it is generated.   Raising the room
> temperature could affect heat exchange slightly because of a
> steeper/shallower T gradient across the walls/floor/ceiling.  For

sure - I'm assuming no heat flux through walls/floor/etc.  but if the
setpoint is close to outside, won't the HVAC do less work and consume
less power?  the ultimate temptation is to dispense with HVAC entirely
and use filtered outside air.

I guess there are two components: the heat-pump efficiency and the
delta-t (setpoint vs outside).  I agree that the former won't care about
fiddling the setpoint; power dissipated that way may dominate.

```