Re: [SAGE] Greener Machine Rooms: Cooling?

[Allan West <allan@cookie.org>]

Mark R. Lindsey wrote:
> It seems like the general consensus on cooling is:
>     (a) It's hard
>     (b) It's non-obvious
> 
> Folks have included lots of interesting tidbits and ideas, but has 
> anybody actually written a book or articles about this? Maybe even one 
> for computer/telecom folks?


I'm a graduate student in Building Construction in my spare time, and
this semester I'm taking the HVAC (Heating, Ventilation and Air
Conditioning) class. I'm going to summarize what I've learned about HVAC
system types for those who aren't familiar, so that you can ask
more/better questions when you talk to your building facilities people.

Within a server room, the careful movement of air through hot and cold
aisles and racks with all the openings blanked off will make the cooling
you have work more efficiently for your server load. Similarly,
minimizing the actual power consumption of the servers will better use
the available cooling.

When it comes to deciding what type of cooling system to put into a
server room, that it usually dictated by the facilities of the building
you're in. Major categories of options include direct expansion (DX) and
chilled water. The varieties of systems are based on how far the
mechanism for chilling is from the heat exhaust, what medium carries the
heat back and forth, and whether there are air ducts between the load
and the cooling unit.

DX (direct expansion): like your car or your house uses. This is the
simplest system to buy as a unit. DX systems move HCFC or HFC
refrigerants between the server room and some outside location, and the
refrigerant carries the heat the whole way.

Chilled water: This is usually a good choice for larger systems. There's
a DX system on the roof/parking lot/etc. which is used to cool water.
The cool water runs to the machine room where air is blown over cooling
coils full of chilled water. A different water loop runs between
the DX system and a large cooling tower to cool the refrigerant. Water
is a more efficient transfer mechanism for heat than air, so it can move
a lot more heat than air.

A big-box retail store will have package DX units on the roof, with all
of the mechanical parts in a single shell, and ducts running down
through the roof. The units are entirely services from the roof, can be
replaced by hoisting a new unit up with a crane, and come pre-charged
and tested from the factory.

Your house probably has a split DX system, with the compressor and
condensing coils outside, and the expansion (cooling) coils and fan
indoors. The ducts are installed for the floor plan of your house, and
the generic indoor and outdoor units are connected and charged by the
A/C technician.

My university campus has chilled water plants around the perimeter and
pipelines which run to individual buildings. My machine rooms have
fan-coil units which blow air over chilled water coils. The cooling work
is done nearly a quarter mile away, in a group of buildings which serve
about 20% of the campus. The pipe network underground allows chiller
plants to cover for one another during maintenance periods.

Some newer machine rooms on campus have a chilled water fan-coil unit
which also has a DX coil for auxiliary/emergency backup when the chilled
water is unavailable or insufficient. When the chilled water is running
the DX coil is idle and its compressor and fans are off. When the
chilled water goes out, the DX system can be powered by building
electricity or the emergency generator.

This week we've started on duct sizing, which is less relevant to
machine rooms since most shops use raised floors to distribute cool air.
If you build a machine room and have control over the duct/plenum/floor
height, larger is better. It allows more capacity growth for the future
and less strain on the fan at every level. Our physics machine room now
has spot coolers and overhead ducts because the 9-inch raised floor is
not raised enough to accommodate the current load.

If anyone has questions about HVAC theory, please feel free to ask. I've
got a textbook and a wide range of charts and graphs to answer most
general questions. I can't say anything about specific hardware, since
I'm in a poor college and we don't get to buy new toys. My machine rooms
are located where fan-coil units already existed.

Allan