Fog Bank in a co-locate

Wed Oct 7 08:34:11 UTC 1998

wb8foz at nrk.COM (David Lesher) writes:
>I'm less sure. IMHE, anything running will not promote condensation.
>It will be warmer than ambient.

The concern is greatest if you have any external doors opening directly
into the co-locate area (no vestibule or 'air lock').  Since it is
"Relative" humidity, what will happen when the warm, humid outside
air enters a cold area?  The supersaturated air will condense
when it hits any relatively cool object, such as the supporting
superstructure and rack rails.  Water will then do what it does best,
start dripping onto the equipment in the racks.  I actually learned
this lesson in high school when I opened a window in the school
computer room on a hot, muggy summer day.  The same thing will happen
in the winter, if you let very, very cold air into a warm area with
40%-50% relative humidity.  The real lesson is avoid external doors
and windows opening directly into the controlled area.

This tends to be a religious issue.  To make matters worse, for a major
data center you really need about four different environmental zones:
people, magnetic media, paper, and electronic equipment.  Most of the
time you compromise and pick a happy medium set of environmental settings.
One misconception is the idea of "Telco environment" being better.  In
fact, most times the telco environment is worse.  The reason you need
more robustly built telco equipment is C.O.'s have a harsh environment.

So, with that in mind, here is one person's opinion on a good comprise
set of environmental parameters for a data/telco/isp co-locate space.
This assumes the space is used for mostly EDP/network equipment, and
only incidental people, tapes, and paper are in the area.

Donelan's Climate Settings:
	8,500 BTU/hr heat dissipation per full rack
		Not all of it must be installed on day 1, but I want to
		see pre-planning how the HVAC will be expanded to meet
		the load (e.g. enough room reserved, aisles wide enough,
		to bring in additional units later, pre-plumbed for the
		outside heat exchangers, compressors, etc).
	Minimum 1 air change per hour
		This is actually a 'clue-check' question.  How the co-locate
		operator answers it will tell you if they really know their
		HVAC systems.  Unoccupied areas can get by with as little
		as 1 air change per 24 hours, but if you are doing an install
		or other work in the area, you'll appreciate more frequent
		air changes.
	Humidity: 35% to 50% R.H., non-condensing, max 2% change per hour
	   Ideal: 40%-45% R.H., measured at three locations with max 5%
		variation.  Humidity is expensive to control, so it is
		common to have R.H. at the high end of the range (e.g.
		45% R.H.) in areas naturally more humid (e.g. northeast)
		and R.H. at the low end of the range (e.g. 40% R.H.) in
		areas naturally drier (e.g. southwest).
	   Maximum (not to exceed 24 hours/annually): 20%-60% R.H.,
		non-condensing
	Temperature: 59f to 77f (15c to 25c), max 4f (2c) change per hour 
	   Ideal: 72f (22c) at my rack, but since it is econmically infeasible
		to control the temperature at the rack level, I normally
		ask for 68f (20c) as a room average, measured at three
		different points, with a maximum of 4f(2c) variation.
		The assumption is at worst I'll end up with 72f(22c) at
		my rack. Anything much colder than 68f(20c) is a bit
		harsh on humans that need to work in the area.  A tech
		thinking about how cold he is, isn't thinking carefully
		enough about the job he is supposed to be doing.
	    Maximum (not to exceed 24 hours/annually): 49f(10c) to 85f(30c)

You'll notice I tend to be metric-centric in my measurements.  These are
much stricter than your typical C.O. environments.

Its more a practical matter of negotiating with the co-locate operator,
and asking for more than you really need on the assumption even if they
deliver just 50% of what they promise it won't be an immediate disaster.
They may have no problem delivering 68f(20c) when the co-locate is 90%
empty.  But when it fills up, I'm betting with the lower the starting point,
the less likely I'll run into my upper limit.

I choose most of the values to give myself enough of a buffer before
exceeding any equipment parameters, so I can try to negotiate a resolution
with the co-locate operator before the levels start to causing equipment
to shutdown.  Otherwise if you set the limits at the maximum your equipment
will tolerate, a co-locate operator may not do anything until after the
temperature goes over 104f(40c), but by then its too late.  Obviously the
co-locate operator wants to write the contract with the largest acceptable
environmental ranges possible, and the co-locate lessee wants the 
narrowest acceptable environmental ranges.  Its a barginning point, how
much do you really want to pay for +- 1 degree, or +- 10 degrees.

In reality, most modern electronic equipment will operate well in a wide
range of human-habitable enviroments, as long as the enviroment is stable.
Its the oscillations that will kill your equipment every time.  And in most
cases, if the price is the same, I am willing to trade a particular value
for tighter tolerances, e.g. 75f+-1f.  But the price is rarely the same,
so I generally choose the bigger range as shown above.
-- 
Sean Donelan, Data Research Associates, Inc, St. Louis, MO
  Affiliation given for identification not representation