Air-cooled chiller economizer

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Does anyone have insight on a relatively easy way to model free-cooling
coils on air-cooled chillers? I thought adding a drycooler might help
simulate this, but I don't even think you can attach a heat-rejection
device directly to a chilled water loop. Is this one of those things
that you have to "estimate" by approximating the total # of available
free-cooling hours and subtracting compressor energy?

Thanks

James Hansen, PE, LEED AP

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For those interested, I think I figured out how to do this (with some
direction from Linda). I set up a "dummy" water-cooled chiller with an
associated drycooler for heat rejection. The chiller EIR was set to 0,
the condenser water circulating pump horsepower was set to 0, and the
EIR of the drycooler was set to the power that I think the air-cooled
chiller condenser fans will use during free-cooling operation.

I then set up two Equipment Control components called "Greater than 38"
and "Less than 38". "Less than 38" has my dummy chiller set up to
handle the load, and "Greater than 38" has my normal air-cooled chillers
set to handle the load. I then set up a Load Management component.
Management Seq 1 references the "Less than 38" Eqpm Ctrl with the Max
OSA Temp set to 38 degrees. Management Seq 2 references the "Greater
than 38" Eqpm Ctrl with the Max OSA Temp set to 110 degrees. (You could
set the switchover point to whatever you prefer).

I didn't spend that much time fiddling with this, but it looks like it
may have worked. My dummy chiller operates 6 hrs in October, 19 in
November, 106 in December, 149 in January, 133 in February, 47 in March
and 4 in April (Washington, DC area). That looks about right.

For reference, the utility bills dropped from $247,931 to $246,744 (for
a 140,000 sq ft office building in DC). This worked out to a 6%
reduction in total space cooling energy for the year, but only about a
0.5% decrease in total utility bills.

And this doesn't take into account the added pressure drop across the
air-cooled chiller condenser fans for the rest of the year. This
relatively minor increase in savings is probably why free-cooling coils
in air-cooled chillers in this climate are not particularly favorable.
And also why the took the economizer requirement out of 90.1-2004 and
the 2006 IECC (for our climate zone).

I welcome any comments if I did something incorrectly....

James Hansen, PE, LEED AP

James Hansen's picture
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Joined: 2011-09-30
Reputation: 200