Cooling Tower Approach

7 posts / 0 new
Last post

I am looking for tips on how to properly compare tower approach temps. Currently, I am comparing a 7 F approach with a 5 F approach. The tower has 4 cells with 44 kW fan energy for each cell and a 10 F range.

Regardless of control parameters, the program shows less tower energy for a smaller approach and I was expecting the opposite.

Can anyone shed some light on this issue?

Thank you!

Judy Peters, PE LEED-AP BEMP
Energy Modeling Engineer
Daikin Applied
763.553.5155
judith.peters at daikinapplied.com | www.DaikinApplied.com

Bldg simulation's picture
Offline
Joined: 2016-04-13
Reputation: 400

An HTML attachment was scrubbed...
URL:

Bldg simulation's picture
Offline
Joined: 2016-04-13
Reputation: 400

In general, a smaller approach means the cooling tower is more efficiently
rejecting heat, i.e. better performance. This overview is pretty good:
http://spxcooling.com/pdf/TR-016.pdf

Best,
Danielle

--
Danielle Bond, Ph.D.
Energy Engineer, eCap Network
http://www.ecapnet.com/

Bldg simulation's picture
Offline
Joined: 2016-04-13
Reputation: 400

Dan and Danielle,

Thank you for your excellent comments. The results make more sense to me now.

Thanks again,
Judy

Judy Peters, PE LEED-AP BEMP
Energy Modeling Engineer
Daikin Applied
763.553.5155
judith.peters at daikinapplied.com | www.DaikinApplied.com

Bldg simulation's picture
Offline
Joined: 2016-04-13
Reputation: 400

Judy,

You didn't specify which software you're using, but in general (and for e+ Specifically) you should pay attention to the difference between design input values and the operation conditions.
In e+ that's the different between what you input in the Cooling tower and the plantloop and sizing objects, versus the setpointManager (setpointmanager:followoutdoorairtemperature set to wetbulb control in particular) you use on the plant outlet to determine the operating conditions at each hour.

(Excuse any typos or misnamed objects above, going off memory here).

In reality and as was said by others, if you design a cooling tower for a smaller approach but with the same fan power you end up with an oversized hear exchanger thus allowing lower fan energy across the full year.

Best,
Julien

Envoy? de mon iPhone

Bldg simulation's picture
Offline
Joined: 2016-04-13
Reputation: 400

An HTML attachment was scrubbed...
URL:

Bldg simulation's picture
Offline
Joined: 2016-04-13
Reputation: 400

It?s a little unclear from this thread if the original post is asking about decreasing the tower DESIGN approach (i.e. evaluating a different tower design), or decreasing the approach of some given tower DURING OPERATION via the control settings of the condenser water temperature.

In eQUEST, and I think in the tower design world, the ?approach? is a design parameter, not just a control parameter.

The post says ?the tower? has a 44 kW fan in both cases. But typically for the same heat rejection, to achieve a closer approach requires either the same physical tower with a ?bigger fan? (more air through the tower, often just by higher rpm) and higher fan rated fan kW, or a bigger physical tower with that higher airflow, but lower velocities and dP, to achieve the same RATED fan kW.

One can separately set the condenser water loop temperature setpoint (in eQ, that?s the tower leaving temperature, aka chiller entering condenser water temperature) or setpoint strategies so any given tower attempts to achieve less than the design approach during some or most hours. That will increase tower fan energy, but will decrease compressor energy in attached systems.

Fred
Fred Porter, BEMP, LEED? AP
Principal Engineer
Sustainability Services
NORESCO

Anonymous's picture
Anonymous