Cooling Tower Variable Speed - Which empirical model to choose?

asked 2016-02-04 04:05:07 -0600

Julien Marrec

29726 ●94 ●76 http://www.effibem.com/

updated 2016-02-05 14:34:56 -0600

__AmirRoth__

4446 ●5 ●17 http://bleedinggreenna...

I'm using a CoolingTower:VariableSpeedthat supplies water to my chiller. You can choose mostly between two empirical models:

YorkCalc
CoolTools (crossflow).

I'm interested in knowing which one I should pick.

I have noted a couple of factual differences. First, YorkCalc uses 27 coefficients while CoolTools uses 35, but that's not a very useful piece of information to have.

More interestingly, the model boundaries differ quite significantly, YorkCalc accepts a broader range of conditions.

From the Engineering Reference, Tower Heat Rejection, Table 64:

Model Boundaries for CT

For the record, YorkCalc is the default in E+.

Does this mean YorkCalc is valid for a broader range of conditions, and would be better?

Do you have any guidance as why I should choose one or the other?

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answered 2016-02-05 12:35:35 -0600

aaron

888 ●1 ●7 https://github.com/aar...

Here's a plot comparing the models over a wide range of conditions. The range that I ran them over was:

Inlet drybulb from 40 to 100 deg-F
Inlet humidity ratio from 0.004 to 0.023
Inlet water temperature from 65 to 100 deg-F
Airflow ratio from 0.4 to 1.0
Waterflow ratio from 0.4 to 1.0

image description

The CoolTools model generally tended to underpredict the heat rejection rate compared to the YorkCalc model. I'm not sure of the reason for this. Here's a comparison of the two models vs a finite difference solution. Again, the CoolTools model tended to underpredict.

image description

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answered 2016-02-05 12:48:13 -0600

Archmage

7185 ●3 ●11

I suggest CoolingTower:VariableSpeed:Merkel instead. Those other formulations have not exactly stood the test of time. As you point out, the ranges are not broad enough, but also the coefficients are too difficult to obtain.

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I would caution that the Merkel model has a few known flaws, of which the largest is that it assumes no water loss through the tower. This causes the model to overpredict the heat transfer rate for a given NTU. The magnitude of this error has been discussed in a number of places (e.g. Braun Methodologies for the Design and Control of Central Cooling Plants, Benton/Feltzin A More Nearly Exact Representation of Cooling Tower Theory...) The Empirical models don't use any of Merke'ls simplifications.