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Multiple cooling towers or one cooling tower with multiple cells?

asked 2016-03-04 07:59:06 -0500

I've often got multiple identical cooling towers in my projects. I'm always wondering whether I should create multiple cooling towers and put each of them on a branch, or use a single cooling tower with multiple cells.

I often end up creating multiple ones just because I'm also not quite sure what I should input in certain fields (the value for one cell or for the whole tower). But reading the engineering reference guide (see link below), I get the feeling I should really be using one tower with multiple cells.

Here's a dummy example to get the ball rolling:

Example with two identical CTs

I have a few questions, but I don't think any warrant an additional separate question:

  • What should I input for Minimum Airflow Ratio? Is the rest correct?
  • My main question: Is there any advantages in using 2 identical CT or one CT with multiple cells? Which are they?
  • Bonus question: if using a multiple cell tower, what the advantages of using MinimalCell or MaximalCell (the engineering reference throws me off... I'd have thought maximal cell would be more efficient due to fan affinity law but I'm not sure anymore)

References:

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answered 2016-03-04 14:01:55 -0500

This only applies to variable speed cooling towers sharing the same basin. If the the towers don't share the same basin the towers should be modeled as separate objects:

  1. Combining the towers and using 0.1 for the minimum flow rate would be fine.

  2. As long as the towers are sharing the same basin and are variable speed I don't imagine there would be any benefit either way. It's probable worth running a test in E+ comparing the two options.

  3. Yes, MaximalCel control saves more energy because of the exact reason you mentioned - the power consumption of the fans depends upon the cube (or a little less) of the fan speed. MaximalCel saves energy under almost all operating conditions if a variable speed fan is used. Further energy savings can be found by floating the tower leaving water temperature control to maintain a constant cooling tower approach temperature.

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Thanks Lincoln this is helpful. For the basin thing, why does it matter? Only for basin heaters, or is there more to it?

Julien Marrec gravatar imageJulien Marrec ( 2016-03-05 05:16:16 -0500 )edit

It just comes down to what load the cooling towers are serving and how they're being controlled. If they share the same basin then they will be controlled/staged in concert to maintain the desired condenser leaving temperature setpoint. Sometimes the cooling towers are serving different loads and can't be modeled as sharing the same basin. I just finished a project where the condenser plant had four cooling towers sitting adjacent to each other with two separate basins and therefore two separate condenser loops. Each condenser loop may have different sequences so they shouldn't be combined.

Lincoln gravatar imageLincoln ( 2016-03-08 17:10:21 -0500 )edit
1

answered 2016-03-05 13:03:10 -0500

301_Hours gravatar image

Couple of other comments to add.

For ASHRAE 90.1 compliance it should be 1 cell per tower and 1 tower per chiller in a separate loop. Ie. don't put 2 separate towers on the same loop serving 2 separate chillers.

In reality towers can only have about 50% water flow in order to get proper distribution of cooling tower water over the tower fill. Please keep this in mind for condenser water pumps running on VFDs.

Running maximum cells vs. minimum cells makes a difference in fan power. One 10 Hp fan running uses more energy than two 10 Hp fans running at 50% due to the fan affinity laws.

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Can you please quote specific sources for the ASHRAE baseline comment?

Julien Marrec gravatar imageJulien Marrec ( 2016-03-25 02:23:14 -0500 )edit

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Asked: 2016-03-04 07:59:06 -0500

Seen: 1,840 times

Last updated: Mar 05 '16