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Modeling Cooling Tower without Chiller in IES-VE

asked 2014-10-19 23:40:25 -0600

bbrannon4 gravatar image

updated 2014-10-20 07:42:53 -0600

I'm trying to test a design that only uses a cooling tower with no chiller (actually there is also an option with a small pony chiller than would only cycle on when the cooling tower can't meet the load). The problem in the VE is that a chiller is necessary to have a cooling tower. But even trying to include the small pony chiller, it is set up that the chiller load conditions are primary. The only half though I have is to come up with a fake chiller curve that would result in a COP of 1 or 100% efficiency and then send the chiller power to a random fuel category to ignore.

Any ideas? Ben

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answered 2014-10-20 11:52:58 -0600

updated 2014-10-20 11:53:42 -0600


Have you considered using a Dedicated Waterside Economizer? It allows you to simulate a chilled water loop serving a cooling coil, and a cooling tower linked to the chilled water loop via a heat exchanger. The setup is limited to one cooling tower per cooling coil, so it might not work with your project; but it does allow for backup cooling from a chiller. See section 2.13 of the ApacheHVAC User's Guide for more info.

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Thanks Eric. That's a good idea I hadn't thought of (the details of the dedicated vs integrated waterside economizers are a little confusing to me). However the chillers are still sequences first in either case, where I want to cooling tower to first take care of all the load it can, and then the chiller only take over to supplement the peaks.

bbrannon4 gravatar image bbrannon4  ( 2014-10-24 01:29:59 -0600 )edit

answered 2014-12-15 03:35:58 -0600

Chienman gravatar image

An old hacky way that can be used for estimation is to use two coils in series. The first coil is the waterside economizer coil and the second is a traditional chiller that comes on when the waterside economizer can't reach capacity.

Since you can specify the capacity of any cooling coil in IES, this will limit the off-coil temperature at some point, and the downstream coil will come on. If you want to get super-hacky, then you can also use a generic cooling source with a high COP to estimate the use of fans and pumps on the waterside economizer.

If you are concerned about shutting off this coil when outdoor conditions (wet bulb or dewpoint) are not favorable, then you can use a second controller monitoring outdoor air that can be hooked up to the first coil's controller to shut it off when outdoor conditions would make it impossible to make sufficiently cold water by evaporation alone. This is a quicker and dirtier way if you are stuck.

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answered 2014-12-08 17:09:30 -0600

EmirPekdemir gravatar image

updated 2014-12-08 17:09:52 -0600

I am pretty sure if you use the pre-cooling tab (not heat rejection), then the cooling towers take care of the load before the chillers. Careful with the outdoor wetbulb temperature input though. Emir

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Do you have a reference for this? It seems that the pre-cooling CT and the chiller operate at the same time. Currently I am using the DWE suggested by Eric along with a backup CHW loop that has a pre-cooling CT. This isn't ideal because the DWE can only be used on a single coil, but then allows the CT to be used on it's own, then when it can no longer meet the load completely, it switches to a CHW loop with CT of equivalent specs.

bbrannon4 gravatar image bbrannon4  ( 2014-12-08 20:37:58 -0600 )edit

answered 2014-12-09 12:26:54 -0600

EmirPekdemir gravatar image

updated 2014-12-09 12:32:58 -0600

yes for the integrated WSE if the cooling tower doesn't have enough capacity OR the wet-bulb temperature you specified is exceeded you will see some chiller energy. If you have enough capacity in the cooling tower AND the wet-bulb temperature you specify is not exceed then there will be zero chiller energy. You will still have "heat rejection fans/pump energy" in both cases. Hope this helps.

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Asked: 2014-10-19 23:40:25 -0600

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Last updated: Dec 15 '14