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# Chiller:Electric:EIR applicable for air-cooled chiller if t_oat < t_chws?

Hello all,

I try to use the performance curves of the Chiller:Electric:EIR model to describe the operation of an air-cooled chiller in a non-linear optimization program.

However, I noticed that the EIR_FT (using the default values of https://www.comnet.org/index.php/382-... for an air-cooled scroll chiller rated at standard conditions) is increasing once the ambient temperature (t_oat) is below the chilled water supply temperature (t_chws) (see figure below with t_chws = 15°C). I was wondering whether this represents the real behaviour of a chiller or whether the EIR_FT curve is not applicable for t_oat below the t_chws, especially since I've seen other approaches assuming the EIR to descrease (or rather the COP to increase) for t_oat < t_chws. Because I'm using the curves for a chiller in an (hypothetical) industrial cooling application, chilled water is required even for low ambient temperatures.

Related to the above, also CAP_FT reaches large values of about ~1.6 for the given application and low ambient conditions and I was wondering if this is still "realistic" or if in general there are limits for t_chws and t_oat using the EIR_FT and CAP_FT curves (other than specified operating conditions by a manufacturer)?

Thanks!

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It’s a rare modeler who actually investigates the characteristics of part load curves. Good for you to have considered that. Chillers can be designed and selected for operation at very cold temperatures, and you need to find or create curves which represent that operation. I suppose you could continue using the hypothetical curve set, since it allows operation at low temperatures, but it may be better to get data for a real chiller which is designed to operate at those temperatures.

ps, you might consider adding a dry cooler which operates (only) at outdoor temperature below ~0C. pps, Since your chilled water will actually be a glycol solution in order to operate at hose temperatures, you should consider making your fluid a glycol concentration instead of standard water.

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