Sizing the chiller

Hi folks, I just rediscovered this site. I really hope there are enough coldwater people in the world by now to keep one forum alive with a few posts per day. And it it looks promising here, for the first time! Anyway I would like to present my little idea about how to size chillers for coldwater tanks. This could be wrong, but it is based on practical experience with my 540 liters freshwater tank and 1000 liters saltwater. And any other method I have seen out there has definitely been wronger.

There are 3 factors deciding the size of the chiller:
1. The warm water chiller's reduced efficiency due to cold water.
2. Energy input from electrical devices.
3. Cold loss to the environment.

Assumptions about the system:
1. Is to run at 5C (41F) in the coldest winter period.
2. Well insulated with only one or two viewing sides with double glass (this is fairly easy to make, a must really).

Efficiency (f): I assume the chiller performs 70% of rated cooling effect. I measured the effect uptake of both of my chillers to be reduced by 22% at 5C. So the efficiency must have been at least reduced by that.

Energy input (Ei). Just add up all the pumps and lighting wattage (corrected for on time. Typically 50% for the lights). You may want to measure with a wattmeter rather than just trusting the specs.

Cold loss (El): This can be hard to estimate. But you can base it on my tank: I estimate the coldloss of my 1000 liters (265 US gallons) tank to be around 450 Watts at 5C. It had insulated walls and double glass in front. The sump was insulated too, but not the piping, which ran for roughly 8 meters through a rather hot environment. It had an external skimmer. Both the skimmer and the piping dripped during the coldest part of the year. Insulation is everything in this context.

The chiller's actual performance must be equal to energy input. If Ecm is the minimum specified performance we have:

Ecm * f = Ei + El
Ecm = (Ei+ El) / f
But this is only the minimum requiring the chiller to run constantly. We would like the chiller to run only half the time, and leave som room for error. I call this preferred capacity Ec:
Ec = 2 * Ecm

For my 1000 liters, this would be Ecm = (440 Watts + 450 Watts) / 0.7 = 1271 Watts. Ec = 2 * 1271 Watts = 2542 Watts. Since 1 HP of cooling is 3500 Watts, I would prefer roughly a ¾ HP chiller. But a ½ HP would work too.

- Jon Olav