First, I would like to preface this post by stating that this project may be the result of insanity on my part, or it will at least lead to it. I first touched on the idea of a chiller a year ago, and after much research and number crunching I'd like to run what I've come up with by folks who are better informed than I am.
I would like to install a chiller in a 4-zone building I own. The maximum cooling load is 10 tons. The maximum cooling loads for each zone are 2 tons, 2.5 tons, 2.5 tons, and 3 tons. There is no 3-phase power. I am planning on using two compressors, water cooled condensers, and a 10-ton capacity Amcot cooling tower.
What I am looking at is using 2 compressors, Compressor A would be a two-stage 5 ton Copeland scroll and Compressor B would be a single-stage Copeland 5 ton scroll. I would like to stage them to better match the load in order to avoid short cycling, particularly when not all zones are calling for air conditioning. The stages would be
Compressor A, stage 1
Compressor A, stage 2
Compressor A, stage 1 & Compressor B
Compressor A, stage 2 & Compressor B
I am looking at the Johnson Controls System 450 to control the compressors, unless any of you think there is a better option.
I have some questions, however, regarding the best way to circulate water through the system:
Initially, I was thinking of using a buffer tank with 4 tappings, supply and return for the chiller, and supply and return for the system. Under this setup, I was thinking of controlling the operation of the compressors based on the temperature of the water in the tank, with the system stepping up through the stages the farther away from the set temperature the water in the tank became. Under this scenario, I would need a circulator between the tank and the chiller, and either a variable speed circulator between the tank and the system and zone valves for each of the 4 zones, or a smaller circulator for each of the 4 zones.
Now, I am thinking of using primary/secondary pumping with the chiller, 2-tapping buffer tank, and a variable speed circulator on the primary loop and smaller circulators for each of the 4 zones. Given the small size of the system, however, would this be overkill? Is there an even better way of doing this?
I have done some math (cower in the basement at this point, folks) in order to determine my gpm requirements for each of the 4 zones.
Assuming a delta T of 12 degrees,
Zone A, 2-tons, 4 gpm.
Zone B, 2.5-tons, 5 gpm
Zone C, 2.5-tons, 5 gpm
Zone D, 3-tons 6 gpm
If the delta T is reduced to 10 degrees,
Zone A, 2-tons, 4.8 gpm
Zone B, 2.5-tons, 6 gpm
Zone C, 2.5-tons, 6 gpm
Zone D, 3-tons 7.2 gpm
As for the cooling tower side...
If memory serves me correctly, when the system is running at its full capacity of 10 tons, the cooling tower would need 30 gpm flow. If the system is running at a lower stage, like 5 tons instead of the full 10, could the rate of flow to the tower be reduced as well?
What might the pros recommend for a good variable speed (variable by delta T) ECM circulator for use on a chilled water system? The pipe runs would be short (going to 4 furnaces in 2 mechanical rooms a few feet away from each other in the basement.
Clear as mud?