chiller - design questions

I am hoping I could get some help with designing a water chiller system. I know this is a boiler forum, but the design idea's are a lot alike and quite honestly can't find anywhere else to ask these questions. I will have a 5 ton AC condensor hooked up to a water coil then have air to water HX on top of each furnace/blower. The design of the house either requires 4-5 small HVAC systems or to seriously chop up the house for HVAC ducting. Using one main chiller will allow one AC condensor to drive all 4-5 of the small HVAC systems without having the need to have 4-5 AC condensors sitting outside.

Due to the low delta temeratures I can't run a water to water heat exchanger, which means I need a pressurized storage. Operating temp of the water will be about 38-50 *F. The AC condensor would be run on a thermostat on the buffer tank, the room thermostats would only turn on their zone circulation pump.


I have narrowed it down to two different designs. The first I do not know the name but it is pictured on page 11 of the following link. I would have a 40-100 gallon water heater inline on the feed to the chiller coil, this would be to prevent the AC condensor from short cycling. Problem with this design is I have to run the chiller circulator pump whenever any zone is running. The plus side is I can use a cheap water heater.
www.caleffi.us/en_US/caleffi/Details/News/.../HydSep102508.pdf

The second design would be to use a very large buffer tank that acts as a hydraulic seperator. All zones as well as the main chiller circuit would dump directly into this tank. This design would allow for the zone pumps to run with the chiller pump being off. The downside is I would need a non-water heater tank to allow either for multiple in/out taps or 1.5" plus taps.

The tank is more of just a buffer, not a storage. 100 gallons will allow for a 8-9 minute recharge from 50 to 40 *F and theoretically a cooling time of the same length without the AC condensor running. I probably would not go over 250 gallons. I had run across this type of tank and figured it might work pretty well for design #2. http://www.ebay.com/itm/330688268931?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

Thoughts, idea's, comments that I am just plain crazy?

PS. I already have a 5 ton AC condensor, a chiller coil (Doucette Industries CC-500, thank god I bought that when copper was cheaper, it is a 110lb chunk of all copper), and Magic Aire water to air heat exchangers. Also have all the pumps (Taco bronze), just need to figure out a final design and find a buffer tank.

hheaddon said:

the chiller coil will need at least 3gpm per ton of ac.I would research air cooled chillers to get a better idea of the controls and theory of operation.also the coils you use to each zone,are they set up chilled water use, as in condensate drain pans!as a hvac guy it could be done.not real easy though. Has anyone done any kind of heat loss or gain design on your house to see what amount of ac you might need.

Click to expand...

Have a Taco 0011 for the chiller coil (0-28gpm) and a Taco 007 for each water-to-air coil. The evaporator coils do have condensate pans. I have oversized the evap coil so that I should be able to run the chiller at a slightly higher target chilled water temperature to gain a little efficency at the condensor, and since there is a large water buffer for the condensor to cool I should't have to worry about an oversized evap coil causing the unit to short cycle. I should aso be able to indirectly control the furnace fan run time (and humidity) by controlling the thermostat for the water temp. Turning the water temp up a couple degree's would be equlivent to having a smaller coil, turning the water temp down would do the opposite. I guess it would be similar to a set-back thermostat for a boiler (if I understand those right, I have never had or run a boiler before).

I currently have 6 tons of cooling, during the heat of the summer two run 100% and another will cycle at about 75% duty cycle.. I have done heat loss calculations myself and come up with the 4.5 to 5.5 ton range (HVAC-CALC and verified with a Manual-J done by myself). I have insulated some and also plan to add conditioned floor space, the calculations say these two will even each other out and the cooling load will remain the same.

I am in the proccess of putting in Bryant Evolution furnaces with the zoned Carrier Infinity/Bryant Evolution controls. I have a large house so if it is undersized slightly it will be easy to turn the target temperature up in a few unused rooms to lessen the cooling load.

Do you have any suggestions/opinions? Also, do you have any sources to research this topic further?

It doesn't feel like I have a handle on it yet, but thank you.

I did thing about the 1 boiler for heat but I was really gunning for the Bryant/Carrier control system, just so happened that I could get the furnaces cheaper then their AHU. I still might do that, nothing preventing me from just using the furnaces as an AHU. I do have dreams of radiant flooring...but soooo expensive to retrofit.

I will re-think the control strategy. What you said makes sense, I don't want the AC condenser running to keep a 100+ gallon tank cool (latent heat gain) without any cooling loads. But on the other hand, if the tank and the system were to warm up too much, cooling it down by 40-50 degree's might take the better part of an hour...too long to wait for the AC system to "start working" in the eyes of my girlfriend. Perhaps have a low outside temp cutoff point, say 55*, that the AC unit will not run below that point. In the 55*+ range, I just eat the extra cost of cooling the system to have it ready to use.

I do have a masonry house with 18" thick brick walls with a lot of south facing windows, sometimes during the year the house starts to get real hot after it gets dark and the outside temp starts to drop, my guess is the masonry has finally equalized in temp and is radiating the heat inside the house. Even with DX units it can get rather annoying. Many times I find myself running the AC units when it is 50* outside, for over a day or two straight, just cause my walls are hot from the 90*+ temps before the cold snap.

I like the idea of the hydraulic separator, simpler design. Probably what I was thinking in my second design option, just didn't know by what name to call it.

For piping, since it is chilled water only, I was contemplating using PVC and/or PEX. Thoughts?

Thank you greatly for the help.

I have found a 250 gallon tank that I am looking at buying to use as a buffer/hydraulic seperator, should give a 20 minute run time for a 10 degree delta.

The tank has a 6" flange on top and bottom. Chiller feed would be at the top, air coil feeds would be at the bottom. I am thinking of making a flange with several pipe fitting hookups on it, should I do anything special on the inside portion of the flange? IE, should I leave the inlets/outlets dumping on the vertical axis or should I put a short extension with a 90* fitting so that they dump into the side of the tank?

I could see the straight intlet/outlet being simpler but it would cause more mixing and less stratification since the returns to the tank would be pointing directly at the feeds at the other side of the tank. The 90* bend inside the tank might cause the tank to stratify a little bit more.

Straight feed to get the cooler water at the bottom for the air coils and hotter water at the top for the chiller coil, then 90* angled returns inside the tank to try and keep the hot on the top and cold on the bottom?

Thoughts?