Cooling water system options

[likesrocks]

Hi, this is my first post here. Looks like there are some great users on here.

I'm looking at options for cooling water from 120 to 80F in a desert climate. As such, there must be basically zero (or minimal) water use. So far, I'm only seeing air cooled chillers as an option. What I am after is basic design data, like kWh/ton-hr cooling so that I can calculate rough operating costs. Unfortunately, I have only found some design rules of thumb of around 0.28-0.3 kW/ton for chillers that I'm assuming are for HVAC use and cool water to ~50F. I read lots about increased condenser temps reducing the energy requirement, so I'm also assuming that my application will be more efficient than the above ROT...? I'm hoping someone here can point me in the right direction. I'm also interested in approximate order of magnitude capital cost differences relative to cooling tower. I'm looking at a very large cooling requirement, approximately 40,000 tons of cooling, so economy of scale is also a factor.

 

[cvanoverbeke]

I suggest calling any of the manufacturers who make air cooled condensers. Based on your comments, it appear that you do not need mechanical (compressor energy) to cool the water. If this is the case, the only kW/ton would from the condenser fans which would be really low, probably less thatn 0.1 kW/ton. I cannot see you cooling 120 to 80 F water with a standard compressor type system. The temps are too high for normal refrigeration pressues

Cvanoverbeke

 

[KiwiMace]

40,000RT warrants some good thinking about this problem.

Rough out some system options and send them to the chiller suppliers for a unit efficiency. Your load profile will also be a huge factor in operating cost.
I would recommend you send your design data to the major chiller manufacturers (Trane, Carrier, York to get started).

The available sizes, numbers reqd and resulting maintenance demands of air-cooled chillers may not prove too useful. 0.3 kW/RT sounds like a dream for air-cooled chillers, but then this is a very high supply temp. As cvanoverbeke pointed out, your required 80degF is well over normal range, but you can make 60degF water and mix it.

If the ambient temps allow, water cooled chillers with closed circuit towers/heat exchangers might work for you, and provide free-cooling operation during night/winter/other cold times.

 

[likesrocks]

Thanks! Kiwi, I have been doing a great deal of thinking about this problem as you suggest! I have learned that a chiller cannot operate at 80F - I think the idea of a tank that is maintained at ~55-60F by the chiller and then blended to achieve the desired final temp is probably the best one.

What I ended up doing for this preliminary operating cost analysis is using an air-cooled heat exchanger sized for the entire heat load up to 70F ambient, then I calculated the heat removed by the ACHE with 105F ambient. The remaining heat is the heat that must be removed by the chiller system during hot weather. I then took the hours that the chiller would need to be operated based on annual ambient temps and used 0.89 kW/RT since my previous 0.3 kW/RT reference was based on best practice water cooled chiller. I'm shocked at how much energy this works out to - 20 MW for the chillers!!! Obviously, my simple operating hours analysis will bound the power requirement on the high side since it assumes that the full load from the chiller is required during all hours that the chiller is required. However, the demand required is astonishing to me.

The issue with anything evaporative water cooled is that this is an area where the dry bulb is very low (Mojave desert) and water is simply not available on site.