Chilled Water Systems for Building HVAC - Achieving High Temp. Rise 5

[JEFF1]

I work at an airport with a large chilled water system, 2.2 million gallons of chilled water storage, 5 chillers @1150 tons, and a design water temperature rise of 18 F (starting with 42 F water). We've learned some valuable things about achieving a high water temperature rise. I'm interested in knowing if anyone else has ideas that they would like to share on this.<br>

 

[cavelle]

What type of heat exchanger designs are being used?

 

[JEFF1]

The only heat exchangers in this system are the heat exchangers in the chillers themselves and the heat exchangers in the air handling equipment. The air handling equipment is, essentially, air conditioners that used chilled water to cool the air. When you're speaking of heat exchangers, I assume that you're referring to the air handler coils.<br>
<br>
These coils are basically similar to coils for more conventional air handling equipment. To achieve the higher temperature rise, the aluminum fins are spaced tighter (say 12 or more fins per inch), and there are a greater number of rows (say at least six) of copper tubes that carry the aluminum fins. The only tricky part about this is that the high water temperature rise means more heat per gallon, and lower GPM. This means that the water velocity in the tube gets lower. When we first got into this, there was concern about laminar flow and poor heat transfer. For the 18 F temperature rise in this system, we haven't seen this as a problem.<br>
<br>
Thanks for your interest.

 

[cavelle]

What shape are the fins?<br>
<br>
I am a student in mechanical engineering and I am looking into starting my own business when I get out of school. Most of my classes were thermodynamics and fluid mechanics classes. I did a lot of cycle analysis with numerical simulations. If you have any advice for me I would greatly appreciate it.

 

[sigmapsi]

What about dehumidification? Is there a loss in dehumidification control?<br>
<br>
I have seen problems with loss of dehumidification control on large chiller systems when the chilled water temperature is reset up only a couple of degrees.<br>
<br>
I also am wondering how much supply temperature loss there is. I assume there is a lot of distribution pipe, and 42 degrees at the ahu coil might require 40 degrees at the chiller?

 

[JEFF1]

The fins on typical air handler chilled water coils are either circular (older style, infrequently sold today)or "plate-fin", meaning that many tubes pass through large fins that are in the shape of a plate. The"plate-fins" are sometimes enhanced in geometry so that the create turbulence and thus increase heat transfer. There is a trade-off; what increases turbulence and heat transfer also increases air friction and the resulting air pressure drop across the coil.<br>
<br>
It's hard to offer career advice. People have different constraints with family, children, etc. Many engineers today have higher degrees, and it seems to me that the people who have higher degrees have a better chance of doing real engineering work as opposed to project management or similar. I recently obtained my professional engineering license, and I think that this also opens the door to better work. But you have to weigh all things against your family constraints, etc.<br>
<br>
One area that I'm interested in that may have some future is magnetic refrigeration. Do you know anything about it? Some people say that someday it may replace vapor compression refrigeration widely. But not all of these things actually work out.

 

[JEFF1]

R.e. dehumdification, yes, when the supply water temperature gets high enough, you can lose control of humidity in the buildings served by the system. Some buildings and some rooms require more dehumidification than others. So chilled water flow cannot be limited as much in these areas. Chilled water temperature rise, as I use the term, is the difference between return water temperature and supply water temperature. When flow is limited, water temperature rise increases. Chilled water reset, which is raising the supply water temperature, generally does not give a higher water temperature rise. This is because the higher supply water temperature has less potential to remove humidity. Hence the latent heat removal capacity is reduced, and less heat enters the water.<br>
<br>
The first step in achieving high water temperature rise in a chilled water system is selecting cooling coils that will give a high water temperature rise.<br>
<br>
Back to dehumidification, our facility is not the one with the highest water temperature rise that dehumidifies acceptably well. Dallas Fort Worth airport is reported to have a chilled water system with a water temperature rise well above 20 F. However, their climate may be drier, and dehumidification may not be as much of an issue.

 

[LeaderP]

Jeff:<br>
<br>
I've been collecting technical articles on this subject for several years. If you'd like me to send them to you, please advise.

 

[JEFF1]

Our facility is pretty well completed, and our primary interest is in making what we have work. I'd still be interested. I'm assuming that they would have to be mailed, as opposed to e-mailed. If you let me know, I'll send a mailing address if required.<br>
<br>
My experience has been that it's not particularly difficult to design a chilled water system that achieves good water temperature rise if you follow some basic principles. But to get the piping built without mistakes and to get the controls doing what they should do is more difficult. My main interest was in practical aspects of getting a system working and keeping it working. But I'd be happy to share what I've picked up r.e. design.

 

[Jasem]

I beleive this is the thermal chilled water storage system.
I would like to have a simple digram of the system if possible.
I also appriciate to get more inforamtion, may be web sites, on the thermal storage system (TES)
address
Projacs
P.O box 93401 Riyadh 11673
Riyadh, Ra
Saudi Arabia

Regards
Jasem

 

[Khaled200]

What about the chillers Kw/ton at the design chilled water supply and return temperatures?