Optimum Condenser Water Temperatures 2

[pbreeden]

While the accepted entering and leaving condenser water temperatures, for as long as I can remember are 29.5 deg C. and 36 deg C. (85 deg F. and 100 deg F.), I can find no justification for the selection of these values, either in terms of first cost or running cost.
They have been accepted for 30 years, there must have been some analysis.

Hoping for an answer

Peter B.

 

[ClydeMule]

Dunno if this is right but here is a SWAG:

1. 85 F. Since most water cooled systems are fed by tower water this is a good general value for available condenser water in the summer.

2. 100F. Accomadates more extreme conditions than 85F, or using a drycooler or closed loop evaporative cooler. With a 20TD between refrigerant and water, this puts the condensing temp @ 120 F. Most R22 systems have a max condensing (at AC conditions) around 140-150 F. Designing the equipment for 120F gives some cost savings (lower amp draw) and some "wiggle room" for extreme conditions.

I don't know where the 20F TD came from. I think what happens is that good working values from the past get turned into "standards". ARI conditions for Water Cooled chillers is 85F. Being written into a test standard makes a value semi permanent.

Just a note from experience, a water cooled system designed for 85-100f tower water installed onto 45-55 F chilled water looped instead may hae some operating issues. Basically your condenser is radically oversized so your water flow regulating valve has a heck of a time keeping constant head pressure.

Hope this helps.

Clyde

 

[Jetpuke]

85 ECWT is based on ARI conditions which use Atlanta Georgia weather. You should consider you local area ambient conditions. In the Northern state of the US we rate cooling towers at 83 ECWT as this is a more realistic ambient. One of my chiller manufacturers reps tells me that I need to rate the tower at 85 F but only because his chiller needs the high head. The three other major manufacturers say their chillers performance improves with the lower ECWT. So I had each rate various chillers at the two different conditions (without telling the reps why I wanted the info) and low and behold the three manufacturers' machines performed better.

Here are a few things to keep in mind. Cooling towers cost about $50 (US) per ton while the chillers they serve run five to ten times that amount. Slightly oversizing a cooling tower (more fill) will allow for a more efficiently operated chiller in most cases. Also a tower size for 83 ECWT with a 5 degree approach is basically the same as a tower rated at 85 degree F and a 7 degree approach.

 

[pbreeden]

Thanks Clydemule and Jetpuke for getting back to me.
What you say confirms my experience
The problem is that from a designers point of view, it takes too much time to establish an optimum delta T for each job and each equipment selection.
If this has not been addressed, it may be a good subject for a bright young person thinking of a subject for his/her final year engineering degree thesis.

PS
While I do speak both languages, my conversions must have been a little rusty. Standard condensing water temperatures used in Sydney are:
ENTERING CHILLER LEAVING CHILLER
29.5 deg. C, (85 deg. F.) 35 deg. C, (95 deg. F.)
COOLING TOWER
Ambient conditions are:
32 deg. C, (90 deg. F.) 24 deg. C, (75 deg. F.)

Hope this makes things a little clearer.

Regards,

 

[Cooky]

When specifying the DT for a chilled / condenser system, there are a couple of points that need consideration.

Firstly, a larger DT results in a lower fluid flow rate, this allows smaller pipe diameters and lower pump ratings. Offering savings to the installation cost.

Secondly, with the lower fluid flow rate, the rate of heat transfer is decreased (as the flow characteristics change from turbulant to laminar). This de-rates the heat exchanger within the chiller, the manufacturer must then select a larger chiller to deliver the specified cooling capacity.

 

[VSSriram]

To add to what Cooky has said. A lower flow rate (GPM/TR) across the condenser, would mean a higher condensing temperature and higher lift requirement for the compressor. If I am right, Trane advocates almost 2 GPM/TR through condenser (especially for centrifugal chillers). I have read some articles from Trane on this - not sure whether this is a marketing gimmick or not. Probably because, Trane chillers are quite efficient at high lift conditions as compared to others like Carrier or York.

More than anything else, one should look at the overall cost including first and running cost of chiller, pumps, cooling tower, pipe sizes, insulation, valves, etc., etc.

 

[simonho]

The 85F (29.4C) cooling water temperature happens to be the ARI rating condition for chiller packages. ARI also stipulates 3 gpm/ton (53L/s/MW) condenser flow for this rating condition. The purpose of the ARI standard (550/590-98) to to ensure that there is a benchmark by which chiller packages can be compared. It is by no means a manual for good design from a first cost or operating cost perspective.

Of late, low flow designs in both the CHW and CDW has been more and more practised due to the pumping power savings that can be had. You need to look at it from an overall chiller plant perspective. For the condenser side, not only will low flow save pumping power, but you then have the opportunity to squeeze more out of the existing tower selection. Lower flow rates gives you a couple of opportunities here to either allow colder condneser water back to the chiller with the same tower, or downsize the tower to meet the same load.

Yes, this will have some effect on chiller performance, but remember to count the total power and energy consumption for all chiller plant components.

 

[abcmex]

pbredeen,
There is no silver bullet as jetpuke said. :-(
Each location has a different maximal enviroment temperature, from this temperature may depend the ECWT. A low ECWT probably will give you a smaller chiller unit or a smaller power bill if you have already an installation.
If Sidney is cooler than 85 F you may use a lower value, your local CT supplier will give you information about a realistic CTW temperature. If you work only for Sidney you may need to do this only once. Check twice if you can get 29.5 deg C out of an enviroment with 32 deg C.

VSSriram/simonho,
Trane argues about 2 gpm/ton and York argues for 3 gpm/ton. You may find more info at the respective Internet site. It is true, you should consider all power consumptions ( CT-Ventilator, CT-Pump, Chiller compressor) to search fo the best performance.

 

[PacificSteve]

Peter B,

You start a very good thread. In the world of "commodity based" low bid design services, it is simple to throw out 85/95 as if something sent down from God and etched in stone. I agree--it is bogus, and primarily useful for specifying equipment in a simple way, quickly.

I would like to add just this one thought to the already many good answers---
Chillers are kind of like electric motors. Electric motors life cycle operating costs are comprised of about 4% initial purchase cost, and 96% future energy cost. A $100,000 chiller may use $2M in electricity over its life, a better $125,000 chiller may use $1.25M in electricity over its life. Especially a chiller that is fed cold CWT, say 75F.

My average wet bulb in Hawaii is 68.6 F, and I like to "right size" the towers to produce a 2F approach, 5F max.

Low bid design companies can't possibly spend the time to optimize. And owners are very attracted by low first cost. Obviously, education of owners, when possible, is the way to go, and then enlist the services of an "energy services company" who is intimately familiar with the proper amount of analysis to optimize the system selection based on design cost, construction cost, electricity cost, the owner's Return On Investment ROI criteria, and control complexity and likelihood of persistence of savings.

Whew! that was a mouthful. Now do that, any you will have a project that you can really be proud of.

PacificSteve

 

[psighi]

From my experience as a former cooling tower representative and now design engineer, my attitude toward most A/C units is: the only true variable in optimization of efficiency is condenser condition. Lower the condensing temperature, the lower the lift and the more efficient the machine. That being said, optimizing the cooling tower or condenser will improve efficiency by 1-2% per degree reduction.
Lowering condenser flow rate will also improve efficiency as its approach is lowered. The lower the approach the lower the condensing temperatures.
Combining both will provide dramatic improvement. I generally like to talk to the chiller manufacture and request larger shells (lower approach) with externally enhanced tubes.
Caution: many screw compressors do not like too cold of a condenser water. Consult the manufacture.

 

[pbreeden]

Thanks so much for all responses. The answers were not exactly what I wanted to hear, as they probably involve more work.
My interpretation of all the responses is that ARI conditions are fine for a normal run of the mill, owner occupied project, as these represent the most efficient operating conditions at local ambients.
On the other hand, in a bidding situation, when operating costs are not an issue, and not part of the client brief, the condensing temperatures can be raised until the increased chiller power consumption equals the reduced pumping and cooling tower fan power consumption.
This can be done by requesting multiple quotes from manufactures, and one would hope the savings generated would cover the expense of doing the optomisation.

As I said before, a good subject for a final year thesis, but too time consuming for us on the coal face.

Thanks to all

 

[alfray]

If you are talking about condenser water for water chillers, the appropriate doc would ARI Standard 550/590. The temps are selected based upon economics with respect to the cooling tower's approach temperature and typical weather data.