Difference temperature in supply and discharge water in a chiller

[amfarhoudi]

Hi
I am a new engineer in a consultant company. A chiller manufacturer company suggest us to Manufacture a chiller which is work with 6 degree celsius as a supply and 15 degree as a return to evaporator and it said that it is goning to be better as efficiency.
That's my question if it is better or not?! And how does it affect on air handling units?!

Thanks

 

[GT-EGR]

It’s not clear what your question or point of comparison is but generally speaking:

The higher the operating temperatures of your chilled water system, especially your return water temperature - the more efficient your chiller will be as far as heat exchanged compared to electrical consumption

But having a higher chilled water temperature and/or a larger delta T (difference in supply and return) will means you will need bigger coils in your air handlers to exchange the same amount of heat and also you may not be able to reach discharge air temperatures from your air handler as cold as you may want

 

[amfarhoudi]

Thanks for reply
As you know according to AHRI the inlet and outlet chilled water temperature to evaporator are 44°F (6°C) and 54°F (12°C) respectively. The question is if the outlet temperature of chilled water will change to 15°C how does it affect on efficiency of chiller, heat exchanger (evaporator), electrical consumption (compressor), pumps, pipes and finnaly air handling units

Thanks

 

[GT-EGR]

This is a complicated question and depends on the chiller you have. Hopefully the manufacturer has performance and efficiency curves showing what your chiller output is when you increase your chiller delta T. You really just have to look through that to find your answer as to whether or not your chiller performance improves by operating at 44-60 EWT/LWT temps, and reducing your flow accordingly.

AHRI conditions you mention are just to provide like for like comparisons between chillers, but not to be used to determine your actual operating efficiency.

You will definitely saving pump energy, since higher delta T means less flow to cover the same load. As far as the chiller goes, I'd defer to the chiller catalogue which hopefully will demonstrate the difference by looking through all their performance tables. Chiller efficiency generally speaking is typically maintained by keeping your chiller or chillers running in the 65-85% capacity range, and is definitely improved by raising your entering water temperature. But as far as delta T goes, I don't have that boiler plate answer for you - since each chiller is unique as far as compressor quantities, compressor modulation capabilities, etc.

 

[jrrocks]

Chiller Outlet temperature should be 44°F (6.7°C)as per AHRI.

Capacity calculation of chiller is based on dT (differential of Supply and return temperature) and manufacturer usually provide part load characteristic of chiller and efficiency at that load. If outlet temperature of your chiller is 15°C (59 F), what will be the return (Inlet) temperature? will this 15°C (59 F) sufficient to meet cooling of building?

AHRI req. 44°F chilled water supply temperature at 2.4 gpm per Ton. I believe, AHRI did required inlet temperature to be 54°F (12°C), which you have mentioned in your statement.

 

[willard3]

A chiller is a carnot cycle and the closer the chiller inlet and outlet temperatures, the greater the efficiency.

 

[jrrocks]

Closer the chiller inlet and outlet temperature means, there is no cooling load (inside the building). So, it is running on 5% or maybe 10% of load (min set-point). How it could be efficient? Please explain.

 

[LittleInch]

jrrocks - Not really - it just means there is more water flowing around the system for the same load surely?

Back to the OP - the vendor says it is going to be "better"... Better than what?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[amfarhoudi]

Sorry I made mistake in my question I should have swapped the inlet and outlet temperature
Outlet temperature, (supply temperature) 44°F (6.7°C)
Inlet temperature, (return water) 59°F (15°C)

 

[lilliput1]

With hospitals that are always expanding we would lower the chilled water supply temperature to 42F and add more chiller capacity (at 42F supply). This is to not have to increase existing pipe sizes. With 42F supply existing coils (sized for 44F supply) would require about 20% less gpm to handle the same cooling load.

 

[IRstuff]

What was the alternative?

For air conditioners, it's often claimed that undersizing relative to general rules of thumb about tons/square foot is "better" because the A/C can run longer and take advantage of the fact that the plumbing is cooled and stays cool longer, resulting in fewer stop/start cycles. For something that is mostly continuously running, it depends on the relative efficiency curve of the chiller vs. cooling capacity. The result ought to be a cost-surface, but it's likely to the relatively flat near the peak efficiency.

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