Why Chilled Water Supply Should be Under 5°C/42°F? 1

[sutan alam]

Hi All,
Currently, I'm working at HVAC system which delivered 3°C of chilled water to the HVAC system with 8-9°C return (5-6°C/11°F delta temperature).
The HVAC spec manual specifies that chilled water supply should be at 6°C/43°F and return at 11°C/53°F (5°C/11°F delta temperature)
though the delta temperature is at standard value, engineers here want to raise the chilled water supply temperature in order to save energy. maybe it sounds plausible but wouldn't be any difference if the delta temperature remain the same?
and what's the effect to have such low operating temperature of chilled water?

 

[EnergyProfessional]

You can design for anything you want. you need to analyze ALL equipment and the needs of the air stream. You need to do an energy simulation and chiller plant analysis to find out the best.
Newer systems tend to have higher dT to save pump-energy. Lower supply temp can increase dT, but requires more chiller energy. If that cancels out pump savings, depends on chiller COP. Or if you have ice storage. Larger dT decreases pipe size, but may increase coil size. Water also needs to be cold enough to dehumidify. It all depends on specifics. Are you cooling desert air to 80°F, or Florida air to 55°F? Are you cooling beer or curing concrete? But the general recommendation for better economics seems to be higher dT.

 

[Viet Mai]

Hi Sutan,

Low chilled supply temperature means LIFT increase therefore reduce chiller efficiency and increase electrical power.

 

[LittleInch]

I'm no HVAAC expert, but my understanding is that to get to 3C without the water icing on the expander, you need more area / less flow as the chiller tempt can't really go below zero for simple water systems. A 6C output allows more efficient use of the chiller to water HX.

At least that's my take on it....

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

 

[BenMacier]

Why do they need to cool the air that much?



* Finding a solution is great * Knowing how to implement it is fantastic * Believing it is the only one and best is naive ?

 

[GT-EGR]

Low operating temperature for chilled water is regularly used when you need want dehumidification capabilities from a CHW coil. Since it is a lower temperature, it can cool air to a much lower temperature and extract more moisture out of the air. If the chiller serves a process and not HVAC the same may be true that it needed the lower temp.

The only way to find out the impact is to analyze all the things it serves - and determine if the outcome they are designed for can still be obtained with higher entering chilled water temps.

The upside will be as they say, better KW/ton, and also you can get more total capacity out of your chiller plant. But if the items served don’t function properly then you shouldn’t do it.

 

[sutan alam]

Thank you for the responses,

I gladly appreciate all of the answers. for more context here we use a falling film chiller with ammonia as refrigerant.
and yes, the AHU is also used to dehumidify the air but the AHUs are designed to receive 6°C of chilled water(I looked up on its original spec sheet) but we supply 3°C due to the initial design of the chiller. that's why people here want to change it.

I have follow up question, Why does cooling the water from 11°C to 6°C would require less compressor work than cooling from 8°C to 3°C even though it extracted roughly the same amount of heat/BTU?

 

[EnergyProfessional]

The chiller has to "lift" and the relevant temperatures are the ambient temperature the heat gets rejected to and the chilled water. The higher the lift (dT), the less efficient. Temperature and pressure in refrigerant are related. the chiller really only cares about refrigerant temperature, but those correlate with the chilled water and ambient temperature. So if ambient is 40°C, the refrigerant is 50°C (you need a dT to transfer heat). and if the chilled water is 3°C, refrigerant needs to be 0°C. At 0°C and 50°C the refrigerant has a certain pressure and pressure difference the compressor needs to provide. So here 50K. If your chilled water is 6°C, the refrigerant on the low side only needs to be 3°C. So only 47K to "lift". I'm making up the numbers, this is just to demonstrate.

that is how a water-cooled chiller saves energy. You ambient drybub temperature at 40°C could mean a wetbulb temperature of 30°C (this depends on your climate, a LOT!). So instead the refrigerant being heated to 50°C, it only needs to be 40°C and yo have less lift. Again, all numbers made up for making a point.

So a given chiller will save energy with less lift. But newer chillers are much more efficient to begin with. But if you increase the chilled water temp, you may have to increase water flowrate and use more pump energy. it all depends on how the system was designed and sized and how it actually works. There are safety margins and you can try out if raising supply to 6°C still meets your demand.

I'm not sure I explained it well. But google some charts that show pressure/temperature of refrigerant, and chiller COP based on lift etc. Any heatpump efficiency will depend on the operating temperatures.

 

[sutan alam]

Thank you so much for the response. you all are so helpful,
You explained it very well. I get the point.

 

[MedicineEng]

If the pumps and pipes are big enough, and since your AHU coils are designed for 6C chilled water, you should be able to raise the chilled water temp to 6C without a problem and with some energy saving along the way.