water chiller-sizing 1

[moideen]

I am design a water chiller for 1320 gallon tank. The ambient will reach to 50c (122F) on summer. The Required water temperature 4c has to be reached within 70 minutes.1320 divided by 70=18 gpm. I assume if ambient is 122F then the water will be 104F.so I calculate the chiller capacity GPM*DELTA T/24(20*65/24=54 ton. Should I consider any other things? Thanks for the advice.

thnx
moideen-dubai

 

[EnergyProfessional]

You should consider the tank insulation and how it gains heat from ambient.
also consider what that tank does, does it buffer chilled water? Or what? You likely want to return the water from top of the tank and supply it to the bottom without mixing up all the water. but also depends on how you draw the water for your actual use.

You say ambient being 122°F makes the water start at 104°F. not sure how you come up with that, abut after some time the water in the tank will equal ambient.

 

[moideen]

Thanks your reply. Tank is insulated. Entering temperature is my guess, because when the ambient reach to 122f the tank water may reach to around 104F.there is no proper calculation. I need help.it is not buffer chilled water. The chilled water outlet connected to the same tank. When reach the 39F the tank will be emptied and refill again. The same process will continue.

 

[EnergyProfessional]

first, i'd stay in the same unit system. you seem to use IP and SI units, use one or the other.
Second, it is dangerous to estimate the temperatures, try to get a good engineering assumption. where does the water come from?

 

[moideen]

Ok I follow SI unit. Municipality water, getting in desert area. Regardless the other specifications what will be the capacity of machine if entering temperature 104F.

 

[LittleInch]

Your calculation is quite basic.

You seem to be working on the assumption that pumping the whole volume through the chiller will gradually lower the temperature. However the inlet condition to your chiller will keep changing, gradually lowering the inlet temperature. Therefore your chiller will gradually work less and less well. You can't lower the outlet temp below 4c and even that is quite difficult as there is only a very low temperature gradient without the water freezing.

I would look art the total gets loss you need to get to 4c and use Ann internal coil.

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

 

[moideen]

LittleInch yes your remark is notable .being same water circulating the internal temperature gradually decreasing.in this condition the chiller size to be varied from the basic formula. I am looking that diversity.

 

[LittleInch]

If your chiller is not variable in area, I think you'll have a curve for temp vs time which will never get to 4C until a lot more than 70 mins. Why is this happening? Do you have a large flow of cold water?

Coming to think of it you need two insulated tanks. One holding your warm water and one holding your cold water. Then your system will work.

Even the best insulation will have some heat transfer...

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

 

[moideen]

LittleInch :I think can depend this formula to find out the required flow rate for pull down time that 1320 gpm divided by seventy minutes =18 gpm. Considering the safety factor flow rate might be 25 gpm. Your thought for two tanks is interesting. Then what you think about the two stage system with series the chilled water splitting with two heat exchanger and two tanks.

 

[LittleInch]

Even at 25 gpm, I don't think the re-circulation method works.

Therefore whatever design you come up with needs to look at it properly.

I can't comment on your design as I don't know what your system is.

I'm curious as to why you want 132gallons of cold water presumably at a high flow, once every 70 minutes or so, adding on time to empty the main tank and time to fill the main tank.

Why don't you cool the water the water on the way into the main tank at say 20GPM or higher? Then your cooling load stays constant.

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

 

[moideen]

LittleInch :-
What you see the basic issues with re circulation system? Then how does typically cool the water tanks? Actually I don’t know the actual usage of this water. I was instructed by management to make a chiller that they want 4c water out only. I don’t know my understanding is correct from your advice for primary cooling the main tank and steady the inlet temperature. Then should I go with two stage system?

 

[show790]

dear

try to follow this

Q = M * Cp * Delta T

Where: Q = The amount of heat transferred to or from the fluid (BTU/hr)
M = Mass flow rate of the fluid (lb/hr)
Cp = Specific heat of the fluid (BTU/lb-°F)
Delta T = The change in temperature of the fluid (°F)

with this you will have the BTU/hr and you can convert it to TR

- After selecting the chiller from suppliers they will give you information for pump and min flow rate of water for this chiller .
- the insulation for tank is to save the that heat .

 

[LittleInch]

The basic issue with taking a tank of water at one temperature, then pumping that water through a chiller and back into the same tank is that the amount of heat being removed from the water / temperature will gradually fall over time in a curve. Hence it is not easy to correctly size the chiller as the duty will gradually reduce over time or the flow rate needs to increase or the size of the chiller needs to increase over time.

None of these things are very efficient or good.

It is therefore much better to have a constant flow with a constant temperature drop from inlet to outlet.

So either put the chiller on the water on the way into your insulated tank ( you'll also need some sort of internal cooling loop to maintain 4 C) or put the chiller on the piping on the way out.

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

 

[HVACMech]

LittleInch is right. To every cooling/heating cycle and device you have an approach temperature. As your target temperature for you fluid approaches the operating temperature of your device the longer it takes to get there. From reading through here it sounds like you have the ambition to try to store cold water, but if I were you I would follow LittleInch's advice and right size your chiller for your actual flow demand. It will work much better and save energy.