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Hot well /Cold well tank sizing for chilled water application

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sridhar1312

Mechanical
Jun 1, 2009
58
Interested to know how to go about sizing the Hot well /Cold well tank for chilled water system.
We want hot well /cold well tank for acting as buffer tank the tank shall have bifurcation up to 75% height so that hot well is around 40% and cold well is 60 % holding capacity.
The chilled water from hot well at 12 degree C is pupmped to chiller and returns to cold well at 7 degree C.
The 7 degree C chilled water is circulated to Air handling units in the Paper Mill and returns to hot well at 12 degree C .
Presently we have sized the tank for 3 minutes storage with 60% for cold well and 40% for hot well.
Is there any guideline or article on sizing of the tank.
 
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The need for a buffer tank arises only when the system load is variable over a broad range and there is a likelihood of system load being lower than the safe minimum chiller load(say 20% of chiller capacity).The tank will ensure stable chilled water temperature and limit chillers starts per hour to healthy limits (recommended 3 or 4 depending on chiller )as required by manufacturer.The minimum recommended chilled water system volume in such a situation is about 7~10 litres/kw depending on chiller manufacturer's requirement.For eg if the chiller capacity is 1000Kw ,minimum system volume required is 7000 L.If the voulme of water in the pipes and equipment is 2000L,you would require a tank of 5000L(7000-2000).The hot and cold well volumes can be equal.But if the flow rates for primary and secondary are different, water should always overflow from cold to hot(pretty much like the bypass pipe in a primary/secondary system)


If the chiller turn down is good due to multiple compressors or variable speed drives,it may not be required.It looks like you may a have a steady load as it is a paper mill.
 
Thanks for the input. The Paper Mill has 1000 TR steam fired Vapor absorption machine(VAM) and 300 TR steam fired VAM. The low side load is around 1000 TR peak.
Presently the tank we have is 40000 Liters with 1000 L for hot well and cold well capacity of 30000 Liters.We have cold well volume more to ensure cold well overflows to hot well so that the chiller unloads depending on the load.The load variation is mainly due to the weather fluctuation and the load factor of the plant.
Now the Paper mill is in expansion and we are adding 600 TR VAM two numbers and all the chilled water shall be in common cold well and hot well tank.We want to combine the tank into a single tank for all the 1000 +300 +600 +600 =2500 TR so that with any chiller all the areas can be covered.
 
I do not get the logic for oversizing the cold well.Assume chiller and primary pump are off.You are transferring water from a bigger compartment (cold well)to a smaller compartment(hot well).In a short while water will start overflowing from hot well into cold well.But if they are of the same size they will balance and there will be no overflow.

On another note with absorption chiller you need to careful with buffer tank sizing. Absorption chillers do not turn down well. Therefore they are meant for steady base loads only. If the chiller trips on chilled water temperature, the machine will go into a dilution cycle which can take about 15 ~20 minutes. It will easily be 30 minutes before the machine can be back on line. Your process may not stand the temperature fluctuation due to such shut downs.
 
Thank you SAK9, you seem to handle your subject pretty well.
My question is: If the absorption chillers do not turn down well and the load is so big in this plant, why use a tank altogether?

If the load is this large, just sequenc eyour chiller plant to the one or two chiller sneedd to meet the load. Why add controls, space, equipment and complications?

I understand the use of a buffer tank (are qe talking about a buffer tank here with hot/cold well language? hope I am still in the subject) would have its application when the system volume is less than the manufcaturer recommended volume (equipment too close to the plant being the case, not enough piping to hold water)

Thanks
 
The hot/cold well (buffer tank) will give the flexibility to cater to huge variations in load. Typically, they are sized 50/50.

Is this purely a comfort AC application or is there some process cooling also done ?

HVAC68
 
The Hot well cold well tank is for Paper mill where AHU FCU's located in all control rooms, I/O Rack Rooms drive Rooms offices and MCC rooms.
There is small quantity for process cooling also.

Earlier they were having only primary pump concept with expansion tank located at top most point of chilled water circuit. The primary constant flow pump had to be oversized and always had problem of chilled water flow in some units or other. Moreover more flow was being pumped than required in chiller jeopardizing the operation.

We have 1300 TR VAM(vapor absorption Machine) with hot well cold well tank of 40000 Litres with partition at 2/3 to ensure overflow is from cold to hot so that the chiller stars operating on part load whenevr the return (hot water temperature is lwo due to mixing with cold water.Currently adding two 600 TR VAM with AHU's etc. We want to go in for Masonry tank combining both existing 1300 TR +1200 (being added) +600 TR provision for standby.We have sized tank as 80000 Litres.
 
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