COOLING TOWER SELECTION WRONG 1

[moideen]

I have a cooling tower; capacity mentioned in submittal that 450 ton. Centrifugal chiller capacity is 450 ton. Very clear cooling tower is undersized. In the summer running two towers for one chiller. As per the same document, the condenser flow rate is 1350 gpm and range 10F. Then capacity will be 1350*500*10=562 ton. I don’t know what happened in design and selection time. I am planning to increase the capacity and improve the efficiency of chiller. Stiil I have a confusion, why the design eng selected 1350 gpm for 450-ton chiller

 

[MintJulep]

It seems that the system designer was unaware of the first law of thermodynamics.

Specifically, a 450 ton chiller produces 450 tons of cooling. It needs to reject more heat than that because, thermodynamics.

The energy that the chiller uses to make cold must also be rejected by the cooling tower.

 

[lukaiENG]

This may be a case of a refrigerant ton (12,000 BTU/hr) vs a cooling tower ton (15,000 BTU/hr). Refer Ashrae Handbook chapter 39 for backup.
The industry decided many moons ago to match the nominal capacity of cooling towers to the nominal capacity of water cooled chillers, this having a 450 ton chiller matched with a 450 ton cooling tower. Why? I guess because the IP system is not complicated enough!
I ignore nominal capacity on most products now and focus on the actual performance data from my selections to avoid such confusion. Nominal capacities are at best ballpark accurate, at worst confusing and misleading - as demonstrated in this instance.

 

[DrRTU]

The designer was lazy. He designed for 3GPM/nominal ton. Depending on local ambient it could have worked. Have you verified the capacity with the manufactures rep selection tools?

 

[Bos88]

I don't think your cooling tower is undersized. The cooling tower suppliers use 15000 Btuh to calculate Equivalent chiller tons. In your example, 1350 gpm *500 * 10 = 6,750,000 buth. Divide this by 15000 = 450 tons or rather cooling tower tons. If you divide the result by 12000 you will get the tons of heat rejection. (562.5 TR).

Best way is to check the chiller selection and cooling tower selection and check if the flows and temp in/out match between the 2. Looking at tons will be confusing as the Chller tons and Cooling towers are based on different calculations.

 

[moideen]

i re-checked the submittal and selection sheet.in selection sheet clearly mentioned 560 ton, but in submittal 450 ton.i think as "boss88" pointed is may be correct. but in actual practice. one chiller cannot run with one tower. find the attachment..

 

[Bos88]

I would not worry too much here. What you have is simply a written process description. They simply called the cooling tower a 450 TR tower same as the chiller capacity. I don't think they even thought of mentioning heat rejection in that description. What you need to look at the actual values which are on the selection sheet from SPX and the chiller as well. Here it shows 1350 GPM at 93-103 and 86F WB which is correct for a 450 TR chiller.

This is a very old selection so I guess it is an existing project which is in operation. Now if you are trying to say that you are not able to operate the plant with 1 chiller + 1 towerand need 2 towers for that, that is a different issue (technical) and discussion. From a selection point of view, your datasheet is correct. From an operation point of view, you might not be able to operate 1 tower with 1 chiller but that would be more a technical investigation you need to look at. Probably better to call SPX Dubai for that.

One more note: if you look at the heat rejection capacity on SPX selections, be aware that they adjust it as well for specific heat and specifc gravity (BTUH = (Flow) X (Range) X 500 X (Specific Gravity) X (Specific Heat)) so it will not be exactly 15000 btuh. This is why you have 6716.1 MBH vs 6750 MBH. Evapco uses 15000 BTU and BAC does not provide this value on their datasheet (i guess to avoid the confusion between chiller tons and cooling tower tons)

 

[DrRTU]

Re-reading your posts and documents, you have two nominal 450 ton towers with two 450 ton chillers and one common condenser water system at a total of 1350 gpm. It looks like the single cell tower was run at 1350 gpm. You have a condenser water issue. Should the tower data have been run at 675gpm?

 

[Bos88]

In my opinion DrRTU, they have a total of 2700 GPM in the system with each tower handling 1350 GPM and each chiller receiving 1350 GPM condenser flow. The Selection Datasheet actually is for the design point for 1 Cell and it shows 1350 GPM. Now if they have installed different towers and have only a total of 1350 GPM for the 2 chillers, they will have a chiller issue. This will be 1.5 GPM/TR or about 19F delta T on condenser. Although the Chiller is a 2-stage, I doubt it will take it especially if it was selected on 3 GPM/TR initially. I am on the opinion that they a total of 2700 GPM in the system as per their process description and tower selection and the data was provided per cell rather than for the total system.

 

[LittleInch]

What exactly is the problem here?

Is the issue that for some reason the original design decided to use an air wet bulb temp of 86 F (30C)? Was this too low? Even in July and August this doesn't seem to go above 25C so it's not that. I can't read the assumed RH but assume it's still valid.

So is the tower simply not working well because the fans are broken or some other issue?


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[DrRTU]

Bos88, I agree 2700 gpm is needed but read the second paragraph, the wording sounds like 2 single cell units makeup to be the “tower” with motorized valves. With a common header they should have stated total flow.

 

[Bos88]

DrRTU, I guess we need Moideen to clarify what is exactly the problem on his system.

In reply to LittleInch, the 86F WB is actually correct for this part of the world. Actually it can get even higher some days. The RH on the cooling tower selelction is 50% by default. SPX Marley uses this value only to calculate the estimated evaporation loss.

 

[IRstuff]

Your document is dated 2008, so the system has possibly been in operation for over 8 years. Lots of things can change in 8 years, fouling, process changes, configuration changes, etc.

You stated that you need both towers to cool 1350 gpm, but you've not said how far off the performance is, i.e., what temperature delta does a single tower achieve, whether both towers are working the same, and whether the conditions are consistent with the original design.

TTFN (ta ta for now)
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[moideen]

see the graphic...in uae some time wet bulb reached 89F. so 86F is common wetbulb design concept in gulf countries.

 

[IRstuff]

And what about my other questions?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[moideen]

As per the site info, past years two tower and two pumps need to work for 460-ton chiller. Otherwise, it will trip on high discharge. flow rate of one pump is 1350 gpm( total 2700)[sup][/sup]Recently, I noticed and involved in this case.at glance in submittal, the tonnages is 460-ton cooling tower, so I thought tower is undersized, and I contacted its contractor who installed the system. I was told there are same issues from beginning. Now all building owners are very cautious in energy saving. Part of this I rechecked and later found spx selection found the total heat rejection is 562 ton. Anyway I suspect the fills and fan blade angles might be something wrong…next month we will teardown the tower and will be checked…..

 

[moideen]

 

[Bos88]

In the original design concept, is one of the chiller a standby? or both are supposed to run. Now I wonder in summer how do they cope with load if they have to run 2 towers for 1 chiller and the 2nd chiller needs to start as well?
If you are getting 89F wet bulb, you will end up with 96F condenser so the chiller might trip and the VFD might not like it either (refrigerant cooled or water cooled type).
If the issues were there from the beginning, why nothing was done to rectify it when the equipemnt were still under warranty?
From the pictures, I can see the plant is surrounded possibly by wall? you could simply have a case of hot air recirculation which increase the inlet wet bulb. Also you need to look at how they control the fan (if on VFD) with respects to condenser setpoint.
I will help if they have logs of wet bulb, condenser water temp....at the time when they experience tripping.
In my opinion you should contact Marley have them do an inspection of the site.

 

[moideen]

Bos88:yiua are correct,tower is surrounded by wall, it also in planning to make ventilation for tower area.here one chiller is standby.ct fan is controlled by vfd, but here vfd manually adjusted to full speed.

 

[LittleInch]

It does look like the system doesn't have much spare in it so any small thing going wrong or out of range (WB temp > 86F, water flow rate not 1350 gpm, fans and systems damaged, dirty or not operating at full speed, non ideal air inlet paths, possible re-circulation of hot humid air) will get to such a level that your chiller can't reject enough heat and trips on one CT.

Lots of things to check...

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