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COOLING TOWER SELECTION WRONG

COOLING TOWER SELECTION WRONG

(OP)
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

RE: COOLING TOWER SELECTION WRONG

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.

RE: COOLING TOWER SELECTION WRONG

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.

RE: COOLING TOWER SELECTION WRONG

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?

RE: COOLING TOWER SELECTION WRONG

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.

RE: COOLING TOWER SELECTION WRONG

(OP)
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..

RE: COOLING TOWER SELECTION WRONG

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)

RE: COOLING TOWER SELECTION WRONG

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?

RE: COOLING TOWER SELECTION WRONG

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.

RE: COOLING TOWER SELECTION WRONG

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?

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

RE: COOLING TOWER SELECTION WRONG

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.

RE: COOLING TOWER SELECTION WRONG

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.

RE: COOLING TOWER SELECTION WRONG

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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RE: COOLING TOWER SELECTION WRONG

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

RE: COOLING TOWER SELECTION WRONG

(OP)
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)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…..

RE: COOLING TOWER SELECTION WRONG

(OP)

RE: COOLING TOWER SELECTION WRONG

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.

RE: COOLING TOWER SELECTION WRONG

(OP)
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.

RE: COOLING TOWER SELECTION WRONG

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.

RE: COOLING TOWER SELECTION WRONG

Cover the basics first with known data. 1. Hire a balance contractor to measure the condenser water flows. Inspect all strainers and pressure gauges first. Verify sump inlets and header nozzles. Run thru combination of pumps. The 6” is undersized for 1350 gpm - tower isolation vlv may not be fully open. 2. Work with SPX rep to inspect tower. Fan and fill must be correct. Verify rpm at fully HZ at drive. 3. At full chiller flow, utilize flow and temps at tower & chiller. Run thru chiller and tower flow combinations.

RE: COOLING TOWER SELECTION WRONG

Consider also adding cooling tower discharge cowl to discharge exhaust above the wall enclosure to avoid recirculation of discharge to intake. Confirm also that the distance between the wall and cooling tower is at least equal to the height of the cooling tower fill.

RE: COOLING TOWER SELECTION WRONG

(OP)
i have a related question. not this tower. another tower brand MEZAN, it's fills are splash fills. is it possible to replace it with film. because film fill is very efficient compare to splash fills.

RE: COOLING TOWER SELECTION WRONG

Start a new post - it gets very messy otherwise and add some more data, drawings, photos etc

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

RE: COOLING TOWER SELECTION WRONG

(OP)
UPDATE
I checked actual water flow with a water flow meter, job done by a company whose main activities are water balancing in the chilled water system, found the flow rate is maximum 600 gpm through one pump. Design 1350 gpm!!!

RE: COOLING TOWER SELECTION WRONG

(OP)
When running two pumps, flow rate of each pump comes down to 300 gpm; I suspect the problem may be any restriction in suction side. still checking..................

RE: COOLING TOWER SELECTION WRONG

I don't think the restriction will be on suction side as this is essentially at atmospheric pressure.

Sounds to me more like someone got theirs sums very wrong when working out the required differential head / pressure required or there is some sort of blockage / partly open valve somewhere in the system.

The obvious ones to check are:
Is the electrical supply what the motor needs ( Voltage, frequency etc)
Are the motors going the right way around?
Is there any VFD drive?
Is there a blocked filter somewhere?
Is the water level correct in the tower collection pond?

Back in your post of 29 Nov you said the pump trips on high discharge. High what? Pressure? temperature? Amps?

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

RE: COOLING TOWER SELECTION WRONG

(OP)
LITTLEINCH:The obvious ones to check are:
Is the electrical supply what the motor needs ( Voltage, frequency etc)-OK
Are the motors going the right way around?-WHAT YOU MEAN
Is there any VFD drive? NO
Is there a blocked filter somewhere? NEED TO BE RE-CHECKED, SUCTION SIDE FILTER AND FILTER BEFOR THE CONDENSER ALREADY CLEANED.
Is the water level correct in the tower collection pond? LEVEL OK

Back in your post of 29 Nov you said the pump trips on high discharge. High what? Pressure? temperature? Amps? HIGH DISCHARGE PRESSURE

RE: COOLING TOWER SELECTION WRONG

If the motors are three phase supply, then depending on how you wire the motors will depend on which way around they spin.

There is usually an arrow on the motor or pump (or both) showing which way they should rotate. When you start and stop the pump you can normally see which way the motor rotates. It is an easy thing to check but a centrifugal pump will work in both directions, but be much less effective the wrong way round. It's not common but does happen.

The degree of lack of flow is a lot and the only other thing is whether someone has mistaken a requirement in metres head for a requirement in feet or some other sort of gross error if it's not a mechanical issue like a blockage or partially closed valve.

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

RE: COOLING TOWER SELECTION WRONG

(OP)
Little Inch: I will recheck the direction of the pump. you are correct that there is no possibility for any blockage. Because ampere is correct as per the motor nameplate.

RE: COOLING TOWER SELECTION WRONG

You also need to start measuring pipe sizes and lengths and number of elbows etc to be able to calculate what the differential pressure needs to be at either 1350 or 2700 gpm and then compare it to your pump curves.

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

RE: COOLING TOWER SELECTION WRONG

(OP)
LittleInch :very true, i have no cad drawings to measure the length and count the number of fittings. Planned to take the measurement physically on site. Thank you

RE: COOLING TOWER SELECTION WRONG

(OP)
pump flow direction is correct, name plate kw is 18.5 and amps draws 30A.

RE: COOLING TOWER SELECTION WRONG

Well then you need to work out if the original design was incorrect or the pump size incorrect for your 1350 gpm.

BTW it is very unlikely that two equal pumps in parallel will actually do 2700 when sized for 1350. The only way is if the pressure loss through the chillers is a large part of the pressure losses and the second one only comes on line when more flow is required.

You seem to have narrowed the issue down to water flow rate not being what was anticipated.

What are you planning to do next?

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

RE: COOLING TOWER SELECTION WRONG

(OP)
LittleInch: now i am trying to measur the actual head loss throughout the loop.

RE: COOLING TOWER SELECTION WRONG

HI Moideen,

Based on your 18.5 kW nameplate, the head should be around 60 feet for the pump (1350 GPM). Using 15 Feet for tower, 20 feet chiller, that gives you about 25 feet for the piping/valves.

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