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
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
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
RE: COOLING TOWER SELECTION WRONG
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
RE: COOLING TOWER SELECTION WRONG
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: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
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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RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
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
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
RE: COOLING TOWER SELECTION WRONG
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
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
RE: COOLING TOWER SELECTION WRONG
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
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
Remember - More details = better answers
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RE: COOLING TOWER SELECTION WRONG
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
Keith Cress
kcress - http://www.flaminsystems.com
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
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?
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RE: COOLING TOWER SELECTION WRONG
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
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.
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RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
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?
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RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
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.
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
Also no pump curve.
And no anticipated losses through the chiller in operation
The schematic shows 4 pressure gauges.
What are the pressures when running?
What is that strange looking DRV? Just a drain valve or something else?
Have you tried isolating the other pump(s) when operating to ensure you're not getting reverse flow past a leaking NRV?
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RE: COOLING TOWER SELECTION WRONG
the pressure in/out condenser is 20/5 psig. DRV fitted at discharge side of the pump.
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
The pressure in out of the pumps will be interesting.
From the data above, the pressure out of the pump looks like circa 20 psi (about 45 ft head)
Also a 15 psi head loss across the condenser is around 30-35 ft, which is a lot more than your 15 feet in the spread sheet
If the inlet line is higher than the cooling water pond and or some way away, you risk getting cavitation if the pump inlet filters start to clog up.
I didn't see any sign of the filter DP in the calculations?
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RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
What is the elevation difference between your pumps and the tower basin? If you have 10+ feet you are probably ok.
If there are NPSH issues, I would put side stream filtration on the tower basin (with basin sweepers) to clear out large debris then install an inline strainer AFTER the pump to take out small debris. This is not the normal location for a strainer, but if you have NPSH issues then it is the best you can do.
RE: COOLING TOWER SELECTION WRONG
That looks good to me.
Key points - How accurate is your 15 ft for the condenser? Your previous post said 15 psi differential which is 30-35 feet?
Your power at 40 BHP = 30kW appears to be higher than your pump motor ( you quote 18.5 kW
Can you post the pump curve please?
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Also: If you get a response it's polite to respond to it.
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
Pretty simple in the end. Your pump is too small for the system you have. If you could run the pumps in series it might work but really it is simply an undersized pumping design.
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RE: COOLING TOWER SELECTION WRONG
RE: COOLING TOWER SELECTION WRONG
Be sure to let us know how you get on.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.