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

 

[LittleInch]

Nice schematic, but no lengths there or how many elbows, tees etc there are to calculate an equivalent length

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?

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

 

[moideen]

As per my preliminary calculation of pump head, i found around 70 ft of head required. see the excel sheet attached,i will go to site to check any fittings pressure drop i missed here.
the pressure in/out condenser is 20/5 psig. DRV fitted at discharge side of the pump.

 

[lukaiENG]

What are the pumps designed for right now? I couldn't find in the thread above.

 

[LittleInch]

Pumps should be 1350 gpm each.

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?

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

 

[moideen]

LittleInch: strainer fitted before the pump inlet line.i recalculated after visiting the site. some value assumed like DRV , Nno data with me. as per the new calculation around will reach 77 ftof head.i put 1350 gpm throut the circuit. is it correct? and if you have fitting loss chart,please share here.

 

[lukaiENG]

Along the line of LittleInch's comment, I would be wary of the effect on NPSH by having the strainers on the inlet to the pump. if there is not enough static pressure at the pump inlet then your risk cavitation. with this in mind, for cooling tower installations you normally want to have tower and the pump as possible. Your pumps will have a minimum NPSH requirement.
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.

 

[LittleInch]

moideen,

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?

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

 

[moideen]

LittleInch :as per the condenser design sheet that the pressure drop is 40pascals(5.8psig or 14feet). i put only the design value in calculation, but the actual drop is 15 psig.bhp calculatioed as per the new head that 78*1350/3960*65%=40 BHP. please see the attached pump curve

 

[moideen]

lukaiENG : as to the past site info, there was no any cavitation issues, because the tower is prperly water treatment and regulatio cleaning of suction strainer. i checked the impeller, there is no any sign of cavitation,and not any report of pump failure.

 

[moideen]

pump curve

 

[moideen]

it is new info that the design pressure drop of condenser is 114kpa,see the attached sheet, my previous info from Mitsubishi dealer that was only 40 pa. But condenseser 3 pass style, so the existing condenser pressure is ok consideiring the design drop, also head is also increased to 105ft /hd

 

[moideen]

condenser selection attached

 

[LittleInch]

Moideen

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.

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

 

[moideen]

Little Inch: yes, finally confirmed, i am on the way to replacement of only one pump first, then I will check the performance difference...

 

[LittleInch]

If you can rig up some temporary pipework to connect two pumps in series it will prove the point but a bigger pump and motor might need new cables and new switchboard/ motor starter as well.

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.