Cooling water pump sizing

[koshyeng]

To calculate cooling water pumps head, do I need to consider staic head and frictional loss of the pipe with high elevation? I mean to say is discharge pipe branches to several elevations and joins to the cooling water return line.

The highes elevation pipe goes to a heat exchnager at 20 psi inlet pressure. Should I consider only pump discharge header and this piping for head calculations?

How about frictional losses in return pipe? Pressure recovered in return piping is good enough to make it back to the cooling tower?

 

[BigInch]

You will need to provide head to the highest point + initial charge pressure head + frictional pressure losses, IF you have no alternate means to fill the loop and must use the circulation pump to do so. If you can fill the system using a temporary pump, then the circulation pump only needs to provide charge pressure + the maximum friction loss in one of the system's loops.

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[koshyeng]

Thanks Biginch.

Highest elevation is 60 feet. Pressured required at exchnager inlet is 10 psi ( pressure head) and frictional losses in the supply line is 60 feet. Do I need to consider frictional losses in return line and header to determine dischrage head for pump?

 

[BigInch]

Yes.

Pump to Exchanger

60 ft * 0.433 psi/ft = 26 psi
+ 10 psi 'required at exchanger
+ supply line flow loss, say 14 psi

= 50 psi Required Minimum Total Pump Discharge Pressure

Pressure at exchanger inlet
= 50 psi - 26 psi static -14 psi frictional
= 10 psi

Loss through Exchanger (say) = 5 psi,
therefore Exchanger outlet pressure = 10 - 5 = 5 psi

pressure at pump suction =
Exchanger outlet pressure 5 psi
- Return line flow loss (say 20 psi)
+ static head of 60 feet from exchanger to pump suction = 26 psi
Total pressure at pump suction = 11 psi

Minimum Required pump differential head =
(50 - 11)psi/0.433 ft/psi = 90.1 ft (39 psi)

When system is not flowing, there is 26 psi (from the 60 ft of static head in each pipe column) so that's both the suction and the discharge sides of the pump, and assuming there is no precharge. If you precharge, add the precharge pressure to all points in the system.

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[koshyeng]

(Pump to Exchanger

60 ft * 0.433 psi/ft = 26 psi
+ 10 psi 'required at exchanger
+ supply line flow loss, say 14 psi

= 50 psi Required Minimum Total Pump Discharge Pressure)


Lets say return line frictional loss is 20 psi then shouldn't the trquired Minimum Total Pump discharge Pressure be 70 psi?? Please clarify for me.

What is precharge pressure?

Thanks Big Inch.

 

[koshyeng]

Big Inch, this is an open cooling tower water system.

 

[BigInch]

? I did the problem using a 20 psi loss in the return line (exchanger outlet to pump suction).

*******************************
pressure at pump suction =
Exchanger outlet pressure 5 psi
- Return line flow loss (say 20 psi)
+ static head of 60 feet from exchanger to pump suction = 26 psi
Total pressure at pump suction = 11 psi
************************************

Precharge pressure might be any pressure that could be necessary to keep pump suction pressure above NPSHR, like you could have in a loop with no elevation change where static head is not recovered in a down-flowing leg, or for any initial minimum pressure requirement you might have for a bladder tank, or to keep the fluid above vapor pressure at the high point... something like that.

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[koshyeng]

BigInch,

Thanks for your patience for clearing my queries.

I did the calculations.

350 feet of 12" pipe at 1800 gpm

Flow loss : 0.275 psi per 100 feet = 2.22 feet
Static head = 60 feet
Frictional losses = 12 psi*2.31 = 28 feet
Pressure required at heat exchanger = 10 psi*2.31 feet = 23 feet
flow and frictional losses in 6 inch pipe to heat exchanger = 23.1 feet

Discharge pressure = 137 feet or 59.3 psig

My pump suction pressure is 2.5 psig.

Total dynamic head = 137 - 6 = 131 feet.

Is this right? I did not include return header friction losses. Frictional losses in return header is 14 psi. In this case, my pump discharge pressure should be 73.7 psig.

Appreciate your valvuable comment.

 

[BigInch]

I have three problems now,

1.) I don't know which frictional loss is 28 ft, where you say, "Frictional losses = 12 psi*2.31 = 28 feet". Is that frictional loss through the exchanger?

2.) You have 350 ft of 12". Is that the supply line to the exchanger? FROM pump discharge TO exchanger?

3.) You have a 6" pipe with a head loss of 23.1 ft, but you are saying it is "to exchanger". Is this a 6" return line FROM exchanger TO the pump suction?

Question? Why a 6" and a 12" in the same loop? velocity in the 6" might be pretty high. What's the flowrate in the 6"?

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[koshyeng]

Biginch,

Frictional loss of 28 feet is due to fittings in 12 inch pipe.

12 inch supply header branches out to several elevations and total supply header flow rate is 1800 gpm.

I mentioned only 60 feet elevation and piping to heat exchanger since this is at the highest elevation. 6 inch line branches out from supply header for this exchanger. flow rate here is 850 gpm and the return line from exchanger is also 6 inch which joins 12 inch return header.

In this case do I need to consider frictional losses in 6 inch return line (850 gpm) and return header frictional losses due to fittings and pipe ( 1800 gpm)


Thank You

 

[BigInch]

Yes you must consider all losses from ALL pipes and fittings; in the supply lines and in the return lines.

Do you have the flowrates in all branches?

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