Head Requirement Question

[ProjectEng]

Water is being pumped from a CT pump pit to 2 large condensers and then the rest of the building needs. Risers from the CW supply header come up to each condenser. The return from each condenser go into the return header back to the cooling tower.

Assume the pressure drop through one condenser is 15psi. Neglect elevation changes. What is the head requirement just for the two condensers?

 

[BobPE]

Asuming the systems were identical before the common manifold going back to the CT then the other condenser loss would by 15 psi. At the point where the flow joined in the manifold, the total loss at that point would be 15 psi, although the flow becomes doubled.

I think that's what you were asking? It was kinda tough to read.

Take care

BobPE

 

[stressriser]

yeah,
I had a hard time reading this too.

I'll just state the rules to support Bob and to see if I understand the question.

Sounds like a parallel system, right?

So:

1. The flow divides in a manner that the head loss in each branch is the same.

2. The head loss between the two headers is the same as the head loss in each branch.

3. The total flow rate is the sum of both branches.

and of course, the friction of all in between.

Now, if these condensers are in series, the head loss is the sum of the two and the flow is the same for each.

 

[BobPE]

stressriser:

I went out on a limb there because I was not sure that's what was asked, but I feel better that you read it the same way.....

Bob

 

[quark]

Projecteng!

You cannot neglect elevation changes if the system is not a closed loop recirculation system. If you are allowing the return water to flow freely into the sump, consider the elevation also. Plus your pressure drop across any of the condenser (if both are of same type or else higher pressure drop)and the frictional losses in the piping.

Regards,

Repetition is the foundation of technology

 

[MortenA]

it sound a lot like a text book question to me

Best regards

Morten

 

[ProjectEng]

Nope, not a textbook question. I know I have to take into account elevation changes and frictional losses due to pipe length and fittings. There is a somewhat complicated piping system on the supply side downstream of the two large condensors.

I was trying to think this through.. It makes sense that less of a head requirement is needed for CW users in parallel as opposed to the same users in series.

But how much less? What if you had 20 identical exchangers in parallel, each with 10psi drop. If I understand what you guys wrote, head requirement for these exchangers would be 10psi total, plus the elevation and frictional losses.

Now, what if you had only 10 identical exchangers in parallel, each with the same 10psi drop. Head requirement is still 10psi?

 

[stressriser]

I knew this is what you were thinking.

Try this, it might help, it might not, but it's how I relate it.

Think of a battery circuit:

A one volt battery in parallel with another one volt battery will give you one volt. But the current (Amps) would double.

A one volt battery in series with another one volt battery will give you two volts. But the current will be the same as a single one volt battery.

The Voltage is the pressure, the current is the flow.

Now, you have to size the wires correctly to carry the power.

Maybe that does not help, but I had to try.

Sr

 

[quark]

You are right stressriser. But when two nonsimilar pressure drops occur then you have to consider the larger one. Repetition is the foundation of technology

 

[BobPE]

quark:

what do you mean, in series pressure losses or parallel? I will give you the benefit of the doubt before I think!!! I have been getting to into thinking lately as you can attest in our conversations in another post!! LOL

BobPE

 

[aliansari]

Is this a problem to be included in fluid mechanics paper, being prepared for second semmester chemical engineering?

Neglect elevation, assume 15 psi pressure drop etc etc. I have never seen such an "ideal" system in real world application.

Mohammad Ali Ansari

 

[Roach]

Aliansari,


Most of the time, when I post a thread, I do not fill it with details of numbers.
Engineering is to first determine the correct approach by understanding the system. I have posted threads with "ideal" systems to comprehend the system, not so that someone else can solve the problem.

"Everyone is ignorant, only in different subjects."

 

[aliansari]

Dear Roach,

Focusing on approach is right, but for total solution, numbers and numerics are essential.

Anyways, required head for your pumps would be

35 foot + piping friction loss + cooling tower elevation, provided tha condensors are located at grade and not at a location higher than the cooling tower.

Regards.

Mohammad Ali Ansari