Pressure drop in condensers.
Pressure drop in condensers.
(OP)
Hi there,
I am looking for a rule of thumb concerning pressure drop in condensers depending on the feeding numnber.
Following is the topic.
Let's assume I have n feedings, and a pressure drop DP1.
in this case pressure drop in proportional to 1/2*v^2
DP1 proportional to 1/2*v^2
Let's assume now that I double the number of feedings, therefore 2n
Since I double the number of feedings, and assuming the same capacity, velocity will decrease to v/2
said this, the new pressure drop will be
DP2 proportional to 1/2*(v/2)^2
said this, I would expect that
Dp1/Dp2 proportional to 4
that is, pressure drop in second case - with the double of circuits - will be something around 4 times less
Please, do you think my argumentation makes sense ?
Thanks
I am looking for a rule of thumb concerning pressure drop in condensers depending on the feeding numnber.
Following is the topic.
Let's assume I have n feedings, and a pressure drop DP1.
in this case pressure drop in proportional to 1/2*v^2
DP1 proportional to 1/2*v^2
Let's assume now that I double the number of feedings, therefore 2n
Since I double the number of feedings, and assuming the same capacity, velocity will decrease to v/2
said this, the new pressure drop will be
DP2 proportional to 1/2*(v/2)^2
said this, I would expect that
Dp1/Dp2 proportional to 4
that is, pressure drop in second case - with the double of circuits - will be something around 4 times less
Please, do you think my argumentation makes sense ?
Thanks





RE: Pressure drop in condensers.
DP1 proportional to 1/2*v^2 and l circuit
DP2 proportional to 1/2*(v/2)^2 and l/2 circuit
This means Dp1/dp2 is something proportional to 8.
Please,
does this make sense to you ?
does anybody now a specific rule ?
Thanks !
RE: Pressure drop in condensers.
RE: Pressure drop in condensers.
I just pulled an old design up at random.
In double pass it is 7.4 ft/sec velocity and 17.9ft of head loss.
In single pass it is 3.7 ft/sec velocity and 2.66ft of head loss.
It does not work out to a factor of 8 because of inlet and outlet losses.
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P.E. Metallurgy, Plymouth Tube
RE: Pressure drop in condensers.
many thanks for your answers.
@mk223
Dp1 is the pressure drop in case 1
DP2 is the pressure drop in case 2 (double of circuits, half of velocity)
@
EdStainless
Yes, more or less.
ok.
so maybe it is an 8^0.9 ?
I don't need an exact number, just a rule of thumb
RE: Pressure drop in condensers.
So for this one condenser you get
14.8 ft/sec 120.4ft
7.4 ft/sec 17.9ft
3.7 ft/sec 2.66ft
For another one
14.0 ft/sec 107.7ft
7.0 ft/sec 16.0ft
3.5 ft/sec 2.14ft
For any real condenser you need to know the tube alloy to select a velocity.
With Cu alloys you stay low, ~6ft/sec for brass and ~7ft/sec for CuNi.
In SS (any stainless) and Ti the modern trend is to run about 9.5-11.5ft/sec.
At these velocities you get better heat transfer and the tubes stay cleaner (which also improves heat transfer).
No one would ever design at <5ft/sec (they would foul) or >15ft/sec (pumping losses would be huge).
You can calculate these, I have found that the results of the HEI method are within 5% of reality. (both deltaP and back pressure)
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P.E. Metallurgy, Plymouth Tube
RE: Pressure drop in condensers.
If I am following you correctly, which I think I am, roughly 8 times is correct as a rule of thumb.
For the record, I am assuming that the double circuit design is two units in parallel, and each unit is half the length but same tube count as the full circuit (1 unit).
RE: Pressure drop in condensers.
RE: Pressure drop in condensers.
Why?
You can gain more by a slight change in tube diameter.
How are you modeling the overall heat transfer?
Nearly all real steam condensers are 2 pass, though 4 pass is common also.
The only single pass that I know of uses very long tubes (114').
What is your goal?
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P.E. Metallurgy, Plymouth Tube
RE: Pressure drop in condensers.
The question is with regards to pressure drop and not overall heat transfer. It was not considered in my example. Obviously more tubes = lower velocity = lower heat transfer coefficient.
RE: Pressure drop in condensers.
I think you should be basically comparing Dp1 with approx. 2 x Dp2 (for 2 x 1/2 circuit). so the factor you are looking at is dp1 / (2 x dp2) and that factor then equates ~2 (instead of 8). Thus your rule of thumb factor is roughly 2.
means same tube diameters, same length, same total capacity, doubling the feeding would reduce the total losses by a factor 2.
Or am I missing something ??
RE: Pressure drop in condensers.
thanks for your answers.
@EdStainless
I am looking for a smart and quick way to evaluate the impact of changing the number of tube feedings in the SAME condenser.
@Afox88
yes, the OP was to get a rule of thumb saying:
what will happen to pressure drop when doubling the number of feedings in the same condenser ?
@rotw
DP1 proportional to 0.5*v^2 and circuit_length
DP2 proportional to 0.5*(v/2)^2 and circuit_length*0.5
thanks
RE: Pressure drop in condensers.
1/2 the flowrate will lead to 1/2 the velocity which will lead to about 1/4 the pressure drop, since it is a function of v^2.
Hope this helps.
RE: Pressure drop in condensers.
please do not forget that the length of circuit will be half of the original.
therefore there is also the circuit_length*0.5
RE: Pressure drop in condensers.
The words circuit, feeds, and same condenser, is not typical lingo for what you are asking.
RE: Pressure drop in condensers.
Many modern condensers with 10-12f/s flows are using 1.125" or 1.250" tubes to reduce pressure drop.
But with higher velocity you still get good heat transfer.
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P.E. Metallurgy, Plymouth Tube