## 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