Coils - Inline vs Staggered tube

[nh3simman]

Coils play a big role in HVAC systems. DX, air cooled condenser, chilled water, hot water, steam heating, energy recovery and the list goes on...

Most of the HVAC coil I have seen have a staggered tube pattern with plate fins.

Despite the importance, there are only a handful of research works that relate the configuration to the performance. In addition, most of the studies I have seen use the typical staggered tube pattern.

So, how do you estimate the plate fin coil that has an in-line tube pattern?

There are plenty documented measurements of in-line vs staggered for bare pipe tubes.

Here is my idea.

To estimate the performance of a plate fin coil with an in-line tube pattern, do the following:

1. Calculate for plate fin with staggered tube.
2. Calculate for bare pipe staggered tube.
3. Calculate for bare pipe in-line tube.
4. Determine ratio of (2)/(3)
5. Correct (1) by multiplying by (4)

I'm interested to hear your comments on this.

[lana]

Here is my idea.

To estimate the performance of a plate fin coil with an in-line tube pattern, do the following:

1. Calculate for plate fin with staggered tube.
2. Calculate for bare pipe staggered tube.
3. Calculate for bare pipe in-line tube.
4. Determine ratio of (2)/(3)
5. Correct (1) by multiplying by (4)

.

Hi nh3simman,

I have never seen such an idea to calculate the coil .
It can be verified or rejected if a few coils are designed and then perform your theory on the and see what happens. But who has the time?
Anyway, in a the real good coil design software, you can configure your pattern and then the calculation will be done accordingly. In in-line pattern there is a huge "air escape" route between tubes, so naturally there would be less efficiency with this pattern.
It would be interesting to know how your theory works.
Cheers

[nh3simman]

...In in-line pattern there is a huge "air escape" route between tubes, so naturally there would be less efficiency with this pattern...

Hi lana, my only concern with this idea is that the fins influence the correction factor. But, since the general air direction is not parallel with the tubes, it may be suitable.

I did try this method and compared it to some of the references that measured both types and got a very good agreement. Question is, how general is the answer?

[US Iceman]

I have a general question.

Why is it important to know the heat transfer of a square tube pitch?

No one I know of uses this anymore because the heat transfer was so much less than a triangular tube pitch.

Or, is this an academic exercise?

[lana]

I did try this method and compared it to some of the references that measured both types and got a very good agreement. Question is, how general is the answer?

Hi,
To be honest, I have no idea
Keep us posted what you found.
Good Luck
Cheers

[nh3simman]

For some strange reason. Baltimore Air Coil use the in-line tube pattern on their ammonia coils.

[US Iceman]

Baltimore Air Coil use the in-line tube pattern on their ammonia coils.

That's only for their condenser coils, right? The few evaporators I have seen are triangular pitch (staggered).

[lana]

I know lots of manufacturers who use in-line pattern (germany, china, ...) .
We also have a few here in Iran.
But waht we manufacture is staggered that is why my calculations are based on this.
Cheers

[nh3simman]

BAC in South Africa make an ammonia evaporator coil with 19mm stainless steel tube and an in-line tube pattern (50mmx60mm).

I'm not sure if the pattern is based on the US design or if it is a local design.

I was surprised to see that coils up to 20 rows are made. To my mind you can leave out at least 5 rows and see no change in suction!

[NoNickName]

Square geometry is preferred for sensible heat transfer, because less dehumidification occurs. In fact, all of our close control units are with square geometry.
Staggered geometry (triangular) is preferred for condensing coil.

[nh3simman]

Square geometry is preferred for sensible heat transfer, because less dehumidification occurs. In fact, all of our close control units are with square geometry.
Staggered geometry (triangular) is preferred for condensing coil.

Thanks, I was not aware of the reasoning.

What do you think of coil rows > 9?

[nh3simman]

When we deal with practical matters like the workings of refrigeration, you will often heat statements like:

Its not practical
Why do we need to know this?
Is this just academic?
...

I would like to make some comment on this

...Why is it important to know the heat transfer of a square tube pitch?...
Or, is this an academic exercise?

Many of the greatest scientific discoveries in history were not accepted by contemporary scientists. They were often seen as wild and incorrect assumptions.

Where would the modern advances in A/C power be if it were not for people like Tesla?

[US Iceman]

Square geometry is preferred for sensible heat transfer, because less dehumidification occurs.

That is interesting. I was not aware of this. It might also dictate why the coil depth might have more than 10 rows deep, which is about as deep as we normally use for refrigeration purposes here in the US.

Is the square pitch used because of an actual design requirement for less dehumidification, or has this been found to serve this purpose based on a specific application with certain equipment?

Many of the greatest scientific discoveries in history were not accepted by contemporary scientists. They were often seen as wild and incorrect assumptions.

Where would the modern advances in A/C power be if it were not for people like Tesla?

You are preaching to the choir. I worked in R & D for almost 5 years and am not known for doing impersonations of ostriches.

All of the research I have read or heard of simply stated the best heat transfer for the cost of the coil was obtained by the use of triangular pitch tubes.

Some manufacturers went so far as to develop an optimum X and Y dimension on the pitch also. And recently, one manufacturer has started to use elliptical tubes.

And, the majority of the refrigeration applications are not concerned with humidity control. Simply temperature control.

I simply asked a question to find out more information.

[nh3simman]

I simply asked a question to find out more information.

Sorry Iceman, no personal attack intended. I'm just a bit over-sensitive about criticism of academic investigation.

[US Iceman]

I'm just a bit over-sensitive about criticism of academic investigation.

That's understandable. It certainly was not intended to be criticism, only a simple question as the statement made was something new to me.

No harm done. But you just found out I can be a little testy too.

[NoNickName]

Sorry for the late answer, I was in Zuerich for helping out our mate engineers.
The square pitch is prone to lower dehumidification, because the coldest point of the whole coil is the tube. When the tubes are inline (in the sense of the air flow) just the first one dehumidifies, while the others are in "shadow".
And for the rows, I don't see any difference (in our applications) from 6 to any more number of rows. The marginal gain is well overwhelmed by the cost increase and the pressure drop.