Heat exchanger area

[Ron10000]

Hi,
This is not a theoretical question
I and designing a heat exchanger.
I am planing to use copper as the material that will exchange the hit.
I'm having trouble deciding about the area of the heat transfer.
I am familiar with a heat exchanger, for the same application, that exchange heat using iron that gets the job done. This heat exchanger area is 0.6 m^2.
Iron thermal conductivity coefficient is about 85 W/m K (correct me if I am mistaken).
Copper thermal conductivity coefficient is about 400 W/m K (again correct me as needed).
Is that mean that I can use about 4 times less area to do the same heat transfer?
I believe that is what the math says, but it doesn't sound like it would work in real world.
Should I somehow take into consideration the materials that transfer and collects the heat?
Would be happy to hear your thoughts.
Thanks,
Ron

 

[EdStainless]

What is the media used for heat transfer? Water?
What style of heat exchanger is this?

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[georgeverghese]

The resistance from the wetted materials used in a heat exchanger is a very small fraction of the total resistance to heat transfer. Most of the resistance comes from tube side fluid, the shell side fluid and the fouling coefficients on shell and tube side.

Hence, switching materials from from steel to copper does not mean 4x less surface area. The total surface area will remain at 0.6m2

 

[LittleInch]

That isn't what the maths say as they are many other heat transfer operations going on.

The resistance to heat transfer of the metal tube is a very small part of the total heat transfer calculation of area.

If indeed you are designing it I suggest you get some help and someone experienced to check your design.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Ron10000]

it's a pipe in pipe heat exchanger.
It should transfer heat from r410a gas to water.

So, in case I know 0.6m2 for iron is enough... How much do you believe switching to copper should save? (If at all?)

Don't over think it, It's not like I'm taking the plans to a factory to build 1000 units
I'm just building 1 for now and I will make some tests and change it a lot.
I am just trying to decide what area to start with.

Many thanks

 

[Latexman]

Don't over think it

That's easy, because you have not given enough information to do it rigorously.

0.5 m^2

Good Luck,
Latexman

 

[LittleInch]

"How much do you believe switching to copper should save? (If at all?)"

My best guess given no other information is less than 5%.

But so much depends on the specifics.

If your DT is small it might make a bigger difference than a large DT.

But even then no more than 10 to 15% IMHO.

Make one, test it then let us know if we're all wrong.

Are you trying to make it smaller?

What else can you change?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

Your value for iron may, or may not, be accurate, depending on temperature/construction/alloying:

https://www.engineeringtoolbox.com/thermal-conductivity-metals-d_858.html

shows iron as low as 52W/m-K at 20C; these were numbers that you could have looked up on the web.

r410a is a sucky heat transfer gas on its own, so are you trying to condense, evaporate, or what?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[Ron10000]

Hi LittleInch,

Sorry if I was rude, I just replied to all posts at once.

I wouldn't say I'm trying to make it smaller, but I don't want to make it ridiculously large for no reason
So your answers are very helpful as I now understand that I shouldn't decrease the area by to much.

Many thanks

 

[EdStainless]

In many cases making the wall thinner has more benefit than changing materials.
In steam condenser service the tube wall is about 3% of the overall resistance.
With higher temp and liquid to liquid the wall is often >2%.
In cases with small delta T and one side gas the wall is a little more important, but I have never seen it over 5%.

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P.E. Metallurgy, consulting work welcomed

 

[georgeverghese]

This "water" you've got as a coolant for R410 may warrant more attention: What is this water? Open loop or closed loop ? Treated or untreated? What treatment?
Water is, generally speaking, corrosive.
a)Check materials suitability to handle corrosion from water
b)Check that fouling coeff for water side suits the type of water

These fouling coeffs alone would more than negate any small increase in heat transfer efficiency from switching over to "copper". There are several grades of copper alloys moreover, some with better corrosion resistance than others at handling water in general.

If you are looking for a compact heat exchanger, there are some compact all welded plate heat exchangers on the internet, these enable much higher heat transfer coeff for a given footprint compared to old fashioned pipe in pipe units. Cooling water for these compact units will have to be filtered, treated and passivated and operate in a closed loop.

 

[Christine74]

My experience matches EdStainless. I've never seen a situation where the tubes accounted for more than 5% of the thermal resistance and 2-3% is typical for tubular heat exchangers. So if you changed to copper you might end up with something like 2% less required surface area.

Rarely does thermal conductivity even factor in when selecting a tube material. Tube metallurgy is almost always chosen based on corrosion resistance and cost.


-Christine

 

[IRstuff]

AND, copper is expensive enough that people steal house wiring from abandoned houses to sell.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[Ron10000]

Hi All,

Sorry for not replying to each one separately.
I will try to reply to all at once

IRstuff, I am trying to build a condenser, the r410a should warm up the water.

georgeverghese, the water that are flowing are in open loop, to say that the water of the tank itself flow trough the heat exchanger.
That is why I chose to got with a pipe in pipe design that will not clog easily and also that is easier to maintain.
I saw a system that contains a plate heat exchanger with a close loop of treated water and a pump to spin these water in a loop, it also requires an expanding tank and a pressure valve. Finally these close loop water go trough an internal heat exchanger inside the tank. It turns out extremely expansive and that is why no one installs it, it would take 10 years to return the initial investment.
I am trying to design an affordable solution, even if it takes a little cost in efficiency.

EdStainless and Christine74, would this also be true for an extremely low conductivity coefficient pipe? For example I know of a pipe that is used for underfloor heating, it is very cheap and very comfortable to handle. It has conductivity coefficient of about 0.45 W/m K. Do you think it is something worth testing?

Thank you all this is very helpful

 

[EdStainless]

Virtually all metals have cond values in the range of 20-200W/mK.
Now you are talking about something that is both fairly thick and two orders of magnitude lower.
That would hurt you a lot.

For water cooled loops people tend to use Cu and brass because it resists the growth of microbes.
Biofouling is one of the greatest long-term reducers of performance.
Just use thin Cu tube and be done with it.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[georgeverghese]

If your water is going through an outdoor evaporative cooling tower, you can expect high maintenance with having to clean up the HE often with microbial fouling. After some time, plain Cu or plain CS tubes may leak also. You may get a little more life with naval brass / admiralty brass or better yet, with Monel. A closed loop treated, passivated cooling water loop is much better.