Steam Heat Exchanger Efficiency

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Main Question or Discussion Point

Heya, id like a little help calculating the efficiency of some steam heat exchangers. They are used in industrial dryers, steam goes through the radiator and then air is blowed to it in order to heat the fabric.

Currently my radiators are made from carbon steel tubes and fins, it has 78 flat tubes of 1 ¾” x 64” so if i calculate my area not considering the fins i get: 1 ¾”/12 x 1 ¾”/12 x 64”/12 x 78 = 121 square feet.

Im planning on having them made from cooper tubes and aluminum fins due to better availability in my area and better thermal conductivity of those materials. Since the cooper tubes are circular the new design will have 144 tubes of 5/8” x 64” so my new area not considering the fins should be something like this: 144 tubes x 5/8”/12 x 3.1416 x 64”/12= 126 square feet.

My questions are basically if those calculations of the area (excluding the fins) are correct and how can i calculate how much more efficient will the cooper radiator be vs the carbon steel radiator in steam usage.

I know cooper has a thermal conductivity of around 400 w/(m.k) and carbon steel is around 43 w/(m.k). Should i just multiply that by the area? or how do i proceed?

Heres a picture on the actual radiator im using at the moment.
https://i.imgur.com/czaSy.jpg

Thanks in advance!


PS: I have never worked with anything similar to this so pardon my ignorance on the subject.
 

Answers and Replies

  • #2
Mech_Engineer
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This can be a difficult question to answer, so before you go through the trouble of trying to calculate fin efficiencies and heat exchanger effectiveness we could use some clairification on the application:
... They are used in industrial dryers, steam goes through the radiator and then air is blowed to it in order to heat the fabric.
I'm curious- how much energy are you currently exchanging from the steam, and how much more you hope to achieve with an improved exchanger? Additionally:
  • What is the inlet and outlet temperature and/or pressure of the steam?
  • Is it condensing after the heat exchanger?
  • Are you hoping to retain a specific amount of superheat to prevent condensation?
 
  • #3
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How much energy are you currently exchanging from the steam, and how much more you hope to achieve with an improved exchanger?

My goal is to get the exchangers to 160 - 180 degrees Celsius, currently the ones im using are in bad shape and barely reach 145c ( They reached around 160-165c when in optimal condition under the conditions stated below). Id like to achieve the target temperature while reducing my overall pressure in the system.

What is the inlet and outlet temperature and/or pressure of the steam?

The Steam enters at around 120 psi and at 175 celsius

Is it condensing after the heat exchanger?

After it passes through the exchanger the steam then goes to the boiler area there i condensates and its reused for other purposes.

Are you hoping to retain a specific amount of superheat to prevent condensation?

Condensation in the machine isnt really a problem.

Additional info:

Heated air hitting the fabric (the fabric is in a conveyor that passes thought the machine, each machine has 5 blowers and five exchangers)

http://www.poloshirt.cn/images/0222.jpg

An example of the airflow:

http://www.poloshirt.cn/images/022.jpg

My Machines look something like this:

http://www.wotol.com/images/thumbs/800x800/500143_8337633faf77aca29a321df51b1e0c14.jpg
 
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  • #4
Mech_Engineer
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My goal is to get the exchangers to 160 - 180 degrees Celsius...
Do you mean this is your goal temperature for the air leaving the exchanger?

... currently the ones im using are in bad shape and barely reach 145c ( They reached around 160-165c when in optimal condition under the conditions stated below). Id like to achieve the target temperature while reducing my overall pressure in the system.
What has changed that reduced their performance over time? Oxidation? Steam flow?

The Steam enters at around 120 psi and at 175 celsius
So this means the maximum temperature you can currently reach for the air leaving the exchanger is 175 celcius. Overall changing to copper will possibly increase your heat transfer, the strongest driver will be the number of fins (overall surface area).

You say you'd like to decrease the pressure of the steam but it comes down to the amount of energy you want to transfer into the air flow. How much air are you blowing over the heat exchangers? Do you know in volumetric flow units like CFM? For a given amount of mass air flow, you can caluclate how much energy is needed to raise the air's temperature by a certain amount, and from there you can calculate how much steam you need to blow through the heat exchangers. The rest is calculating approximately how many fins you need and what material you want to make them out of.

Basically what I'm trying to get at is changing the material of the heat exchanger doesn't automatically increase its performance, if you're limited by the amount of steam available for example...
 
  • #5
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Do you mean this is your goal temperature for the air leaving the exchanger?
Around that yes


What has changed that reduced their performance over time? Oxidation? Steam flow?

The exchangers had gotten holes over the years (6 years), ive had to eliminate some tubes in all of them. The quality of my steam wasn't that good so ive had trouble with all my steam related equipment due to oxidation, but that's solved now.



How much air are you blowing over the heat exchangers? Do you know in volumetric flow units like CFM?

Sadly that info i cant get

I can reduce or increase the pressure of the system if needed, but as i said the idea was to see if it was possible to reduce 10 or 5 psi and still get the same performance.
 
  • #6
Mech_Engineer
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I seriously doubt you'll be able to reduce the pressure of the steam while still blowing the same amount (or even more) air over the exchanger. The pressure drives the mass flow rate, which in turn drives your energy flow rate for the system.

Changing the heat exchanger to copper and adding more fins may increase the effcieincy of your heat exchanger, but in the end it's the steam flowing through it that defines how hot your air gets and how much energy you're imparting in the dryer. Keep in mind that copper is pretty good at corroding as well, are you going to plate it or something?
 
  • #7
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Keep in mind that copper is pretty good at corroding as well, are you going to plate it or something?

The steam contains chemicals to prevent that.

And i am increasing the amount of fins as well.
 
  • #8
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When you say the steel tubes have holes, I feel you may have a steam trap issue. If you are not ridding the system of air, condensate, and non- condensables you will create carbonic acid. Carbonic acid is very corrosive and will eat away at steel tubes. It's imperative that the steam traps be properly sized and installed for the specific application. This leads me into the next point of your outlet air temperature. At 120 psig, you have very hot steam and 180 degree air shouldn't be hard to achieve assuming a conservative delta T and consistent cfm. If air is not eliminated, you will trap it in the tubes and you will find that your heat tranfer rate goes down considerably.

To me, the most important thing is your production. Copper tubes & aluminum fins have very good heat transfer rates. Steel & steel does as well. It's my opinion that you splitting hairs trying to calculate the efficiency between the two because when you do an overall cost analysis you may find the cost of the "more efficient" configuration will not have a high enough ROI. Install the steam traps correctly, vent air & non-condensables, get the condensate back to the boiler and you'll be efficient in both energy and production.

I have items available for download (good, useful engineering information) on my LinkedIn profile...http://www.linkedin.com/in/JoeyTenuta [Broken]
 
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  • #9
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You are very certain my damage was due to carbon dioxide, we solved that issue and our steam is top notch now. We do return our condensate back to the boilers, our traps are well placed and in working order and the copper and the aluminum exchanger is around USD 2,000 cheaper. I just want to know if its worth while buying the copper and aluminum one (besides the lower unit cost).
 
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