25 replies to this topic

[mastergii]

Hey guys,

 

I'm a student designing some heat exchangers. I'm using Hysys, and I get a UA-value from the data. I  then fine typical U-values in literature, to calculate A.

 

What I am struggling with is to find how big plot area this A value, say 15 000 m^2, occupies. I'm finding it difficult to find any numbers, so if anyone of you could point me in the right direction? I guess I need to use some numbers from vendors?

 

I'm designing air coolers and Shell and tube HX. 

 

 

[curious_cat]

Sounds like a really huge S&T HEX. 15,000 m2 area? 

[mastergii]

Sounds like a really huge S&T HEX. 15,000 m2 area? 

I guess I can divide this total area into many HEX. Just need to find out how much area one HEX can handle, to get  a realistic number of HEX

[srfish]

Wow !!, that is a huge installation. For shell-and-tube heat exchangers, that much area would take approximately 25 exchangers of bare tubes. You might take another look at that overall heat transfer coefficient.

[mastergii]

Wow !!, that is a huge installation. For shell-and-tube heat exchangers, that much area would take approximately 25 exchangers of bare tubes. You might take another look at that overall heat transfer coefficient.

i have used 600 W/m^2 C

[Bobby Strain]

If you supply the basis you are more likely to get help.

 

Bobby

[mastergii]

If you supply the basis you are more likely to get help.

 

Bobby

Okey,

 

I have an air cooler in Hysys cooling water.

Water inlet temperature is 45 C

Water outlet temperature is 30 C

Water mass flow: 4.639e6 kg/h

Air inlet temperature is 25 C

Air outlet temperature is 35 C

 

This gives me a total air flow: 2.552e+7 m3/h

Overall UA : 5.481e+7 kj/C-h

 

Any other data I should provide?

[curious_cat]

It's a shell and tube air cooler? I'm confused. 

 

And you really want to 5 million kgs of water every hour?

[mastergii]

It's a shell and tube air cooler? I'm confused. 

 

And you really want to 5 million kgs of water every hour?

 

This is just the Air cooler, not the S&T HX.

And yes, I want 5 million kgs water. It's for process cooling of a LNG Plant in a closed circuit

Edited by mastergii, 04 December 2013 - 01:52 PM.

[Pilesar]

Have you considered a direct contact heat exchanger to transfer heat from the water to the air? Look up "cooling tower" for more information.

[mastergii]

Have you considered a direct contact heat exchanger to transfer heat from the water to the air? Look up "cooling tower" for more information.

I have. But this is a problem I'm assigned to solve. So looking for other solutions are good, but I still need some numbers for my air cooler. 

So back to my problem. I tried this calculator, which gives me a number. But then I won't use my UA-value. 

http://www.gearainey...Calculator.html

 

Any suggestions on how I can solve this?

[srfish]

Since you are a student, I don't want to supply the answer for you. Excuse my British Units but you could use the following information:  A 7 ft. wide section with 4 rows has 154 tubes. This is based on 1 inch tubing with 5/8 inch fins and 10 fins/inch that give 5.58 ft2/ft.

[PingPong]

Please note that the U-value of 600 W/m² does not refer to the finned area, but to the bare outside tube area.

 

To estimate plot space one needs to know the tube spacing (pitch) and the number of tube rows. Number of fins is not relevant.

 

I tried this calculator, which gives me a number. But then I won't use my UA-value. 

http://www.gearainey...Calculator.html

 

Any suggestions on how I can solve this?

You should enter what the calculator asks, nothing else.

Select Water Cooling and you will get an estimated plot area of almost 5000 m², which is H U G E.

Edited by PingPong, 04 December 2013 - 03:23 PM.

[mastergii]

Please note that the U-value of 600 W/m² does not refer to the finned area, but to the bare outside tube area.

 

To estimate plot space one needs to know the tube spacing (pitch) and the number of tube rows. Number of fins is not relevant.

 

 

 

I tried this calculator, which gives me a number. But then I won't use my UA-value. 

http://www.gearainey...Calculator.html

 

Any suggestions on how I can solve this?

You should enter what the calculator asks, nothing else.

Select Water Cooling and you will get an estimated plot area of almost 5000 m², which is H U G E.

So what U-value should I use?

This was much harder than I thought...

[PingPong]

In that calculator you do not need to enter the U. At Description simply select Water Cooling in the pull down menu. The calculator then uses its own built-in U value for that.

 

If you want to calculate plot space all by hand, you could use your U = 600 W/m² although it is probably a bit high.

Use a tube pitch of 60 mm (due to the fins) and 25 mm OD tubes in 3 or 4 rows above each other.

 

Note however that your UA : 5.481 * 10⁷ kJ/^oC-h is not correct for an aircooler.

Duty = 81 MW and CMTD = 6.5 ^oC for 2 tubepasses (F = 0.90) or 6.8 ^oC for 3 tube passes (F = 0.95),

so UA is roughly 12000 kW/^oC = 4.3 * 10⁷ kJ/^oC-h

[mastergii]

 

Note however that your UA : 5.481 * 10⁷ kJ/^oC-h is not correct for an aircooler.

Duty = 81 MW and CMTD = 6.5 ^oC for 2 tubepasses (F = 0.90) or 6.8 ^oC for 3 tube passes (F = 0.95),

so UA is roughly 12000 kW/^oC = 4.3 * 10⁷ kJ/^oC-h

Okey, my UA-value is straight from Hysys, why isn't this correct?

[PingPong]

Read my calculation again.

 

I don't know what you specified in Hysys with regard to exchanger type, et cetera, but for an aircooler with water flowrate and temperatures as in message #7 your UA is much too high.

It is very simple to calculate it yourself by hand. Try it so that you learn something.

 

Note also that your UA = 5.481 * 10⁷ kJ/^oC-h and your U = 600 W/m².^oC would result in A = 25000 m² ,

not 15000 m² as you seemed to think in message #1.

Edited by PingPong, 04 December 2013 - 06:11 PM.

[mastergii]

PingPong: 

 

Okey, thanks!

 

So what about a S&T HEX. I have an U-value from hysys saying 9.45e+7, I guess that woulld mean a huge A as well. I have understood that I cant use the U value from the program, so what U value should I use?

 

the HEX is cooling down water with seawater

Seawater inlet : 10C

seawater Outlet: 25

Water inlet:27C

Water oulet: 15C

Water massflow: 6e+6

Seawater(Using adjust in hysys) :4.84e+6

 

And how would one calculate the totalt plot area?

 

Thank you for your time and help

[PingPong]

I have understood that I cant use the U value from the program

What program do you mean?

Problem with "typical U's" is that every source gives different values, because U depends on velocity of both hot and cold media, and depends on fouling factors considered.

 

To cool cooling water with sea water Plate and Frame heat exchangers are used:

http://www.cheresour...liminary-design

[mastergii]

PingPong:

 

Hysys

[mastergii]

I dont see the point in making a new thread, so I'll just try here:

 

I have a question I hope you can help me with.

 

What will the inlet velocity of the air on an air cooler be? I cant seem to find any sources.

[PingPong]

That depends on the design of the aircooler.

 

Run that GEA calculator and in the output you will see the Inlet Face Velocity.

 

Note that for your aircooler GEA uses a much lower Exhaust Air Temperature than you did, and consequently the air flowrate is double that of your simulation.

[mastergii]

PingPong:

 

I'm just bringing this up again.

 

 

Found a U-value on 120-145 on this page : http://www.hudsonpro...infan/tech.html

It also says the number is based on :outside bare tube surface for 1-inch OD tubes with 10 plain extruded aluminum fins per inch, 5/8 inch high, 21.2:1 surface ratio.

 

So If I divide my UA-value on the a U-value from this internet page, what area will I get? The plot area? 

[PingPong]

When you divide UA by U then you obviously get A, which is the bare outside tube area of the aircooler, not the plot area.

 

When I wrote that U = 600 W/m².^oC is a bit high, I meant that it is probably closer to 500 than 600.

That GEA calculator seems to use something in the range of 450 - 500 W/m².^oC for water cooling by air.

 

The U-values in that hudsonproducts website are not in W/m².^oC but probably in British units: Btu/hr.ft².^oF

[mastergii]

What material and how thick would a plate and frame hex be? Need this information to fina the thermal conductivity

Edited by mastergii, 07 December 2013 - 10:44 AM.