12 replies to this topic

[shin29]

Dear all,

In our plant we have a air cooled heat exchnger with two fans , and finned tubes .

 there is some need for enahancement of existing duty

what are the ways through which duty of the existing exchanger can be imcreased.

 

I have read some material on tube insertion but no sure how they work and how their calculaiton is done.

 

Kindly advice me .

regards,

Sahil

 

 

[thorium90]

blade pitch, variable speed motor?

[markymaark]

Obvious answers are

1) Increase heat transfer area, A

2) Increase driving force, deltaT

3) Increase heat transfer film coefficient on the air side, h

4) Switch from co-current to counter-current flow

 

Concerns

1) I'm assuming Increasing A is not possible and is not what you are asking about

2) I'm assumingLowering T inlet of air may not be possible for you

3) h can be increased by increasing the flowrate among other methods.  Also, more humid air will raise the heat capacity.

 

Just some thoughts.

[thorium90]

If you really have so much excess heat to waste, perhaps you can consider putting that heat to some better use. Would need more thinking about where to put it and how, but remember heat is energy after all...

Edited by thorium90, 15 February 2013 - 12:38 PM.

[SSWBoy]

Some sites I've been at spray water over the finned tubes in summer to help heat loss. Obviously the location of the fan and if there's anything under it might push you away from this

[srfish]

There are various devices that can be used to insert inside the tubes to improve the heat transfer. A common type is the twisted tape. Some of the air-cooler manufacturers market these. I believe a calculation procedere is in the Heat Exchanger Design Handbook.

[shin29]

blade pitch, variable speed motor?

No sir it is not a variable speed motor

[shin29]

There are various devices that can be used to insert inside the tubes to improve the heat transfer. A common type is the twisted tape. Some of the air-cooler manufacturers market these. I believe a calculation procedere is in the Heat Exchanger Design Handbook.

Dear sir ,

thanks for your quick respose , can you suggest me the name of the book in which such calculation are provided

 

regards,

sahil

[shin29]

Obvious answers are

1) Increase heat transfer area, A

2) Increase driving force, deltaT

3) Increase heat transfer film coefficient on the air side, h

4) Switch from co-current to counter-current flow

 

Concerns

1) I'm assuming Increasing A is not possible and is not what you are asking about

2) I'm assumingLowering T inlet of air may not be possible for you

3) h can be increased by increasing the flowrate among other methods.  Also, more humid air will raise the heat capacity.

 

Just some thoughts.

Sir

thanks for your response

1 . increasin A is not possible unless it is small modification only . if there is some way

2. Sir i simulated  the case on pro II , but by lowring the air inlet is not helpeng in increasing duty as temperature differande remain same . i.e 40 deg c inlet increase to 47 deg C and 20 deg C to 27 ..

3.  Sir from your experiance , can you suggest me same ways to increse air inlet flow.. i quite new to this field

[thorium90]

Increasing the blade pitch might just be the simplest way for you, granted the additional power requirement doesnt overload the motor. The second but more expensive way is to buy a higher rated motor or a variable speed motor. These two methods means no tinkering of the tubes, no complicated calculation etc.

 

Your air inlet is like it says, air, you cant really dictate the conditions of nature nor do you want it do dictate your exchanger operations...

You cant change the humidity of air too, same like above, its part of nature...

I assume your air cooled exchanger is cross flow: with the process fluid in the finned tubes and air on the outside and there is no such thing as changing from cocurrent to countercurrent flow for your air cooled fan exchanger. You also did not quite specify if it is forced or induced draft fan.

Spraying water is not an option if clean water is more expensive than air... its inefficient and a waste of water with unsubstantial improvement in cooling capacity.

Increasing heat transfer area by adding tubes is typically not easy as there normally isnt much space to do that or it be a big job to reweld and reconfigure the tubes. Definitely an expensive idea.

The idea by srfish is feasible but requires some contractor to do it for you unless you are confident of DIY... Also, your tubes are already finned, not sure how much improvement you can get out of tube inserts. It is typically used for shell and tube exchangers though.

Edited by thorium90, 16 February 2013 - 02:07 AM.

[gegio1960]

It depends on the magnitude of duty increase to be managed.

What I've seen, in order of increasing costs:

1) cleaning (are the current performance in line with the original ones?)

2) optimisation of air side (motors, fans). Just for reference: http://www.howden.co...stemdefault.htm

3) change of tube bundles.

As temporary measure, in very hot hours and very critical services, cooling by water spraying (!)

Good luck!

[srfish]

You asked for the name of the book and it is HEDH ( Heat Exchanger Design Handbook).

 

It would help to know what fluid is inside the tubes. If the fluid is in laminar flow, what is changed on the air side will have very little effect on increasing the heat transfer. If the tube side is in laminar flow then some kind of insertion inside the tubes would would be of great benefit. In some cases inserting reinforcing rods inside the tubes gives an inexpensive form of increased heat transfer. But a more beneficial method is to use twisted tape inserts.There are published articles that discuss twisted tape inserts. One is "Heat Transfer and Pressure Drop

Correlations for Twisted-Tape Inserts in Isothermal Tubes: Part 1-Laminar Flows" by Manglik and Bergles (1993). If you would like to know more references let me know.

[breizh]

Shin 29 ,

 

What about the outside of the tubes? Do you perform some cleaning time to time. It's obvious that dirt will affect the heat transfer .

My 2 cents

 

Breizh