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Ache Performance


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#1 sargon 1979

sargon 1979

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Posted 14 August 2017 - 07:35 AM

Hi all;

I have a datasheet for air cooled heat exchanger (ACHE). In this document i found that the vendor has calculated the product UA (Overall heat transfer coefficient multiply by heat transfer area), this product is calculated according to a fixed set of design conditions such as mass flow rates of air and process fluid, air inlet/outlet temperature, process fluid inlet/outlet temperature, pressure drop,etc...

 

If i apply the equation Q=U.A.∆Tlm having the in/out temperatures then i can calculate the duty.

 

 

can i use this product (UA) always to calculate the duty? regardless whatever temperatures i have

Is the duty mentioned in the datasheet considered as the max. duty



#2 Bobby Strain

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Posted 14 August 2017 - 09:17 AM

Probably. But remember that the calculated value uses a fixed fouling factor. I presume that the service is single phase. But you should calculate the duty based on the process fluid enthalpy change, presuming you know the flow.

 

Bobby



#3 sargon 1979

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Posted 15 August 2017 - 08:46 PM

Ok, what happened if i enter the datasheet figures in ACHE unit in Hysys? What figures should i enter?
My headache comes from different results.
For example, the duty is 891 kw at air inlet temp. Of 45℃, air outlet temp. Of 47.87 ℃,air flow rate 144.36 m3/s, tubes area is 32462 m2, U=21
If i enter the duty and inlet air temp. Then, air outlet temp. Will not be 47.87 ℃
And vice versa? Also what is the right result if i change air temp. From 45℃ to 25 ℃. Please help

#4 rdcrags

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Posted 15 August 2017 - 10:56 PM

It may not apply in your case, but remember the transfer rate in the tubes varies with velocity.



#5 sargon 1979

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Posted 20 August 2017 - 06:00 PM

Hi all
How can i establish a heat balance for air cooled heat exchanger? How can i calculate air heat flow required by a specific cooling duty? If i have a design case and some rating cases for an ACHE With different UA values then, is there a fixed value of UA can be always used to calculate the duty?
Thank you

#6 srfish

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Posted 21 August 2017 - 11:07 AM

A fixed UA cannot be used to calculate a process heat duty regardless of operating temperatures on the tube side. The U will vary because of those temperatures.  The air flow calculation uses the process heat duty divided by the product of 0.24 times air temperature rise.






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