Hello All,

I was going through Relief Philosophy prepared by our company where I have found something which I was unable to digest. For sizing PSV mounted on Fractionation column, several cases were considered. In that one of case was partial power failure. Partial power failure for that considers air cooler fans will not work but the reflux pump (Turbine driven) will be working.

Following points were consider for partial power failure:

1) For induced draft fans will have 30 % of normal cooling duty
2) For Forced draft fans will have 15 % if normal cooling duty

My question is now for partial power failure the cooling would be by natural draft only then why there is difference for induced draft and forced draft because any way the fans will not be working ???

Thanx n Regards,

Gunjan

Gunjan,
I guess...this mainly due to construction of induced draft compare to forced draft arrangement.

Induced draft fan arrangement have following advantages :
- Better air distribution across the bundle. (no disturbance by fan blade when it stop)
- Less possibility of hot effluent air recirculating into the intake

Hence a higher cooling effect can be achieved by a induced draft arrangement.

Above only from qualitative point of view...

Please read...API Std 521 section 5.6...the quoted % (initial guess) can always be validated with actual configuration.

Hope this help.

JoeWong

Thanx JoeWong...

I agree with you that there is difference in arrangement for forced draft and induced draft. But my question is when the fans are not working (partial power failure), cooling is due to natural draft only.
So, at that point it is mentioned that cooling is 30 % for induced draft and 15 % for forced draft of total cooling in normal condition. why there is difference when cooling mode will be same i.e. fans are not operational ????

Natural draft involve air movement but with lower magnitude (15%-30%) compare normal operation with fans...

JoeWong

Gunjan,
Works by Berryman (1993) and Henry (1988) has indicated that heat removal from induced draft and forced air cooler in the range of 20-40% and 5-15% respectively...

Reference ;
1) "The Natural convertion performance of an air colleed heat excahnger', Berryman (1983)
2) "Behaviour of an air-cooled heat exchanger natural convection performance", Henry (1988)


JoeWong

Well JoeWong..

I dont have access to the books you quoted. I am looking for the reason why there is difference in cooling rate for forced draft & induced draft when the cooling will be by natural draft for both in case of power failure...??????

Cheers,

Gunjan

Gunjan:

I think I have an understanding of what is troubling you and preventing Joe from helping you directly through his references. The source of your dilemma, I believe, is due to what I consider the worse insidious problem in engineering: bad engineering communication due to bad writing. And the bad writing I’m referring to is that found in your company’s “Relief Philosophy”.

I have been through many Hazops, Design Reviews, PSV “corrective” projects, and have written quite a few operations and maintenance manuals during my 47-yr career. One outstanding obstacle to converting the information discussed and found in each engineering effort has been the correct, accurate, and understandable English employed in writing the minutes, the manuals, and the Management of Change documentation involved. It seems as if most Chemical Engineers have been totally asleep during their English or writing courses in High School and through University. And I don’t refer strictly to English Courses. I know that Universities in other countries teach their equivalent of English in their native language – but the engineering graduate produced still comes out with an inability to communicate correctly and accurately in their native language as well as in other languages. It’s not about the language – it’s about the thinking and mental process of organizing and transmitting your thoughts accurately.

Now let me explain what is happening, in my opinion, in your particular case. You are correct in trying to understand what to take into consideration when you have a partial (as opposed to a 100%) power failure. Your situation is a very practical and rational one: you need to know what the cooling duty in your induced fan coils is when the electric power fails to those fans. The engineering directive you find in your “Relief Philosophy” states:

1. For induced draft fans will have 30 % of normal cooling duty
2. For Forced draft fans will have 15 % if normal cooling duty

The above, in my opinion, is bad communications. What should have been correctly written is the following:

1. In the case of an induced fan coil application, approximately 30% of the normal cooling duty should be assumed to be transferred during a partial power failure when the coil’s fans cease to operate;
2. In the case of a forced fan coil application, approximately 15% of the normal cooling duty should be assumed to be transferred during a partial power failure when the coil’s fans cease to operate.

I believe that my description is pretty explicit and explains what you need to know. Joe Wong was also trying to explain this – but by explaining the mechanical details involved between an induced and a forced draft installation.

Note how easy it is to mis-communicate. I don’t know if the following is literally found in your “Relief Philosophy” or if you committed a typo error, but the word “if” should be replaced with “of” because the meaning changes:

“For Forced draft fans will have 15 % if normal cooling duty”

Joe Wong correctly addressed your basic question: “why there is difference for induced draft and forced draft because any way the fans will not be working”. He answered by directing you to explanations over the difference between the induced and forced draft fan designs – which is what you asked for. The explicit answer is that when a coil is left without a fan to force air across its heat transfer area, the coil still retains an ability to cool its internal fluid through the creation of natural draft air currents that are created due to the temperature differences between the atmospheric incoming air and the coil surface – very similar to a thermosyphon effect. There is a difference in the cooling rate between both because of the physical positions of the fans are different and also because of possible baffles and louvers that are used.

Mr. Montemayor,

Good explanation...I would like to take this oppurtunity to thank you for your kind assistance.

Appreciate.




JoeWong

Thank you very much Mr. Montemayor for your clear explanation. I have understood now what Mr. JoeWong was trying to explain. I am really very grateful for your efforts to explain the same. We are really very fortunate to have Veteran Engineer around us to clarify our doubts.


Regards,

Gunjan

Hello,
I am new to this site. I am Chemical Engineer working in refinery.
I am really Inspired by Mr.Art Montemayor reply.

Great.

You better understand.

how to take help from mechanical engineering design and how to weigh percentage to mechanical engineering design.

Your job is to check the type of fan installed and consider the percentage accordingly.

Regards