13 replies to this topic

[jprocess]

Dear All:
For by-passed air cooler, the design temperature of the downstream equipment, if any, shall be the maximum upstream operating temperature of the by-passed exchanger.The maximum operating temperature downstream from a cooler is achieved in case of loss of cooling.So it is equal to the maximum temperature upstream from the cooler.For the air cooler without by-pass, natural draft account for 20% of design duty.
As you see the decision of having by-passed air cooler or not have a great impact on downstream equipment design temperature and material.
So which parameters should be considered to make this decision?
Thanks in advance.
Cheers.

[pleckner]

I'm not sure where you are getting this 20% reduction in design duty from, perhaps API's recommendation when doing relief calcs? This is not a hard and fast rule, only an allowance by this body.

I would go with the upstream temperature as if it were a by-pass, but that's me. By the way, I might not add my usual 50 degress F "fat" factor in this case but I would have to think this one through a little more.

[jprocess]

Dear Phil,
I stated that we use only 20% of design duty and not, reduce the design duty by 20% !
This is a common practice that I found in lots of design criterias.
But my question is not about setting design pressure that is I want to know when we use by-passed air cooler and when we ignore it?
Cheers.

[pleckner]

My mistake. I did mean to paraphrase your statement of "...20% of design duty".

You say you found this in "lots of design criteria". Where? Individual facilities have their own design criteria but they don't develope their own. The reason they seem to all be the "same" is that they just adopt the same one and call it their own. Therefore, it is only one design criteria multiplied several times so we should be cautious on how much we want to rely on these as being cross-checks against each other.

Enough said about that.

I go back to what I said about design "temperature". I would use the temperature of the stream prior to the air cooler as the basis for the design temperature of the immediate downstream equipment.

[Padmakar Katre]

Dear JProcess and Pleckner,
Just during reading this thread one doubt I came i.e.Should we consider the temperature fall due to natural air circulation in case of the air cooler failure (which is similar to cooling water failure case in case of other coolers/condensers-or does it mean something different i.e. air cooler bypass means the literal meaning of it ) while deciding the design temperature of the Air cooler downstream system?

[pleckner]

That is the question from JP, isn't it?

My take is no, do not take into account any drop in temperature due to natural air circulation when determining downstream equipment design temperature.

[JoeWong]

That is the question from JP, isn't it?

My take is no, do not take into account any drop in temperature due to natural air circulation when determining downstream equipment design temperature.

Phil & Padmakar,
Sorry to step in...

JP,
Your first post stated...

"As you see the decision of having by-passed air cooler or not have a great impact on downstream equipment design temperature and material.
So which parameters should be considered to make this decision?"

The question is opinion related to design temperature with or without air cooler bypass

Your third post stated...

But my question is not about setting design pressure...

Phil responded your post with design temperature instead design pressure...

...that is I want to know when we use by-passed air cooler and when we ignore it?

This very much subject your operation and design intend. I doubt any one can advise with the limited information.

Your questions in first post and third post are not really tally...



I guess your question is

If i there is a bypass across air cooler, should the downstream system design temperature equal to upstream design temperature ?

My opinion is the pressure drop across bypass valve (or air cooler) is low. JT effect will be minimum. I would make my downstream system design temperature same as upstream system design temperature.

If there isn't any bypass across air cooler, should the downstream system design temperature equal to upstream design temperature ?

Phil has taken a conservative approach by ignoring the allowance the natural cooling. Phil also clearly stated that API has provided the allowance (nominal 20% of design duty) to take into account of natural cooling.

Let me provides you additional information :
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 cooled heat exchanger', Berryman (1983)
2) "Behaviour of an air-cooled heat exchanger natural convection performance", Henry (1988)


Personally i would think you may consider natural cooling (during design phase) if
- design to API Std 521
- design basis has not prohibited you doing this
- considering fin in good condition
- provide protection system (high-high temperature trip with shutdown valve) against continuous feeding of hot fluid to downstream equipment

You have to reconfirm this allowance (magnitude compare to what your have considered) during Detailed design phase and confirm protection system is sufficient with SIL study.

Now back to real application experience, i have the feeling that there will be not much differences by taking or not taking the allowance. If your upstream design temperature is 100 degC and with natural cooling, the downstream design temperature is 70 degC. I don't see much difference by setting downstream design temperature at 70 or 100 degC. Of course there are exceptional combination...If your upstream design temperature is 200 degc and with natural cooling, the downstream design temperature is 70 degC. then it may be worth to consider the allowance from optimum design perspective.


JoeWong

[jprocess]

Dear All:
Thanks a lot for your valuable comments.
To padmakar:
This is a convenient rule of thumb that for by-passed air coolers we can include cooling due to natural heat transfer.The proposed value is 20% of design duty which is near to values indicated by JoeWong.
I think my question is clear.
Deciding to use by-passed air coolers or not, shows its effect on downstream equipment design temperature.Because for by-passed type we can consider 20% cooling due to natural convection and for those which are not by-passed we should use the maximum upstream operating temperature.
We can also follow the Phil's recommendation but as JoeWong said it can be too conservative in some instances.
When I read this part of design criteria I do not know that who determine to have by-passed air cooler?And when to use by-passed and when not?
I want to add an additional question:
In any configuration the maximum upstream operating temperature should be known.How to specify it?
Should we decrease the efficiency of upstream compressor in simulation file to a value lower than normal efficiency?
Your valuable comments are appreciated.

[pleckner]

@JP:

.... I do not know that who determine to have by-passed air cooler?And when to use by-passed and when not?

It is the process engineer's job to determine this. When and whys are based on such factors as operability, controllability, safety, etc. It is no different than determining the design of any system. You're also not going to always get the same opinion about how to design certain things and no one is right or wrong in many cases.

In any configuration the maximum upstream operating temperature should be known.How to specify it?
Should we decrease the efficiency of upstream compressor in simulation file to a value lower than normal efficiency?

No, I wouldn't throw an artibrary reduction in equipment efficiency just to get a higher operating temperature. This is why you take your maximum operating temperature you can ever expect to get or an upset temperature and still add the "fat" to that number to come up with your design temperature. The "fat" should cover the unexpected.

One last thing, I don't think my criteria is over-conservative and I think you may be misreading what Joe said. I doubt you'll get a 200 C stream cooled to 70 C by natural air flow.

[JoeWong]

JP,

Taking conservative approach does not mean BAD. There is no right or wrong in design approach...It's also subject to other factor i.e cash rich or poor, schedule driven or not, operating and maintenance philosophy, etc.

In situation where project is
- cash rich
- schedule driven, can't afford to lose time
- project request less rework, less optimization
- VERY low air cooler capacity which lead to low temperature gain and minimum incentives for optimization,
- etc

i will make the downstream design temperature same as upstream design temperature...


Phil,
"I doubt you'll get a 200 C stream cooled to 70 C by natural air flow."...
I hope you (and other readers) understand and accept this is a bad example ("joke") created (without reasonable technical thinking) just to shows magnitude of incentive for optimization...

My apologize for this bad example...

Thanks for "knocking my head".

JoeWong

[jprocess]

Dear Phil,
About your statement:
"When and whys are based on such factors as operability, controllability, safety, etc. It is no different than determining the design of any system."
This is a general advice.Could you please explain more about it?

[pleckner]

@JP:

Your question was:

.... I do not know that who determine to have by-passed air cooler?And when to use by-passed and when not?

And I responded: It is the process engineer's job to determine this. When and whys are based on such factors as operability, controllability, safety, etc. It is no different than determining the design of any system.

It is you who decides how you are going to configure your process. You can use go-bys from other sources and that is OK if the appllication is the same or similar. Perhaps in one case you can use a by-pass and perhaps in another you can't. Only the Process Engineer for the system knows this. Some people like to control downstream temperaure using a hot-gas by-pass and others would rather throttle the cooling medium (change fan positions or shut them off for air coolers). I don't know what your specific problems may be so I can't judge which is better. Is your sysem big so it has limited turndown? Is the air cooler you bought equiped with adjustable fan angles, louvers, what?

So it is up to you.

[jprocess]

Dear Phil,

Is your sysem big so it has limited turndown? Is the air cooler you bought equiped with adjustable fan angles, louvers, what?

The system turndown is 30%.
The project is in engineering stage so there is no air cooler at this time.
By my question I wanted to know about your experiences about by-passed air coolers.

[pleckner]

Projects I've worked on where we used air coolers didn't have by-passes. It's been a long time so I can't remember the details.