11 replies to this topic

[curious_cat]

Are there any broad guidelines or heuristics to evaluate where an Air Cooled HEX might make sense over a conventional Cooling Water condenser loop?

 

Currently, all our distillation-condensors are CW cooled and none air cooled. We are planning some capacity expansion and this might be a good time to change over to air cooling, if it makes economic sense. 

 

Any tips? In typical plants others on the forum have seen, which sort of situations had air cooling. What are the capital / running cost implications for both options? The heat transfer coefficients I've seen make me think the Air Cooled option has a significantly larger Cap Cost due to the larger heat transfer area needed?

 

I don't know what else is relevant data but some items: Our ambient air is fairly dry and 10 C to 30 C. 

 

Most of our condensers are Stainless Steel and somewhere between 80 C and 140 C and need  a fairly small area (10 m2 to 40 m2). We are a fine chemicals facility. Does air cooling only make sense for large setups?

 

Would love opinions.

[Padmakar Katre]

Hello,

Please let us know what is the driving force to change from cooling water to air coolers? Any operating issues with these exchangers? Air coolers are mainly used when you have CW related issues at higher process side inlet temperatures. Looking at the area requirement a double pipe or a hair-pin exchanger will work better for you. Addition to this installation of an air cooler in an existing plant may have space issue, piping routines. I don't deny the possibility of using air coolers for your case, you can have single common unit with different bundles catering to each column ovhd vapor condensation.

[PingPong]

We are planning some capacity expansion

What does that mean exactly?

Are you adding new columns and condensors?

Or are you debottlenecking existing columns?

 

In general CW exchangers are cheaper than aircoolers, provided that cooling water availability is not a problem.

 

If, due to the capacity expansion, your existing CW system would in future be too small, then you could add aircoolers upstream some of the existing (largest duty) CW exchangers, and use aircoolers for new condensers.

[Sharma Varun]

Pingpong here has a point and my limited knowledge tells me availability of cooling water is one of the major deciding factors, for a refinery we had all fin fan cooler condensers due to same.

 

Edited by Art Montemayor, 22 September 2013 - 03:09 PM.

[curious_cat]

 

We are planning some capacity expansion

What does that mean exactly?

Are you adding new columns and condensors?

Or are you debottlenecking existing columns?

 

Both.  Debottlenecking some columns using various strategies (new packings, pre-heaters, etc.) is feasable. In some cases we will need to add a second parallel column and split flows. 

 

 

In general CW exchangers are cheaper than aircoolers, provided that cooling water availability is not a problem.

 

If, due to the capacity expansion, your existing CW system would in future be too small, then you could add aircoolers upstream some of the existing (largest duty) CW exchangers, and use aircoolers for new condensers.

 

Understood. This is exactly what I wanted to know. Thanks!

[curious_cat]

Hello,

Please let us know what is the driving force to change from cooling water to air coolers? Any operating issues with these exchangers? Air coolers are mainly used when you have CW related issues at higher process side inlet temperatures. Looking at the area requirement a double pipe or a hair-pin exchanger will work better for you. Addition to this installation of an air cooler in an existing plant may have space issue, piping routines. I don't deny the possibility of using air coolers for your case, you can have single common unit with different bundles catering to each column ovhd vapor condensation.

 

Thanks for the comments! 

 

Driving forces are several:

(1) I know Air Cooled HEX is an option & don't want to skip it from consideration this time.

(2) Current CW condensors will not suffice post-expansion

(3) Our cooling towers will be a bottleneck post expansion if we use conventional HEX for all duties. So the cost tradeoff is between going for additional CW capacity versus adding in some Air Cooling loops. 

 

We are having no abnormal issues with our current CW condensors. Our process inlet temperatures are not too hot; hottest condensor is ~140 C. 

[abhi_agrawa]

Curious_cat,

 

A. What is the condensation temperature for the coldest Air Cooler that you intend to use? The temperature approach between process outlet and air outlet is important

 

B. Please be advised that the pressure drop for an air cooler is generally higher than for a shell and tube exchanger, do verify that this does not become an issue.

 

abhishek

[curious_cat]

A. What is the condensation temperature for the coldest Air Cooler that you intend to use? The temperature approach between process outlet and air outlet is important

 

80 C. On the hottest summer day our air would be ~40 C. Looking at the comments, it might be wiser to use it for the hottest condensors which are ~140 C. 

 

 

B. Please be advised that the pressure drop for an air cooler is generally higher than for a shell and tube exchanger, do verify that this does not become an issue.

 

The  process side pressure drop? If so, I might have to exclude any vacuum condensors. Those are pressure drop sensitive. 

[abhi_agrawa]

Curious_cat,

 

A. Usually I have seen that the process outlet temperature for air cooled condensers is limited to be at least 15-20 deg C above the maximum design air inlet temperature. Most of the people use 5 deg C margin on air inlet temperature also. That would mean if your highest air inlet temperature is 40 deg C (in summer), the process outlet should be at least 60 - 65 deg C.

 

B. Air coolers usually have a higher pressure drop (process side) as compared to shell and tube. However, if there is a big concern on pressure drop, then the same can be lowered by using more tube / larger tubes. But this has economic implications as the cost would go up.

 

It is best to get in touch with an air cooler manufacturer for your application and work with them to see what would be a good design for the air cooler and then evaluate any impact on the process as well to determine if air cooler would be suitable for you application.

 

abhishek

[Bobby Strain]

If it makes economic and enviromental sense, do it.

 

Bobby

[PingPong]

The  process side pressure drop? If so, I might have to exclude any vacuum condensors. Those are pressure drop sensitive.

Pressure drop is a matter of design. An aircooled condensor can usually be designed for the same pressure drop as a CW exchanger.

They can also be used for vacuum condensors, usually in a sloped down position so as to minimise liquid hold-up and associated pressure drop.

They can also be in the form of an A-frame, which are often used as steam turbine exhaust (vacuum) condensors:

                                   

Anything is possible, provided you are prepared to pay for it.....
 

Best make a list of all CW users, with duties, in- and outlet temperatures and CW flowrates. That gives you an idea where the really big CW users are, and where the most can be gained in terms of debottlenecking the CW system.

Replacing vacuum condensors by aircoolers is best left as a last resort, but if necessary it is not impossible.

[gegio1960]

curious cat,

in your situation I'd suggest the following (of course teh advices are based on my experience, that could be very different from the one useful for your plant):

- go with ACHE for all the new columns. If you find space for the column, you'll find it (more easily) for the ACHE too;

- consider the impacts of the revamps on the existing S&THE. Maybe you can

• "easily" revamp them (eg bundle replacement) or
• you can shift part of the excess duty to the new ACHEs you're already considering (parallel use) or
• you can put a new ACHE upstream of the ex S&THE (series use) or 
• totally replace the ex S&THE with a new one

- (never adopted, just for brainstorming) since your air seems to be quite "fresh", you can also consider to go up with the CW outlet temperatures from the single S&THE and then remove the excess heat in a common ACHE before entering the existing CT.

 

That seems an interesting study to be made,

good luck!