8 replies to this topic

[daraj]

Most of the time as an amateur process engineerwhen we develop CONCEPTUAL process flow sheets or designs for a future plant or when doing a  competitor assessment type study and when we are modeling columns alonmg with associated heat exchangers we are told to keep the heat exchanger design simple, like force the software(aspen EDR or hysys) to be able to find a design with just one shell for a BEM type exchanger and avoid multiple shells in series or parallel.But in case, the capacity of the plant has to be enhanced or doubled and another design need to be developed, then I guess it would be difficult to keep heat exchanger design to just single shells and will have to consider multiple shells in series or parallel.

 

In this context, my questions are as follows: 

 

1. when do you or how do you decide to switch from a single shell BEM(or BEU) to 2 more or shells in series or parallel? what decides this?is it the shell dimensions or the velocity/pressure drops? are there any thumb rules or values for making this decision?

 

2. How common is it in reality to find heat exchangers that have shells in series or parallel?are they uneconomical or are difficult to handle an dare preferred less as compared to single shells?

 

3. when do you go series and when do you use parallel configuration? which between the two is preferred and what are the pros/cons of each type of configuration?

 

Thanks in advance

[fallah]

Hi,

 

In brief,

 

Parallel HXs: When HX exceeds allowable diameter for big heat duty

 

Series HXs: Having temperature cross

[srfish]

In counterflow type applications, exchangers are added in series in order to increase the tube velocity and thus the heat transfer coefficient. The decision on when to add exchangers is a matter of ecnomics.

[daraj]

fallah and srfish, thanks.

 

How common are these parallel configurations? For instance when I have to stretch an existing plants's capacity or develop a conceptual design for the same plant but with twice as much capacity, then I might have to use parallel configuration, it looks like, to work around the heat exchanger diameter restrictions that fallah mentioned and to handle more heat duty. Are there any drawbacks here?

i.e do you look for an alternative TEMA type that can accomplish the heat transfer in a single shell whenever possible or do you keep existing TEMA type and settles for parallel shells? does this decision depend on just economics? These are the kind of decisions to make on a high level, at the conceptual design stage

 

also for turndown purposes(after you have scaled up the exchanger) is a single shell preferable to having multiple shells or is it the reverse?

[srfish]

The first thing I would do is to check the capacity of the existing heat exchangers. There may of been some changes in the plant operation since the exchangers were installed. There also may have been excess surface in the original exchangers.

[daraj]

srfish, this is a conceptual design for a future plant, assume. Nothing to do with any existing plant. how to proceed? I have a huge duty, large vapor load for a condenser(as well as reboilerZ). If I use single shell i get a huge surface area in simulation tool(>2000 m2), so iam thinking of putting shells in parallel

[srfish]

Wow!, Those will be very large heat exchangers. There needs to be more information available in order to make a decision. For example: The type of industry, design pressures, materials,etc.

 

The type of industry such as gas processing,refinery or chemical plant. This will determine the maximum allowable tube length and type of heat exchanger.

 

1000 m2 surface is considered the maximum amount for a single heat exchanger.

[daraj]

srfish, thanks. type of industry=chemical,not refinery. type of exchanger i used in simulation= BEM. this is for reboiler. Using BKT reduces area. That is another thing, when there are more than 1 type of TEMA that can be used in a conceptual design, how will you pick? especially if you need to scale these up in the future?  I can just keep adding shells in parallel to increase capacity but they willincrease cost of BEM.I also might have to worry a bit about turndown(but not a lot, still this is just conceptual stage)

[srfish]

If there is "more than 1 type of TEMA that can be used...how wll you pick?". Pick the least expensive type. For example a BEM type is less expensive than an AEM or a NEN.

 

Earlier I posted that the maximum amout of surface in a single shell-and-tube is 1,000 m2. Yesterday I was checking a quote that a manufacturer made that had a lot more surface than 1,0000 m2. it was over 2000 m2.