Shell & Tube Exchangers in Series

[PetroBob]

We are proposing to install a new shell and tube exchanger physically close to an existing shell & tube exchanger. Both existing and new exchangers use hot glycol solution to heat process water. The existing exchanger #1 has a high enough glycol discharge temperature to serve as inlet to new exchanger #2. My question is: Are there any particular process considerations or concerns with installing exchangers in series (on the glycol side)? Most designs I've seen have hot glycol always from hot glycol header and cold glycol always discharge to cold glycol header (ie all in parallel. Why is this?

NB - We would control the 2nd exchanger water outlet temperature by using a bypass flow control on the glycol side.

 

[MechAg06]

I see two immediate areas of concern. Here are the questions that I would ask if it were proposed to me.

1. Can over-performance (e.g. clean condition or extra-cold process supply temp) occur in the first shell that will reduce the heating medium temperature to the second shell? Can the associated impact on LMTD be incorporated into the design of this new service? Is it feasible that an upset could exceed your predicted bounds right when maximum performance is need out of the new shell? What risk is associated with this?

2. You mention a glycol header. Is the heating medium supplied off of a larger shared utility? If so, have you done a hydraulic analysis of the utility system? The new shell will add additional hydraulic resistance to this specific branch of the utility network. Since all branches must see the same pressure drop across them, the glycol flow will rebalance as it seeks this equilibrium. Unless some means exists to externally redirect the flow, a portion the glycol will be redistributed to the other branches. This stands to 'starve' the existing exchanger of its heating medium. The degree to which this occurs depends upon how much additional hydraulic resistance you introduce and the relative size of this branch to the larger network.

 

[davefitz]

One issue related to series configuration is if one HX is out of service or not transferring heat at its design rate then the duty of the downstream HX may increase to a level greater than anticipated by the designer. This would lead to higher temperature differentials across the tubesheet ( if it is a U-tube design) than the designer assumed, leading to tube leaks at the tubesheet joint.

"Whom the gods would destroy, they first make mad "

 

[PetroBob]

Very useful replies - good points. Thanks MechAg & Dave.

 

[GenB]

What you are doing is superheating. As long as you control the primary heat input and if the HX is sized correctly I do not see any problems.