Falling Film Heat Exchanger

[homerphish]

What would some of the advantages and disadvantages be of running pumped liquid ammonia to a falling film heat exchanger instead of going flooded with a surge drum?

 

[sterl]

Not Much in terms of overall efficiency....

A really well designed Flooded Drum system can operate at 12 or 15 to 1 recirc ratios at fairly high heat flow rates.
The peak rate is only achieved at one set of conditions, and if everything is working right, it will coincide with your peak load condition. A really critical design will only work well within a limited range of the Peak, but that is not "industrial" design, most arrangements are much more tolerant than that. For a falling film chiller on well water, that means it works well for those days when water is high in temperature and consumption is steady...So if you've got a seriously "lumpy" load cycle, you may need some flywheel, as in a sotrage tank....

And speaking of steady: the large volume of already cold refrigerant in the Drum gives you a high tolerance for short term load swings. SHORTCOMING: Pipe has to be done right or the HEX will accumulate oil...
By Contrast:
Most pumped recirc arrangements are adjusted for more like 4 or 5:1 on R-717. They will work just as well at Low Loads as they do at high loads, as long as there is no vertical risers on the vapour side. Control schemes have been devised to address what happens in a tall riser (pipe) at low loads, but the brief version is: Evaporating pressure rises to higher than suction pressure due to the column of liquid in the riser, once the pipe starts acting as a separator rather than a conduit for 2-phase flow.

Lots of data out there on how to avoid the accumulation of liquid in the riser, but the only performance condition that would cause me to consider replacing an already in place surge drum would be an impossible oil return/oil extraction arrangement, which a well designed recirc arrangement would displace. Even a very good gravity flooded system is occasionally going to need something to address oil extraction....at these temperatures, stills and transfer arrangements can work, but at lower temperatures and high oil viscosities: You will encounter some very detailed problems getting oil to move.

Hope this helpful. If you have an identifiable problem condition, with compressor proximities, pipe sizes, orientations, relative elevations and so on we'll try to get more specific.