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These days i'm interesting in the shell and tube thermosyphon evaporator, it is new for me. could sb tell me some information? for example: what is the optimal recriculation rate? which side is annomia in? is it the same to thermosyphon oil cooler in designing?
where can i get more details about this type of evaporator??
thanks
cygbob
In a shell & tube thermosiphon heat exchanger the ammonia is usually on the tube side. The liquid being cooled is on the shell side.
You design these as you would a thermosipon oil cooling system. Except in this case, the surge drum is normally mounted on top of the heat exchanger. The recirculation rates are normally designed for 3 or 4.
Thanks, US_Iceman!
is there any other difference between this type and the DX type execpt for the recirculation ?
In DX it can have more tube passes and is it also do for thermosiphon evaporator? I think it may have some resistence problems .
Thanks again
The thermosiphon on the tube-side is normally a single pass arrangement. DX chiller usually have multiple passes.
The shell-side is similar to a DX design. The baffle spacings are adjusted for the heat transfer and shell-side pressure drop.
As noted, the ammonia (or tube side) is kept a single pass design to minimize pressure drop. Often the bundle is tilted a few degrees with the outlet end a few inches higher than the inlet to aid in establishing a positive flow in the right direct and to prevent the nit from getting gas bound.
One particular problem with thermosiphon designs is extended operation at low loads. There will not be sufficient velocity to carry the oil up to the thermosiphon drum. They can become oil logged and suffer from poor performance.
Ken
if the tube side is just one pass, i think the heat transfer codfficient just has a little inprovement, and it also much lower than the flooded type, what other advantages?
Thermosiphon designs are best used when the cooled process is viscous such as high concentration and cold glycol solutions or like applications. You also need to look at the heat transfer coefficients and determine if the shell or tube side is controlling.
A flooded cooler will not perform well on the viscous fluids like a thermsiphon.
One pass on the tube side is used to keep the pressure drop down and to aid is keeping the flow moving. You typically design an ammonia thermosiphon chiller to operate around 3 or 4 to 1 circulation ratio.
The thermosiphon design is actually better heat transfer due to the turbulence as compared to a pool boiling design.
The trick to TS design is to get adequate static head to have it over come liquid feed losses plus the heat exchanger losses and the liquid vapor return losses.
Ken
TXiceman raises a good point. Since the viscous fluid is on the shell-side the baffle spacing is adjusted for optimum heat transfer and pressure loss.
If the viscous liquid was on the tube-side the only thing you can "adjust" is the number of passes or do some surface enhancements of the tube ID.
I got that, in some projects we designed the Te<-15℃,we mostly took the DX evaporators because of the flow resistance.
Comparing with the DX, what difference in the calcluating of flow boiling heat transfer coefficients and the controling evaporating temperature, mass flow?
So much questions, I really appreciated your kindness and reply, thanks!
cygbob
Here are some sources I found quickly.
http://www.rit.edu/~taleme/Papers/Jo...apers/J038.pdf
http://www.aseanenvironment.info/Abstract/41012196.pdf
http://www.osti.gov/bridge/servlets/...able/12396.pdf
http://www.wlv.com/products/databook...ta/db3ch10.pdf
That should keep you busy.:D
The Wolverine paper is pretty good. It is often less expensive to use DX in smaller projects, but TXV valves top out on size and limit the design.
One thing to consider is that you loose some of your heat transfer surface in a DX to superheating the refrigerant. Heating gas into the superheat region is far less efficient than using the full tube length for cooling.
To decided on thermosiphon, or flooded deign, you have to decide which side of the exchanger will be controlling. Here you look at the inside area ratio to the outside area ration as you inside and outside heat transfer coefficients.
Depending on the fluid being cooled, you can sometimes do better with a larger OD prime surface tube than a smaller finned surface tube. DX does not do well with large diameter tubes so you need to stay with the smaller diameters.
There are many papers written on two phase flow and boiling in a tube and pool boiling of refrigerants. Another source of data is HTRI, Heat Transfer Research Institute.
You do have to be a member to use the data. B-JAC has gotten much better data on refrigerant exchangers than they had say 10 or 20 years ago.
Ken
It's been a while since I have done the calc's but it seems the larger tube OD's offered a higher Reynolds number (and inside film coefficient) with the benefit of a lower pressure loss through the tubes. Does that sound about right Ken?Quote:
Originally Posted by TXiceman
For a long time, almost all of the NH3 flooded chillers used 1.25" tubes for this reason on viscous fluids.