Design of Vertical Falling Film Evaporator

There may be some features in common, but mostly it is different. (And plate/frame heat exchangers are also somewhat different.)

thanks for your reply..what about a falling film evaporator with tubes inside..do the design procedure is the same with shell and tube exchanger?

meaning, using the typical design procedure as the shell and tube, using the Kern's method..i'm quite new to this equipment and are somewhat confused..i need to use this equipment for my design project ;to concentrate ammonium nitrate..

I have done quite a bit of heat exchanger design work from the standpoint of selecting a suitably sized unit from manufacturers' published data, and some years ago I designed shell-and-tube exchangers "from the ground up." In those days we used some conservative "tried-and-true" K values.

A common basis for all designs is Q = K A (lmtd). The thermal conductivity K varies widely, depending on things like fluid velocity/turbulence, boundary layer or film thickness, etc. In Btu/h-ft²-°Flmtd units, I have seen K values from around 50 to 400, which is a large range. There are now micro-channel HX designs that probably exceed 400. Shell/tube exchangers tend somewhat to the lower end of the range, and plate/frame to the higher. I think falling-film tends to lie between; I'm not sure.

Any of these designs can constitute a pressure vessel or pressure piping component on one or both sides, although most falling-film exchangers are atmospheric on the outside. The pressure design varies on which side is a pressure vessel versus a pressure pipe structure versus atmospheric.

I apologize if this is vague, but it is hard to summarize a large field in just a few words.

LMTD (Logarithmic Mean temperature difference) and NTU ( Number of Transfer Units ) -Effectiveness are the two methods which are used to design heat exchanger. You may need to combine it with the design of pressure vessel.

it seems that there are various ways to calculate Area..which are more accurate, by material & heat balance of the evaporator (Geankoplis) or LMTD, using LMTD, velocity is so very2 small.. which causes Reynolds Number to be so laminar..i understand that for vertical FFE the Re should be turbulent..

Dear Friend,

There is difference in the DESIGN PHILOSOPHY for the design of FFE and and shell Tube Exchanger.

As I understand your question it INDICATES CLEARLY the shell tube exchanger perforforms only EXCHANGE OF HEAT or HEAT TRANSFER between the Hot and Cold Medium, and HEAT TRANSFER is governed by SEVERAL FACTORS, where EVAPORATION WILL NOT OCCUR.

If you mean Evaporation is taking place in the Heater then the Vapour formed due to Evaporation is to be removed and then it is NO LONGER a SHELL TYPE HEAT EXCHANGER.

The FFE WHILE HEAT TRANSFER TAKES PLACE PARTIAL EVAPORATION TAKES PLACE,depending upon several factors such as CIRCULATION RATE, FEED TEMPERATURE, PERMISSIBLE VAPOURPRESSURE, and DRAWAL, etc and in this case BOILING POINT ELEVATION due to HYDRO STATIC COLUMN will not arise.

DHAYANANHDAN.S

I'm sorry, but most of that is not really correct. I work every day with shell/tube, plate/frame, and falling film heat exchangers in which condensation or boiling occurs on one side. (It's a medium-large ammonia refrigeration fish-freezing plant.)

yes..for my case the evaporation took place inside the tube..and the steam providing the heat outside the tube..since i couldn't find any design guidelines to design this type of evaporator..i mean the internal (no of tubes, baffles and so on) therefore i thought could the design be the same with shell and tube..

sir,

i am in need of a falling film evaporator for a dairy. kindly advice

Sure. Go out to the market and make a commercial enquiry to equipment suppliers.

Evaporating milk is not rocket science.

Falling film evaporators are constructed like a shell and tube heat exchanger, but operated much differently. The fluid being evaporated is very viscous, such that it can be made to form a film on the inside surface of the tube. This is done by controlling the flow into the tube by means of weirs and flooding the surface of the top tubesheet up to the level of the weirs. The shellside of the FFE is condensing steam, so it provides a constant temperature heat source. The falling film would have a zone where the fluid heats up to the boiling point (as it falls) and after that a boiling zone. I don't know, but I suspect that these act as concentrators of fluid, because as the film fluid falls, some will pass out of the tubes and drip into the bottom head, having some component boiled off.

You would need to find a correlation for boiling of a thin film of a viscous fluid. Heat transfer programs like HTRI's Xist or ASPEN's Shell and Tube Thermal perform this.

how to design the vapor-liquid separator inside this type of evaporator?is the design calculation step same with typical vapor-liquid separator..how to combine both design..shell and tube with the separator?

There are too many design possibilities to give any simple answers.

Area calculations are just geometry. The internal and external area of tubes will differ, but I think one usually chooses whichever is the more "sensitive" side; i.e., wherever the boundary conditions, turbulence, and thermal conductivity are less favorable. (I'm not sure about this, but at least it is reasonable.)

For a S/T exchanger with with evaporation on the shell side, the separator can be an auxilary pipe or vessel mounted above, with one or more connecting pipes. If there is evaporation in the tubes, the separator will be a separate vessel, sometimes called a "knockout drum," "surge drum," or "accumulator."

Most of the FFE's I know of have the vaporizing/condensing fluid on the inside of the tubes (or formed passages), with falling-film liquid outside at atmospheric pressure. However, the whole business can be enclosed in an outer vessel, in which you could have pressure on either or both sides, with equal or different pressure ratings.

(On the other hand, there are also older-style vertical refrigerant S/T condensers with condensing refrigerant in the shell and falling-film cooling water inside the tubes.)

As you may begin to see, many many variations are possible.

A couple of clarifications are warrented. In a FFE, the 'film' is the important term. Just any S/T HX with cold fluid descending the tubes is not an FFE. The tubeside fluid has to be applied to the wall of the tube ID as a thin film.

I had designed and fabricated FFE many decades back for making spice oleoresins. As the name implies, the liquid flows down the iner surface of the tbue as a film. The centre of the tube is free of liquid. The flow is by gravity.Since evaporation takes place as the film travels down, the viscosity increases and the thickness buils up. Since velocities are small, the flow is laminar and heat transfer rather poor and not much controllable.

The falling film finally collects at a sump in the base from where it it can be recirculated or taken off for downstream processing.. There can be a foced flow of a suitable gas (air for most aqueous solutions) that can help remove the vapours.

Usually this technique is used for heat labile materials. A wiped film evaporator is an improvement in which the film is actively spread on the inner surface of larger diameter tubes (20 cms) by a system of vertical wiper blades. Heat transfers and so evaporation can be quite high in hthese and a sigle pass is usually sufficient to achive the desired result.

Bioramani

interesting, we make liquid smoke use WFE (wiped film evaporators) and use oleoresins in our flavoring columns