Expert: Daniel Mazur - 10/20/2007

Question
I'm a bt older. I have never been to uni nor 12th year but my twin daughters, who we home schooled to year 10, are both at uni if that helps.
I am a farmer trying to do a bit of desalination of sea water and am aware that vapporisation will occur at much lower temps in a reduced pressure vessel.
My question is ; If I accessed warm seawater of 30Deg C and misted this water into a vessel at pressure of 1 psi a certain portion of the water would stay as a vapour. 16Deg C, is the temperature of my condensing chamber, which is cooled by salty groundwater, also at 1psi. How much pure water will I be able to draw off as vapour compared to the brine that will be left in the bottom? This of course will be drawn out and the cycle condinued.
If the feed water was brought up to 50DegC by solar heaters, how much could I expect then?
I have 4' x 20'long fireglass water high pressure pipes for vessels etc
Thankyou in anticipation
Tim

Answer
Hello Tim,
I have been thinking about your task for a couple of days, for I only have some general advice on the physics behind the process. For detailed advice you need an engineer, because the actual water draw (e.g. in liters per minute) depends on many design parameters.
I can say that you are on the right track. You will definitely increase the evaporation rate substantially (about 3 times), when you increase the temperature from 30 to 50 degC, so I do recommend to do that. Remember that when you reduce the pressure, you are always reducing it both in the evaporator and in the condenser. While the lower pressure helps you in the evaporation, it impedes the condensation rate. However, I checked the water phase diagram and it is perfectly sensible to have the whole system at 1 psi, if your evaporator is at 50degC and condenser at 16degC. Should you use 30degC for the evaporator, it will work more efficiently at about a third of psi (0.33 psi).
The rate of clear water gain is going to be proportional to the area of evaporation surface and the area of the condensation surface. It is advisable that you set the condenser's surface/volume ratio to a similar value as you see in the freon condenser in your fridge - the optimal ratio for water will not be too different. You will need an efficient water drainage, so that the condensed water immediately vacates the wall to allow the next incoming water molecules to condensate.
You will need to optimize the length and cross section of the duct between the evaporator and condenser - too short and wide may make it too hard to achieve the temperature difference, too long and narrow will limit the water gain rate.
Quite essentially, the evaporation on one end and condensation on the other create so called "diffusive flow" of vapor, which is a flow driven by a difference in the vapor concentration. Your pumping on the system should be done on the condenser end and must be placed carefully, so that you minimize the loss of vapor into your pump (vapor pumped away is lost for condensation). By putting the pump on the condenser you will also induce a small pressure difference between evaporator and condenser, which produces "convection flow", i.e. flow of vapor driven by a difference in pressures. This will also help the efficiency, if designed well.
To the actual value of the clear water gain rate I cannot give a number, just an experience. At high school we distilled at 100degC, condensed at ~10degC, the evaporation and condensation surfaces were ~10cm^2 and the rate was maybe 1 liter/hour. Roughly speaking, you are going to use ~2.5-times smaller difference in temperatures, so you must increase the evaporation and condensation surfaces 2.5 times to get the same rate. Whatever you desired rate is, you must increase the surface areas. When the areas get to be too large (hundreds of square meters) you will need to employ more clever engineering ideas.
Good luck.
Daniel