Fresh Water Generation

[Dawn6]

I work for a sim company and need to mathematically describe a fresh water generator that would be on a ship. A similar model is shown here

http://www.brighthub.com/engineering/marine/articles/66786/image/69487/.

The issue is that I only have a couple known variables, and too many unknown. I know that the input temp of the sea water is 30C, and I know that the steam heater has an energy of 7892kW, but I can change that if I choose. I also know that the pressure of the sea water going into the generator is 4bar. The only other known is that the generator can produce 740m3/24h. There is a vacuum being pulled via venturi ejectors, and that pressure is whatever I decide.
I am unsure how to calculate the overall energy transfer of the system. I need to know the temperatures out of the FWG into the steam heater, out of the steam heater back into the FWG, and the temp of the brine out of the FWG. I also need to decide how much vacuum to pull. From the energy balance, I am hoping that I can get an equation describing how much fresh water is made as a function of input temperature, heat transfer from the steam heater, and the decrease in pressure from the venturi.
Anybody have any help?

 

[chicopee]

I would draw a cross view of the desalinization tank with all of its piping, compartments, pumps; inflows and outflows with their associated mass rates and temperatures including brine concentrations between compartments would need to be identified in this diagram. Heat inputs and surface areas of the evaporators should be listed and use the manufacturer's specification to determine evaporation rates. Then you can calculate the enthalpies of the water at each compartment to get an approximate idea on the required energy.
That would be my approach.

 

[racookpe1978]

Input water temperature is going to vary from 2 deg C to 30 degree C...

 

[IRstuff]

You already have most of the information you need. The majority of the heat input is driven by the evaporation energy and thermal mass of the water. The coldest your output stream can be is the temperature of your inlet.

TTFN

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[rmw]

You don't get to decide how much vacuum to pull, the water temperature in your last effect does. You can specify what pressure you want your vacuum equipment to pull down to, but it will only pull down to the saturation pressure corresponding to the temperature of the water in the effect where it is pulling from.

The vacuum jet/pump does not create the vacuum, the condensation of vapor in the effect does. The vacuum equipment merely removes the non-condensables so that they don't blanket the heat transfer surface needed to condense the vapor to distillate.

Your temperatures in/out will be a function of the approach temperature to the respective inputs/outputs. The approach temperature is a function of the design of the heat exchange equipment.

rmw