Vacuum steam reboiler and condenser

[NoNickName]

I have a design here of a steam boiler/condenser with a heat pump system which evaporates water under vacuum at 45/50°C and then re-condenses it away for concentrating grey and black waters or for saving water from industrial effluents.
Has anybody got a white paper or any technical publications concerning such application?

[NoNickName]

Anyone would give it a try?

[mbc]

Is it for concentration of fruits or tomatoes? And you want to re-condenser it with un-direct cooling system(( In direct system we use water and steam from vacuum batches solved in it ))(( In Un-direct system we use shell and tube to re- condenser this steam waters )) Am I right
What is you capacity of systems ? >>

[NoNickName]

No, it's for concentrating sewage and all sort of dirty and venomous liquids, so to reduce the costs of transportation and destruction.
Indirect system, exactly right.
Capacity varies between 30 and 200kW depending on size of unit.

[mad fridgie]

I slightly different way may be an option, this really depends upon many factors. (freezing point and fluid density solution, and density of suspended solids)
Seperate the water out by freezing.
Using a scraped surface heat exchanger, bianary ice is formed. (very small pure balls) within the liquid.
The liquid is pumped to a vertical settling chamber. The ice rises to the top, you scrape off. (use the ice for pre-cooling or liquid sub-cooling)
No need to worry about vacuums and of course low energy levels required for latent change.

[mbc]

If your goods is acidity you have to use stainless steel 316 or 304 depends of your acidity
If not, You can use plat heat exchanger ( open type or block) . I suggest you use Shell and tube or open type.
So you need 1- concentration system 2- water vacuum pump 3- cooling tower with all pumps
For more effency it is better you use two stage concentrations system fist one cools by cooling tower water and second one by cold water . but this system is expensive and to cool water you have to spend money.
What is your area temperature if is hot you have to use this system because of 45/50 ‘c in vacuuming
If you could go up about 65 ‘C , use one stage with high capacity with collection water under it if you want your system runs continues use two collection water under that

[mbc]

mad fridgie
good idea but how about if your system is greasy or you have color in your water , might is slow icing you could separate color or salt in block ice but in fast cooling I am not sure you can could>>

[mad fridgie]

mad fridgie
good idea but how about if your system is greasy or you have color in your water , might is slow icing you could separate color or salt in block ice but in fast cooling I am not sure you can could>>

.

Yes you are correct if greasy, then likely the grease would be less dense than the ice, and would float to the top first.
The actual process is continuos, you can either concentrate in single pass, or re-circle through a tank where the tank becomes more concentrated over a period.
For this process to work your freezing point of the solution needs to be below 0C, As you remove the ice the freexing point drops.
Not perfect for everything, but can have it uses.

[mbc]

nonickname
what you mean of 30 up to 200 KW
KW is a your heating capacity you are useing to heat up ? OR..

what is your sewage and all sort of dirty and water Capacity in M3 /24 h

[NoNickName]

Ok, sorry if I wasn't too clear. What I would like is a general typical system of the type I described, in order to put down some sort of guideline for a boiler-condenser of water from exhausts defluents.
These liquids cannot be iced, as they may contain substances soluted with water and that have a higher freezing point. I have to make sure I only boil off and condense water.
The chamber is under vacuum and the boiling temperature is around 45 to 55°c depending on vacuum and type of liquid.
The capacity specified is heating capacity.
The defluents may be sewage, dirty water from car washing plants, or waste water from chemical plants which may contain acids, alkalines, toxic compounds, etc.
I'm not much interested in the specs of the boiling chamber. I am more interested in putting down a typical refrigeration circuit as reference design.

[Peter_1]

We service some Watropur sludge dryers (water compactors) but not working the way you describe it.

[NoNickName]

We service some Watropur sludge dryers (water compactors) but not working the way you describe it.

How do they? Warm air?

[Peter_1]

The sludge first passes through a hydraulic press to squeeze out as much as possible the water.
This is anyhow the cheapest way to remove excess water.
It then is transported to a steel container - in fact a slightly modified roll off container - where they installed a heatpump/dryer on it. Air blown on the perforated bottom grille and retracted on top of the silo.
Condenser and evaporator in line with a R404a tandem Copeland of +/- 120 Hp between it.
Dimensions: +/- 2 x 2 x 10 m (you can walk in the machine)
Additional subcooler/condenser on top of the dryer in case temperature gets too high (>40°c Air off)
I think I started here once a thread about these machines.
If you need more info, give a shout but I think you gets the picture now how they do it.

[NoNickName]

Do you have a piping schematic of it?

[Peter_1]

Not an original from the manufacturer, I should need to make a drawing myself.

[mbc]

as you know water vacuum in normal condition goes vacuum about -0.7 mbar
so your should rise you temperature up to 60 -65 to have good result
I have another idea for heating your waste use shell and tube stainless steel for condenser up to 65 ‘C and from 65 up to area temperature use another air condenser or you can use you re –condenses water by another shell and tube condenser . also you can use this water for pre cooling your water vapor , So you save money for heating and cooling.

[NoNickName]

uh? Water boils at 52°C at a pressure of 0.14 bar or 126°F at 2 psi.

[mad fridgie]

Are you hoping to directly flash off in your vacuum chamber (internal heat exchanger) or are you going to heat the liquid under pump pressure, via an external heat exchanger, then return to chamber then flash off, or do you intend to heat and flash off in a seperate heat exchanger.

[mad fridgie]

You have not indicated if this is a continuous process or a batch process.
If it is a batch process, then getting the process to work soley by the refrigeration is problem, additional energy will be required to sensible heat first, then some for latent.
Also you need consider, servicing, if are are directly flash off on your heat exchanger then fouling will become a major issue. Also when the process is running you may also need to reject heat external from the process (depends upon system losses)
I would suggest that as far as fluid heating you use a secondary loop from your refrigerant plant. The secondary loop has a large thermal store.
Relieves the 2 earlier issues.
The condensing of the steam I would go direct expansion. Allow for a large vapor pipe between vacuum chamber and evaporator (minimise pressure drop)
Ideally the evap need to be raised to a point higher than the vacuum, to allow for a liquid head on the drain, other wise you will need to control and pump

[NoNickName]

The water is vaporized (condenser side of refrigeration circuit) and recondensed (evaporator of the refrigeration circuit) in the same chamber. The falling drops of condensed water are trapped in a recess and later drained in the flush.
New dirty liquid is added in the chamber at the same flow rate the clean water is drained out of it.
Progressively, the chamber raises the level of sediments, until the unit is stopped (one a week), and the sediments removed from the bottom of the chamber.
Despite clean water is drained and new sewage is loaded, the chamber is always kept at low pressure by means of a venturi.

[mad fridgie]

What fluid static head are you looking at in side the chamber, are you going to break the surface boundry layer (stirring or spraying) have you taken into consideration the effect of the impurities on the boiling point.
Are you using steam or air for you venturi.
How are you going to handle the excess heat of rejection?

[NoNickName]

The fluid in chamber just sits and is being boiled off. There may be a stirrer, but this is not mandatory.
I'm not interested in the water boiling point, as the refrigeration circuit condensing temperature will settle around the water boiling point.
I'm using water for the venturi. In the reduction of section, the water pressure reduces and air is sucked out of the chamber. Air is then released outside.
There is no excess heat of rejection. All the heat available is always used for boiling water. The more water boils off, the more dirty fluid is loaded.

[mad fridgie]

The fluid in chamber just sits and is being boiled off. There may be a stirrer, but this is not mandatory.
I'm not interested in the water boiling point, as the refrigeration circuit condensing temperature will settle around the water boiling point.
I'm using water for the venturi. In the reduction of section, the water pressure reduces and air is sucked out of the chamber. Air is then released outside.
There is no excess heat of rejection. All the heat available is always used for boiling water. The more water boils off, the more dirty fluid is loaded.

I presume the venturi is only being used to remove non condensables, (not a primary source of warer vapor extraction), If this is the case you will have excess heat of rejection (how much depends upon system losses, is your vacuuum chamber insulated etc) So for example we are condensing 'X" water vapor 200Kw (SST load) lets say for instance COP 4
Then total heat of rejection 250Kw (give or take a bit) you will end up boiling of 25% more vapor (give or take), your internal vapor pressure will rise (boiling point will increase) SCT will increase, reducing nett refrigeration effect and increasing power draw reducing COP, Over the period you are looking at you may become out range on your refrigeration unit.
As far as the boilng point due to the impuriteis, yes the the system will balance at some point, but are you not trying to design a process for optimum performace.
The fluid just sat there, OK if only very shallow, but if is deep the static head will increase the boiling point, or are you looking at natural convection current to raise the the heat fluid to the surface (flash off at this point) If this is the case then fluid become more viscose (thicker) then these currents will reduce along with reduced heat transfer co-efficients (raising head pressure even futher)

[mad fridgie]

On the practical side, I would use some form of fluid agitation, and install a additional water coil (this depends upon local water temperature) either to condense the water within the chamber, or to keep the fluid a controlled temperature. Either way is removing excess energy from the system
If I am wrong with my presumption that you have access heat, then Ok you waste very little capital on a coil, but if I am right, then this will save you a lot of onsite greif.
Mad

[NoNickName]

The venturi is just used for evacuating air from the chamber and reduce the boiling point of the fluid.
I won't be able to boil water at 100°C, so I have to evacuate the chamber to reduce the boiling point.

There are a number of these systems in operation, and I'm only trying to optimise the transients. The inherent problem of such system is that during startup it takes all the heat to boil water, but no steam condenses on the evaporator, so I have virtually zero heat load, and lots of subcooling. This ends up in liquid and oil floodback, that even suction accumulators can't stop.

[mad fridgie]

I can certainly agree with starting the plant (no load, as stated earlier) Again I suggest that you add a secondary coil in your fluid, use an external thermal store (hot water tank), use your excess energy HOR (de-superheater) to heat this tank. On start up use this energy via the second coil to start the boiling process.
Your issues are fundemental in just about all drying proccesses. to achieve maximum efficeiency you need to keep your SST as high as possible and your SCT as low as possible. Ido not see any reason for not getting a COP of between 5-6 on this type of plant.
Remember, just because it has been done before does not mean it is right!

[NoNickName]

I would say that in the starting time, an air evaporating coil can be used in place of the steam condenser, until any steam is available as heat load.

[mad fridgie]

I would say that in the starting time, an air evaporating coil can be used in place of the steam condenser, until any steam is available as heat load.

Yes, this another option, "Many ways to skin a cat"
If you were to go this way you could use this coil to give some liqud sub-cooling (rejecting excess energy) an increasing nett refrigeration effect (more water condensing)
Mad