Thermodynamics calculation: Flashing water at 75C

In summary, a water vaporizer for a vacuum flash cooling system produces 1.4% of the liquid vaporized.
  • #1
Keeskwaak
11
2
Homework Statement
need to figure this out and why
Relevant Equations
m = Q / (h_vap - h_liq)
Hello all,
For a project I really need to know some number i can't seem to produce myself.

we are flashing water @75 degrees Celsius, the water after flash-cooling is 67 degrees constant pressure is 0.27 bar (absolute) flow rate is 15000 KG/per hour.

how much water is vaporized?

Help is much appreciated
 
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  • #2
Do you mean in a vacuum flash cooling system like this one?

1690989787174.png

https://www.dekkervacuum.com/flash-cooling/
 
  • #3
Yes, though its i think somewhat bigger, it features a vacuum pump and a condenser to maintain a low pressure, Basic goal is oe eliminate dissolved oxygen and air bubbles, i need to figure out how much volume is lost
 
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  • #4
What is the pressure before the flash, atmospheric?
 
  • #5
Chestermiller said:
What is the pressure before the flash, atmospheric?
Yes its almost atmospheric
 
  • #6
Applying the open system (control volume) version of the 1st law of thermodynamics and assuming an adiabatic flash, we have $$\Delta h=0$$
State 1: 1 kg liquid water at 75 C

State 2:
x kg saturate water vapor at 0.27 bar and 67 C
(1-x) kg saturated liquid water at 0.27 bar and 67 C

From the steam tables, what is the enthalpy of liquid water at 75 C and 67C?
What is the enthalpy of saturated water vapor at 67 C?
 
  • #7
enthalpy for Liquid water @75C is 313.97, @67C is 280.45
the enthalpy of saturated water vapor at 67C is 2620.96
 
  • #8
Keeskwaak said:
enthalpy for Liquid water @75C is 313.97, @67C is 280.45
the enthalpy of saturated water vapor at 67C is 2620.96
So what is the parameter x equal to ?
 
  • #9
I'm so sorry but I don't get it
 
  • #10
$$\Delta h=280.45(1-x)+2620.96x-313.97=0$$
 
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  • #11
but how does that work and how to translate it to 15000kg/h?
 
  • #12
Keeskwaak said:
but how does that work and how to translate it to 15000kg/h?
This tells you the fraction of water that is lost to vapor.
 
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  • #13
Chestermiller said:
This tells you the fraction of water that is lost to vapor.
As much as i appreciate your help and answers, I still can't make the equation
 
  • #14
Keeskwaak said:
As much as i appreciate your help and answers, I still can't make the equation
x=0.014, so 1.4% of the liquid vaporizes.
 
  • #15
Chestermiller said:
x=0.014, so 1.4% of the liquid vaporizes.
I still cant make the equation but I see what you are doing, you make a energy balance.
Next question to make it more interesting: what if the water was milk? I don't know the Enthalpy numers for milk. but I do know the water in milk is 87%, and because the condensate is clear (mostly) is it safe to asume the water vapor will also be 87% of 1.4%?
The water inlet is 87% of 15000kg? or am i making a to short cut?
 
  • #16
Keeskwaak said:
I still cant make the equation but I see what you are doing, you make a energy balance.
Next question to make it more interesting: what if the water was milk? I don't know the Enthalpy numers for milk. but I do know the water in milk is 87%, and because the condensate is clear (mostly) is it safe to asume the water vapor will also be 87% of 1.4%?
The water inlet is 87% of 15000kg? or am i making a to short cut?
To be conservative, I would just use the 1.4%.
 
  • #17
Chestermiller said:
To be conservative, I would just use the 1.4%.
yes to be on the safe side, but that is no science
 
  • #18
Keeskwaak said:
yes to be on the safe side, but that is no science
Then go out and spend some money to do some VLE experiments on milk to get a 13% more accurate answer. Have you considered the effect off Jupiter's gravity on the system since "science" tells us that that might have some (tiny) effect too.
 
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  • #19
Chestermiller said:
Then go out and spend some money to do some VLE experiments on milk to get a 13% more accurate answer. Have you considered the effect off Jupiter's gravity on the system since "science" tells us that that might have some (tiny) effect too.
Hi,
No pun and no disrespect intended, as I mentioned before: "Next question to make it more interesting", its just that.
i was not asking for a accurate answer, maybe just how someone else looks at this.
 

Related to Thermodynamics calculation: Flashing water at 75C

What is the definition of flashing in thermodynamics?

Flashing in thermodynamics refers to the rapid vaporization of a liquid when its pressure is suddenly reduced below its vapor pressure, causing a portion of the liquid to instantly convert into vapor.

How do you calculate the amount of water that flashes at 75°C?

To calculate the amount of water that flashes at 75°C, you need to know the initial and final pressures, the initial temperature, and the properties of water. Using the energy balance equation and steam tables, you can determine the fraction of water that vaporizes by comparing the enthalpy before and after the pressure drop.

What is the significance of the initial and final pressures in flashing calculations?

The initial and final pressures are crucial because they determine the extent of flashing. The larger the difference between the initial pressure and the final pressure, the more significant the flashing effect. The final pressure must be below the vapor pressure of water at the given temperature for flashing to occur.

What are the typical applications of flashing water in industry?

Flashing water is commonly used in various industrial processes such as steam generation, chemical processing, refrigeration, and power plants. It is particularly useful in pressure relief systems, cooling processes, and in the design of evaporators and condensers.

How does temperature affect the flashing process of water?

Temperature plays a critical role in the flashing process. Higher temperatures increase the vapor pressure of water, making it easier for flashing to occur when the pressure is reduced. At 75°C, water has a specific vapor pressure, and the extent of flashing will depend on how much the pressure is reduced relative to this vapor pressure.

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