Thermodynamics calculation: Flashing water at 75C

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Discussion Overview

The discussion revolves around calculating the amount of water vaporized during a flash cooling process at 75 degrees Celsius, with specific conditions including a post-flash temperature of 67 degrees Celsius and a pressure of 0.27 bar. Participants explore thermodynamic principles and calculations related to this process, including the implications of using milk instead of water.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Exploratory

Main Points Raised

  • One participant asks for help in determining how much water is vaporized during the flash cooling process.
  • Another participant clarifies that the system involves a vacuum flash cooling setup aimed at eliminating dissolved oxygen and air bubbles.
  • Participants discuss the pressure conditions before the flash, with one confirming it is almost atmospheric.
  • A participant applies the first law of thermodynamics to the system, assuming an adiabatic flash and setting up an energy balance equation.
  • Enthalpy values for liquid water at 75°C and 67°C, as well as for saturated water vapor at 67°C, are provided by participants.
  • One participant expresses confusion about the calculations and how to relate them to the flow rate of 15000 kg/h.
  • Another participant calculates that 1.4% of the liquid water vaporizes, but expresses difficulty in understanding the equation setup.
  • A hypothetical scenario is introduced regarding the vaporization of water from milk, with considerations about the enthalpy of milk and the percentage of water it contains.
  • Participants discuss the implications of using a conservative estimate for the vaporized water when considering milk.
  • One participant humorously suggests conducting experiments to obtain more accurate data, while another emphasizes that they are not seeking precise answers but rather different perspectives.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the calculations, with some confusion remaining about the energy balance equation. There is no consensus on the implications of using milk instead of water, and the discussion includes both exploratory reasoning and humor regarding the accuracy of estimates.

Contextual Notes

Limitations include potential missing assumptions about the system, the dependence on specific enthalpy values, and the unresolved nature of the calculations related to the flow rate and the transition from water to milk.

Keeskwaak
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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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Do you mean in a vacuum flash cooling system like this one?

1690989787174.png

https://www.dekkervacuum.com/flash-cooling/
 
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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What is the pressure before the flash, atmospheric?
 
Chestermiller said:
What is the pressure before the flash, atmospheric?
Yes its almost atmospheric
 
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?
 
enthalpy for Liquid water @75C is 313.97, @67C is 280.45
the enthalpy of saturated water vapor at 67C is 2620.96
 
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 ?
 
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.
 

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