How Much Water for Evacuated Tube Solar Oven to Stay Below 120 PSI?

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Discussion Overview

The discussion revolves around the design considerations for an evacuated tube solar oven, specifically focusing on the amount of water needed to maintain pressure below 120 psi while achieving high temperatures. The conversation includes aspects of thermal dynamics, pressure ratings, and the behavior of water in vapor and liquid phases.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant describes their setup using half inch type K copper pipes and a vacuum of 100 mbar to lower the boiling point of water, aiming to superheat steam for higher temperatures.
  • Another participant inquires about the maximum operating temperature, which is stated to be 400F.
  • A suggestion is made to refer to steam tables to understand the relationship between temperature and pressure, indicating that keeping vapor pressure below 120 psi allows for any volume of water.
  • It is proposed that limiting the temperature to 340F could keep the pressure below 120 psi at any volume, while also maintaining a liquid phase.
  • The original poster acknowledges the need for saturated steam to capture latent heat, indicating a shift in understanding regarding the phase of water used.

Areas of Agreement / Disagreement

Participants express differing views on the optimal operating conditions, particularly regarding temperature limits and the implications for pressure and water volume. The discussion remains unresolved with multiple competing perspectives on how to achieve the desired outcomes.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the relationship between temperature, pressure, and water volume. The calculations and specific conditions under which these relationships hold are not fully explored.

aluminumholds
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Hey everyone,

I'm building my first evacuated tube solar oven and I seem to run into a problem I can't get around. First of all, I am going to use half inch type K copper pipes with water as a heat transfer fluid. I am putting a vacuum (100 mbar or so) on the tubes to lower the boiling point of the water. The tubes are about 10 feet long, giving me a volume of 23.56 in^2. I'm doing this in order to have the water in vapor form most of the time in order to superheat the steam and achieve temperatures higher than 212F. If my highest pressure can be 120 psi, according to Copper.org which describes pressure rating for brazed joints using saturated steam, what is the amount of water I need to add to the pipes in order not to exceed that pressure?

Thanks in advance for your help and advice, it'll really help me out :)
 
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What is the maximum temperature you are looking to operate at?

BoB
 
rbelli1 said:
What is the maximum temperature you are looking to operate at?
400F would be my max temperature in the heat pipes.
 
This http://www.slideshare.net/muhammad_anam/steam-tables
might be helpful. I don't know how to calculate for your situation. I just know that the temperature is important.

If you keep the vapor pressure below your 120psi then any amount of water is OK. Could you limit the temperature to 340F? If I am reading the linked tables correctly you then stay below 120psi at any volume. You would also keep a liquid phase at all times.

BoB
 
rbelli1 said:
This http://www.slideshare.net/muhammad_anam/steam-tables
might be helpful. I don't know how to calculate for your situation. I just know that the temperature is important.

If you keep the vapor pressure below your 120psi then any amount of water is OK. Could you limit the temperature to 340F? If I am reading the linked tables correctly you then stay below 120psi at any volume. You would also keep a liquid phase at all times.

Thanks for the table BoB, they'll definitely be useful. As of know I don't have a way to limit temperatures...however I've done some more reading and know that I need to have saturated steam, not superheated steam; to capture the latent heat of water condensing.
 

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