Thermodynamic question: first law

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Discussion Overview

The discussion revolves around estimating the power required for a pump to elevate liquid water through a height of 100 meters, specifically focusing on the application of the first law of thermodynamics in an open system context. Participants are seeking clarification on the appropriate formulas and concepts to apply in this scenario.

Discussion Character

  • Homework-related
  • Technical explanation

Main Points Raised

  • One participant expresses confusion about the formula needed to calculate the required pump power, indicating they have tried various approaches without success.
  • Another participant suggests considering the energy required to raise 1 liter of water by 100 meters and asks about the relationship between power, energy, and time.
  • A later reply reiterates the need to use the open system steady flow version of the first law of thermodynamics, providing the equation that includes mass flow rate and specific enthalpy.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the specific formula to use, as one participant is still seeking clarity while another provides a suggested approach. The discussion remains unresolved regarding the application of the first law in this context.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the system, such as the adiabatic process and negligible changes in temperature and kinetic energy, which may affect the calculations.

Superhawkkodaka
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Estimate the kilowatt pump you would need to pump 450 L/min of liquid water at 20c through a rise in elevation of 100m. Assume the water is pumped adiabatically and with negligible changes changes in temperature and kinetic energy.

answer given to us is -7.35kw..

Question.. what formula should i use?.. I am lost in this one.. please be specific guys.. I've used all i know and still can't get the answer..
 
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Hints:

How much energy would it take to raise 1L (=1kg) up by 100m?
What is the equation that relates power, energy and time?
 
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Superhawkkodaka said:
Estimate the kilowatt pump you would need to pump 450 L/min of liquid water at 20c through a rise in elevation of 100m. Assume the water is pumped adiabatically and with negligible changes changes in temperature and kinetic energy.

answer given to us is -7.35kw..

Question.. what formula should i use?.. I am lost in this one.. please be specific guys.. I've used all i know and still can't get the answer..
You are supposed to use the open system steady flow version of the first law:

##mΔh+mgΔz=Q-W_s##

where Ws is the rate of doing shaft work, m is the mass flow rate, and h is the specific enthalpy.

Chet
 
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Chestermiller said:
You are supposed to use the open system steady flow version of the first law:

##mΔh+mgΔz=Q-W_s##

where Ws is the rate of doing shaft work, m is the mass flow rate, and h is the specific enthalpy.

Chet
weew thanks!
 

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