Thermodynamic question: first law

Tags:
1. Apr 3, 2015

Superhawkkodaka

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?.. im lost in this one.. please be specific guys.. i've used all i know and still cant get the answer..

2. Apr 3, 2015

CWatters

Hints:

How much energy would it take to raise 1L (=1kg) up by 100m?
What is the equation that relates power, energy and time?

3. Apr 3, 2015

Staff: Mentor

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

4. Apr 3, 2015

Superhawkkodaka

weew thanks!!!