Question

A piston/cylinder contains 2 lbm of liquid water at 70 F, and 30 lbf/in.². There is a linear spring mounted on the piston such that when the water is heated the pressure reaches 300 lbf/in.² with a volume of 4 ft³. Find the final temperature and plot the P-v diagram for the process. Calculate the work and the heat transfer for the process. 

Answer 1

Take CV as the water. 

 m₂ = m₁ = m ; m(u₂ − u₁) = ₁Q₂ - ₁W₂

State 1: Compressed liquid, take saturated liquid at same temperature. 

 v₁ = v_f(20) = 0.01605 ft³/lbm, u₁ = u_f = 38.09 Btu/lbm 

State 2: v₂ = V₂/m = 4/2 = 2 ft³/lbm and P = 300 psia

=> Superheated vapor T₂ = 600 F ; u₂ = 1203.2 Btu/lbm 

Work is done while piston moves at linearly varying pressure, so we get