1-ft of air is contained in the spring-loaded piston-cylinder device shown in figure. The spring constant is 5 Ibf/in, and the piston diameter is 10 in. When no force is exerted by the spring on the piston, the state of the air is 250 psi and 460° F. This device is now cooled until the volume is one-half of its original size. Determine the change in the specific enthalpy of the air in Btu/lbm.