3. 300 lb/hr of CO2 enters a heat exchanger as a saturated liquid at 100 psia and emerges

at 75 psia, and 20°F. This evaporation and superheating process is accomplished by

passing dry air through the heat exchanger. Air enters at 70°F and atmospheric pres-

sure, and emerges at 50°F. There is no pressure drop on the air side of the heat

exchanger. What is the required air flow rate, and the heat transfer rate within the

heat exchanger?

4. R134-a is used as the propellant for a small portable jet rocket for maneuvering parts

during the assembly of a space station. The device consists of a small spherical tank

containing saturated R134-a at 25°C, a valve, and a nozzle. The user attaches the jet

to the package, opens the valve momentarily, and lets some gas escape through the

nozzle. Heat transfer from the walls to the R134-a keeps the pressure constant in the

tank during a short burst and the flow through the nozzle may be considered steady.

The valve throttles the R134-a to 140 kPa, and the nozzle discharge state is 14 kPa

and -60°C. The thrusting force is approximately given by F = mV, where m is the

mass flow rate and V the exit velocity. User requirements call for a thrust of 50 N.

Determine the specific impulse of this device, Isp = F/gom. By comparison a chemical

rocket has an Isp between 350-460 seconds. For extra credit, find the Mach number at

the nozzle exit.