Why would you want to maximize the length of tubing you coul

Why would you want to maximize the length of tubing you could pack into a heat exchanger? How does the energy of a flowing fluid differ from that of a stationary fluid? Define all terms describing the energy of each. The kinetic energy of a fluid increases as it is accelerated in an adiabatic nozzle. Where does this energy come from? Why would you expect the temperature of a fluid to drop during a throttling process? Why use a valve to get the pressure reduction required in a typical vapor cycle refrigeration process rather than a turbine, which could employ the pressure drop to generate power, thereby reducing the power required to operate the system? How can a heat pump or refrigerator have a COP of > 1 without violating the first law of thermodynamics? Is it possible for a heat engine to operate without rejecting any waste heat to a low temperature reservoir? Explain why, or why not.

Solution

>> Solution 1.

We do want to maximize the length of tubing because more the length of tube, more the distance fluid travelled to exchange heat, more the time available for heat exhanging, more efficiency of heat exchanger....

>> Solution 2.

Stationary fluid have Pressure energy and Potential Energy

Pressure Energy in Stationary fluids = Force Acting/Cross Sectional Area

and, Potential Energy head = Height from reference at which fluid is kept

>> Flowing fluid in addition to Pressure Energy & Potential Energy also have Kinetic Energy

and, Kinetic Energy = (1/2)m*v²

>> Solution 3.

>> In Adiabatic Nozzle, there is no heat loss.

Now in a nozzle, as Cross Section gets decreasing and as Flow Rate, Q , i.e. Q = A*V , has to remains constant.

So, As Area Decreases , Velocity increases and in turn, Kinetic Energy of Fluid Increases....

>> Solution 4.

Because during throttling process, high pressure is converted to low pressure, thus lossing the pressure energy.

As, fluid is losing energy, until and unless it gets energy from some outside source, its temperature will drop