Ammonia enters the compressor of an industrial refrigeration plant at 2 bar, −10°c with a mass flow rate of 15 kg/min and is compressed to 12 bar, 140°c. heat transfer occurs from the compressor to its surroundings at a rate of 6 kw. for steady-state operation with negligible kinetic and potential energy effects, determine (a) the power input to the compressor, in kw, and (b) the rate of entropy production, in kw/k, for a control volume enclosing the compressor and its immediate surroundings such that the heat transfer occurs at 300 k.

a) W=85.225 kW

b)

Explanation:

First, consider the energy balance for the compressor: The energy that enters to the system (W and enthalpy of the feed flow) is equal to the energy that goes out from it (Heat Q and enthalpy of the exit flow):

Consider the enthalpy data from van Wylen 6th edition, Table B.2.2. According to that, ,  

So, the power input to the compressor is:

b) The differential entropy change dS for a reversible heat transfer dQ at a temperature T is:

This equation can be integrated if the heat transfer surface temperature remains constant, which is the case, giving as a result:

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air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.