Energy Transfer homework

In summary, the problem is asking for the rates of energy transfer by mass into and out of the compressor in a refrigeration system, neglecting potential energy. The compressor takes in saturated vapor at 0.14MPa and discharges superheated vapor at 0.8MPa and 50 degrees celsius at a rate of 0.04kg/s. The suction and discharge areas of the compressor are 10cm^2 and 5cm^2 respectively. It is not necessary to know the mass, as the problem is asking for energy on a per unit mass basis. The kinetic energy per unit mass can be calculated using the velocity (V) and the specific enthalpy (h) and internal energy (
  • #1
onemoretomorrow
1
0
Hi guys i really need some help on this question.

Refrigerant 134a enters the compressor of a refrigeration system as saturated vapour at 0.14MPa and leaves as superheated vapour at 0.8MPa and 50 degrees celsius as a rate of 0.04kg/s. The suction area of the compressor is 10cm^2 and the discharge area 5cm^2. Determine the rates od energy transfer by mass into and out of the compressor, neglecting potential energy. Note the order of magnitude of the enthalpy and kinetic energy terms.

I can't seem to be able to get the value for the mass. can someone help? thanks
 
Physics news on Phys.org
  • #2
Determine the rates od energy transfer by mass into and out of the compressor
One does not need mass, since the problem seems to be asking for energy on a per unit mass basis.

The kinetic energy per unit mass is simply V2/2. Thermodynamic properties are usually given on a per unit mass basis, e.g. specific enthalpy, h (J/kg), or internal energy, u (J/kg).

Mass flow rate is given - 0.04kg/s - and assuming the continuity equation, as steady-state, mass rate (in) = mass rate (out), otherwise there would be an accumulation (if mass flow in exceeds mass flow out) of the system.
 
  • #3


Sure, I would be happy to help with your question. In order to calculate the rates of energy transfer, we need to use the first law of thermodynamics, which states that energy cannot be created or destroyed, it can only change forms. In this case, we are looking at the energy transfer by mass into and out of the compressor.

To calculate the mass, we can use the ideal gas law equation, PV = mRT, where P is the pressure, V is the volume, m is the mass, R is the gas constant, and T is the temperature. We have the pressure and temperature values, as well as the volume, which is the area of the compressor. We can also assume that the gas is an ideal gas, since it is a refrigerant.

Using the given values, we can calculate the mass as follows:

m = (PV)/(RT) = (0.14MPa * 10cm^2) / (8.314 kJ/mol*K * 323.15K) = 0.00017 kg

Now, to determine the rates of energy transfer, we need to calculate the change in enthalpy and kinetic energy between the inlet and outlet of the compressor. The change in enthalpy can be calculated using the ideal gas equation, h = u + Pv, where h is the enthalpy, u is the internal energy, P is the pressure, and v is the specific volume.

For the inlet conditions, we have:

h1 = u1 + P1v1 = 0 + (0.14MPa * 0.0017m^3/kg) = 238 kJ/kg

For the outlet conditions, we have:

h2 = u2 + P2v2 = 0 + (0.8MPa * 0.00034m^3/kg) = 272 kJ/kg

Therefore, the change in enthalpy is:

Δh = h2 - h1 = 272 kJ/kg - 238 kJ/kg = 34 kJ/kg

Next, we need to calculate the change in kinetic energy. Since the mass flow rate is constant, the kinetic energy change can be calculated using the following equation:

ΔKE = (m * (V2^2 - V1^2)) / 2

Where m is the mass flow rate, V2 is the outlet velocity, and V1 is the inlet
 

1. What is energy transfer?

Energy transfer refers to the movement of energy from one object or system to another. This can occur through various processes such as conduction, convection, and radiation.

2. Why is energy transfer important?

Energy transfer is essential for sustaining life and for the functioning of our everyday devices. It allows us to power our homes, transport goods, and fuel our bodies.

3. How does energy transfer impact the environment?

Energy transfer can have both positive and negative impacts on the environment. For example, the burning of fossil fuels for energy release harmful greenhouse gases, contributing to climate change. However, the use of renewable energy sources can help reduce these negative impacts.

4. What are some real-life examples of energy transfer?

Some common examples of energy transfer include the transfer of heat from a stove to a pot, the conversion of sunlight into electricity through solar panels, and the movement of electricity from power plants to homes through power lines.

5. How does energy transfer relate to the laws of thermodynamics?

The laws of thermodynamics govern energy transfer and state that energy cannot be created or destroyed, only transferred or converted from one form to another. Energy transfer obeys the second law of thermodynamics, which states that in any energy transfer, some energy will always be lost as heat.

Similar threads

  • Engineering and Comp Sci Homework Help
Replies
10
Views
1K
  • Engineering and Comp Sci Homework Help
Replies
1
Views
2K
  • Mechanical Engineering
Replies
8
Views
2K
  • Engineering and Comp Sci Homework Help
Replies
1
Views
3K
  • Engineering and Comp Sci Homework Help
Replies
2
Views
3K
  • Engineering and Comp Sci Homework Help
Replies
17
Views
2K
  • Engineering and Comp Sci Homework Help
Replies
2
Views
4K
  • Engineering and Comp Sci Homework Help
Replies
1
Views
3K
  • Engineering and Comp Sci Homework Help
Replies
2
Views
3K
  • Engineering and Comp Sci Homework Help
Replies
1
Views
1K
Back
Top