Thermodynamics Polytropic Process Compressor

In summary, according to the question, may be the surrounding temperature is above refrigerant‘s temperature.which caused heat transfer.
  • #1
raniero
42
0
I was revising for my course exam and encountered a question in my book (Applied Thermodynamics for Engineering Technologists, p 66 example 3.3) which stated that a compressor follows a polytropic process to compress air; I understood the calculation process which resulted in heat being transferred to the refrigerant. The problem is that I cannot understand how physically a compressor would be absorbing heat from the environment. I always thought that any type of compression would give off heat.

Theoretically I know that for a polytropic process if n=γ the process is adiabatic, if n is smaller than γ heat is given off and if n is bigger than γ heat is received into the working gas (), but I would like to know how this is physically possible.

The question I talked about above is the one in the image below.

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Thanks in advance.
 
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  • #2
It never went cross my eyes that n has anything to do with heat absorption and heat rejection. Simple is that, when a gas is compressed above its critical temp, its temperature increases. You are giving energy in the form of electricity which is converted into mechanical energy by a piston or impeller,this increases kinetic energy of the fluid.where would this energy go? It would increase its temp.
 
  • #3
According to the above problem, may be the surrounding temperature is above refrigerant‘s temperature.which caused heat transfer.
 
  • #4
:-)
Thanks
 
  • #5
engnr_arsalan said:
According to the above problem, may be the surrounding temperature is above refrigerant‘s temperature.which caused heat transfer.

Maybe that is true, but I'm not sure about it. Thanks :)
 
  • #6
Energy is transferred to a system either by work or by heat transfer,in above problem they have already found out the portion of energy tranfered by work,rest is heat transfer,which is obviously due to temperature difference.since Q is positive,it means heat is added to the system,thus surrounding/casing temp was greater then refrigerant temp.
 
  • #7
engnr_arsalan said:
Energy is transferred to a system either by work or by heat transfer,in above problem they have already found out the portion of energy tranfered by work,rest is heat transfer,which is obviously due to temperature difference.since Q is positive,it means heat is added to the system,thus surrounding/casing temp was greater then refrigerant temp.

Oh, okay I understood the idea now. Seems so simple after the explanation, I was blind folded by the fact that a compression always releases heat. Thanks for your input :)
 

1. What is a polytropic process?

A polytropic process is a thermodynamic process that involves a change in temperature and pressure in a system while also maintaining a constant ratio between the two variables. In other words, it is a process where the pressure and volume of a gas change, but the ratio of pressure to volume remains constant.

2. How does a compressor work in a polytropic process?

A compressor is a device that increases the pressure and decreases the volume of a gas. In a polytropic process, a compressor works by taking in a gas at a certain pressure and volume, and then compressing it to a higher pressure while maintaining a constant ratio between the two variables.

3. What is the role of thermodynamics in a polytropic process compressor?

Thermodynamics plays a critical role in a polytropic process compressor as it helps us understand the relationship between temperature, pressure, and volume in a gas system. By applying thermodynamic principles, we can accurately predict the behavior of a gas during a polytropic process and optimize the performance of the compressor.

4. How is efficiency measured in a polytropic process compressor?

The efficiency of a polytropic process compressor is typically measured by the polytropic efficiency, which is the ratio of the actual work done by the compressor to the work that would be done if the process were isentropic (no heat transfer or friction losses). A higher polytropic efficiency indicates a more efficient compressor.

5. What are some common applications of a polytropic process compressor?

Polytropic process compressors are commonly used in various industries such as oil and gas, chemical, and refrigeration. They are also used in air compressors, gas turbines, and other machinery that requires compression of gases. Additionally, polytropic process compressors are essential in refrigeration systems, where they compress and expand refrigerant gases to maintain the desired temperature in a space or device.

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