Increasing Compressor Efficiency Question

[cskib21]

I reference this forum all the time and you guys are always good at breaking down confusing questions so I decided I needed to make an account and get some help with a mental struggle I have been having for quite some time!

Two commonly discussed ways of improving a compressor's efficiency in a standard refrigeration cycle are:

1) Increase suction pressure- Less dp across compressor, less work.
2) Reduce suction gas temperature- Denser gas, less work.

This is confusing to me and I am hoping someone can clarify. My thoughts:
1) When the pressure is increased, the temperature must also increase as well, right? Pv=nRT.
2) And when the temperature of the gas is decreased, the pressure must also decrease as well right?
-assuming v,n and R stay constant in the closed cycle...

So how can both of these measures be true? You are negatively effecting one by improving the other so can't only one of these be correct? Or is it a balance of both?

I can't wrap my head around it, any help is appreciated.

 

[jack action]

I'm no expert on refrigeration cycle, but I agree with a reduction on suction gas temperature increasing efficiency since COP = 1 / (T_H / T_L - 1).

For the other one, sure, reducing the pressure drop across the pump will reduce the pump work, but it will also reduce the temperature differential, thus the efficiency. That would be amazing if one could reduce the pressure drop to zero while maintaining the same heat transfer!

 

[russ_watters]

I reference this forum all the time and you guys are always good at breaking down confusing questions so I decided I needed to make an account and get some help with a mental struggle I have been having for quite some time!

Two commonly discussed ways of improving a compressor's efficiency in a standard refrigeration cycle are:

1) Increase suction pressure- Less dp across compressor, less work.
2) Reduce suction gas temperature- Denser gas, less work.

This is confusing to me and I am hoping someone can clarify. My thoughts:
1) When the pressure is increased, the temperature must also increase as well, right? Pv=nRT.
2) And when the temperature of the gas is decreased, the pressure must also decrease as well right?
-assuming v,n and R stay constant in the closed cycle...

So how can both of these measures be true? You are negatively effecting one by improving the other so can't only one of these be correct? Or is it a balance of both?

I can't wrap my head around it, any help is appreciated.

You're right, but there are other variables in that equation you can change...

 

[cskib21]

Won't v,n and R stay constant in the closed cycle...?

 

[russ_watters]

Won't v,n and R stay constant in the closed cycle...?

Why be closed?

Or, rather, why make a different cycle, with some parameters different, but not others?

 

[jack action]

Won't v,n and R stay constant in the closed cycle...?

n will not necessarily be constant.

If you used the form p = ρRT for the equation, and you know either p, ρ or T, then you are stuck with one equation and two unknowns. You are missing an equation to find a unique solution. For gases, you use pv^k = constant (polytropic process assumption) and when phase changes are involved, thermodynamic properties tables are used.

 

[Randy Beikmann]

cskib21, you need to think about how a chunk (a given mass) of the gas changes as it travels through the system. You can't meaningfully use Pv=nRT on the whole system at once, because pressure, volume, and temperature vary from location to location. To increase the density (m/v) of the gas before it enters the compressor, you can run it through a heat exchanger. Its volume will decrease, ignoring any pressure change, per Pv=nRT, assuming it acts like an ideal gas.
So the main point is that P, v, and T of the chunk of mass are all variables, dependent on each other.