Wish to understand the Venturi Effect WRT cooling

[slay2k]

Not a physics student -- but was thinking about how the A/C system works in a vehicle.

First surprise was that pressure drops when a tube narrows. Seemed counter-intuitive as I was picturing taking a balloon and squeezing it -- would the pressure in the middle be lower than the ends? Then again that's a closed system, but it affected my thought process.

Perhaps a garden hose is a better example. Sure, the water speeds up, but it certainly doesn't feel any easier to squeeze a garden hose (say a wide but flexible one) with water running through it. The more you squeeze, the harder it would get because you're constricting the flow -- is it not inner pressure making it harder and harder to squeeze?

Lastly, I understand that the refrigerant (R134a) in a cooling system needs to expand to cool, but I definitely do not understand how constricting it into a tiny section allows it to expand and become hyper-cooled gas.

A) Where is it expanding to? How is there sufficient room to expand in the constricted area? How does this not increase pressure?
B) If the total pressure in the system remains the same (dynamic increases, static drops?) then why is the refrigerant phase changing? In other words, if the total pressure remains the same then why does the boiling point decrease at all?

Apologies if the questions are stupid. Would really love to understand the most fascinating part of what makes A/C possible.

 

[russ_watters]

The refrigerant expands as it passes the constriction: its a nozzle and the tube after the constriction is larger than the constriction. Also, total pressure does not determine whether the phase (and temperature) changes: static pressure does.

 

[slay2k]

The refrigerant expands as it passes the constriction: its a nozzle and the tube after the constriction is larger than the constriction. Also, total pressure does not determine whether the phase (and temperature) changes: static pressure does.

But it expands within the constriction where the static pressure is lower, right? If so, how can it expand when the amount of space is fixed?

Also, is there an intuitive way to understand why only the static pressure affects the phase and temperature?

 

[rcgldr]

There's at least one pump in the A/C system where the intake of the pump takes in lower pressure gas from the region where the lower pressure and cooler gas is removing heat from the interior of the vehicle, and the pump outputs higher pressure and hotter gas to the radiator section where the heat is dissipated via the radiator to the ambient air outside. In this case, Venturi effect isn't being utilized. The constriction between the radiator and vehicle section opposite the pump restricts the flow (like a reverse pump), working in conjunction with the pump to maintain the low pressure and high pressure regions.

 

[slay2k]

There's at least one pump in the A/C system where the intake of the pump takes in lower pressure gas from the region where the lower pressure and cooler gas is removing heat from the interior of the vehicle, and the pump outputs higher pressure and hotter gas to the radiator section where the heat is dissipated via the radiator to the ambient air outside. In this case, Venturi effect isn't being utilized. The constriction between the radiator and vehicle section opposite the pump restricts the flow (like a reverse pump), working in conjunction with the pump to maintain the low pressure and high pressure regions.

I'm familiar w/ the compressor (pump) and the condenser (radiator) components of the system. The Venturi effect exists in all of these systems, sitting between the condenser low-side and the in-cabin evaporator either in the form of a) a thermal expansion valve (dynamic flow control + Venturi) or b) orifice tube (Venturi only).

Without this drop in pressure and subsequent vaporization of the refrigerant liquid, A/C would not be possible.

I still don't have a full grasp of the process from a physics standpoint, however, specifically the questions in my last comment.

 

[slay2k]

Something like flash evaporation makes sense to me, because there is space in the drum for the liquid to expand into. I don't know if this is what exists in the A/C system though?

 

[rcgldr]

I don't see why Venturi effect is needed, and it's not mentioned in these wiki articles (or I missed it):

http://en.wikipedia.org/wiki/Thermal_expansion_valve

http://en.wikipedia.org/wiki/Flash_evaporation

The reduction in pressure is due to the valve restricting the flow sufficiently for the intake side of the compressor to to create and maintain the reduced pressure. The reduction in pressure and the design of the flow path assist with the flash evaporation.

 

[boneh3ad]

But it expands within the constriction where the static pressure is lower, right? If so, how can it expand when the amount of space is fixed?

Also, is there an intuitive way to understand why only the static pressure affects the phase and temperature?

Static pressure decreases as velocity increases and for a subsonic flow, the velocity increases through a constriction. So yes, the static pressure decreases in the constriction. This is not where the expansion and cooling takes place, however. The flow of refrigerant is a highly compressible process. The pressure downstream of the constriction is much lower than upstream and the flow is choked by the constriction. It therefore expands rapidly once it leaves the constriction, causing it to cool.

 

[rcgldr]

To follow up on boneh3ad's post:

If so, how can it expand when the amount of space is fixed?

The expansion is from the small diameter constriction to the much larger diameter tubing just downstream of the constriction. In this case there's an external force involved, the intake side of the compressor that is reducing the pressure downstream of the constriction, and the output side of the compressor that is increasing the pressure upstream of the constriction. So just downstream of the constriction, the highly compressible flow expands, cools, and slows down (versus it's velocity through the constriction).

 

[russ_watters]

I'm familiar w/ the compressor (pump) and the condenser (radiator) components of the system. The Venturi effect exists in all of these systems...

The venturi effect is not well applied to any of these situations. The venturi effect doesn't deal with significant density changes, much less phase changes. It deals only with slight changes in pressure due to the velocity change and the low-speed version of Bernoulli's principle.