# Dumb question on auto ac



## noquacks (Jun 5, 2010)

Why do they call it the suction side of the compresor(the side that receives the expanded gas from the evaporator) to be compressed in the comp, when it shows psi of 30 or so. I mean, if it was sucking from the evaporator, wouldnt it have a vacuum?? I dont get it.

Thnaks!


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## ukrkoz (Dec 31, 2010)

c/p

_I will use the simplest form of compressor to illustrate its working principle; the reciprocating compressor, also known as piston compressor. _
_As the driver, which is usually an electric motor, rotates; the piston is set to move on a continuous linear motion between two positions, within the casing._ 

_As the piston moves away from the valves quickly, a space of low pressure is created within the chamber. Due to the negative pressure difference between the chamber and outside it, the *suction* valve will open, allowing refrigerant gas* from suction side* to rush into the enclosed space. _
_This happens as the force from the pressure difference overcomes the spring force. _
_Due to the construction of the compressor’s shaft, the piston will be on a momentary stop at the end of the suction cycle. During this moment, pressure difference between suction line and gas chamber is very little, allowing the spring force to pull the suction valve towards closed position. _
_Now what happens? The piston starts to move towards the valves again. _
_Compression will happen, because both valves are closed; and the piston is quickly reducing the volume of the gas. _
_Déjâ vu! I know you would say that. Compression takes place, and positive pressure difference between the gas chamber and outside it, will force open the *discharge* valve. _
_As a result, air conditioner's refrigerant gas will be forced out through the discharge line. 
_


that you still have a relatively "high" pressure in suction line means not much, comparing to the "high pressure" side of a/c.



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http://www.air-conditioner-selection.com/how-air-conditioners-work.html
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## noquacks (Jun 5, 2010)

Thanks, UK. Also, I guess it sucks refrigerant from a can since even though there may be 30psi on a guage, the can itself has way more, I guess about 100psi or so?? Then, the HIGHER psi in the an overcomes a psi much lower in the suction line(?) Does that shed light on this UK?


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## fabrk8r (Feb 12, 2010)

noquacks, think of the compressor in comparison to an internal combustion engine. They both have an intake and an exhaust...the refrigerant compressor's suction port is like the intake port on an engine. The same comparison can be made for the discharge port of the compressor, likening that to the engine's exhaust.

The only time there should be a true vacuum in the system is when a vacuum is pulled to remove air and moisture prior to adding refrigerant.


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## ukrkoz (Dec 31, 2010)

noquacks said:


> Thanks, UK. Also, I guess it sucks refrigerant from a can since even though there may be 30psi on a guage, the can itself has way more, I guess about 100psi or so?? Then, the HIGHER psi in the an overcomes a psi much lower in the suction line(?) Does that shed light on this UK?



well, "suction" line is called suction cuz it's connected to suction port on compressor; it is *relatively* low pressure line, as compared to the high side of a/c system. 
freon mix is sucked from can by compressor. you have a specifically designed system to maintain fluids flow in one direction. when you have can connected, freon is being sucked out of it by suction action from compressor. simple proof is, if compressor does not work, can will not release into system. should you have had a much higher pressure in a can, it would have released by itself. it does not, and on some cars, one has to trip compressor into action for freon to flow into the system during refill. 
indeed, just like fabr said - air flows into combustion chambers, and quite violently, due to suction created by pistons.


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## noquacks (Jun 5, 2010)

Thanks, guys. I thionk I get it now. Its an edjucashun. Seriously, sometimes one has to ask questions for answers. Patient forum members are just great people.

That example of the comp being like an engine with the valves is excellent.


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