# Liquid line or suction line confusion!

Hello, I’m reaching out to the community to help clear something up for me! I’m currently reviewing the HVAC specifically air conditioning. So I understand that the liquid refrigerant flows into the expansion device then into the evaporator coil where it then boils and vaporizes and receives heat from the interior air. Afterwards the heated vapor then moves to the compressor and condenser where the heat gets released by the air blowing through it and the refrigerant re-condenses into liquid and the cycle repeats. So my question is when the vapor receives heat from the interior wouldn’t the suction line be warm from receiving the interior heat and not the liquid line because it just released the heat? InterNachi says the liquid line will be warm which is why I’m confused because it doesn’t sound logical.

Thanks for the help in advance!

Short answers is high (liquid line) vs low (suction line) pressure of the refrigerant gas.

When the refrigerant gas in its vapor form gets to the compressor it is compressed and heats up due to compression. at this point it is the hottest (high pressure). It then enters the condenser where it begins to give off heat to the outside and turn into a liquid. When it exits the condenser, it is in liquid form and is still warm (liquid line), and still under high pressure (high side). Then it gets to the expansion device and starts to turn into vapor and pick up heat as it goes through the evaporator. When it exits the evaporator, it’s in vapor form, lower pressure (low side) and is of lower temperature because of low pressure vs the liquid line which is under higher pressure. Hope this makes sense.

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You must regulate the metering device so that the refrigerant changes state (liquid to a vapor) in the evaporator only. The gas after the evaporator also must have enough capacity to cool the compressor (in the state of a gas). It should come out of the evaporator around 40 degrees F (50-55 degrees at the compressor). Any liquid in the suction line could damage the compressor. Look up “Super heat Adjustment”.

Because your hand is in the 90’s, the suction line should always feel cool.

The liquid line should not be hot or warm for that matter. Your hand is in in the 90’s and the liquid line should be close to the outdoor air temp, which is close to being in the 90’s whenever the a/c is being used. You should not feel heat if the coil is clean.

Now if you grab the compressor discharge line, you won’t be holding on for long!

If the liquid is hot, you’ll loose capacity as some of the liquid must flash off to cool the refrigerant down to about 70 degrees F before it enters the evaporator.

You never want the change of state to happen outside the evaporator and you never want refrigerant as a gas after the condenser.

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Metering device is on the evaporator coil (piston or TXV most all R22systems use pistons, 410A uses txv, but both use either or) when refrigerant enters metering device it is compressed into gas ( that’s when it’s cold) evaporator turns cold when that happens. Warm air passing thru the coil cools off, it also pulls moisture out of the air, and you get crisp cool air blowing in the house. The refrigerant than continues thru the suction line ( big line) back into condenser unit, at this point it’s still in gas state that’s why big line is always cold. In the condenser unit it turns back to liquid and the presses repeats.

Why would you write this?

You are right that was not correct term to use, my apologies.

The A/C cycle relies on state transition to move heat. Converting a gas to a liquid, as happens in the compressor, is exothermic. It releases heat. While the heated liquid is in the condenser coil the fan blows air across it and extracts heat from the refrigerant, because the refrigerant is significantly hotter than the ambient air (or liquid in a hydronic system), because heat moves from warmer to cooler areas. This process extracts significant thermal energy from the coolant and transfers it to the outside air

Now the liquid refrigerant, which is still “hot” but has significantly reduced thermal energy moves through the liquid/pressure line to the evaporator coil, where it passes through a metering device which reduces the pressure and the liquid flashes to a gas (it skips the boiling state and evaporates directly from liquid to gas - like your blood would do in the vacuum of space). Transferring a liquid to a gas is an endothermic reaction. It absorbs heat making the gaseous refrigerant cold. The gaseous refrigerant is routed through the evaporator coil while to house air is blown across the coil. This extracts a great deal of thermal energy from the air passing across the coil because, again, heat moves from warmer to cooler, thus cooling the air in the house.

The system, by repeatedly cycling the refrigerant between liquid and gaseous states while extracting thermal energy from or absorbing thermal energy into into it effectively “pumps” thermal energy (heat) from one location to another. A “heat pump” heats the house by simply reversing the roles of the two coils compared to cooling mode.

Remember transitioning from gas -> liquid releases heat; transitioning from liquid -> gas absorbs heat (like when your sweat evaporates on a hot day).

No, the metering device meters. It does not compress anything. Compression happens in the compressor (no pun intended). The metering device is where there is a pressure differential. The pressure change is what causes the state of the refrigerant to start to change. Some of the gas will flash off (depending on how hot the high pressure liquid is coming out of the condenser). This flash gas cools the liquid on the low pressure side of the metering device entering the evaporator.

What you wrote is a little off Chuck. The refrigerant is not passing through the evaporator as a gas, it is part gas and part liquid (depending where you measure it). As you stated;

.There is actually liquid refrigerant all the way through the evaporator. The change of state must happen in the evaporator, not in the metering device or in the suction line. Thus Super Heat adjustment is required. We want the gas at the outlet of the evaporator to be 10 degrees F higher than the low side saturation temperature to insure there is no change of state happening outside of the evaporator. Under charged units will cause the metering device to freeze into a snow ball. Over charged, or improper adjustment of the metering device causes the suction line to freeze up. Both are bad news for the compressor as well as inefficient cooling of the building.

If you see frost on the suction line, the first thing to check is air flow. Refrigerant pressure can not be evaluated to determine refrigerant charge if the air flow through the coil is not correct (ie. dirty coil, fan blade, bad motor).

Keep in mind that a “Pump” must suck in order to pump. Suction is as important in the refrigerant cycle as pumping.

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Thank you for your response, you summed it up nicely and made everything clear!