Why does a refrigerating system need an evaporator?

Im studying Ac refrigeration and I do not understand what this book keeps showing. Why have an evaporator at all? If colder vapor is going into a compressor and it's churning out hotter vapor, then that hotter vapor has to cool to become liquid(which has less temperature) why can't the compressor pump the low vapor at a lower psi? Wouldn't that equal hot liquid instead if hotter vapor?


The answers still not answering the question. Can someone tell me why a compressor turn cold vapor into hot liquid. Why can't it skip the hot vapor stage?

10 Answers

  • M.
    Lv 7
    5 months ago

    The refrigeration system moves heat from one place to another. 

    The CONDENSER sheds heat. 

    The EVAPORATOR absorbs heat. 

    Very simple. 

    The refrigeration principle is centered on the state change of the refrigerant.  The liquid state and the vapor state. 

    Going from liquid state to vapor state ABSORBS heat. 

    Going from vapor state to liquid state SHEDS heat. 

    You made a slip in your explanation that shows why you don't understand.  I have met trained HVAC technicians who had the same idea as you. 

    The "compressor" is not a "pump"! 

    The compressor has vapor IN and vapor OUT. 

    Cold, low pressure vapor IN, and hot, high pressure vapor OUT. 

    The compressor can't shed heat.  The heat of compression makes heat, plus the friction of the mechanism makes a little more heat.  It all travels to the condenser via the hot refrigerant vapor. 

    p.s.  You thumb down dingbats!

  • fuzzy
    Lv 4
    5 months ago

    First you can't compress a liquid.

    Second what would be the use of no evaporator? that's the part we get the cold air from, no cold air/liquid = no use for the system. 

    I think you are getting condenser and evaporator mixed up.

    Lets start with liquid Freon which would be at the bottom of the condenser (it has been condensed) we meter it to to the evaporator where it expands (evaporates) and removes heat from the process (air/liquid - whatever) now we have a hot vapor (it absorbed the heat) we send it to the compressor (because it's already expanded)

    we need to compress it down again, now we have a hot gas that needs to be turned back into a liquid (hence the condenser) which condenses it back to a liquid giving off the hot air/liquid from the evaporator back to some cooling source (air/liquid - whatever)

    and the process begins again.

  • 5 months ago

    The evaporator is the part of the refrigeration system that is doing the actual cooling. Because its function is to absorb heat into the refrigeration system (from where you don't want it), the evaporator is placed in the area to be cooled. ... The refrigerant vaporizes from the heat it absorbs heat in the evaporator.

    The reason 'why' is because that's the way it works.

  • 5 months ago

    The evaporator is sort of like a heater core but has the 134a refrigerant circulating through it to make it cold instead of hot. The blower motor blows air through the evaporator fins to cool the air coming out of your vents. Do a search for "how do air conditioners work" and you'll get a video showing the reason for all the components and what they do.

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  • 5 months ago

    When you compress gas. it gets Hot. Law of physics, the work has to go somewhere/.

    Allowing it to expand back into gas  absorbs heat, gets cold. Like using  bug spray on plants, there is a warning that getting too close will freeze plants.

    An evaporator uses ;large tubes to allow gas to expand efficiently and surface area of tubes adds to the heat transfer/efficiency. So do the fins and fan. It is made of Al or Cu  that aides  heat transfer, both metals transfer heat quickly.,  do not retain it like  iron or steel.

    A compressor is Certainly A Pump. Heat pump. It    takes lower pressures and produces higher pressures. Put yore hand over inlet, Sucks, Don't it? Put thumb over outlet, it blows it off. . 

    R134A?  He didn't ask strictly  about Modern car AC's. Some still  use R12, houses  and industrial units use R22 or  something even newer. Their pressures are much higher than a car's system.

  • 5 months ago

    This dude enplanes refrigeration very well:


    Youtube thumbnail

    You should watch this video. 

    When the compressor compresses the refrigerant, whitch is currently in a gas, the refrigerant  condenses, or converts to a liquid, because of the high pressure. When a gas condenses, it releases heat energy, which causes the condenser to get hot. The job of the condenser is to transfer the heat to the surrounding air, and cool off the refrigerant, to ensure it condenses to a liquid. If the condenser gets too hot. It may not condense.

  • 5 months ago

    Compressed vapor contains a lot of heat that gets radiated off, so then the it gets evaporated it contains a lot less heat than it had.

  • Anonymous
    5 months ago

    " why can't the compressor pump the low vapor at a lower psi?"  Because high school physics will tell you it cannot.  Get a tire hand pump (your compressor) and start pumping.  Now feel the hand pump body after 10 minutes.  There is your answer.  When a gas is compressed, it becomes hot.  If it does not become hot, then all heat engines will cease to operate.

    The liquid coming out of the condenser is about 100degF.  It cannot do any cooling.  It has to "flash" to about 75 percent liquid and 25 percent vapor.  That is the purpose of the metering device.  Now this refrigerant of 75 percent liquid and 25 percent vapor at about 35 to 40 degF has to be in a heat exchanger to do any cooling.  This heat exchanger (your evaporator) does two things.  It cools the air passing through it (or removes the heat) and it "boils" the refrigerant so it becomes a gas before it goes back to the compressor. 

  • Anonymous
    5 months ago

    The cooling take place when there is a change of state from liquid to vapor. The evaporator is the low pressure part of the system where this happens. The cold vapor from the low side turns to a hot liquid in the high side due to the high pressure created by the compressor. The hot liquid loses heat in the condenser and returns to the low side through an orifice that creates a pressure drop and allows it to turn into cold vapor again. Without the changes of state from liquid to vapor and back again caused by the variance in pressure between the high side and the low side of the system there would be far less (almost no) cooling.

  • Rick
    Lv 7
    5 months ago

    when the compressed gas finishes cooling what it is suppose to cool, it is still under SOME compression, so it has to be 'warmed-up' to go back into a liquid state for the next pass at the compression cycle.

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