How does air conditioning cause gas to expand?

Gas cools as it expands.

I am reading a book by the cosmologist Michio Kaku and he was elaborating on how if the universe kept of expanding for infinity, temperature would plunge to absolute zero as gases expand.

He compared it to air circulating in pipes in air conditioners and refrigerators. How does that work?

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• Sean
Lv 5
8 years ago

The compressor actually works in conjunction with a metering device to maintain two separate pressures inside a refrigeration/AC system. The theory is the same for both. The compressor continually pumps gas from the low side to the high side and the metering device only allows so much refrigerant to pass from the high side to the low side thus creating a pressure differential between the two coils. At no point does the compressor allow the refrigerant to expand causing it to absorb heat. The pipe sizing throughout the system pretty much remains uniform to two different sizes. It is in fact the theory of maintaining 2 different pressures on either side of the system that allows the refrigerant to absorb or reject heat in the 2 differing sides.

In addition to 'refrigeration cycle' research thermodynamics as well. According to thermodynamics pressure and temperature follow each other, meaning if you add heat to a substance it wants to expand, if the area it is contained in is being controlled then it can't expand and the pressure increases. By maintaining a pressure differential between the low and high sides they are able to lower and raise the temperature of the refrigerant through heat absorption and rejection. Referring to AC on a R-22 system they aim to lower the pressure to roughly 60psi at about a temperature 34'F leaving the metering device as a liquid refrigerant. By passing room air at say 75'F across the low side coil they are able to absorb heat from the air into the refrigerant dropping air temp to roughly 55'F supply air. The heat absorption raises the Temperature of the refrigerant causing it to boil off into a vapor since it is being contained and can't expand. The vapor is pumped through the compressor and into the high side where depending on ambient temps and manufacture design it would likely be between 170psi and 225psi. The refrigerant at this drastically higher pressure increases drastically in temperature as well. As the refrigerant passes through the outdoor/condenser coil it rejects heat to ambient air being pulled across the coil. Through thermodynamic properties as it becomes cooler it also condenses back into a liquid heading back to the metering device. The cycle continues until the controller or thermostat declare that set point has been reached and it shuts down the compressor.

What Michio Kaku explained is a sound hypothesis or theory of how the universe is working and widely accepted and there is proof to support this. However his example of comparing it to what happens in an air conditioner is some what misleading when looking at the actual operation of an AC.

• 8 years ago

In refrigerators, gasses are kept pressurized into liquid form in tiny capillary pipes that have a very short diameter. As they flow through the system, the pipes increase in diameter, and the liquid can depressurize and evaporate into a gas. This process removes heat from the system. Air conditioners work on a similar premises, but also include humidity control, unlike most refrigerators.