Phase Transition, Expansion Valve, Evaporator, Compressor, Condenser and Air Conditioning

Posted by PITHOCRATES - April 3rd, 2013

Technology 101

We can use Volume, Pressure and Temperature to change Water from a Liquid to a Gas and back Again

Liquids and gasses can do a lot of work for us.  If we can control three variables.  Volume.  Pressure.  And temperature.  For example, internal combustion engines work best when hot.  But excessive heat levels can damage the engine.  So we use a special anti-freeze/anti-boil liquid in the cooling system.  A pump circulates this liquid through the engine where it absorbs some of the excess heat of combustion that isn’t used in pushing the piston.  After leaving the engine it flows through a radiator.  Air blows across tubes in the radiator cooling this liquid.  Ejecting some of the heat of combustion into the atmosphere.  Lowering the temperature of the cooling liquid so it can flow through the engine again and absorb more heat.

Our first cars used alcohol in the winter for a lower freezing point.  So this liquid didn’t freeze in the engine and crack the block.  Letting the coolant flow out.  And with no cooling available the excessive heat levels would damage the engine.  In the summer time we used plain water in the cooling system.  And kept the cooling system sealed and under high pressure to prevent the water from boiling into steam.  But the high pressure often caused a hose or a radiator cap to fail.  Releasing the pressure.  And letting the cooling water boil out leaving the engine unsafe to operate.

If this happened on a hot summer’s day and you got a tow to a gas station you may have sat there waiting for them to complete the repairs.  Sipping on a cool bottle of soda from a refrigerated soda machine.  Soon drops of water would condense onto your cold bottle.  The cold bottle cooled the water in gas form (the humidity in the air) and turned it back into a liquid.  So in these examples we see how we were able to use pressure to keep water a liquid.  And how removing heat from water as a gas changed it back into a liquid.  This phase transition of a material has some very useful applications.

The High-Pressure Refrigerant Liquid from the Condenser loses Pressure going through the Expansion Valve

The phase transition between a liquid and a gas are particularly useful.  Because we can move liquids and gases in pipes and tubing.  Which allows us to take advantage of evaporation (going from a liquid to a gas) in one area.  While taking advantage of condensation (going from a gas to a liquid) in another area.  By changing pressure and volume we can absorb heat during evaporation.  And release heat during condensation.  Allowing us to absorb heat inside a building with evaporation.  And release that heat outdoors with condensation.  All we need are a few additional components and we have air conditioning.  An expansion valve.  An evaporator.  A compressor.  A condenser.  A couple of fans.  And some miscellaneous control components.

We install the expansion valve and the evaporator inside our house.  Often installed inside the furnace.  And the compressor and the condenser outside of the house.  We interconnect the indoor and the outdoor units with tubing.  Inside this tubing is a refrigerant.  Which is a substance that transitions from liquid to a gas and back again at relatively low temperatures.  As the refrigerant moves from the evaporator to the condenser it is a gas.  As it moves from the condenser to the evaporator it is a liquid.  The transition between these stages occurs at the evaporator and the condenser.

The refrigerant leaves the condenser as a liquid under high pressure.  As it passes through the expansion valve the pressure drops.  By restricting the flow of the liquid refrigerant.  Think of a faucet at a kitchen sink.  If you open it all the way the water flowing in and the water flowing out are almost equal.  But if we just open the faucet a little we get only a small trickle of water out of the faucet.  And a pressure drop across the valve.  With the full force of city water pressure pushing to get out of the faucet.  And a low pressure trickle coming out of the faucet.

As the Warm Air blows across the Evaporator Coil any Humidity in the Air will condense on the Coil

As the liquid leaves the expansion valve at a lower pressure it enters the evaporator coil.  A fan blows the warm air inside of the house through the evaporator coil.  The heat in this air raises the temperature of the refrigerant.  And because of the lower pressure this heat readily boils the liquid into a gas.  That is, it evaporates.  Absorbing heat from the warm air as it does.  Cooling the air.  Which the fan blows throughout the ductwork of the house.

As the gas leaves the evaporator it travels through a tube to the condenser unit outside.  And enters a compressor.  Where an electric motor spins a crankshaft.  Attached to the crankshaft are two pistons.  As a piston moves down it pulls low pressure gas into the cylinder.  As the piston moves up it compresses this gas into a higher pressure.  As the pressure rises it applies more pressure on the spring holding the discharge valve closed.  When the pressure is great enough it forces open the valve.  And sends the high-pressure gas to the condenser coil.  Where a fan blows air through it lowering the temperature of the high pressure gas enough to return it to a liquid.  As it does it releases heat from the refrigerant into the atmosphere.  Cooling the refrigerant.  As the liquid leaves the condenser it flows to the expansion valve to repeat the cycle.  Over and over again until the temperature inside the house falls below the setting on the thermostat.  Shutting the system down.  Until the temperature rises high enough to turn it back on.  A window air conditioner works the same way.  Only they package all of the components together into one unit.

There is one other liquid in an air conditioning system.  Water.  As the warm air blows across the evaporator coil any humidity in the air will condense on the coil.  Like on a cold bottle of soda on a hot summer day.  As this water condenses on the evaporator coil is eventually drips off into a pan with a drain line.  If the evaporator is in the furnace this line will likely run to a sewer.  If the evaporator is in the attic this line will run to the exterior of the house.  Perhaps draining into a gutter.  If it’s a window unit this line runs to the exterior side of the unit.  These simple components working together give us a cool and dehumidified house to live in.  No matter how hot and humid it gets outside.

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