Engineering Refrigeration The tunnel requires a freezer unit at the entryway to protect it against summer heat.  Refrigeration is a simple process that takes advantage of the thermal "heat sink" properties of ammonia or freon when it phase changes from a gas to a liquid and back.    Freon gas is cooled outside the tunnel by rapidly blowing air across a condenser radiator.  The freon liquefies and is pumped through a plumbing system into another fan assembly inside the tunnel.  The tunnel fan blows tunnel air across the tunnel evaporator radiator.  This warms up the freon (it actually boils) until it is whisked outside by the plumbing system, taking heat from the tunnel with it.  The outside condenser cools the freon into a liquid again and the process repeats continuously as determined by the thermostat inside the tunnel.  The entire system is powered by local electricity.   The internal evaporator system and double doorway looking out.  The fans blow warm tunnel air across a radiator filled with cool neon liquid.  This cools the tunnel air and boils the neon liquid into a gas.  The neon gas is pumped outside taking tunnel heat with it.  The evaporator fans (above the tunnel doorway) are set inside the blue radiator. *Note the ice stalagmites that form on the floor from moisture dripping off the warm evaporator pipe before it takes the heat outside the tunnel.

Arctic Entryway An imperfect double-door system helps insulate the tunnel entrance (left).  The exterior condenser sits in the hut on the right.   Condenser It looks more complicated than it is.

http://www.crrel.usace.army.mil/permafrosttunnel/1e5_Refrigeration.htm

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