Outdoor refrigeration

The refrigeration cycle is pretty nifty and its development began in the late 1700s. The earliest use of commercial refrigeration occurred sometime in the mid 1800s by, you guessed it, breweries. The refrigeration cycle is pretty, well, um, cool, and has four main parts to the cycle.

The cycle begins by compressing a refrigerant gas, such as ammonia; this results in high-pressure, hot, refrigerant gas. The hot gas flows from the compressor into the condensing coil where a coolant, normally air from a fan (this is the thing outside of your house that blows hot air in the summer), removes heat from the hot gas, thereby converting it into high pressure liquid. This phase change occurs because the gas condenses when cooled.

The third step is where the cool begins; the high pressure gas flows through an expansion valve, which is essentially a nozzle providing backpressure to the liquid side of the system. The nozzle creates pressure drop in the system and when the high pressure liquid expands the result is a mixture of cold refrigerant gas and liquid (for more on why the mixture becomes cold read about adiabatic expansion). This mixture then flows to the heat exchanger responsible for chilling, often called “the coil” in a refrigeration system, where the refrigerant absorbs energy from the environment and is evaporated into gas. The energy absorbed by the refrigeration is removed from the system as the warm gas enters the compressor. The cycle now repeats itself and the energy removed in the condenser represents the energy absorbed in the evaporator coil.

Depending on how this system is set up, in terms of what the compressor and thermal expansion valve are doing to the refrigerant, and what type of refrigerant is being used this type of refrigeration loop works great as either a refrigerator or a freezer. The system starts and stops by activating the compressor and this is triggered using a thermostat. The bottom line is that the unit is designed to remove heat from the system, in other words make the inside of the cooler cold, when running. And when the unit is not running the inside of the cooler becomes warmer if there is a heat source inside of the cooler, such as a fermenting batch of beer, or if warm air from the outside environment infiltrates the cooler, or as the cold from inside the cooler is transferred out across the walls of the cooler.

So, what happens in the winter time when a refrigerator is placed outside and the thermostat is set to 68 °F (20 °C)? If the temperature inside is warmer than 68 °F (20 °C), the compressor kicks on and stays on until the temperature reaches 68 °F (20 °C). However, if the outside temperature is cold, let’s say it’s 20 °F (-7 °C) out and the cooler begins to drop below 68 °F (20 °C), nothing happens with respect to the compressor. The temperature inside the cooler will continue to drop as the cold from outside the cooler transfers inside. This is no different than what happens inside your house during the winter. Eventually, the contents of the refrigerator will freeze if the outside temperature stays cold for long enough. This process is slowed by insulation, but insulation does not stop it.

If you really want to have a refrigerator that does not drop below the set point, then you must install a heater and use two thermostats. The simplest heater is to use a heat lamp, but if you ferment in glass carboys you need to be careful not to damage your beer with light. Refrigerator heater kits are sold by some refrigerator manufacturers specifically to keep refrigerators located in garages from becoming too cold.