This is a project which started off as something of a joke, but I ended up actually making it. It is a refrigerator for cooling a single beverage as fast as possible. Most of the device is from a window air conditioner, but the capillary tube, evaporator, and cool air impeller have all been removed. What remains is a compressor, condenser with cooling fan, and a base to put everything on. In place of the capillary tube is a metering valve, which allows the expansion impedance to be adjusted. A coil of quarter inch copper tubing acts as the evaporator, which is placed inside an insulated container. Valves and 1/4" flare connectors are also included to the high and low pressure sides for charging the system. When the refrigerator is running, a beverage bottle or can is placed in the coil, and a coolant (such as a mix of alcohol and water) is used to provide good thermal contact between the cooling coil and beverage. The coolant must be non-toxic, and non-corrosive to the copper tubing, and should not freeze.

The refrigerant used is ordinary propane. In most respects, Propane is a very good refrigerant. It is cheap and easily available, has a zero ozone depletion potential (ODP), and a negligible global warming potential (GWP) of 3 (the most common zero ODP refrigerant is the HFC R-134a, with a GWP of 1300). Hydrocarbons are also miscible with most refrigerant oils, such as mineral and polyolester oils. Propane's major drawback is it's flammability, and the added safety considerations around this are why it is not used as often in commercial products.

Propane (R-290 is it's refrigerant designation) is similar to R-22 in many respects (the original refrigerant used in the air conditioner in this project), and for this purpose it can be used as a low cost and easy to obtain drop-in replacement. The volumetric heat capacity of R-290 is comparable to R-22, while the enthalpy of vaporization roughly three quarters larger. The vapor pressures are very similar, but the vapor and liquid densities of R-290 are approximately half of R-22. This means the volume of R-290 required to charge a system is about equal to the volume of R-22 required, but the weight of R-290 used is roughly half. The coefficient of performance (COP) of R-290 over R-22 depends on the amount of superheat (temperature difference in the vapor coming in to the suction port of the compressor and the temperature of the refrigerant at a vapor pressure equal to the suction port pressure). At greater superheat, the COP of R-290 is greater than R-22 because of R-290's greater vapor heat capacity.

Below is an image of a test setup running, before I had enough copper tubing to complete the project. The entire low pressure side quickly frosts up. The manifold gauges are shown connected to the high and low pressure ports of the refrigerator, with the service line of the manifold going to a valve that switches between a vacuum pump and propane tank. The temperature of the high and low pressure sides can be inferred from the pressure readings on the manifold gauges, by matching the temperature of R-290 to the measured vapor pressure.

Once additional copper tubing was acquired, everything was arranged to fit within the profile of the base. All connections are brazed with a copper phosphorus alloy, except for the compression fittings on the valves.

The completed cooler is shown below. The low pressure side tubing is covered with insulating foam, and an insulated container approximately the size of the evaporator coil is used to hold the coolant between the coil and beverage.