Chilling Out — The Fridge, That Is

There’s a Southern phrase, “I’m fixing’ to …” that usually means “I am preparing to ….” Since it’s Spring, Lisa and I have been thinking about cruising again. In other words, we are “fixing’ to go cruising.”

Unfortunately, that phase has take on a whole new meaning for us. We’ve literally been fixing Pearl for the six weeks. We installed an air conditioner, we’ve replaced the batteries and charger, changed Pearl’s registration, replaced the fluxgate compass and recalibrated the autopilot, and now we are messing with the refrigeration system. The refrigeration system failed before we tried to leave the last time. Guess what? It did it again.

Having determined that there should be no magical boxes on Pearl, we’ve dug into the refrigeration system. It’s both simple and interesting. It is has an air cooled compressor/heat exchanger (unlike our air conditioner which has a water cooled heat exchanger) and two cold plates, one each in the fridge and the freezer. The way the system works is the compressor and heat exchanger cool the refrigerant, pass it through the first cold plate in the freezer, then through a constant pressure valve, into the second cold plate, and back. The system is controlled by a thermostat on the cold plate in the fridge — it really doesn’t matter how cold the freezer gets as long as the fridge stays at temperature (about 35F). There are no fancy electronics here, just tables mapping temperature (remember Boyle’s Law?) and pressure and a mechanical control in a closed system.

There are two pressure readings that apply, one is the “low side” pressure for the cooled refrigerant where it exists the compressor/heat exchanger, and the “high side” for where it returns to it. The magic here is balancing the high and low sides with a combination of adding refrigerant and twiddling the constant pressure valve until it equalizes out. It’s not quite rocket science, but the same principles that apply in the  combustion chamber and nozzle of a rocket engine apply here. Hmmm. Maybe it IS rocket science.

The base line pressure for the refrigerant on the low side should be 2-3 psi. The high side pressure depends on the ambient temperature.

Where we’ve been running into a problem is getting the pressure balanced between the high and the low sides. So far, this has been the result of a faulty constant pressure valve, and or some debris in the low pressure line when the valve was last replaced. Also, there may be a very small leak in the system that causes refrigerant loss whenever the freezer is defrosted. To find that we need to pressurize the system with Nitrogen to about 500 psi. I hope we can skip that for the moment.