Hydrostatic Pressure – Short 108

In this short podcast episode, Bryan explains how hydrostatic pressure can build up in refrigerant cylinders and present a hazard to technicians.

Your refrigerant cylinders have tare weight and water capacity values stamped on the tank. You’ll want to use these when weighing the refrigerant you recover because you don’t want to exceed 80% capacity. However, capacity changes when the liquid density changes; that density will change with pressure and temperature.

Hydrostatic pressure builds up when you have overfilled refrigerant vessels. When those vessels get warm, the density will decrease, and the liquid refrigerant expands. At some point, the vessel will contain 100% liquid and can no longer expand, so hydrostatic pressure will build. When that happens, you have a dangerous situation on your hands; the vessel may even explode.

AHRI recommends using 77 degrees as a guideline for figuring out the vessel capacity. However, we recommend using 130 degrees out of an abundance of caution; the back of your van probably won’t get much hotter than that, so we use it as an operational maximum.

We only get hydrostatic pressure when we recover refrigerant as a full liquid. When we recover refrigerants like R-410A in the liquid phase, we get a 45-PSI increase for each degree (Fahrenheit) of temperature increase. For R-22, that number is about 60 PSI; with R-134A, that number is about 40 PSI. When we get temperature swings from an ice bucket (~32 degrees) to the back of a hot van (~130 degrees), the pressure can build up within the vessel.

We also need to think about hydrostatic pressure when pumping down systems with microchannel coils. Hydrostatic pressure can build up in the receiver, and liquid can fill your condenser.

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