Charging a cascade system

Q On a cascade refrigeration system using R-503 and a remote condensing unit, what is the proper way to charge it?

A When talking about a cascade system, you charge using a standby charge, sometimes referred to as a static charge. The internal volume of the components and piping have been calculated so that at 70 F there will be a specific pressure in this unit.

Ultra-low temperature refrigerants such as R-503 are above their critical point at this temperature, therefore, you have absolutely no liquid present. The refrigerant is in its vapor state.

The service technician needs to know the static charge or standby pressure. By charging to this pressure when the unit is placed into operation and the refrigerants change state to a liquid for example, there will be sufficient pressure to provide both the needed net refrigeration effect and the required liquid seals.

This standby pressure varies from unit to unit and from manufacturer to manufacturer. It is generally between 70 and 130 psi as the maximum low-side design pressure is 150 psi. Once the system has been placed into operation and fine tuned to its proper operational superheat and subcooling, the static pressure should be recorded at the unit for future reference.

Question from Ratib Baker, CMS, Rising Sun, Md. Answer by Garth Denison, CMS, past RSES International president, senior product application engineer, Sporlan Valve Canada.