Which of the following is not considered a secondary coolant?

In refrigeration and cooling systems, secondary coolants are fluids that are used to transfer heat away from an area, often used in conjunction with primary coolants like refrigerants. Secondary coolants can include substances that can absorb heat and lower temperatures effectively.

R-134a is a refrigerant, not a secondary coolant. It is primarily used within the refrigeration cycle as the main fluid that absorbs and releases heat in the evaporator and condenser. Its role is distinct from that of secondary coolants, which typically circulate within a system to transfer heat to the primary refrigerant or directly to the environment.

In contrast, sodium chloride, calcium chloride, and glycol are examples of secondary coolants. These substances are often used to adjust the freezing point of water or serve as a heat transfer medium without participating in the refrigeration cycle itself. They play a crucial role in various applications such as brine systems, where they help maintain low temperatures in cooling processes. This differentiates them clearly from a refrigerant like R-134a, affirming why R-134a is the correct choice for this question.