When topping off an operating system containing a blended refrigerant, how should the refrigerant be introduced?

When topping off an operating system containing a blended refrigerant, how should the refrigerant be introduced?

• A. Introduce as a vapor into the low side
• B. Introduce as a liquid into the high side without throttling
• C. Introduce as a liquid into the low side using a throttling valve
• D. Introduce as a vapor into the high side

Answer: (C)

When topping off a system that uses a blended refrigerant, the goal is to preserve the delicate mixture and avoid fractionation. Blended refrigerants (like R-407C, R-410A) have components that boil at different points, so introducing them as vapor or into the wrong side can alter the blend’s ratio inside the system. Introducing the refrigerant as a liquid into the low side and using a throttling (metering) valve provides controlled, gradual addition. The liquid entering the low-pressure side will flash to vapor as it passes through the evaporator, absorbing heat and mixing with the system refrigerant without disturbing the existing blend. The throttling valve meters the flow to prevent large surges, minimizing the risk of liquid slugging, improper superheat, or oil return issues. Adding liquid into the high side or introducing vapor risks fractionation, uneven distribution, and performance problems, so the best practice is liquid into the low side with a throttling valve.

When topping off a system that uses a blended refrigerant, the goal is to preserve the delicate mixture and avoid fractionation. Blended refrigerants (like R-407C, R-410A) have components that boil at different points, so introducing them as vapor or into the wrong side can alter the blend’s ratio inside the system.

Introducing the refrigerant as a liquid into the low side and using a throttling (metering) valve provides controlled, gradual addition. The liquid entering the low-pressure side will flash to vapor as it passes through the evaporator, absorbing heat and mixing with the system refrigerant without disturbing the existing blend. The throttling valve meters the flow to prevent large surges, minimizing the risk of liquid slugging, improper superheat, or oil return issues.

Adding liquid into the high side or introducing vapor risks fractionation, uneven distribution, and performance problems, so the best practice is liquid into the low side with a throttling valve.