Expansion devices in small vapour-compression systems Where is a capillary tube commonly used as the expansion device?

Introduction / Context:
In vapour-compression refrigeration, the expansion device throttles high-pressure liquid refrigerant to the low-pressure evaporator. For small and medium capacities, the simplest device is a capillary tube. Knowing where capillary tubes are appropriate is key for design, troubleshooting, and exam questions.

Given Data / Assumptions:

• Systems considered: domestic refrigerator, water cooler, and room air-conditioner.
• Typical capacities are relatively low (from a few hundred watts to a few tons of refrigeration).
• Steady operation with modest load variation.

Concept / Approach:
A capillary tube is a long, small-bore tube providing fixed flow resistance. The pressure drop depends on length, internal diameter, and refrigerant mass flow. It is inexpensive, has no moving parts, and works well when the evaporator load is fairly steady and when on-off or reciprocating compressors are used.

Step-by-Step Solution:
Identify applications that fit small, steady-capacity duty: domestic fridges, water coolers, room A/C.Recognize capillary tube advantages: low cost, simplicity, self-compensating during off-cycle due to pressure equalization.Conclude that all listed small systems routinely employ capillary tubes as standard expansion devices.

Verification / Alternative check:
Manufacturer service manuals for refrigerators, direct-expansion water coolers, and room A/C units routinely list capillary tubes in parts diagrams, confirming standard usage in these categories.

Why Other Options Are Wrong:
Singling out only one of the three would ignore the widespread practice across all these small-capacity systems. Very large chillers, however, typically use thermostatic or electronic expansion valves to cope with large load variations.

Common Pitfalls:
Confusing capillary tubes with thermostatic or electronic expansion valves; assuming capillary tubes can regulate superheat dynamically (they cannot).

Final Answer:
All of these