Question

Describe the function of a capillary tube. Write its advantages and disadvantages.

Hint:

It is a thin long copper tube that drops pressure and meters the flow. List at least four advantages and disadvantages.

Answer 1

Step 1: Understanding the Concept:
A capillary tube is a long, thin, hard-drawn copper tube of very small bore. It is a fixed-type expansion device used in small refrigeration systems. It has no moving parts.

Step 2: Function of a capillary tube.
It is fitted between the condenser (or the drier) and the evaporator.
The liquid refrigerant at high pressure flows through the narrow bore. The friction in the tube and the speed of the flow cause a big drop in pressure.
The pressure falls from the condenser pressure to the evaporator pressure. A small part of the liquid flashes into vapour, and the temperature falls.
The tube also controls the flow of refrigerant into the evaporator. A longer or narrower tube gives more resistance and less flow.
When the compressor stops, the pressures equalise through the tube, so the compressor starts easily with no load.

Step 3: Advantages.
1. It is very simple and cheap.
2. It has no moving parts, so there is almost no maintenance.
3. No receiver is needed, so the refrigerant charge is small.
4. Pressures equalise during the off period, so a low starting torque motor is enough.

Step 4: Disadvantages.
1. Its flow cannot be adjusted when the load changes.
2. It can get choked by dirt or moisture, so a filter-drier is needed.
3. The refrigerant charge must be exact. An extra charge can send liquid into the compressor.
4. It is suitable only for small units and gives poor control for large or varying loads.

Final Answer:
The capillary tube reduces the pressure of the liquid refrigerant and meters its flow to the evaporator. It is simple and cheap but cannot be adjusted and chokes easily.