The selection of a capillary tube for a 1/4 HP compressor depends on fundamental factors such as the type of refrigerant gas and the evaporator temperature for the specific application.
Selecting the Capillary Tube
It is crucial not to choose the capillary tube solely based on the motor’s horsepower (HP). Instead, the best practice is to use the cooling capacity of the compressor, typically measured in BTU/h or kcal/h.
Sometimes, the cooling capacity data might not be available for the compressor. In such cases, careful calculations are required to determine the appropriate dimensions of the capillary tube.
Problem with Using HP:
Using HP as a reference can be inaccurate since it only reflects the motor’s electrical power, which is influenced by the motor’s efficiency. This approach does not account for the compressor’s efficiency, potentially leading to incorrect selection.
Key Factors for Selecting a Capillary Tube
| Condition | Evaporation Temperature | Condensation Temperature |
|---|---|---|
| LBP (Low Back Pressure) | -23.3 °C / -10 °F | 54.4 °C / 130 °F |
| MBP (Medium Back Pressure) | -6.7 °C / 20 °F | 54.4 °C / 130 °F |
| HBP (High Back Pressure) | 7.2 °C / 45 °F | 54.4 °C / 130 °F |
Source: Tecumseh
Capillary Tube Dimensions for 1/4 HP Compressors
Capillary tube performance depends on its length and diameter, which determine its level of restriction. Below are some important rules and considerations:
- Diameter: Changes in diameter affect the flow more significantly than equivalent changes in length.
- Length:
- Longer tubes reduce flow, but excessively long lengths can be inefficient.
- Shorter tubes increase flow, eventually acting more like an orifice rather than a capillary tube.
General Recommendation: Keep the length between 5 feet (1.5 m) and 16 feet (4.9 m).
Specification Tables by Refrigerant
R134a (1/4 HP):
| Condition | Diameter (in) | Length (m) |
|---|---|---|
| HBP | 0.042 | 1.8 |
| MBP | 0.042 | 2.0 |
Source: Tecumseh
R410A (1/4 HP):
| Temperature (T) | Diameter (in) | Length (in) |
|---|---|---|
| -10 °F (-23.3 °C) | 0.031 | 73 |
| 25 °F (-3.89 °C) | 0.031 | 40 |
| 45 °F (7.2 °C) | 0.042 | 101 |
Source: Industrias JB
R404A (1/4 HP):
| Evaporation Temperature | Condition | Diameter (in) | Length (ft) |
|---|---|---|---|
| -23 °C | Cooling Capacity: 1000 BTU/h | 0.031 | 4½ |
| -6.7 °C | Cooling Capacity: 1000–3000 BTU/h | 0.052 | 8½ |
R22 (1/4 HP):
| Condition | Diameter (in) | Length (m) |
|---|---|---|
| HBP | 0.050 | 3.0 |
| MBP | 0.036 | 2.5 |
Source: Tecumseh
R407C (1/4 HP):
| Condition | Diameter (in) | Length (in) |
|---|---|---|
| Low Temperature | 0.031 | 56 |
| Medium Temperature | 0.031 | 31 |
| High Temperature | 0.042 | 79 |
