How To Design a Capillary Tube: The Simplest and Most Reliable Throttling Device

Amongst those components that manage the flow of refrigerants through air conditioning and refrigeration equipment, the simplest, lowest-priced most straightforward and most certain one is a capillary tube. Other expansion valves and fineries are partly brilliant but of limited capacity when there remain cases in which the unpretentious plain old humble capillary tube continues as an enclave item, just plumbed in with its customary behavior.

If you're going to design a capillary tube for refrigeration, you'll need to acquaint yourself with its principles, governing factors, and method of designing it. In this blog, we'll walk you through the entire process of designing a capillary tube, expose the underlying working principles of it as mentioned by the Capillary Tube Supplier, and inform you about how to personalize it according to your need.

What Is a Capillary Tube?

A capillary tube is one that is small and narrow with a minor bore, utilized as a throttling device in refrigeration plants. It is used to decrease the pressure of liquid refrigerant from condenser to evaporator pressure so that it will evaporate and absorb heat in the evaporator.

The mechanism is as follows: with the high length and low diameter, friction and pressure drop occur to the refrigerant while it travels through the tube. Pressure drop leads to flashing of the refrigerant to liquid-vapour upon approaching the evapourator.

Why a Capillary Tube?

Capillary tubes are usually employed for low-capacity applications like domestic refrigerators, water coolers, window air conditioners, and deep freezers. The following is the reason:

• Simplicity: It has no moving parts, and thus it is highly reliable.

• Low Cost: It is less expensive than electronic or thermostatic expansion valves.

• Maintenance-Free: It is made to need less maintenance after installation.

• Small Size: especially made to be utilized in applications where there is limited space.

But they are employed for constant-load operation and steady-state operation since they cannot dynamically change the flow.

Key Design Considerations

A capillary tube is not choosing some random diameter and length tube. There are quite a number of parameters that determine its performance:

1. Type of Refrigerant: Various refrigerants are characterized by having various fluid and thermodynamic characteristics. R134a, R22, R410A, and R600a conduct different behaviour as they change in pressure and temperature, respectively, upon influencing tube length and diameter.

2. Refrigerating Capacity: It refers to the system's capacity to cool, typically tons of refrigeration (TR), watts, or BTUs/h. Increased capacity entails an increased mass flow rate of refrigerant, which will determine the tube sizes.

3. Condenser and Evaporator Pressures: These are a function of the operating conditions required. Pressure drop across the capillary tube should be equal to the pressure difference between the condenser outlet and the evaporator inlet.

4. Condenser Exit Subcooling: Subcooling enhances refrigerant density at the inlet of the capillary tube with subsequent influence on flow behaviour. More subcooling allows tubes to be shorter in length.

5. Ambient and Evaporator Temperatures: They establish pressure levels to be achieved in the system and influence refrigerant flash rates and flow resistance.

6. Tube Material: Mainly copper due to thermal conductivity, corrosion resistance, and workability.

Capillary Tube Design Procedure

The following are step-by-step design guidelines for a capillary tube:

Step 1: List System Parameters You will need to know:

• Refrigerant type (e.g., R134a)

• Cooling load (e.g., 500 W)

• Evaporator temperature (e.g., -10°C)

• Condenser temperature (e.g., 50°C)

• Subcooling (i.e., 5°C).

Mass flow rate through the compressor (a function of the cooling load and enthalpy difference).

Step 2: Calculate Pressure Drop

Pressure drop across the tube is high-side (condensing) minus low-side (evaporating) pressures, as found in the refrigerant properties tables.

Example:

• High-side pressure (P₁) = 13.5 bar at 50°C

• Low-side pressure (P₂) = 2.8 bar at -10°C

• ΔP = P₁ - P₂ = 10.7 bar.

Step 3: Select Internal Diameter

Select a few common average internal diameters of capillary tubes, e.g., 0.6 to 2.0 mm. Use graphs or computer software (e.g., Capillary Tube Selection charts) to select from the options.

Lower diameters provide higher pressure drop and smaller length. For instance

• 0.9 mm ID will accommodate 2.5 m

• 1.2 mm ID will accommodate 3.5 m

Step 4: Calculate Required Length

This is the most important step. Use refrigerant flow simulation computer programs or empirical pressure drop plots, tube diameter, refrigerant, and subcooling vs. tube length.

• Use ASHRAE or manufacturer data tables to estimate an approximate calculation. For instance, for R134a:

• With ID 0.9 mm, 5°C subcooling, and 500W capacity, the length would be approximately 2.7 m.

Installation Tips

• Coiling: Coil the capillary tubes to save space. Steer clear of kinks or sharp corners that inhibit flow.

• Brazing: Leak-proof brazing should be done while brazing for sealing the evaporator and condenser.

• Cleanliness: Dry and dust-free the tube; water or dust clogs the tube at once.

• Filter/Drier: Always incorporate the filter-drier before the capillary to avoid clogging.

• Compressor pairing: The size of the tube corresponds to the capability of the compressor. Oversizing devices or undersizing devices will work.

Advantages and Limitations

Advantages:

• It costs little and is a simple one.

• It contains fewer moving parts.

• Compact and easy installation

• Sealed system is acceptable.

Disadvantages:

• Not ideal for cyclic load applications

• Prone to clogging Hard to service or replace Charge the system to full.

Design Errors to Avoid

• Oversizing: A Too short or too-thick tube will see a poor pressure drop and lead to flooding the evaporator.

• Under sizing: Too short or small tubing will impede flow, starving the evaporator and lowering efficiency.

• Inadequate Subcooling Accounting: Inadequate accounting for subcooling may cause incorrect sizing.

• Undercharging of Refrigerant: Capillary systems are highly charge-sensitive. Always charge by weight, never by pressure.

Conclusion

Designing a capillary tube is not quite as easy as it might seem, but it does take the use of a reasonable amount of physics, thermodynamics, and engineering common sense. Well-designed capillary tubes can provide good performance, life, and reliability in micro-refrigeration systems. The system should be properly designed and installed since the system is non-adjustable when sealed.

When designing your initial system, start with off-the-shelf charts and increasingly use simulation programs to refine your designs. With time, you will get a subconscious sense of how to select the correct tube length and diameter for the application. As an engineer or as a hobbyist, capillary tube design is an excellent way to design reliable and efficient cooling systems.

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