Process 5 - Mechanical Expansion of Copper Tubes
- The image shows actual Gang Bullet Expansion machine installed in the factory.
It can expand upto 41 " X 30 " wide X 6 row coil.
- After the circuit of copper tubes and aluminum fins is done there is a gap of about 0.4 mm between the outer surface of the copper tube and inner surface of the hole punched in aluminum fin
- As a air gap is present between the two metal surfaces the heat transfer between the primary and secondary heat exchangers is affected to a large extent. To remove this drawback the operation of mechanical bullet expansion of the copper tubes is done.
- As mentioned above there is about 0.4mm clearance fit between the two metals which affects the heat transfer considerably .so to convert this clearance fit into a interference fit about 0.08 mm to 0.1 mm between copper tube and whole aluminium fin.
There are two types of bullet expanders used in the industry:
- Gang bullet expander-this is used upto 41" height
- Flexible bullet expander-this is used for larger size of heat exchangers
For the coils having fin lenght more than 41 inches or breadth more than 30" or rows more than 6 a flex bullet expander having two carbide bullets is used, which works on the same principle as gang bullet expansion and causes the same interference as that by gang bullet expansion.
Mechanical expansion is better than water jet expansion due to following reasons :
- Uniform expansion and interference of 0.08 mm to 0.1 mm between tubes and fins.
- No water traces .which are harmful to system.
- Discharge pressure of bullet expanded coil is 10 - 20 psi lesser than waterexpanded coil assisting heat transfer.