Punching of Aluminum Foils to Produce Fins-Process 1
32 Ton pneumatically operated press mounted with 12 row 3/8 inch progressive draw die enabling machine to achieve 9 fins per inch to 14 fins per inch with automatic feeder, automatic fin collector CNC based fin cutting.
Punching is carried out by a three phase electro-pneumatic, 32 tones max, 160 rpm and 4.5 bar minimum pressure punching machine.
Fins are created from the aluminum sheet by the shearing of aluminum sheets 1100 grade soft aluminum.25.4 mm breadth and (0.1 mm to 0.14 mm thick) by the punch force produced in the punch-die arrangement.
The aluminum sheets are first taken in the elastic region by the force applied. As the force increases gradually the sheets move into the plastic region and finally fracture is caused. This creates a specific pattern of holes on the 25.4 mm breadth aluminum sheets .
Straightening Bending and Cutting of Copper Tubes-Process 2
Imported copper tubes from China usually come in jumbo rolls which are straightened by straightening machine one tube at a time by bi-directional rollers.
As copper tubes are soft annealed straightening occurs without cracking .
After straightening the copper tubes are cut into smaller tubes of required length by a rotational cutter causing burrless cutting.
These copper tubes are then bent in 'U' shape tubes to avoid increase in no. of joints.This is done by an automatic copper tube bending machine, which can bend 4 copper tubes simultaneously.
Assembling Circuit-Process 3
The copper tubes and the fins which act as a primary and secondary heat transfer means are then assembled according to the design required. The circuit is based on the requirement of the customer or in-house design developed by the company taking into account all the theoretical heat transfer aspects.
Mechanical Expansion of Copper Tubes-Process 5
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 length 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 water expanded coil assisting heat transfer.
Brazing and Testing and Purging Process-Process 6
Depending upon the circuit to be done the u-turns are brazed on the copper tubes. Also the inlet and outlet pipes made of copper are brazed according to the circuit on the copper tubes. The brazing is done with oxy acetylene flame and selected copper brazing rods/silver rods depending upon the application.
We use gas flux resulting in brazing done in an inert atmosphere around the joint, this prevents the oxidation process near the joints.
The entire coil is tested at 400 - 450 psi for any leakages at the joints, 'U' bends, tubes, distributors and headers.
During the process of brazing the coils are purged by nitrogen to ensure the cleanliness and quality of coil.
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Shree Heat Exchangers
Ameya Gole ( Manager )Shree Heat Exchangers
Chandrasakha Warehouse, Gala No. 38, Ambegaon Bk Survey No. 1, Pune - 411046, Maharashtra, India