Brazing Handbook - Paste Application

Paste application

Brazing pastes are used to form joints between the tubes and the fins as well as to join the tubes to the header. Tank to header and other kinds of joints can also be brazed
For tube-to-fin joints, brazing paste can be applied either on the tube surfaces or on the fin tips, see figure 12
figure 12
A layer of paste can be applied on the surfaces of the heat exchanger parts by many different methods, such as spraying, dipping, roll coating etc.
Except for the thermoplastic pastes, the pastes have to be dried, normally with warm air. Good temperature control during heating and drying is needed to prevent overheating and subsequently poor brazing results. The brazing properties of the pastes can be destroyed above 130ºC. Instructions from paste suppliers must be followed. Most of the brazing pastes involve some kind of solvent, which can form effluents during drying. Contact the paste-manufacturer for more information.

Important: Do not overheat the paste during drying.

Paste on tubes-Spraying

The method most commonly used today is spraying with commercial spray guns. As an example, tube coating, manually as well as automatic spraying is illustrated in figures 13 and 14. Due to the drying time for the paste, it is so far not possible to have the spray coating in line with the tube welding equipment. By turning the tubes 90º, it is possible to match the coating speed with the welding speed.
As pastes from different manufacturers can have different spraying properties as well as drying time, the settings of the applying parameters sometimes have to be changed. The coating is not a homogenous brazing metal, instead it is formed with particles and binder and after drying can also be porous, which means that the coating thickness for a certain weight could differfrom spraying methods as well as type of paste. Therefore the amount of brazing metal should be measured as weight/area.
Coatings should be evenly applied with a coating weight of 150-250 g/m2 after drying, formula 1 shows a calculation of the amount of paste on tubes related to these values. Table 6 shows these amounts of paste related to some tube dimensions. The better the tolerances of the tubes and fins, the thinner the layer can be. It is recommended to start with the thickest (heaviest) coating.

A = L x 2(H + W)x P [1]

Where
A: the amount of dried paste on the tube
L: the length of the tube
H: the height of the tube
W: the width of the tube
P: the preferred amount of paste/m2
tabel 6
Pastes which have a high metal content and which leave a smooth surface after drying are preferred.
It is recommended not to coat the tube ends. If the tube ends are coated it can sometimes have a negative influence on the joint quality of the tube-header joints.
figur 13 figur 14
Depending on the storage area atmosphere, the coated tubes can be stored for days up to several months. The storage should be clean and with no contamination of the coating.
This coating method consumes more paste than needed for the tubes (over-spraying). Work is going on to minimize this amount.

Development of tube coating methods

Work is going on to develop not only the paste spraying but also other coating processes, e.g. thermal spray. The new developments will probably be commercially available during 2007, with options for coating online tube-mill or offline, whichever is preferred.

Important : The amount of filler metal for all tube coating methods shall always fulfill the amounts shown in table 6.

Paste on fin tips

Consumption of brazing paste could in most cases, depending of the fin density, be lowered by applying the paste on the tips of the fins rather than on the tubes. Thermoplastic pastes, as well as some solvent-based pastes, are suitable for fin-tip application. The coating thickness is measured by weighing; the recommended amount of paste on each tip is 0.3 mg/mm to 0.5 mg/mm of fin width. Again, the better the fit between fin and tube the smaller the amount of paste application on the tips. Table 7 shows some examples of amount of paste for some fin widths and density. In the table fpi is the number of fin tips per inch.
The fin tips are coated by contact with paste coated rolls. When using thermoplastic paste, the fins pass through a pair of rollers and are coated in one step. This principle is shown in figures 15 and 16. When using solvent-based pastes, the tips of the fins are coated on one side at a time with drying between.
Depending of the storage atmosphere, the coated fins can be stored for days up to several months. For all types of storages, it should be clean and with no contamination of the coating.
tabel 7 figure 15 figure 16

Tube-to-header joints

A dedicated paste (sometimes called slurry) is recommended for tube-to-header joints. This paste has a lower viscosity than the pastes for tube or fin-tip coating and can be applied by pouring or spraying. Normally the paste is applied on the airside of the header. The volume of the paste is around double the volume of the filler metal in the joints after brazing. The visible amount of paste should therefore be more than what the joints are expected to be after brazing. The amount of paste required is typically 0.5 g (dry weight) per tube end for 16mm wide tubes and 1.8 g per tube end for 60x6mm tubes. These quantities are just guidelines, as the amount of paste is influenced by the geometry of the joint. Formula 2 shows guideline for calculating the amount of paste for other tube dimensions.

E = (H + W)/36 [2]

Where
E is the amount of dried paste on the tube end
H is the height of the tube
W is the width of the tube
And for a total amount on a header

N x (H + W)/36

where N is the quantity of tubes in the header
For multi row cores it is important that the paste is evenly distributed at all joints.
In practice, a small amount of flux can be added to this kind of paste to make the brazing process more forgiving when slightly oxidized components are used. Developments are in hand to find a flux-less paste for this application, too. The paste application methods are well suited for automation. Figure 17 illustrates the principle of paste application to headers. For small production the paste can be applied manually as shown in figure 10. When coating and drying manually, do not overheat the paste. The brazing properties will be destroyed if the paste is heated above 130ºC.
It is recommended to braze the parts within 1-2 days after applying header paste, depending on the storage atmosphere.
figur 18

Important: The brazing properties will be destroyed if the paste is heated above 130 ºC during drying.