Bonnets

Bell & Gossett Heat Exchanger Bonnets

Tube-Side Closure, Connections and Pass Partitions

A bonnet bolts to the tubesheet and closes the tube side. It carries the tube-side inlet and outlet connections and, on multi-pass units, the partition plates that route the fluid back and forth through the bundle.

Materials across the range. Cast iron is standard on most catalogued units. Cast brass appears where the tube-side fluid or the environment favours it, and some SU units carry steel bonnets. The part number establishes which a given unit has, and substituting materials changes both the corrosion behaviour and the weight.

Pass partitions make the bonnet arrangement-specific. A 2-pass bonnet and a 4-pass bonnet for the same shell diameter are different castings, because the partition geometry differs. This is the same reason head gaskets are pass-specific, and the two parts have to match each other as well as the unit.

Both tube connections are on the bonnet. Because a U-tube bundle returns the fluid to where it started, the tube-side inlet and outlet are both on the same end. That end is also the one you have to clear to withdraw the bundle, so the piping arrangement and the maintenance access are the same problem.

Condition matters. A cast iron bonnet that has been in service for decades may be sound, or it may have thinned enough at the gasket face that a new gasket will not seal. When a joint keeps weeping despite correct gaskets and torque, the bonnet face is worth measuring.