The front-end heads are primarily of two types, the channels and the bonnet. The
bonnet head B is cast in one piece and has either a side- or an end-entering nozzle.
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Although the bonnet head is less expensive, inspection and maintenance requires breaking
the pipe joints and removing the bonnet. Hence, the bonnet head is generally used for
clean tube-side fluids. The channel head can be removable, as in the TEMA A head, or
can be integral with the tubesheet, as in TEMA C and N heads. There is a removable
channel cover in these front-end heads for inspection and maintenance without disturbing
the piping. The nozzles are side entering in these types. Notice that while the shell is
welded onto the TEMA Nhead, it is flanged to the TEMA C head. In the TEMA Nhead,
no mechanical joint exists (all welded joints) between the channel and tubesheet and
between the tubesheet and the shell, thus eliminating leakage between the shell and the
tubes. The TEMA D head has a special high-pressure closure and is used for applications
involving 2100 kPa gauge (300 psig) for ammonia service and higher pressures for other
applications.