The present invention relates to a heat exchanger and in particular to an end bonnet for use in a shell and tube heat exchanger. 
Shell and tube heat exchangers generally provide a counter or cross flow arrangement for the cooling of a first fluid in the shell body by a second or coolant fluid passing through the tubing within a shell body, which is frequently cylindrically shaped.  This tubing provides communication between sealed opposite ends of the cylindrically shaped configuration and defines a flow path for communication of the cooling fluid from end to end of the shell structure.  The tubes terminate at an end plate or flange at either end of the shell and bonnet is provided at either end of this shell to define a transfer chamber for fluid communication between successive sets of tubes at each end of the shell. 
Heat exchangers are generally utilized for cooling various fluids, which may be either gaseous or liquid, by coolant fluid transferred through the tube arrangements.  As it picks up heat from the fluid to be chilled, the coolant fluid will boil or vaporize as it flows through the tubing network extending between the bonnets.  Initially during the cooling cycle, the cooling fluid is generally a liquid. 
The tubes provide a tortuous path encompassing multiple passes of the coolant fluid through the shell and, as it continues to increase in temperature, the cooling fluid expands.  As the cooling fluid proceeds through each successive or sequential pass, there will be a change of state for the fluid from liquid to the gaseous state.  This change of state requires an expanded tube volume to accommodate the expanding cooling fluid.  Therefore, subsequent cooling passes require an increased number of tubes or larger cross-sectional area tubes to transfer the initial fluid volume through the heat exchanger network of tubes.  Failure to provide this increased fluid transfer volume, as the coolant fluid temperature increases until it attains the vapor state, would result in high fluid velocities in the tubes and large back pressure.  In addition, problems relating to the fluid distribution result from these pressure-temperature changes. 
Abrupt increases in flow areas causes large pressure drops within the heat exchanger and results in decreases in pressure and thus reduction in the boiling point of the refrigerant or cooling fluid.  This characteristic indicative of a phenomenon referred to as flashing.  Flashing refers to the transition from liquid to the gaseous phase due to the drop in saturation temperature.  Therefore, it is desirable to limit the loss of cooling capacity due to flashing. 
Bonnets of varying designs have been provided for aiding and improving fluid flow, which designs include the utilization of U-shaped return passages and inlet and outlet passages in alignment with the tubes within the housing for providing a continuous flow path through the tubes.  These U-shaped passages may be provided in a flat-plate type end bonnet.  However, such U-tubes are very expensive and difficult to maintain.  Other prior art heat exchangers employ hemispherically shaped bonnets that are subdivided by partitions or baffle plates between the flange plates and the contoured inner surface of the bonnet.  These baffle plates thus provide transfer chambers in the bonnet between successive tube bundles of the tube network.  However, the abrupt increase in flow area in the bonnets causes undesirable pressure drops.