In a liquid-to-liquid heat interchanger, the fluid to be heated is passed only once through the tubes before it gets discharged, i.e. single pass. The heat transfer in this case is not efficient. When few tubes per pass are desirable, the double pipe heat exchanger is employed.
Construction of Double Pipe Heat Exchanger
The construction of a double pipe heat exchanger is shown in Figure 1.1. In this, two pipes are used: one is inserted in the other. The inside pipe (or tube) is used for the pumping of cold liquid to be heated. The outer pipe acts as a jacket for the circulation of the hot liquid. All jacketed sections are inter-connected.
Normally, the number of pipe sections is few. The length of the pipe is also less. Glass tubes, standard iron pipes, and graphite constructions are available. Standard metal pipes are assembled with standard return bends. A proper number of such pipes are connected in parallel and stacked vertically. The pipes may have longitudinal fins on their outer surface.
Working
The hot liquid (heating medium) is pumped into the jacketed section. The hot fluid is circulated through the annular spaces between them and carried from one section to the next section. Finally, it leaves the jacket. In this process, the pipes get heated, while the hot fluid loses its temperature.
The liquid to be heated is pumped through the inlet provided on the right side. The liquid gets heated up and flows through the bent tubes into the next section of the pipe. The liquid further gets heated. The same liquid continues to flow and finally leaves the interchanger through the exit point on the right side.
Uses: These heat exchanger is useful when not more than 0.9 to 1.4 meter square of the surface is required.
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