In this article we'll describe two less-talked about aspects of a heat exchanger: cross temperature and the number of passes.
The terms describe the scenario where the outlet temperature of the cold fluid is higher than the outlet temperature of the hot fluid. The cross temperature point is where the two fluids have the same temperature.
In order to get a temperature cross, fluids need to flow in opposite directions. That means that the inlet point of one is the outlet point of the other. Also, the number of passes in a heat exchanger must be limited to one.
In a heat exchanger, the fluid flowing on the tube side can make either one pass or two or more, before reaching the outlet valve. The number of back and forth passes can be up to eight.
The number of passes normally depends on the fluid temperatures (if there's a temperature cross, there can only be a single pass), and the strength of the internal flow. If the quantity of the fluid is not large, its speed inside the exchanger is low, and that reduces the capacity for exchange. With two or more passes inside the exchanger, the available space for each pass is reduced, the speed of the flow increases, and that generates more thermal exchange.
The disadvantage of multiple passes is load loss, or the different of pressure between the inlet and the outlet.
Examples of single, double, and triple passage on the tube side