A heat exchanger works by allowing heat from one fluid to pass another, cooler, fluid without them mixing or coming into direct contact.
For example, imagine a pipe with another pipe around it. The inner pipe could allow a hot fluid to pass through it while a cooler fluid is passed simultaneously through the outer pipe. This would allow the cooler fluid to reduce the temperature of the warmer one, as the warmer fluid simultaneously increased the heat of the cooler one. Of course, this is a very basic example ofheat exchange, and there are a number of other factors to consider when investigating heat exchangers:
1. Passes
By curving the pipes, for example into an 'S' shape, you can allow the fluids to make more than one 'pass' before leaving the heat exchanger. A single pass is a straight pipe, where the fluid enters at one end and exits at the other end of the heat exchanger fairly quickly. A double pass uses a U-shape, so that the fluid enters and leaves the heat exchanger at the same end, prolonging the time that the fluids are passing one-another in the heat exchanger. A triple pass uses an 'S' shaped formation that allows the fluid to travel along the length of the heat exchanger three time before exiting. The greater the number of passes, the greater the amount of heat transfer is available – simply because the fluids are together in the system for longer – although this can also lead to pressure drops and a loss of velocity.
2. Temperature Cross-Over
Temperature cross-over occurs when the heat of the cooler fluid begins to cross over with the temperature of the hot fluid in the heat exchanger. For example, oil entering a heat exchanger at 80⁰C alongside water at a temperature of 30⁰C, could see their temperatures cross-over should the oil reduce to 50⁰C as the water reach 51⁰C. At this point the cooler fluid (water) has become hotter than the oil. Temperature cross-over can significantly reduce the efficiency of a heat exchanger, particularly when cooling. This can be avoided by increasing the coolant's flow rate (see, 'Flow Rate' below), so there is more coolant in the system. Where temperature cross-over is unavoidable, using a plate heat exchanger (see 'Types of Heat Exchanger, below) is the best solution.
3. Temperature differential
This refers to the difference in temperature between the coolant and the hot fluid, which is important in a heat exchanger, as shown by 'temperature cross-over' (above). The coolant should be kept at a lower temperature than the hot fluid, and, the colder the coolant, the more effective it will be at taking heat out of the hot fluid.
4. Flow Rate
This is the amount of fluid that is passing through a cross-section of a pipe in a specific period of time. It is worked out as the volume of fluid per the time the fluid has flowed - with a greater flow rate potentially increasing the heat exchanger's capability to transfer heat. However, more fluid also means a greater mass to transport, as well as increasing pressure loss and velocity.