Heat Exchangers

One of mankind’s largest current challenges is to avoid global warming, due to greenhouse gases, exceed the point when the effects will be devastating to our way of life. There are several things that can and should be done. Among them is to utilize as much energy as possible from the fuels we consume. Besides developing more energy efficient engines, this can be done by “harvesting” the low-grade energy from the combustion, i.e. extract the excess heat from the exhaust. The latter has been done in process industry for ages, hot process streams which need to be cooled are used to heat up cold streams which need to be heated.

The equipment used for this is simply called a heat exchanger. In such two streams of fluids exchanges heat, i.e. the cool stream gains energy, whereas the hot becomes cooled.

A heat exchanger can have very different shapes and sizes, depending on the materials, manufacturing, operational demands, and the physical state of the heat exchanging fluids. In common for all of these is that the two fluids are separated by a heat conducting material and that the energy exchange through the separating barrier must be in balance, i.e. the energy lost on one side must correspond to the energy gained on the other. The laws of thermodynamics tell us that there will be a natural flow of heat from the hot side to the cold side. This means that if sufficient heat transfer area is provided it should be possible to transfer energy until both fluids have the same temperature. This implies that the effectivity of a heat exchanger depends on how large heat exchanging area it can provide. There is another important fact that can be drawn from the laws of thermodynamics, the fluid streams leaving the heat exchanger cannot become hotter or colder than the entering streams.

Figure 1. Example of heat exchanger types.

There are other factors which govern the efficiency of a heat exchanger. Beside area are the most crucial factors the fluids mass flow, physical state and properties of the fluids, and the ability of the separating material to conduct heat. Since one would like to perform the heat exchange as a continuous process, e.g. while the engine is running, the flow speed influences the heat transfer. At too low speed there is no turbulence in the fluid, so it is only the parts of the fluid which are in contact with the barrier which gains/lose energy. At too high speed the fluids will not have sufficient time to exchange their energy. Which flow speed that is suitable for a certain heat exchange is governed by physical properties of the fluids, e.g. how well can the substance conduct the heat, i.e. how much turbulence is needed to ensure a good heat transfer. The physical properties are quite different between the physical states, e.g. a liquid is generally a much better conductor than a gas is since the liquid's molecules are more densely packed than the molecules are in a gas.

Let us assume that we have found a heat exchanger with sufficient area and the proper design to handle a heat exchanging application which has two continuous fluid streams. Unless the heat exchanger is a cross-flow model (more on this below) there are two ways the heat exchanger can be operated; the stream can either have the same flow direction, concurrent flow, or the opposite direction, countercurrent flow. The flow direction will affect the end result of the heat transfer, if concurrent flow is used the exit temperature of the two streams from the heat exchanger will be nearly the same for both fluids, see figure 2 for an ideal temperature graph, whereas in countercurrent flow the fluids can attain exit temperatures close to the entering temperature of the opposite fluid stream, see figure 3.

Figure 2. Ideal representation of the fluid temperature evolvement in a heat exchanger operated in a concurrent flow. Red colour represents the hot stream and the blue the cold stream.

Figure 3. Ideal representation of the temperature evolvement in a heat exchanger operated in countercurrent flow. Red colour represents the hot stream and the blue the cold stream.

The reason for the difference in behaviour lies in the fact that if the two fluid streams have the same direction it will be the same part of the fluids which exchanges energy through the heat exchanger, whereas the fluids in countercurrent flow meet new hotter/colder fluid when it flows through the heat exchanger.

In a cross-flow heat exchanger, the two fluids flow direction cross each other, whereof the name. The temperature distribution in the two fluids as they pass through the heat exchanger is more complex than the previously described. The entering fluids will heat exchange against its counterpart at a different temperature depending on its entrance point; the hot fluid will meet the coldest fluid at/close to the inlet of the cold fluid, whereas the hot fluid at the outlet of the cold fluid will meet the “hottest” cold fluid. Figure 4 shows a 3-D representation of the temperature distribution in a cross-flow heat exchanger.

Figure 4. 3-D representation of the temperature evolvement in a cross-flow heat exchanger. Upper surface represents the hot fluid and the lower surface represents the cold. Image is taken from “MATHEMATICAL MODEL FOR SINGLE-PASS CROSSFLOW HEAT EXCHANGER”, T. Eirola et al.

By choosing a heat exchanger with a suitable design and size for the application at hand you can utilize energy that otherwise would have been wasted. By doing so, you will reduce your energy consumption and thereby help the global environment. The selection of heat exchanger can be aided by heat transfer calculations. Even though such calculations involve flow dynamics, heat fluxes, and thermodynamics, are the basic equations quite simple. In a coming article, these equations will be described and explained.