Basic Concepts of Heat Exchangers
A heat exchanger is a device designed for the purpose of allowing heat transfer from one medium to another at different temperatures. Most commonly, these mediums consist of two fluids that flow close to each other and are separated by a material, often metals, with good heat transfer properties. The fluids are primarily characterized by their temperatures at the entrance to the heat exchanger. The hot (warm, in figure below) fluid, the fluid with the highest temperature initially, transfers heat to the cold fluid as they both pass through the heat exchanger, thus lowering the temperature of the hot fluid and raising the temperature of the cold fluid.
Heat exchangers are generally categorized by the flow of the two fluids in relation to one another. Three categories consist of parallelflow , counterflow , cross flow. In a parallelflow heat exchanger, the fluids pass through parallel to each other and the change in temperature of both fluids is characterized by the below curve.
A counterflow heat exchanger is one in which the two fluids pass through the heat exchanger in opposite directions. An example of this design and the change in temperature curve may be seen below.
A cross flow heat exchangers fluids pass perpendicular to one another. The most common configuration for the cross flow design consists of fins which evenly distribute the free flowing fluid across tubular pass-thourghs that contain the second fluid. Often this design utilizes ambient air as the free flowing fluid to remove heat from the fluid contained within the closed tubes. This configuration is shown in the following figure.
Physics of basic heat exchangers
Two methods are readily used to physically and mathematically explain the purpose of heat exchangers. These two techniques are known as the Log Mean Temperature Difference (LTMD) method and the Effectiveness-NTU method. However, for these techniques to be useable, several assumptions must be made.
- Uniform flow
- Steady flow
- All of the heat transferred from the hot stream is deposited into the cold stream
- No phase change
- Constant specific heats
- Negligible kinetic and potential energy
- U (over heat transfer coefficient) is constant
The log mean temperature difference relates the inlet temperatures, outlet temperatures, the overall heat transfer coefficient U, and the area separating the two mediums. However, if only the inlet temperatures are known, the LMTD method is quite complicated to use. This is when the effectiveness-NTU method is preferable. A brief derivation of both methods is provided through the following link.
Derivation of LMTD and effectiveness-NTU
Heat Exchanger Principles in Automobiles
Most automotive heat exchangers are similar to shell and tube cross flow design, with multiple tube passes. But instead of having a defined shell around the tubes, with another controlled fluid forced across the tubes by means of a pump, there is no limited control volume for the shell. The tubes are open to the air and are dependant upon outside conditions.
Q = h A ΔT
Q = U A LMTD
The cold flow “inlet” temperature varies dramatically and the mass flow rate is limited by the speed of the vehicle. In the previous two equations used to calculate heat transfer, this means that both “h” and “ΔT” values are subject to external conditions. To aid in more constant heat transfer rates, however, many automotive heat exchangers use fans to deliver constant cold fluid supply (longitudinal-mounted engines) or engage when working fluid temperatures reach the maximum of their operational range (transverse-mounted engines). The tubes on these heat exchangers are also aided by fins, further increasing the surface area “A” in the equations.
While the purpose of heat automotive heat exchangers is to remove heat from the mechanical systems, the goal is not merely maximum heat transfer from the system. There are other secondary purposes as well. If only maximum heat transfer were the objective, then efforts would be to only concentrate on designing the cooling system/ heat exchangers around the working fluid with the highest thermal conductivity, which would be water (water & ethylene glycol mixture) which has a k value of around .6 W/mK instead of motor oil or transmission oil which has a k value of around .2 W/mK. The reason that heavy-duty automobiles and racecars use oil or transmission coolers is for not only local heat dissipation and more uniform temperatures across multiple systems, but also to inhibit thermal breakdown of the working fluid itself. If temperatures are too high in oil, this leads to chemical breakdown and degradation of the oil and additives in the oil. This changes its viscosity and other physical properties.
After years of experience and testing over the years, automobiles have developed into very complex and functional machines. The materials of each part and the working fluids are chosen for very specific reasons. Most heat exchangers in today’s automobiles are made from aluminum for its light weight, relatively high availability and its very high thermal conductivity. Radiators are filled with water because of its high heat capacity and thermal conductivity. Although ethylene glycol is added to aid the cooling system, it actually lowers the k value of the water mix. Its purpose is to increase the boiling temperature and lower the freezing temperature of water.
Types of Vehicle Heat exchangers
Some types of Automotive Heat Exchangers include but are not limited to radiators, oil coolers and intercoolers. It is possible to use heat exchangers for almost any of the fluids in a vehicle. Air conditioners and heaters are also examples, however they are not restricted to vehicles.
A radiator is a cooling device used in the engine in which hot liquid flows through exposed pipes and transfers heat to the air by fans. Fins are used to conduct the heat from the tubes and transfer it to the air. The fluid used is typically a mixture of ethylene glycol, water and a small amount of corrosion reducer.
Oil coolers are used mainly in transmissions to keep the oil temperatures within safe limits.
Finally intercoolers are air-to-air or air-to-liquid heat exchangers. They are used on turbocharged internal combustion engines to cool down the hot compressed air coming from the turbocharger.
Sources:
- http://www.hukseflux.com/thermal%20conductivity/thermal.htm
- http://www.engineersedge.com/heat_exchanger/cross_flow.htm
- http://me1065.wikidot.com/local--files/handouts-and-links/ht_deriv.pdf
- Incropera,DeWitt,Bergman,Lavine. Introduction to Heat Transfer, Fifth Edition. John Wiley & Sons, 2007
