Heat Transfer: - Conduction and Convection and Heat Exchangers

 

Heat Transfer: - Conduction and Convection and Heat Exchangers

Even though it's 93 million miles away the Sun is capable of producing temperatures in excess of       130 degrees Fahrenheit here on Earth. It provides the heat needed to support life and to continue many natural cycles.  

We get the Sun's heat by a process called radiation. That's just one way that heat energy can move from one place to another. Here at the manufacturing and processing plant and refineries, we use two processes to transfer heat in heat exchangers conduction and convection.

 Using a heat exchanger like this one makes our operations more energy efficient.

In this blog will discuss two topics heat transfer specifically conduction and convection and heat exchangers.

As you know warm materials tend to lose heat to cooler ones when the materials are in direct contact with one another the process of heat transfer is called conduction.

 Another way to transfer heat is by convection. This is caused by a change in fluid density that sets up currents.

 

 The following example will illustrate both processes.

 If a metal container of hot water is touching another filled with cold water the temperatures of both will change because of heat transfer.

The greater the temperature difference the faster the transfer takes place in this example. The heat is carried through the metal walls by conduction.

The cold water closest to the wall absorbs the heat directly from the metal and increases in temperature.  This warm less dense water rises and is replaced by cooler water , then a convection current transfers heat to all parts of the water.

This transfer results in a change in temperature for both containers.

Conduction and convection take place almost simultaneously to move heat from one fluid to     another.

In conduction an important factor affecting the rate of heat transfer is the type of conductor used

Conductors vary widely in their ability to transfer heat as you may expect.

 Metals are better conductors than glass or wood materials as shown in below chart.

Let's see how conduction and convection work together in a simplified heat exchanger.

Because of the difference in temperature the heat from hot oil inside the tube is conducted through the tube wall to the cooler liquid in the shell

 The hot oil becomes cooler while the cool water absorbs heat.

 In this heat exchanger the fluids are being pumped through the vessel.  This causes turbulence a disturbance in flow, Turbulence results in a faster more efficient flow of energy.

 Velocity also influences the rate at which heat moves between two fluids of different temperatures

 Water that travels too quickly can't absorb enough heat. The optimum velocity represents the most efficient heat transfer.

 Another consideration for efficiency is the area of contact.

 The greater the surface area the more heat will be transferred. To ensure a large surface area most heat exchangers have a system of tubes inside.

 The liquid in these tubes is referred to as tube side flow.  Another liquid is directed through the space around the tubes. This is called the shell side flow.

 By having many tubes more heat can be transferred.

 So far we've discussed conduction heat transfer by direct contact.

 Convection heat transfer by current movement and several factors that influence the efficiency of heat exchangers Including temperature difference, Conducting material, fluid turbulence, fluid velocity and surface area.

 In addition to these another factor that affects efficiency is the direction of flow.