Thermal Insulation

When we put hot water inside thermos, water will still hot although it has been several hours. This also happens when we put cold water inside thermos. After hours, water will still cold. How come?

Some may think how thermos knows that inside is hot or cold thus thermos keep the water hot and cold. The answer of this question is Thermal Insulation. As we know that heat is transferred spontaneously from higher temperature to lower temperature. But if between hot and cold temperature is blocked by thermal insulation, heat transfer will be minimized. Actually it is impossible to makes heat transfer rate is zero. It means that although we put thermal insulation between two temperatures, heat transfer still occurs, but very small. By this explanation, we can say that hot water inside thermos also becomes cold but it needs long time.

When hot water is put in room temperature, water will transfer heat to the surrounding whether by conduction, convection or radiation. But when hot water is inside thermos, heat transfer is minimized. Thermos wall is made by thermal insulation material which has very small coefficient conductivity. This cause heat transfer from water to surrounding is blocked. So the temperature of water will stay hot until long time (longer than we put without thermos).

It also happens when cold water is inside thermos. Heat transfer should be from outside thermos to inside thermos because temperature water inside thermos is colder than surrounding. But this heat transfer is minimized by thermal insulation material. Therefore temperature water inside thermos will be kept cold for long time.

Heat pipes

Heat pipes are often used for cooling system. Heat pipe is defined as a vapor-liquid phase-change device that transfers heat from a hot reservoir to a cold reservoir using capillary forces generated by a wick or porous material and a working fluid (The CRC Handbook of Thermal Engineering). How does heat pipe work?

Heat pipe has three basic components, those are container, working fluid, and capillary structure. When heat pipe receives heat, temperature will increase including working fluid. If working fluid has already saturated, it will change its phase to be vapor. This vapor will through capillary structure and transfer heat to the wall of heat pipe. Then after release heat, working fluid becomes liquid and goes to the heat source again. This phenomenon is repeated again.

Heat pipes has many advantages, such as high heat transfer capacity, precise isothermal control, functional independence of evaporator and condenser, quick thermal response, remote applications, high reliability, small size and light weight. Because of its advantages, heat pipe is often used in cooling system such as electronic cooling system.

Besides electronic cooling system, heat pipe also is used for water heater using solar energy. Because it has very high transfer capacity, heat radiation from sun is received and used to heat water. This hot water is used for residential needs.

The simplest heat pipes that can be made is using pipe (copper) and water as working fluid. To adjust with the requirement, the pressure inside heat pipes should be decreased using vacuum pump. So, boiling temperature of water will be decreased and can be used for below 100 Celsius degree. Other working fluid can be used for low temperature are acetone, ammonia, methanol.

For high temperature, heat should use another working fluid. Based on operating temperature, heat pipes usually use cesium, potassium, sodium, Lithium. Cesium is used in range 300 to 600 Celsius. Potassium is used in range 400 to 1000 Celsius. Sodium is used in range 500 to 1200 Celsius. And Lithium is used in range 900 to 1700 Celsius.

Heat transfer

What is Heat transfer??

Talk about heat transfer, there are two words heat and transfer. The definition of heat is the form of energy that can be transferred from higher temperature to lower temperature. So, Heat transfer is heat energy transfer from one system to another system due to temperature difference.

Heat can be transferred by three ways, conduction, convection, and radiation. What is the difference?

Conduction

Conduction is kind of heat transfer without any movement of molecules. When we burn rod of metal at one of its edge, after a while we will feel the other edge is also hot. This is conduction phenomena. Heat transfer by this phenomena is affected by several parameters, those are surface area, thermal conductivity coefficient, temperature difference and length. As increasing surface area, thermal conductivity coefficient and temperature difference, heat transfer will also increase. As increase of length, heat transfer will decrease. We can say heat transfer by conduction is directly proportional to surface area, thermal conductivity, and temperature difference, But inversely to the length. Mathematically we can write

Q=(k . A. ∆T) / L

Convection

Convection is kind of heat transfer that involves movement of molecule. For example, when we heat water on a pan by stove, the highest temperature is on the bottom of pan. Water temperature at bottom is also higher than at the top. Density of matter is lower if the temperature is high. So, density of water at bottom lower than at the top, thus water from the bottom moves up and water from top moves down.

Convection heat transfer is affected by surface area, convection coefficient, and temperature difference. Mathematically we can write:

Q = h . A .   ∆T

Talk about convection heat transfer, the “game” of this phenomena is at convection coefficient. To determine this coefficient need complicated correlation between fluid mechanics, friction factor, geometry, pressure, etc.

Radiation

Radiation heat transfer is heat transfer without medium. Heat from sun to the earth is transferred by radiation because no atmosphere in outer space. This heat transfer can be written mathematically:

Q = ε . σ . A . T⁴

Where

ε : emissivity coefficient

σ : Stefan-Boltzmann constant

Thermal Resistance in Electronics

Thermal resistance is the ratio between temperature difference and power dissipated. In electronics, Thermal resistance is a great interest for engineer. It is due to every electronic equipment produce heat and need to be cooled. If they cannot be cooled properly, it will be harmed because of overheat problem.

Thermal resistance can be analogized by electrical resistance. Current represent heat flow, voltages represent temperature differences, and Resistor represent thermal resistance. It can be simplified: R_th=∆T/Q is analogized by R=V/I.

There are several kind of thermal resistance, such as conductive thermal resistance, convective thermal resistance, and spreading thermal resistance. Thermal resistance is basically needed to calculate heat transfer from one point/surface to another point or surface.

In electronic, Thermal resistance is the parameter that informs how effective heat dissipated can be transferred to the ambient. Moreover, it can be used to determine heat source temperature, usually chip junction temperature. In other words, heat source temperature can be known if thermal resistance is already known.

Let’s consider a simple electronic device with heat sink. Heat transfer is started from junction to casing and finished at ambient. This construction results several thermal resistances, those are junction to casing thermal resistance, casing to ambient thermal resistance through heat sink. Total thermal resistance of this system is R_total= (T_j-T_amb)/Q= R_JC+R_CA

Power dissipated from electronic device is assumed that all electricity is converted to heat. Therefore heat dissipation is : Q=VxI, where Q represent heat dissipation, V represent Voltage, and I represent Current. For LED heat dissipation, sometimes optical power from LED is considered, so heat dissipation becomes, Q= P_electrical-P_optical = (V*I)-P_optical.

Heat Sink in Electronics cooling

Electronics technology has been developing rapidly since the first transistor was invented. Semiconductor technology is the most significant part for electronics technology development. At the first time one transistor with very small size could replace cathode tube. Now, thousand or even million transistors can be operated in one single chip. However, electronics have maximum temperature to be operated properly. A survey showed that the most cause of electronic failure is temperature. Thus electronics cooling cannot be separated in electronics technology.

Many types of cooling system for electronics have been introduced. The most common electronics cooling system is heat sink. Heat sink system can be divided into two parts, forced convection and natural convection.

Natural convection is convection heat transfer without any force applied to heat sink. This convection occurs because of buoyancy force naturally. Fluid (e.g air) has lower density if its temperature is high, this causes air moves up.  In electronics, heat dissipated from chip causes increasing temperature surrounding then air density becomes low, thus air moves up. Since natural convection only uses buoyancy force, usually natural convection heat sink is attached vertically.

Forced convection need additional force to move air flows on heat sink. Fan is usually used to support air flow on heat sink. Heat transfer rate at this heat sink is bigger than natural convection. Many electronic equipments use this type.

Heat sink is attached to heat source to enhance heat transfer rate. When heat dissipated from electronic device cannot be overcome by heat sink, it needs additional system or even different cooling system. Heat pipes or thermoelectric cooler may be an option for additional system in heat sink.