Heat Transfer

Heat transfer, or heat exchange, is a process of (what some people nickname it as) "heat migration" from a point, to another point.

Come again? From what point to what point?

From a point of higher temperature to a point of lower temperature.

Heat transfer occurs from a point with higher temperature, to a point with lower temperature

Tell me about heat transfer from the beginning, please.


What if, I start at energy?

Energy, as defined by my Physics teacher, Mr Ismail, back in high school, is the ability to do work.

Work, in simple terms, is defined as force (F) applied to move any object, by a distance (x).

Hence, if the force required to move the object is larger, i.e. moving a train (due to its steel-laden weight), then the work done per distance moved is much larger than moving a bag of cotton.

That was just an example.


The nature of energy:

As explained in the compression section, every atom has an internal energy.

This is due to the continuous motion of electron/s orbiting the neutron/s and proton/s. Electron has its own mass to move, so force is required. Electron has its own distance to move. Hence, energy is existent.

As long as the basic of all building blocks, namely electron, proton, and neutron exist, energy can neither be destroyed nor created by humans.


Therefore, energy can be transformed from one state, to another:

Consider a sample of gas, with a specified internal energy.

Consider another sample of gas with lower internal energy.

Now, due to the internal energy from the electrons, atoms or molecules will move in a specified random motion. Speed of which these particles (atoms or molecules) move, will depend on the internal energy. The higher its internal energy, the higher the speed of these particles.

Back to our samples of gas.

When these gases are brought into contact, the gas particles will start to collide with each other. Gas particles with higher internal energy, will collide with gas particles with lower internal energy.

Speed of these particles will be traded. The lower particles’ speed will gain its speed, and the higher gas particles’ speed will loose its speed.

Analogy

It is analogous to what happens when a white ball in snooker game hits a pack of snooker balls. Initially, the white ball will have a very high speed. As it impacts the pack of balls (which are initially at a standstill), the white ball will loose its speed tremendously, and the rest of the pack will gain some speed.

This is an example of how energy is transferred from one state, to another.


So, how can you say that heat is a form of energy?

A specific sample of gas, with temperature above 0 Kelvin, and a finite mass; has particles vibrating, rotating, or travelling at a high speed. In other words, these particles are moving.

Since these particles are in motion, the gas particles are doing work! Therefore, heat is a form of energy.


This brings me to the definition of heat:

It is the work done by the continuously in motion particles of a specific matter, with a specific mass.

Heat and heat transfer equations

And since particles in motion will have a specific kinetic energy, temperature, that is proportional to the kinetic energy will present. The higher the kinetic energy (or the speed of these particles), the higher the temperature will be.

Therefore, heat is the multiplication of the mass (m) in kg, the heat capacity of the specific matter (c) in Joules kg-1 K-1, and temperature (T) in K (Kelvin).

The heat capacity exists, as it would take more energy, to heat up a kilogram of water, by one degree Kelvin or Celsius, than to heat the same mass of air, by same temperature increment – without change in phase (e.g. from liquid to gas)

All because of the difference in the internal energy of the particles – again.

Lets use Refrigerant 12 and water as an example, both at 1 bar pressure and at 25 oC (77 oF).

Since Refrigerant 12 will be in gaseous state and water will be in liquid state for the given condition, we will expect that the internal energy of Refrigerant 12 to be higher than water.

Thus, we will also expect the heat capacity for Refrigerant 12 to be lower than water, i.e. it is easier to heat up Refrigerant 12 by one degree Celsius or Kelvin, than raising the same temperature for water, at the same mass.

R12 with lower specific heat value will require less heat, per same temperature increase, compared to water at the same massYep, proven true from Rogers' and Mayhew’s “Thermodynamic and Transport Properties of Fluids”. Refrigerant 12 will have value for c of 0.98 kJoules kg-1 K-1, whereas water will have value for c of 4.18 kJoules kg-1 K-1.


Weaving into heat transfer:

We have known that,

  • energy is the ability to do work

  • energy can be transformed from one form to another

  • heat is a form of energy

  • heat content depends on the mass of the matter, the matter itself, and the temperature at which the matter is held

Therefore, we can say that heat is a form of energy, heat transfer can occur, and heat is also a function of temperature.

As defined earlier, heat transfer occurs from a point with higher temperature, to another point of lower temperature.

This happens as the point with higher temperature, with higher particle kinetic energy, will transmit the energy, to the point with lower kinetic energy, i.e. lower temperature.

Analogy

Well, you don’t see stationary billiard balls "pull" in the cue ball. You see the cue ball with high speed impacting the stationary balls.

Similarly, you don’t feel the stationary soccer ball physically pulling your foot to do penalty kick. The ball takes in your foot’s high speed impact and gains speed.

In short, particle’s kinetic energy, which is closely related to temperature, is transferred from a larger value, to a lower value.

This explains why heat transfer occurs from a higher temperature, to a lower temperature.

If temperature has increased for a matter, then we say - it gained heat.

Example is in evaporation and boiling or refrigerant.

If the reverse happens we would say it lost heat.

My favourite example is condensation of refrigerant.


But, higher temperature does not necessarily mean higher heat:

Yes, we should not be confused between heat, and temperature.

Heat is energy contained by a specific mass of matter, at a temperature. It is a combination of all three elements.

Whereas, temperature is more of a localised measure of the kinetic energy of a matter’s particles.

A drop of Refrigerant 134a at 1 oC (33.8 oF) will give the same temperature reading as a bulk of Refrigerant 134a at 1 oC (33.8 oF).

However, the drop will boil much much quicker than the bulk, as energy required from warmer air is lower, due to the small mass. The bulk will take a longer time to boil, as there is a lot of mass to deal with.


Reaching equilibrium in heat transfer:

Consider a glass, half filled with water at 20 oC (68 oF).

Then put in some more water at 1 oC (33.8 oF).

You wont see the water go down in temperature to 1 oC (33.8 oF). You wont also see the water stay at 20 oC (68 oF).

In fact, the heat transfer between the water will occur and reduce the temperature to a new point, in between those two temperatures. Now, this new temperature value will be,

  • closer to 20 oC (68 oF) if, the original water is more than the added water

  • closer to 1 oC (33.8 oF) if otherwise occurs

  • at mid point between 20 oC (68 oF) and 1 oC (33.8 oF), if the amount added is the same as the original amount

Equilibrium point during heat transfer process varies with the masses involved

The example explains that equilibrium temperature of two materials subject to heat transfer, depends on the mass, and the type of material (if the materials are different).


Heat transfer application in our air conditioners:

Air conditioners work through,

to initiate boiling by heat transfer from warmer air, at the evaporator.

This process of heat transfer happens continuously until equilibrium temperature reached between the evaporator, and the room.

And that equilibrium is reached by removing heat from your space, thus reducing the air temperature.

Heat into your room, must be balanced by heat out of your room, in an air conditioned spaceA room with low temperature is also known as, the COOL ZONE.

Wrapping up.

As long as there is a cooler zone, and warmer zone, heat transfer will continue to happen. It’s like, a never ending destiny.





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