Unit C - heat transfer

How is heat transferred ?

Higher temperature to lower temperature

Name the three methods by which heat is transferred ?

1. Conduction

2. Convection

3. Radiation

Heat transfer in conduction , give example

Occurs in a solid body and still fluids

Heat a metal rode with gas , heat felt at end of rod

Heat transfer in convection, give example

Occurs through moving fluids

boiling

Heat transfer in radiation , example

Occurs between surfaces , ie between gases

sun rays penetrating earth

Describe what occurs in conduction

1. Temperature increase and molecules gain more energy and collide with slower moving neighbors

2. Energy of thermal motion passed from one molecule to next

Why are metals good conductors of heat ?

Free electrons so when molecules vibrate free electrons can detach and can share energy from one molecules to the next

Steady-state in relation to heat transfer

Temperature remains constant with time

- heat transfer always from high temp to low temp

Equation for Heat conduction through walls in a state state

Q = k x A( T1 - T2 ) / L

k = coefficient of thermal energy

A = area

T1-T2 = temp difference

L = wall thickness

Equation for heat transfer in a non-steady conditions

Fourier's equation

Q = -k x A dT/dx

k = coefficient of thermal energy

X/k = thermal resistance

A = area

dT/dx = temp gradient

If we increase thermal resistance , what will happen ?

Heat loss will be reduced

Why is stainless steel a good conductor of heat ?

Free electrons so when molecules vibrate free electrons can detach and can share energy from one molecules to the next

Little thermal resistance

Idea for use of lagging jack to retain heat ?

- jacket had a fibre air mixture which prevented heat loss to external enviornment -

increases thermal resistance which decreases heat flow and increases heat retention

Why is cork a poor conductor of heat ?

It is a porous material and has a high thermal resistance , decrease heat flow and increases heat retention

What is natural or free convection ?

Heated materials flow due to difference in density

buoyancy forces induced by this

Describe the process of natural convections

Example with kettle for making tea

1. Surface below the kettle is heated

2. Heat conducted from surface to first layer of water causing it to decrease in density and expand

3. First layer rises upwards

4. New fresh water comes in contact same occurs

- continuous process

5. First layer becomes cooler and then reaches heated surface and rises agin

In natural convection what drives flow ?

Temperature differences

What is forced convection ?

Heated fluid is forced to move by an external force blower , stirrer oven

What factors affect the equation for heat transfer in convection

1. Flat or curved surface

2. Horizontal or vertical surface

3. Is fluid a gas or liquid

4. Density,viscosity,heat and thermal conductivity of the fluid

5. Is there laminar or turbulent flow

6. Evaporation or condensation

Equation for convection

Q/t = h A (T - Ts)

H = convection coefficient

A = area of surface

T = temperature of many body of fluid

Ts = temp of surface

When is conduction seen in heat transfer in fluids ?

- natural free convection : heat the surface and transfer to water in kettle

Explain the heat exchange between fluids across a solid boundary

Pool boiling

Boiling caused by a heater in contact with pool of water

Forced circulation boiling

Movement of boiling water helped by external agitator which forces liquid over heating surface

What is a pan evaporator ?

- made of stainless steel and heat is supplied by a jacket

- allows for pool boiling and vapour removed by pipe

Draw a diagram of heat transfer in boiling

[boiling heat transfer curve] Explain what occurs at A-> B

- First sign of boiling : bubbles

- Bubbles move to surface by natural convection so little surface disturbances

- low heat flux

Explain what occurs at B-C

Nucleate boiling : higher temperature causes bubbles to form more rapidly

Explain what occurs at C

Peak flux - mass heat transfer

What occurs beyond C

Vapour film forms at surface and at bottom with poor conductivity

What occurs if you continue boiling after D ?

Films boiling but this can cause damage to evaporator

·       In pool boiling, what is the critical temperature drop and why is it important to know what it is?

From the boiling heat transfer graph in the slides:

As the temperature difference (ΔT) increases, heat flux rises through natural convection and nucleate boiling.

• It reaches a peak heat flux at point C, where “ΔT = critical temperature drop”.

• Beyond this point, a vapour film forms, which has poor conductivity, causing heat flux to decrease sharply (film boiling).

Knowing the critical temperature drop is essential because exceeding it causes the formation of a vapour film that drastically reduces heat transfer efficiency, which can lead to overheating of heating surfaces.

·       Explain the process of heat transfer between fluids across a solid boundary. Where might you see this approach used in pharmaceutical processing?

When fluids are separated by a solid wall heat transfer occurs in 3 steps

1.      Convection from the hot fluid to the surface of the solid wall

2.      Conduction through the solid wall

3.      Convection from the other side of the wall into the cooler fluid 

We can see this approach in pharmaceutical processing in jacketed vessels, tubes and processes such as evaportation, distillation and crystallisation

·       Is conduction involved in heat transfer in fluids? If so, explain where and how.

Yes. Conduction occurs in fluids at the fluid–solid boundary, where a thin laminar layer of almost-still fluid transfers heat only by conduction. Because fluids have low thermal conductivity, this layer has a high temperature gradient. Even during convection, the initial heat transfer from the hot surface to the first layer of fluid happens by conduction before the fluid begins to move