HEAT TRANSFER

by

Aruna c.p

INTRODUCTION SPECIFIC HEAT CAPACITY CONDUCTION CONVECTION RADIATION THERMAL CONTACT RESISTANCE HVAC INTRODUCTION HISTORY BASIC–AC SYSTEM ECONOMIZER CYCLE CHOSING AN AIR CONDITIONING SYSTEM REFRIGERATION

CONTENTS

Heat always moves from a warmer place to a cooler place.

Hot objects in a cooler room will cool to room temperature.

Cold objects in a warmer room will heat up to room temperature.

INTRODUCTION

WHY WE NEED HEAT TRANSFER

A thermodynamic simply tells us how much amount of heat transfer from one equilibrium state to another equilibrium state.

Heat transfer plays major rule design of many devices,such as radiators, solar collectors, various components of power plants, even space craft.

HOW DOES ENERGY AFFECT MATERIALS?Do different materials need the same amount of energy to increase their temperature by the same amount?

To increase the temperature of 1 kg of water by 1°C, requires 4200 J.

To increase the temperature of 1 kg of copper by 1°C, requires 390 J.

Water and copper require different amounts of energy because they have different values for a property called specific heat capacity.

It is the amount of energy required to increase the temperature of 1 kg of a material by 1°C.

So, the specific heat capacity for water is 4200 J/kg°C

and for copper is 390 J/kg°C.

WHAT IS SPECIFIC HEAT CAPACITY?

The specific heat capacity of a material is the amount of energy required to raise 1 kg of the material by 1 °C.

It can be used to work out how much energy is needed to raise the temperature of a material by a certain amount:

Energy = mass *specific heat capacity* temperature change

Energy is measured in joules (J).

Mass is measured in kilograms (kg).

Temperature change is measured in °C.

Specific heat capacity is measured in J/kg°C.

SPECIFIC HEAT CAPACITY EXAMPLE

Using the specific heat capacity of water (4200 J/kg°C), how much energy is needed to increase the temperature of 600 g of water by 80°C in a kettle?

Note: mass = 600 g = 0.6 kg

Energy = mass *specific heat capacity* temperature change

energy = 0.6 x 4200 x 80 = 201 600 J

HEAT TRANSFER MODES

Conduction

Convection radiation

WHAT IS CONDUCTION?

solids liquids gases

Conduction is the transfer of energy from more energetic particles of a substance to adjacent less energetic ones result of interaction between

particles.

How are the particles arranged in a solid, a liquid and a gas?

FOURIER LAW

In 1822 Fourier postulated that the rate of heat transfer is proportional to the temperature gradient present in a solid.

THERMAL CONDUCTIVITY

CONVECTIONWhat happens to the particles in a liquid or a gas when you heat them?

The particles spread out and become less dense.

Convection heat transfer

Fluid movement

Cooler, more d____, fluids sink through w_____, less dense fluids.

In effect, warmer liquids and gases r___ up.

Cooler liquids and gases s___.

ensearmer

ise

ink

TYPES OF CONVECTION

forced convection ex: pump Natural convection ex: wind

HOW DOES HEAT TRAVEL THROUGH SPACE?

The Earth is warmed by heat energy from the Sun.

How does this heat energy travel from the Sun to the Earth?

infraredwaves

There are no particles between the Sun and the Earth, so the heat cannot travel by conduction or by convection.

The heat travels to Earth by infrared waves. These are similar to light waves and are able to travel through empty space.

WHAT ARE INFRARED WAVES?

Heat can move by travelling as infrared waves.

These are electromagnetic waves, like light waves, but with a longer wavelength.

They can travel through a vacuum.

This means that infrared waves act like light waves:

They travel at the same speed as light – 300,000,000 m/s. They can be reflected and absorbed.

EMISSION EXPERIMENT

Four containers were filled with warm water. Which container would have the warmest water after ten minutes?

Shiny metal

Dull metal

Dull black

Shiny black

The __________ container would be the warmest after ten minutes because its shiny surface reflects heat _______ back into the container so less is lost. The ________ container would be the coolest because it is the best at _______ heat radiation.

shiny metalradiation

dull blackemitting

ABSORPTION EXPERIMENTFour containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes?

The __________ container would be the warmest after ten minutes because its surface absorbs heat _______ the best. The _________ container would be the coolest because it is the poorest at __________ heat radiation.

dull blackradiation

shiny metalabsorbing

Shiny metal

Dull metal

Dull black

Shiny black

RADIATION QUESTIONS

Why are houses painted white in hot countries?

White reflects heat radiation and keeps the house cooler.

Why are shiny foil blankets wrapped around marathon runners at the end of a race?The shiny metal reflects the heat radiation from the runner back in, this stops the runner getting cold.

STEADY V/S TRANSIENT HEAT TRANSFER

Steady; no change with timeTransient: change with time

THERMAL CONTACT RESISTANCE

The thermal contact resistance will be greater for rough surfaces because an interface with rough surfaces will contain more air gaps whose thermal conductivity is low.

thermal contact resistance is significant and can even dominate the heat transfer for good heat conductors such as metals, but can be disregarded for poor heat conductors such as insulations.

The thermal contact resistance can be minimized by applying

• a thermal grease such as silicon oil • a better conducting gas such as

helium or hydrogen • a soft metallic foil such as tin, silver,

copper, nickel, or aluminum

25

THERMAL CONTACT RESISTANCE

Temperature distribution and heat flow lines along two solid plates pressed against each other for the case of perfect and imperfect contact.

HVAC

INTRODUCTION

Heating, Ventilating and Air Conditioning, HVAC, is a huge field. HVAC systems include a range from the simplest hand-stoked stove, used for comfort heating, to the extremely reliable total air-conditioning systems found in sub marines and space shuttles.

HISTORY

In 1851 Dr. John Gorrie was granted U.S. patent 8080 for refrigeration machine.

Cooling the New York Stock Exchange, in 1902, was one of the first comfort cooling systems. Comfort cooling was called “air conditioning

SCOPE OF MODERN HVAC

Greenhouse gas emissions and the destruction of the earth’s protective ozone layer are concerns that are stimulating research

Energy conservation is an ongoing challenge to find novel ways to reduce consumption in new and existing buildings without compromising comfort and indoor air quality.

AIR CONDITIONING

“air conditioning,” when properly used, nowmeans the total control of temperature, moisture in the air (humidity), supplyof outside air for ventilation, filtration of airborne particles, and air movementin the occupied space.

There are seven main processes required to achieve full air conditioning

1. Heating2. Cooling 3. Humidifying4. Dehumidifying5. Cleaning6. Ventilation7. Air movement

BASIC AIR-CONDITIONING SYSTEM

ECONOMIZER CYCLE

CHOOSING AN AIR-CONDITIONING SYSTEM

Building design; if there is very little space for running ducts around the building, an all-air system may not fit in the available space

Location issues; site conditions peak summer cooling conditions summer humidity peak winter heating conditions wind speeds sunshine hours typical snow accumulation depths

Utilities: availability and cost; The choice of system can be heavily influenced by available utilities and their costs to supply and use

…CONT

Indoor requirements and loads;

The thermal and moisture loads Outside ventilation air Zoning Client issues; Buildings cost money to construct and to use. Therefore, the designer has to consider the clients’ requirements both for construction and for in- use costs

REFRIGERATION

Heat flows in direction of decreasing temperature, i.e., from high-temperature to low temperature regions. The transfer of heat from a low-temperature to high-temperature requires a refrigerator and/or heat pump

COP

IDEAL VAPOR COMPRESSION REFRIGERATION CYCLE

Vapor compression cycle

REFERENCES

1. Yunus A. Cengel “Heat and mass transfer” McGraw-Hill Education(India) Pvt Limited, 2007.

2. www.board works.com.3. ASHRAE Learning Institute’s Fundamentals of HVAC&R e-Learning System.

THANK YOU