Note -introduction_of_heat

AIR CONDITIONING & REFRIGERATION TECHNOLOGY

1. Heat Transfer Terminology

1.1 The difference between heat and temperature.
1.2 The heat and work
1.3 The three modes of heat transfer.
1.4 The following terms as they relate to heat transfer:
(a) Heat flux
(b) Thermal conductivity
(c) Convective heat transfer coefficient
(d) Stefan Boltzman coefficient.
(e) Overall heat transfer coefficient (u-value)
(f) Log mean Temperature Different (LMTD)
(g) Number of Transfer Unit (NTU)
1.5 Energy Balance (Equilibrium Temperature)
1.6 Heat Transfer Terminology Summary

Dept: HVACR Subject: Heat Transfer & Heat Exchanger

Subject Code: FRD 20303 Page: 1 of 8


1.1 The difference between heat and temperature.

In describing heat transfer problems, students often make the mistake of interchangeably
using the terms heat and temperature. Actually, there is a distinct difference between the two.

Temperature is a measure the heat intensity or heat level of a substance. It is a relative
measure of how hot or cold a substance is and can be used to predict the direction of heat transfer.
The symbol for temperature is T. The common scales for measuring temperature are the Fahrenheit,
Rankine, Celsius, and Kelvin temperature scales.

Heat is a form of energy in transit. The transfer of energy as heat occurs at the molecular
level as a result of a temperature difference. Heat is capable of being transmitted through solids and
fluids by conduction, through fluids by convection, and through empty space by radiation. The
symbol for heat is Q. Common units for measuring heat are the British thermal unit (Btu) in the
English system of units, the calorie in the conventional metric system and Kilo Joule (KJ) in SI
system (International System of Units).

1.2 The Work and Heat

Work is the transfer of energy resulting from force acting through a distance. Common units
for measuring heat are Joule (J).
Heat is energy transferred as the result of a temperature difference. Neither heat nor works
are thermodynamic properties of a system. Heat can be transferred into or out of a system and work
can be done on or by a system, but a system cannot contain or store either heat or work. Heat is
related to the molecular motion or vibration. A molecule is the smallest particles in which any
substance can be broken down and still retain its chemical identity. As a substance is warmed,
molecules move more rapidly. As a substance is cooled, they slow down. If all heat is removed from
a substance, all molecular motion stops.




1.3 DESCRIBE the three modes of heat transfer.

Heat is always transferred when a temperature difference exists between two bodies. Heat
always flows from a warmer substance to a cooler one. What happen is that the faster moving
molecules give out some of their energy so slower moving molecules. Therefore, the faster
molecules slow down a little and the slower ones move a little faster. There are three basic modes of
heat transfer:

a) Conduction involves the flow of heat among parts of substance. The flow can also be from one
substance to another substance in direct contact.

b) Convection involves the moving of heat from one place to another place by way of fluid or air.

c) Radiation, or radiant heat transfer, where heat transfer in the form of wave motion similar to light
waves in which energy is transmitted from one body to other body without need of intervening
matter.

The three modes of heat transfer will be discussed in greater detail in the subsequent chapters of this
module.

Figure 1: Three methods of heat transfer




1.4 DEFINE the following terms as they relate to heat transfer:

a) Heat flux
The rate at which heat is transferred is represented by the symbol. Common units for heat ˙Q
transfer rate are Btu/hr. Sometimes it is important to determine the heat transfer rate per unit area, or
heat flux, which has the symbol. Units for heat flux are Btu/hr-ft2. The heat flux can be ˙Q
determined by dividing the heat transfer rate by the area through which the heat is being transferred.

Where, q = heat Flux, (W/m2. or BTU/hr.ft2.)
Q = Heat flow rate (Watt or BTU/hr)
A = Area to the direction of heat flow (m2 or ft2)

b) Thermal conductivity
The heat transfer characteristics of a solid material are measured by a property called the
thermal conductivity (k) measured in Btu/hr.ft.0F and w/m.0C. It is a measure of a substance’s ability
to transfer heat through a solid by conduction. The thermal conductivity of most liquids and solids
varies with temperature. For vapors, it depends upon pressure.

& = kA T1 − T2 → k = Q / A
&
Q
L L(T1 − T2 )

q
=
L (T1 − T2 )




c) Convective heat transfer coefficient
The convective heat transfer coefficient (h), defines, in part, the heat transfer due to
convection. The convective heat transfer coefficient is sometimes referred to as a film coefficient and
represents the thermal resistance of a relatively stagnant layer of fluid between a heat transfer surface
and the fluid medium. Common units used to measure the convective heat transfer coefficient are
Btu/hr.ft2.0F and w/m2.0C.

d) Stefan Boltzman
A body that emits the maximum amount of heat for its absolute temperature is
called a black body. Radiant heat transfer rate from a black body to its surroundings can
be expressed by the following equation.

σ = Stefan-Boltzman constant
0.1714 x 10-8 Btu/hr-ft2.R4 and = 5.676 x 10-8 W/m2.K4

e) Overall heat transfer coefficient (u-value)
In the case of combined heat transfer, it is common practice to relate the total rate of heat
transfer ( ), the overall cross-sectional area for heat transfer (Ao), and the overall temperature ˙Q
difference (To) using the overall heat transfer coefficient (Uo). The overall heat transfer coefficient
combines the heat transfer coefficient of the two heat exchanger fluids and the thermal conductivity
of the heat exchanger tubes. Uo is specific to the heat exchanger and the fluids that are used in the
heat exchanger.

Watt

w / m2.C.

m2

0
C




f) Log mean temperature difference (LMTD)
In heat exchanger applications, the inlet and outlet temperatures are commonly specified
based on the fluid in the tubes. The temperature change that takes place across the heat exchanger
from the entrance to the exit is not linear. A precise temperature change between two fluids across
the heat exchanger is best represented by the log mean temperature difference (LMTD), defined in
Equation below

g) Number of Transfer Unit (NTU)

The Number of Transfer Units (NTU) Method is used to calculate the rate of heat transfer
in heat exchangers (especially counter current exchangers) when there is insufficient information to
calculate the Log-Mean Temperature Difference (LMTD).Method is preferred when only the inlet
temperatures are known.

The method proceeds by calculating the heat capacity rates (i.e. flow rate multiplied by
specific heat) Ch and Cc for the hot and cold fluids respectively, and denoting the smaller one as Cmin.




1.5 Energy Balance (Equilibrium Temperature)
In heat transfer analysis, we are usually interested only in the forms of energy that can be
transferred as a result of a temperature difference, that is, heat or thermal energy in such cases it is
convenient to write a heat balance and to treat the conversion of nuclear, chemical and electrical
energies into thermal energy as heat generation. The energy balance in that case can be expressed as

Energy Balance for Closed Systems (fixed mass)
A closed system consist of a fixed mass. The total energy, E for most systems encountered in
practice consists of the internal energy, U. This case for stationary systems since they don’t involve
any changes in their velocity or elevation during a process. The energy balance relation in that case
to reduce to

where we expressed the internal energy change in terms of mass, m the specific heat at constant
volume, CV and the temperature change ∆T oh the system. When the system involves heat transfer
only and no work interactions across its boundary, the energy balance relation further reduces to

Where Q is the net amount of heat transfer to or forms the system. This is the form of the energy
balance relation we will most often dealing with a fixed mass.

For a steady-flow system with one inlet and one exit, the rate of mass flow into the control volume

must be equal to the rate of mass flow out of it. That is When the changes in
kinetic and potential energies are negligible which is usually the case, and there is no work
interaction, the energy balance for such a steady flow system reduces to

Where Q is the rate of net heat transfer in to or out of the control volume




1.6 Heat Transfer Terminology Summary

Heat is energy transferred as a result of a temperature difference.
Temperature is a measure of the amount of molecular energy contained in a substance.
Work is a transfer of energy resulting from force acting through a distance.
The Second Law of Thermodynamics implies that heat will not transfer from a colder to a
hotter body without some external source of energy.
Conduction involves the transfer of heat by the interactions of atoms or molecules of a
material through which the heat is being transferred.
Convection involves the transfer of heat by the mixing and motion of macroscopic portions
of a fluid.
Radiation, or radiant heat transfer, involves the transfer of heat by electromagnetic radiation
that arises due to the temperature of a body.
Heat flux is the rate of heat transfer per unit area.
Thermal conductivity is a measure of a substance’s ability to transfer heat through itself.
Log mean temperature difference is the T that most accurately represents the T for a heat
exchanger.
The local heat transfer coefficient represents a measure of the ability to transfer heat through
a stagnant film layer.
The overall heat transfer coefficient is the measure of the ability of a heat exchanger to
transfer heat from one fluid to another.
The bulk temperature is the temperature of the fluid that best represents the majority of the
fluid which is not physically connected to the heat transfer site.



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