Fundamentals of Refrigeration and Air-Conditioning (course)


 Refrigeration: The process of removing heat.
Air-conditioning: A form of air treatment whereby temperature,
humidity, ventilation, and air cleanliness are all controlled within
limits determined by the requirements of the air conditioned
enclosure.


1.1 Basic physics – temperature

The general temperature scale now in use is the Celsius scale, based
nominally on the melting point of ice at 0°C and the boiling point
of water at atmospheric pressure at 100°C. (By strict definition, the
triple point of ice is 0.01°C at a pressure of 6.1 mbar.) On the
Celsius scale, absolute zero is – 273.15°C.
In the study of refrigeration, the Kelvin or absolute temperature scale
is also used. This starts at absolute zero and has the same degree
intervals as the Celsius scale, so that ice melts at + 273.16 K and
water at atmospheric pressure boils at + 373.15 K.




1.2 Heat

Refrigeration is the process of removing heat, and the practical
application is to produce or maintain temperatures below the
ambient. The basic principles are those of thermodynamics, and
these principles as relevant to the general uses of refrigeration are
outlined in this opening chapter.
Heat is one of the many forms of energy and mainly arises from
chemical sources. The heat of a body is its thermal or internal
energy, and a change in this energy may show as a change of
temperature or a change between the solid, liquid and gaseous
states.
Matter may also have other forms of energy, potential or kinetic,
depending on pressure, position and movement. Enthalpy is the
sum of its internal energy and flow work and is given by:
H = u + Pv
In the process where there is steady flow, the factor Pv will not
2 Refrigeration and Air-Conditioning
change appreciably and the difference in enthalpy will be the quantity
of heat gained or lost.
Enthalpy may be expressed as a total above absolute zero, or any
other base which is convenient. Tabulated enthalpies found in
reference works are often shown above a base temperature of
– 40°C, since this is also – 40° on the old Fahrenheit scale. In any
calculation, this base condition should always be checked to avoid
the errors which will arise if two different bases are used.
If a change of enthalpy can be sensed as a change of temperature,
it is called sensible heat. This is expressed as specific heat capacity,
i.e. the change in enthalpy per degree of temperature change, in
kJ/(kg K). If there is no change of temperature but a change of
state (solid to liquid, liquid to gas, or vice versa) it is called latent
heat. This is expressed as kJ/kg but it varies with the boiling
temperature, and so is usually qualified by this condition. The
resulting total changes can be shown on a temperature–enthalpy
diagram (Figure 1.1).



Example 1.1 For water, the latent heat of freezing is 334 kJ/kg and
the specific heat capacity averages 4.19 kJ/(kg K). The quantity of
heat to be removed from 1 kg of water at 30°C in order to turn it
into ice at 0°C is:
4.19(30 – 0) + 334 = 459.7 kJ
Example 1.2 If the latent heat of boiling water at 1.013 bar is 2257
kJ/kg, the quantity of heat which must be added to 1 kg of water at
30°C in order to boil it is:
Fundamentals 3
4.19(100 – 30) + 2257 = 2550.3 kJ
Example 1.3 The specific enthalpy of water at 80°C, taken from
0°C base, is 334.91 kJ/kg. What is the average specific heat capacity
through the range 0–80°C?
334.91/(80 – 0) = 4.186 kJ/(kg K)


1.3 Boiling point

The temperature at which a liquid boils is not constant, but varies
with the pressure. Thus, while the boiling point of water is commonly
taken as 100°C, this is only true at a pressure of one standard
atmosphere (1.013 bar) and, by varying the pressure, the boiling
point can be changed (Table 1.1). This pressure–temperature
property can be shown graphically (see Figure 1.2).



The boiling point is limited by the critical temperature at the upper
end, beyond which it cannot exist as a liquid, and by the triple point
at the lower end, which is at the freezing temperature. Between
these two limits, if the liquid is at a pressure higher than its boiling
pressure, it will remain a liquid and will be subcooled below the
saturation condition, while if the temperature is higher than
saturation, it will be a gas and superheated. If both liquid and
vapour are at rest in the same enclosure, and no other volatile
substance is present, the condition must lie on the saturation line.
At a pressure below the triple point pressure, the solid can change
directly to a gas (sublimation) and the gas can change directly to a
solid, as in the formation of carbon dioxide snow from the released
gas.
The liquid zone to the left of the boiling point line is subcooled
liquid. The gas under this line is superheated gas.



1.4 General gas laws

Many gases at low pressure, i.e. atmospheric pressure and below for
water vapour and up to several bar for gases such as nitrogen, oxygen
and argon, obey simple relations between their pressure, volume
and temperature, with sufficient accuracy for engineering purposes.
Such gases are called ‘ideal’.
Boyle’s Law states that, for an ideal gas, the product of pressure
and volume at constant temperature is a constant:
pV = constant


1.5 Dalton’s law

Dalton’s Law of partial pressures considers a mixture of two or
more gases, and states that the total pressure of the mixture is equal
to the sum of the individual pressures, if each gas separately occupied
the space.


Example 1.7 A cubic metre of air contains 0.906 kg of nitrogen of
specific gas constant 297 J/(kg K), 0.278 kg of oxygen of specific
gas constant 260 J/(kg K) and 0.015 kg of argon of specific gas
constant 208 J/(kg K). What will be the total pressure at 20°C?

pV = mRT
V = 1 m3
so p = mRT
For the nitrogen pN = 0.906 × 297 × 293.15 = 78 881 Pa
For the oxygen pO = 0.278 × 260 × 293.15 = 21 189 Pa
For the argon pA = 0.015 × 208 × 293.15 = 915 Pa
—————
Total pressure = 100 985 Pa
(1.009 85 bar)



1.6 Heat transfer

Heat will move from a hot body to a colder one, and can do so by
the following methods:
1. Conduction. Direct from one body touching the other, or through
a continuous mass
2. Convection. By means of a heat-carrying fluid moving between
one and the other
3. Radiation. Mainly by infrared waves (but also in the visible band,
e.g. solar radiation), which are independent of contact or an
intermediate fluid.
Conduction through a homogeneous material is expressed directly
by its area, thickness and a conduction coefficient. For a large plane
surface, ignoring heat transfer near the edges:






The exception to this is the effect of solar radiation when
considered as a cooling load, such as the air-conditioning of a building
which is subject to the sun’s rays. At the wavelength of sunlight the
absorptivity figures change and calculations for such loads use
tabulated factors for the heating effect of sunlight. Glass, glazed
tiles and clean white-painted surfaces have a lower absorptivity, while
the metals are higher.


1.7 Transient heat flow

A special case of heat flow arises when the temperatures through
the thickness of a solid body are changing as heat is added or
removed. This non-steady or transient heat flow will occur, for example,
when a thick slab of meat is to be cooled, or when sunlight strikes
on a roof and heats the surface. When this happens, some of the
heat changes the temperature of the first layer of the solid, and the
remaining heat passes on to the next layer, and so on. Calculations
for heating or cooling times of thick solids consider the slab as a
number of finite layers, each of which is both conducting and
absorbing heat over successive periods of time. Original methods of
solving transient heat flow were graphical [1, 5], but could not
easily take into account any change in the conductivity or specific
heat capacity or any latent heat of the solid as the temperature
changed.
Complicated problems of transient heat flow can be resolved by
computer. Typical time–temperature curves for non-steady cooling
are shown in Figures 16.1 and 16.2, and the subject is met again in
Section 26.2.



1.8 Two-phase heat transfer

Where heat transfer is taking place at the saturation temperature of
a fluid, evaporation or condensation (mass transfer) will occur at
the interface, depending on the direction of heat flow. In such
cases, the convective heat transfer of the fluid is accompanied by
conduction at the surface to or from a thin layer in the liquid state.
Since the latent heat and density of fluids are much greater than
the sensible heat and density of the vapour, the rates of heat transfer
are considerably higher. The process can be improved by shaping
the heat exchanger face (where this is a solid) to improve the drainage
of condensate or the escape of bubbles of vapour. The total heat
transfer will be the sum of the two components.
Rates of two-phase heat transfer depend on properties of the
volatile fluid, dimensions of the interface, velocities of flow and the

extent to which the transfer interface is blanketed by fluid. The
driving force for evaporation or condensation is the difference of
vapour pressures at the saturation and interface temperatures.
Equations for specific fluids are based on the interpretation of
experimental data, as with convective heat transfer.
Mass transfer may take place from a mixture of gases, such as the
condensation of water from moist air. In this instance, the water
vapour has to diffuse through the air, and the rate of mass transfer
will depend also on the concentration of vapour in the air. In the
air–water vapour mixture, the rate of mass transfer is roughly
proportional to the rate of heat transfer at the interface and this
simplifies predictions of the performance of air-conditioning coils
[1, 5, 9].






in the next section we will discuss the The refrigeration cycles




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