2. Definitions
Refrigeration is the branch of science which deals with
the process of reducing and maintaining the
temperature of a space or material below the
temperature of the surrounding.
Air conditioning refers to the control of temperature,
moisture content, cleanliness, air quality and air
circulation as required by occupants, a process or a
product in the space.
3
3. 4
Applications of Refrigeration
and Air Conditioning
Industries
Food and Beverage Industries
Textile industries
Electronics Industries
Horticulture – transportation of
flowers, fruit...
Temperature and humidity control
Air cleanliness
Cold truck
Textile Industry
4. 5
Applications of Refrigeration
and Air Conditioning [Cont…]
Household and offices
Refrigerators
Air conditioning systems
Split AC for single rooms
Household refrigerator
7. 8
HISTORY OF REFRIGERATION
Natural Refrigeration
In olden days, refrigeration was achieved by natural means such as the
use of ice or evaporative cooling.
In earlier times, ice was either:
1. Transported from colder regions,
2. Harvested in winter and stored in ice houses for summer use
or,
3. Made during night by cooling of water by radiation to
stratosphere.
Materials like sawdust or wood shavings were used as insulating
materials in these icehouses. Later on, cork was used as insulating
material.
10. 11
HISTORY OF REFRIGERATION [Cont…]
1. Art of Ice making by Nocturnal Cooling:
In this method, ice was made by keeping a thin layer of water in a
shallow earthen tray, and then exposing the tray to the night sky.
Compacted hay of about 0.3 m thickness was used as insulation. The
water looses heat by radiation to the stratosphere, which is at around -
55°C and by early morning hours the water in the tray freezes to ice.
12. 13
HISTORY OF REFRIGERATION [Cont…]
2. Evaporative Cooling:
Evaporative cooling is the process of reducing the temperature
of a system by evaporation of water.
Human beings perspire and dissipate their metabolic heat by
evaporative cooling if the ambient temperature is more than
skin temperature.
Animals such as the hippopotamus and buffalo coat
themselves with mud for evaporative cooling.
13. 14
HISTORY OF REFRIGERATION [Cont…]
Evaporative cooling has been used for centuries to obtain cold
water in summer by storing the water in earthen (clay) pots.
The water permeates through the pores of earthen vessel to its
outer surface where it evaporates to the surrounding,
absorbing its latent heat in part from the vessel, which cools
the water.
14. 15
Question : Discussion among students
What is the difference and similarity between
evaporative cooling and drying?
Is it possible to dry without using evaporation? Discuss
15. 16
HISTORY OF REFRIGERATION [Cont…]
3. Cooling by Salt Solutions:
Certain substances such as common salt, when added to water dissolve in
water and absorb its heat of solution from water (endothermic process).
This reduces the temperature of the solution (water + salt).
Sodium Chloride salt (NaCl) can yield temperatures up to -20°C and
Calcium Chloride (CaCl2) up to -50°C in properly insulated containers.
However, as it is, this process has limited application, as the dissolved salt
has to be recovered from its solution by heating.
16. 17
HISTORY OF REFRIGERATION [Cont…]
4. Artificial Refrigeration
Refrigeration as it is known these days is produced by artificial means.
Though it is very difficult to make a clear demarcation between natural
and artificial refrigeration, it is generally agreed that the history of
artificial refrigeration began in the year 1755, when the Scottish
professor William Cullen made the first refrigerating machine, which
could produce a small quantity of ice in the laboratory.
Based on the working principle, refrigeration systems can be classified
as vapor compression systems, vapor absorption systems, gas cycle
systems etc.
17. 18
HISTORY OF REFRIGERATION [Cont…]
Vapour Compression Refrigeration Systems:
The basis of modern refrigeration is the ability of liquids to absorb
enormous quantities of heat as they boil and evaporate.
Professor William Cullen of the University of Edinburgh
demonstrated this in 1755 by placing some water in thermal contact
with ether (air) under a receiver of a vacuum pump.
The evaporation rate of ether increased due to the vacuum pump
and water could be frozen.
This process involves two thermodynamic concepts, the vapor
pressure and the latent heat.
18. 19
HISTORY OF REFRIGERATION [Cont…]
A liquid is in thermal equilibrium with its own vapor at a
pressure called the saturation pressure, which depends on
the temperature alone.
If the pressure is increased for example in a pressure
cooker, the water boils at higher temperature.
The second concept is that the evaporation of liquid
requires latent heat during evaporation.
If latent heat is extracted from the liquid, the liquid gets
cooled.
19. 20
HISTORY OF REFRIGERATION [Cont…]
The temperature of ether will remain constant as long as the
vacuum pump maintains a pressure equal to saturation
pressure at the desired temperature. This requires the
removal of all the vapors formed due to vaporization.
If a lower temperature is desired, then a lower saturation
pressure will have to be maintained by the vacuum pump. The
component of the modern day refrigeration system where
cooling is produced by this method is called evaporator.
20. 21
HISTORY OF REFRIGERATION [Cont…]
If this process of cooling is to be made continuous the vapors have to be
recycled by condensation to the liquid state.
The condensation process requires heat rejection to the surroundings.
It can be condensed at atmospheric temperature by increasing its
pressure.
Hence, a compressor is required to maintain a high pressure so that the
evaporating vapours can condense at a temperature greater than that of the
surroundings.
22. 23
HISTORY OF REFRIGERATION [Cont…]
A refrigeration system can also be used as a heat pump, in which the
useful output is the high temperature heat rejected at the condenser.
Alternatively, a refrigeration system can be used for providing cooling in
summer and heating in winter. Such systems have been built and are
available now.
23. 24
Principles of Refrigeration
Our focus here are refrigerators.
In order for refrigerators to function, they need at
least the following components;
- Evaporator
- Compressor
- Condenser
- Expansion Valve
- Refrigerant; to collect the heat from the product
25. 26
Evaporator
The purpose of the evaporator is to remove unwanted
heat from the product.
It is basically a heat exchanger (coil/pipe).
Refrigerant contained within the evaporator is boiling at a
low-pressure. The level of this pressure is determined by
two factors:
- The rate at which the heat is absorbed from the product to the
liquid refrigerant in the evaporator
- The rate at which the low-pressure vapor is removed from the
evaporator by the compressor
When leaving the evaporator coil, the liquid refrigerant is
in vapor form.
26. 27
Compressor
The purpose of the compressor is to draw the low-
temperature, low-pressure vapor from the evaporator via
the suction line.
When vapor is compressed, its temperature rises.
The compressor transforms the vapor from a low-
temperature vapor to a high-temperature vapor, in turn
increasing the pressure.
27. 28
Condenser
The purpose of the condenser is to extract heat from the refrigerant
to the outside air.
Fans mounted above the condenser unit are used to draw air
through the condenser coils.
The temperature of the high-pressure vapor determines the
temperature at which the condensation begins.
As heat has to flow from the condenser to the air, the condensation
temperature must be higher than that of the air.
The high-pressure vapor within the condenser is then cooled to the
point where it becomes a liquid refrigerant once more, whilst
retaining some heat.
28. 29
Expansion Valve
The expansion valve is located at the end of the liquid
line, before the evaporator. The high-pressure liquid
reaches the expansion valve, having come from the
condenser.
The valve then reduces the pressure of the refrigerant
as it passes through the orifice, which is located inside
the valve.
On reducing the pressure, the temperature of the
refrigerant also decreases to a level below the
surrounding air.
This low-pressure, low-temperature liquid is then
pumped in to the evaporator.
30. Objectives
1. Have an understanding of laminar and turbulent
flow in pipes and the analysis of fully developed
flow
2. Calculate the major and minor losses associated
with pipe flow in piping networks and determine
the pumping power requirements
31. Introduction
Average velocity in a pipe
Recall - because of the no-slip
condition, the velocity at the walls
of a pipe or duct flow is zero
We are often interested only in Vavg,
which we usually call just V (drop
the subscript for convenience)
Keep in mind that the no-slip
condition causes shear stress and
friction along the pipe walls
Friction force of wall on fluid
32. Introduction
For pipes of constant
diameter and
incompressible flow
◦ Vavg stays the same down the
pipe, even if the velocity
profile changes
◦ Why? Conservation of Mass
same
Vavg Vavg
same
same
33. Introduction
For pipes with variable diameter, m is still the same due
to conservation of mass, but V1 ≠ V2
D2
V2
2
1
V1
D1
m m
35. Laminar and Turbulent Flows
Critical Reynolds number (Recr) for
flow in a round pipe
Re < 2300 laminar
2300 ≤ Re ≤ 4000 transitional
Re > 4000 turbulent
Note that these values are approximate.
For a given application, Recr depends
upon
◦ Pipe roughness
◦ Vibrations
◦ Upstream fluctuations, disturbances
(valves, elbows, etc. that may disturb
the flow)
Definition of Reynolds number
36. Laminar and Turbulent Flows
For non-round pipes, define the hydraulic
diameter
Dh = 4Ac/P
Ac = cross-section area
P = wetted perimeter
Example: open channel
Ac = 0.15 * 0.4 = 0.06m2
P = 0.15 + 0.15 + 0.5 = 0.8m
Don’t count free surface, since it does not
contribute to friction along pipe walls!
Dh = 4Ac/P = 4*0.06/0.8 = 0.3m
What does it mean? This channel flow is
equivalent to a round pipe of diameter 0.3m
(approximately).
37. The Entrance Region
Consider a round pipe of diameter D. The flow can be
laminar or turbulent. In either case, the profile
develops downstream over several diameters called the
entry length Lh. Lh/D is a function of Re.
Lh
38. Fully Developed Pipe Flow
Comparison of laminar and turbulent flow
There are some major differences between laminar and
turbulent fully developed pipe flows
Laminar
◦ Can solve exactly
◦ Flow is steady
◦ Velocity profile is parabolic
◦ Pipe roughness not important
It turns out that Vavg = 1/2Umax and u(r)= 2Vavg(1 - r2/R2)
39. Fully Developed Pipe Flow
Turbulent
Cannot solve exactly (too complex)
Flow is unsteady (3D swirling eddies), but it is steady in the mean
Mean velocity profile is fuller (shape more like a top-hat profile, with very sharp
slope at the wall)
Pipe roughness is very important
Vavg 85% of Umax (depends on Re a bit)
No analytical solution, but there are some good semi-empirical expressions that
approximate the velocity profile shape. See text
Logarithmic law (Eq. 8-46)
Power law (Eq. 8-49)
Instantaneous
profiles
40. Fully Developed Pipe Flow
Wall-shear stress
Recall, for simple shear flows u=u(y), we had
= du/dy
In fully developed pipe flow, it turns out that
= du/dr
Laminar Turbulent
w w
w,turb > w,lam
w = shear stress at the wall,
acting on the fluid
41. Fully Developed Pipe Flow
Pressure drop
There is a direct connection between the pressure drop in a pipe and the shear
stress at the wall
Consider a horizontal pipe, fully developed, and incompressible flow
Let’s apply conservation of mass, momentum, and energy to this CV (good
review problem!)
1 2
L
w
P1 P2
V
Take CV inside the pipe wall
42. Fully Developed Pipe Flow
Pressure drop
Conservation of Mass
Conservation of x-momentum
Terms cancel since 1 = 2
and V1 = V2
43. Fully Developed Pipe Flow
Pressure drop
Thus, x-momentum reduces to
Energy equation (in head form)
or
cancel (horizontal pipe)
Velocity terms cancel again because V1 = V2, and 1 = 2 (shape not changing)
hL = irreversible head
loss & it is felt as a pressure
drop in the pipe
44. Fully Developed Pipe Flow
Friction Factor
From momentum CV analysis
From energy CV analysis
Equating the two gives
To predict head loss, we need to be able to calculate w. How?
Laminar flow: solve exactly
Turbulent flow: rely on empirical data (experiments)
In either case, we can benefit from dimensional analysis!
45. Fully Developed Pipe Flow
Friction Factor
w = func( V, , D, ) = average roughness of the
inside wall of the pipe
-analysis gives
46. Fully Developed Pipe Flow
Friction Factor
Now go back to equation for hL and substitute f for w
Our problem is now reduced to solving for Darcy friction factor f
Recall
Therefore
Laminar flow: f = 64/Re (exact)
Turbulent flow: Use charts or empirical equations (Moody Chart, a famous plot of f vs. Re and /D, See Fig. A-12, p. 898 in
text)
But for laminar flow, roughness
does not affect the flow unless it
is huge
47.
48. Fully Developed Pipe Flow Friction Factor
Moody chart was developed for circular pipes, but can be used for
non-circular pipes using hydraulic diameter
Colebrook equation is a curve-fit of the data which is convenient for
computations (e.g., using EES)
Both Moody chart and Colebrook equation are accurate to ±15% due
to roughness size, experimental error, curve fitting of data, etc.
Implicit equation for f which can be solved
using the root-finding algorithm in EES
49. Types of Fluid Flow Problems
In design and analysis of piping systems, 3 problem
types are encountered
1. Determine p (or hL) given L, D, V (or flow rate)
Can be solved directly using Moody chart and Colebrook equation
2. Determine V, given L, D, p
3. Determine D, given L, p, V (or flow rate)
Types 2 and 3 are common engineering design
problems, i.e., selection of pipe diameters to minimize
construction and pumping costs
However, iterative approach required since both V and
D are in the Reynolds number.
50. Types of Fluid Flow Problems
Explicit relations have been developed which eliminate
iteration. They are useful for quick, direct calculation,
but introduce an additional 2% error
51. Minor Losses
Piping systems include fittings, valves, bends, elbows, tees,
inlets, exits, enlargements, and contractions.
These components interrupt the smooth flow of fluid and cause
additional losses because of flow separation and mixing
We introduce a relation for the minor losses associated with
these components
• KL is the loss coefficient.
• Is different for each component.
• Is assumed to be independent of Re.
• Typically provided by manufacturer or
generic table (e.g., Table 8-4 in text).
52. Minor Losses
Total head loss in a system is comprised of major losses
(in the pipe sections) and the minor losses (in the
components)
If the piping system has constant diameter
i pipe sections j components
