Title: ISAT 413 - Module III: Building Energy Efficiency
1ISAT 413 - Module III Building Energy Efficiency
Topic 3 Refrigeration, Heat Pumps and Air
Conditioning
- Evaporating Cooling
- Vapor-Compression Refrigeration
- Vapor-Absorption Refrigeration
- Other Refrigeration Systems
- The Heat Pump
- Air Conditioning
2- Evaporative Cooling (Eastop, Example 4.8)
In a cooling tower the water to be cooled enters
near the top of the tower and flows down through
packing in counter flow to an air stream as shown
diagrammatically in Fig. 4.34. Air is induced
from the base of the tower by a fan mounted on
the top of the tower. Using the data given below
and neglecting heat losses and pressure losses,
calculate (i) the required mass flow rate of air
induced by the fan for the given cooling
duty (ii) the mass flow rate of make-up water
required.
3Cooling Tower (Evaporative Cooling)
4Data Water Mass flow rate, 15 kg/s temperature
of water at entry, 27oC temperature of water at
exit, 21oC mean specific heat, 4.18
kJ/kg.K. Air Ambient conditions, 1,01325 bar,
23oC dry bulb (DB), 17oC wet bulb (WB) air at
exit, saturated with water vapor at 25oC mean
specific heat of dry air, 1.005 kJ/kg.K fan air
power input, 5 kW. Assumptions For air at
standard atmospheric pressure it can be assumed
that the vapor pressure is given by For water
vapor at low vapor pressure it is a good
approximation to assume that the enthalpy is
equal to the saturated value at the same dry bulb
temperature.
5(i) the required mass flow rate of air induced by
the fan for the given cooling duty
6(No Transcript)
7- Vapor-Compression Refrigeration
P-h Diagram of an Ideal Vapor-Compression
Refrigeration Cycle
8Schematic and T-s Diagram for Ideal
Vapor-Compression Refrigeration Cycle
9Schematic and T-s Diagram for Actual
Vapor-Compression Refrigeration Cycle
10- The performance of refrigerators and heat pumps
is expressed in terms of coefficient of
performance (COP), defined as
11- The standard of comparison for refrigeration
cycles is the reversed Carnot cycle. A
refrigerator or heat pump that operates on the
reversed Carnot cycle is called a Carnot
refrigerator or a Carnot heat pump, and their
COPs are
12- The transfer of heat from lower temperature
regions to higher temperature ones is called
refrigeration. Devices that produce refrigeration
are called refrigerators, and the cycles on which
they operate are called refrigeration cycles. The
working fluids used in refrigerators are called
refrigerants. Refrigerators used for the purpose
of heating a space by transferring heat from a
cooler medium are called heat pumps.
13- The most widely used refrigeration cycle is the
vapor-compression refrigeration cycle. In an
ideal vapor-compression refrigeration cycle, the
refrigerant enters the compressor as a saturated
vapor and is cooled to the saturated liquid state
in the condenser. It is then throttled to the
evaporator pressure and vaporizes as it absorbs
heat from the refrigerated space.
14- Very low temperatures can be achieved by
operating two or more vapor-compression Systems
in series, called cascading. The COP of a
refrigeration system also increases as a result
of cascading.
15- Another way of improving the performance of a
vapor-compression refrigeration system is by
using multistage compression with regenerative
cooling. A refrigerator with a single compressor
can provide refrigeration at several temperatures
by throttling the refrigerant in stages. The
vapor-compression refrigeration cycle can also be
used to liquefy gases after some modifications
Q Cycles? Components? Working Fluids?
16Working Fluids Refrigerants
- The designation of CFCs (ChloroFluoroCarbons)
- is by a two or three digit number
- The digit on the right represents the number of
fluorine (F) atoms - The middle digit represents one more than the
number of hydrogen (H) atoms - The digit on the left represents one less than
the number of carbon (C) atoms and omitted when
there is only one carbon atom.
CHClF2 (R22) or called HCFC 22
17Some Common CFC, HCFC Refrigerants
CCl3F (R11), CCl2F2 (R12), and CHClF2 (R22), HFC
134a, CCl2FCClF2 (R113) and CClF2CClF2
(R114) R502 which is an azeotropic mixture (acts
as a single substance and cannot be separated
into its components by distillation.) of R22 and
R115 The commonly accepted designation for
refrigerants other than halocarbons (compounds
containing carbon, fluorine and chlorine in
varying proportions, known as CFCs) is to put the
digit 7 in front of the relative molecular mass
(e.g. NH3 (R717), CO2 ( R744).)
(Notice that researches are now underway to
develop acceptable alternatives.)
18- Vapor-Absorption Refrigeration
Ammonia Absorption Refrigeration Cycle
19- Another form of refrigeration that becomes
economically attractive when there is a source of
inexpensive heat energy at a temperature of 100
to 2000C is absorption refrigeration, where the
refrigerant is absorbed by a transport medium and
compressed in liquid form. The most widely used
absorption refrigeration system is the
ammonia-water system, where ammonia serves as the
refrigerant and water as the transport medium.
The work input to the pump is usually very small,
and the COP of absorption refrigeration systems
is defined as
20- The maximum COP an absorption refrigeration
system can have is determined by assuming totally
reversible conditions, which yields -
- where T0, TL, and Ts are the absolute
temperatures of the environment, refrigerated
space, and heat source, respectively.
21- Other Refrigeration Systems
Schematic of Simple Thermoelectric Power Generator
22A Thermoelectric Refrigerator
10-13
23- A refrigeration effect can also be achieved
without using any moving parts by simply passing
a small current through a closed circuit made up
of two dissimilar materials. This effect is
called the Peltier effect, and a refrigerator
that works on this principle is called a
thermoelectric refrigerator.
24Refrigerator and Heat Pump Objectives
The objective of a refrigerator is to remove heat
(QL) from the cold medium the objective of a
heat pump is to supply heat (QH) to a warm medium.
25Heat Pump Heats a House in Winter and Cools it
in Summer
26- Air Conditioning (Eastop, Example 4.11)
An air conditioning plant is designed to maintain
a room at 20oC, percentage saturation 50, which
an air supply to the room of 1.8 kg/s at 14oC,
percentage saturation 60. The design outside air
conditions are 27oC,percentage saturation 70.
The plant consists of a mixing chamber for
re-circulated and fresh air, a cooling coil
supplied with chilled water, heating coil, and a
supply fan. The ratio of re-circulated air to
fresh air is 3 the cooling coil has an apparatus
dew point of 5oC, and the refrigeration unit
supplying the chilled water has an overall
coefficient of performance 2. Neglecting all
losses and fan and pump work, calculate
27(i) the total air conditioning load for the
room (ii) the required total energy input (iii)
the required energy input if the energy to the
heating coil is supplied from the refrigeration
plant condenser cooling water.
28(i) the total air conditioning load for the room
29(ii) the required total energy input
30(iii) the required energy input if the energy to
the heating coil is supplied from the
refrigeration plant condenser cooling water.