ISAT 413 - Module III: Building Energy Efficiency

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ISAT 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

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• 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.
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Cooling Tower (Evaporative Cooling)
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Data 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.
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(i) the required mass flow rate of air induced by
the fan for the given cooling duty
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• Vapor-Compression Refrigeration

P-h Diagram of an Ideal Vapor-Compression
Refrigeration Cycle
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Schematic and T-s Diagram for Ideal
Vapor-Compression Refrigeration Cycle
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Schematic and T-s Diagram for Actual
Vapor-Compression Refrigeration Cycle
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• The performance of refrigerators and heat pumps
  is expressed in terms of coefficient of
  performance (COP), defined as

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• 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

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• 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.

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• 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.

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• 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.

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• 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?
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Working 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
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Some 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.)
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• Vapor-Absorption Refrigeration

Ammonia Absorption Refrigeration Cycle
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• 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

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• 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.

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• Other Refrigeration Systems

Schematic of Simple Thermoelectric Power Generator
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A Thermoelectric Refrigerator
10-13

• (Fig. 10-28)

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• 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.

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Refrigerator and Heat Pump Objectives

• The Heat Pump

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.
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Heat Pump Heats a House in Winter and Cools it
in Summer
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• 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
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(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.
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(i) the total air conditioning load for the room
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(ii) the required total energy input
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(iii) the required energy input if the energy to
the heating coil is supplied from the
refrigeration plant condenser cooling water.