REFRIGERATION, HEAT PUMP CYCLES

1
REFRIGERATION, HEAT PUMP CYCLES
2
REFRIGERATORS AND HEAT PUMPS
The transfer of heat from a low-temperature
region to a high-temperature one requires special
devices called refrigerators. Refrigerators and
heat pumps are essentially the same devices they
differ in their objectives only.
for fixed values of QL and QH
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.
3
THE REVERSED CARNOT CYCLE
The reversed Carnot cycle is the most efficient
refrigeration cycle operating between TL and
TH. However, it is not a suitable model for
refrigeration cycles since processes 2-3 and 4-1
are not practical because Process 2-3 involves
the compression of a liquidvapor mixture, which
requires a compressor that will handle two
phases, and process 4-1 involves the expansion of
high-moisture-content refrigerant in a turbine.
Both COPs increase as the difference between the
two temperatures decreases, that is, as TL rises
or TH falls.
Schematic of a Carnot refrigerator and T-s
diagram of the reversed Carnot cycle.
4
HISTORY

• In Egypt (2 century) cooling effect -
  vaporization water
• 1755 - William Cullen produced ice using vacuum
  pumps and phase transformation
• 1777 Walther Hermann Nerst added to water
  H2SO4
• 1834.a. Jacob Perkins the first prototype as
  today we use
• 1844.a. Jon Corienair conditions
• 1864.a. absorber effect, Littman

5
THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE
The vapor-compression refrigeration cycle is the
ideal model for refrigeration systems. Unlike the
reversed Carnot cycle, the refrigerant is
vaporized completely before it is compressed and
the turbine is replaced with a throttling device.
This is the most widely used cycle for
refrigerators, A-C systems, and heat pumps.
Schematic and T-s diagram for the ideal
vapor-compression refrigeration cycle.
6
The ideal vapor-compression refrigeration cycle
involves an irreversible (throttling) process to
make it a more realistic model for the actual
systems. Replacing the expansion valve by a
turbine is not practical since the added benefits
cannot justify the added cost and complexity.
Steady-flow energy balance
An ordinary household refrigerator.
The P-h diagram of an ideal vapor-compression
refrigeration cycle.
7
ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE
An actual vapor-compression refrigeration cycle
differs from the ideal one in several ways, owing
mostly to the irreversibilities that occur in
various components, mainly due to fluid friction
(causes pressure drops) and heat transfer to or
from the surroundings. The COP decreases as a
result of irreversibilities.
DIFFERENCES Non-isentropic compression Superheated
vapor at evaporator exit Subcooled liquid at
condenser exit Pressure drops in condenser and
evaporator
Schematic and T-s diagram for the actual
vapor-compression refrigeration cycle.
8
The Compressor

• The compressor is the heart of the system. The
  compressor does just what its name is. It
  compresses the low pressure refrigerant vapor
  from the evaporator and compresses it into a high
  pressure vapor.

9
The Condenser

• The Discharge Line leaves the compressor and
  runs to the inlet of the condenser.
• Because the refrigerant was compressed, it is a
  hot high pressure vapor (as pressure goes up
  temperature goes up).
• The hot vapor enters the condenser and starts to
  flow through the tubes.
• Cool air is blown across the out side of the
  finned tubes of the condenser (usually by a fan
  or water with a pump).
• Since the air is cooler than the refrigerant,
  heat jumps from the tubing to the cooler air
  (energy goes from hot to cold latent heat).
• As the heat is removed from the refrigerant, it
  reaches its saturated temperature and starts
  to flash (change states), into a high pressure
  liquid.
• The high pressure liquid leaves the condenser
  through the liquid line and travels to the
  metering device. Sometimes running through a
  filter dryer first, to remove any dirt or foreign
  particles.

10
Metering Devices

• Metering devices regulate how much liquid
  refrigerant enters the evaporator .
• Common used metering devices are, small thin
  copper tubes referred to as cap tubes,
  thermally controller diaphragm valves called
  TXVs (thermal expansion valves) and single
  opening orifices.
• The metering device tries to maintain a preset
  temperature difference or super heat, between
  the inlet and outlet openings of the evaporator.
• As the metering devices regulates the amount of
  refrigerant going into the evaporator, the device
  lets small amounts of refrigerant out into the
  line and looses the high pressure it has behind
  it.
• Now we have a low pressure, cooler liquid
  refrigerant entering the evaporative coil
  (pressure went down so temperature goes down).

11
Thermal expansion Valves

• A very common type of metering device is called a
  TX Valve (Thermostatic Expansion Valve). This
  valve has the capability of controlling the
  refrigerant flow. If the load on the evaporator
  changes, the valve can respond to the change and
  increase or decrease the flow accordingly.
• The TXV has a sensing bulb attached to the outlet
  of the evaporator. This bulb senses the suction
  line temperature and sends a signal to the TXV
  allowing it to adjust the flow rate. This is
  important because, if not all, the refrigerant in
  the evaporator changes state into a gas, there
  could be liquid refrigerant content returning to
  the compressor. This can be fatal to the
  compressor. Liquid can not be compressed and when
  a compressor tries to compress a liquid,
  mechanical failing can happen. The compressor can
  suffer mechanical damage in the valves and
  bearings. This is called liquid slugging.
• Normally TXV's are set to maintain 10 degrees of
  superheat. That means that the gas returning to
  the compressor is at least 10 degrees away from
  the risk of having any liquid.

12
The Evaporator

• The evaporator is where the heat is removed from
  your house , business or refrigeration box.
• Low pressure liquid leaves the metering device
  and enters the evaporator.
• Usually, a fan will move warm air from the
  conditioned space across the evaporator finned
  coils.
• The cooler refrigerant in the evaporator tubes,
  absorb the warm room air. The change of
  temperature causes the refrigerant to flash or
  boil, and changes from a low pressure liquid to
  a low pressure cold vapor.
• The low pressure vapor is pulled into the
  compressor and the cycle starts over.
• The amount of heat added to the liquid to make
  it saturated and change states is called Super
  Heat.
• One way to charge a system with refrigerant is by
  super heat.

13
(No Transcript)
14
Refrigerant

• A liquid that has a low boiling point.
• There are several refrigerant manufacturers.
• Heat pumps still use R22 refrigerants. R22
  performs well over
• the range of temperatures that heat pumps operate
  at.
• R22 is known as a hydrochlorofluorocarbon (HCFC)
  refrigerant
• and has an ozone depletion (ODP) factor of 0.05.
• Many heat pumps today use R-407C or R-410A, which
  are hydrofluorocarbons (HFC). Both R-407C and
  R-410A have zero ozone depletion potential (ODP),
  and slightly lower global warming potential (GWP)
  in the case of R-407C, than R-22. R410A has a
  slightly higher GWP than R22.
• Performance (heating capacity and efficiency) is
  about the same with R-407C and about 4 better
  with R410A compared to R-22.
• R-22 will be phased out for new equipment by
  January 1, 2010.

15
SELECTING THE RIGHT REFRIGERANT

• Several refrigerants may be used in refrigeration
  systems such as chlorofluorocarbons (CFCs),
  ammonia, hydrocarbons (propane, ethane, ethylene,
  etc.), carbon dioxide, air (in the
  air-conditioning of aircraft), and even water (in
  applications above the freezing point).
• R-11, R-12, R-22, R-134a, and R-502 account for
  over 90 percent of the market.
• The industrial and heavy-commercial sectors use
  ammonia (it is toxic).
• R-11 is used in large-capacity water chillers
  serving A-C systems in buildings.
• R-134a (replaced R-12, which damages ozone layer)
  is used in domestic refrigerators and freezers,
  as well as automotive air conditioners.
• R-22 is used in window air conditioners, heat
  pumps, air conditioners of commercial buildings,
  and large industrial refrigeration systems, and
  offers strong competition to ammonia.
• R-502 (a blend of R-115 and R-22) is the dominant
  refrigerant used in commercial refrigeration
  systems such as those in supermarkets.
• CFCs allow more ultraviolet radiation into the
  earths atmosphere by destroying the protective
  ozone layer and thus contributing to the
  greenhouse effect that causes global warming.
  Fully halogenated CFCs (such as R-11, R-12, and
  R-115) do the most damage to the ozone layer.
  Refrigerants that are friendly to the ozone layer
  have been developed.
• Two important parameters that need to be
  considered in the selection of a refrigerant are
  the temperatures of the two media (the
  refrigerated space and the environment) with
  which the refrigerant exchanges heat.

16
HEAT PUMP SYSTEMS
The most common energy source for heat pumps is
atmospheric air (air-to- air systems).
Water-source systems usually use well water and
ground-source (geothermal) heat pumps use earth
as the energy source. They typically have higher
COPs but are more complex and more expensive to
install. Both the capacity and the efficiency of
a heat pump fall significantly at low
temperatures. Therefore, most air-source heat
pumps require a supplementary heating system such
as electric resistance heaters or a gas
furnace. Heat pumps are most competitive in areas
that have a large cooling load during the cooling
season and a relatively small heating load during
the heating season. In these areas, the heat pump
can meet the entire cooling and heating needs of
residential or commercial buildings.
A heat pump can be used to heat a house in winter
and to cool it in summer.
17
INNOVATIVE VAPOR-COMPRESSION REFRIGERATION SYSTEMS

• The simple vapor-compression refrigeration cycle
  is the most widely used refrigeration cycle, and
  it is adequate for most refrigeration
  applications.
• The ordinary vapor-compression refrigeration
  systems are simple, inexpensive, reliable, and
  practically maintenance-free.
• However, for large industrial applications
  efficiency, not simplicity, is the major concern.
  
• Also, for some applications the simple
  vapor-compression refrigeration cycle is
  inadequate and needs to be modified.
• For moderately and very low temperature
  applications some innovative refrigeration
  systems are used. The following cycles are
  generally employed
• Cascade refrigeration systems
• Multistage compression refrigeration systems
• Multipurpose refrigeration systems with a single
  compressor
• Liquefaction of gases

18
Gas for Heat Pumps

• Heat pumps fired by natural gas have been
  commercially developed.
• One type uses the absorption cycle, where the
  energy for refrigerant compression is provided by
  a gas burner.
• Another variation is the engine-driven heat pump
  cycle. Here a natural gas engine is used to drive
  the compressor. During operation, heat is
  recovered from the engine jacket cooling water
  and engine exhaust.
• Gas heat pumps are less common than electric
  heat pumps.
• Performance compared to electric heat pumps is
  lower, with lower COPs for both absorption and
  engine-driven units than for conventional
  electric heat pumps.
• They promise to reduce global warming through
  more efficient conversion of natural gas and
  reduced emissions from electric power plants as
  they do not use electricity to drive the heat
  pump.

19
Gas engine driver
20
District heating and heat pump
Oslo