Introduction to Vapor Compression Refrigeration Systems – Part II

ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

ME340A: Refrigeration and Air Conditioning Department of Mechanical Engineering Instructor: Prof. Sameer Khandekar Indian Institute of Technology Kanpur Tel: 7038; e-mail: [email protected] Kanpur 208016 India Introduction to Vapor Compression Refrigeration Systems – Part II

Sameer Khandekar Sir M. Visvesvaraya Chair Professor Department of Mechanical Engineering Indian Institute of Technology Kanpur Kanpur (UP) 208016 INDIA

Sameer Sameer Khandekar Webpage: home.iitk.ac.in/~samkhan/ 1

In this presentation…

◉ Performance measurement ◉ Ideal vapor compression system ◉ Real/Practical system ◉ Refrigerators and Heat Pumps

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 1 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Measuring performance of 1 refrigeration systems Rating, Carnot cycle, Ideal and real cycles

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Refrigeration effect

Refrigerating effect (N): It is defined as the quantity of heat extracted from a cold body or space to be cooled in a given time. N = Heat extracted from the cold space/(time taken)

Note: 1 Metric ton = 1000 kg (also referred to as ‘tonne’, to distinguish it with a ‘short ton’ 1 Short ton = 2000 pounds = 907 kg (in US a short ton is called as ‘ton’)

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 2 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Refrigeration Capacity or Tonnage rating

◉ Capacity of refrigerating machines are expressed by their cooling capacity. The commonly used unit for expressing the capacity of a refrigerating machine is Ton of refrigeration. ◉ One ton of refrigeration is defined as the quantity of heat extracted (refrigerating effect) to freeze one metric ton of water at 0°C into ice in 24 hours. ◉ Latent heat of ice = 334 kJ/kg i.e., 334 kJ of heat should be extracted from one kg of water at 0°C to convert it into ice. ◉ One ton of refrigeration = 334 x 1000 kJ/24 hrs. = 336 x 1000/(24 X 60 X 60) kJ/s = 3.86 kW (or = 3.5 kW if we use a short ton)

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Coefficient of performance

◉ How to estimate the efficiency of the system?

for fixed values of QL and QH

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 3 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

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 liquid–vapor mixture, which requires a compressor that will handle two phases, and, Schematic of a Carnot refrigerator and T- ○ Process 4-1 involves the expansion of s diagram of the reversed Carnot cycle. high-moisture-content refrigerant in a turbine.

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COP of reversed Carnot cycle

◉ 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.

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 4 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

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

Sameer Sameer Khandekar vapor-compression refrigeration cycle. 9

Ideal vapor compression cycle

◉ 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.

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 5 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Ideal vapor compression refrigeration cycle

Steady-flow energy balance

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A refrigeration cycle drawn on T-s diagram

1-2: Compression 2-3: Condenser 3-4: Expansion 4-5: Evaporator

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 6 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Refrigeration cycle on a P-v diagram

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Example: A household refrigerator

General Electric: 1927 Sulphur dioxide/ Methyl formate) Modern: 21st century Fridge advert of 1905

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 7 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Example: A household refrigerator

◉ −18 °C (0 °F) (freezer) ◉ 0 °C (32 °F) (meat zone) ◉ 5 °C (41 °F) (cooling zone) ◉ 10 °C (50 °F) (crisper)

An ordinary household refrigerator.

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Actual vapor compression 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. Schematic and T-s diagram of actual vapor compression refrigeration cycle.

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 8 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Actual vapor compression cycle

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 of actual vapor compression refrigeration cycle.

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Practical refrigeration cycle

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 9 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Practical refrigeration cycle

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Practical refrigeration cycle

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 10 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

2 The Heat Pump Extracting heat from cold space

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A heat pump

◉ A heat pump is a device that transfers heat energy from a “source” of heat to a "heat sink".

◉ Heat pumps move thermal energy in the opposite direction of spontaneous heat transfer, by absorbing heat from a cold space (usually atmospheric air/ground water) and releasing it to a warmer one (conditioned space/house).

◉ A heat pump uses a small amount of external power to accomplish the work of transferring energy from the heat source to the heat sink.

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 11 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

A heat pump

◉ When a heat pump is used for heating, it employs the same basic refrigeration cycle used by an air conditioner or a refrigerator, but in the opposite direction – releasing heat into the conditioned space rather than the surrounding environment. ◉ In heating mode, heat pumps are three to four times more effective at heating than simple electrical resistance heaters using the same amount of electricity. ◉ However, the typical cost of installing a heat pump is also higher than that of a resistance heater.

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A heat pump

◉ 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.

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 12 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Same machine – cooling/heating

◉ A heat pump can be used to heat a house in winter and to cool it in summer.

◉ 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.

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Reversing valve design

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 13 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Winter/Summer operation of the same machine

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Practical layouts

Between atmospheric air to room air

Between ground and room air

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 14 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Practical layouts: Geothermal heat pump

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3 Gas Refrigeration Cycle Reversing the Brayton Cycle…

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 15 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Brayton cycle

◉ In general, the Brayton cycle describes the workings of a constant- pressure heat engine. Today, modern gas turbine engines and air breathing jet engines are also a constant-pressure heat engines

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Reversing the Brayton cycle

◉ A Brayton cycle that is driven in reverse direction is known as the reverse Brayton cycle. It is similar to the ordinary Brayton cycle but it is driven in reverse, via net work input. ◉ This cycle is also known as the gas refrigeration cycle or Bell Coleman cycle. ◉ It is widely used in jet aircrafts for air conditioning systems using air from the engine compressors. ◉ It is also widely used in the LNG industry where the largest reverse Brayton cycle is for subcooling LNG using of the order of 100 MW of power from a gas turbine-driven compressor and nitrogen refrigerant.

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 16 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Gas refrigeration cycles

◉ The reversed Brayton cycle (the gas refrigeration cycle) can be used for refrigeration.

Simple gas refrigeration cycle.

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Gas refrigeration cycles

◉ The gas refrigeration cycles have lower COPs relative to the vapor-compression cycles or the reversed Carnot cycle. ◉ The reversed Carnot cycle consumes a fraction of the net work (area 1A3B) but An open-cycle aircraft cooling system. produces a greater amount of refrigeration (triangular area under B1). ◉ Despite their relatively low COPs, the gas refrigeration cycles involve simple, lighter components, which make them suitable for aircraft cooling, and they

Sameer Sameer Khandekar can incorporate regeneration 34 Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 17 ME340A: Prof. Sameer Khandekar http://home.iitk.ac.in/~samkhan

Gas refrigeration cycles

◉ Without regeneration, the lowest turbine inlet temperature is T0, the temperature of the surroundings or any other cooling medium. ◉ With regeneration, the high- pressure gas is further cooled to

T4 before expanding in the turbine. Gas refrigeration cycle with regeneration.

◉ Lowering the turbine Tinlet Extremely low temperatures can automatically lowers the turbine be achieved by repeating Texit, which is the minimum regeneration process.

Sameer Sameer Khandekar temperature in the cycle. 35

Any questions ?

You can find me at ◉ [email protected]

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Department of Mechanical Engineering Indian Institute of Technology Kanpur 208016 Kanpur India 18