Refrigeration Cycles

Learning Outcomes Demonstrate understanding of basic vapor-compression refrigeration and heat pump systems. Develop and analyze thermodynamic models of vapor-compression systems and their modifications, including sketching schematic and accompanying T-s diagrams. evaluating property data at principal states of the systems. applying mass, energy, entropy, and exergy balances for the basic processes. determining refrigeration and heat pump system performance, coefficient of performance, and capacity.

Learning Outcomes Explain the effects on vapor-compression system performance of varying key parameters. Demonstrate understanding of the operating principles of absorption and gas refrigeration systems, and perform thermodynamic analysis of gas systems.

History of Refrigeration  First successful commercial refrigeration system was produced in 1857 by James Harrison and D.E. Siebe using ethyl ether as refrigerant.

…..how does refrigeration work?

Let’s try to define… Refrigeration makes thing cold....

Refrigeration is cooling of a system below the temperature of the surroundings

Refrigeration is REMOVAL OF HEAT QL TH >> TL QL TH TL Refrigeration is REMOVAL OF HEAT

1000C -100C QL QL TH >> TL TH TL 1100C 00C Water at pressure…? at atmospheric condition QL QL TH >> TL TH TL 1100C 00C

Saturated Temperature (0C) Pressure (bar) Saturated Temperature (0C) 1 100 0.75 91.78 0.5 81.33 0.25 64.97 0.1 45.81 0.075 40.29 0.05 32.88 0.025 21.08 0.01 6.98

Boiling point temperature of some Important Refrigerants at atmospheric condition Freon – 12 -300C Freon – 22 -400C R-134a -260C Ammonia -320C

R-X W 350C QH vapour, pH, 450C liquid, pH, 450C 450C Condenser vapour, pH, 450C liquid, pH, 450C 450C Compressor R-X W Capillary tube vapour, patm, -300C Vapour, plow, -300C Liquid, patm, -300C Liquid, plow, -300C -300C QL Evaporator QL TH >> TL TL -200C 350C TH

Refrigeration is Removal & Relocation of Heat

Basic components & typical operating conditions:

Refrigeration -vs- Heat Pump

Searching for Ideal Cycle….

Searching for Ideal Cycle….

Compare the COP of CARNOT refrigerator in summer and winter. Comparing the CARNOT Refrigeration Cycles…. Assignment: 01 Compare the COP of CARNOT refrigerator in summer and winter. TH/high TH/low TL TL

Comparing the CARNOT Refrigeration Cycles…. Assignment: 02 Compare the COP of CARNOT refrigerator & air conditioner in same weather condition. TH TL/high TL/low

Vapor-Compression Refrigeration Cycle Most common refrigeration cycle in use today Two-phase liquid-vapor mixture There are four principal control volumes involving these components: Evaporator Compressor Condenser Expansion valve All energy transfers by work and heat are taken as positive in the directions of the arrows on the schematic and energy balances are written accordingly.

Vapor-Compression Refrigeration Cycle The processes of this cycle are: Process 4-1: Two-phase liquid-vapor mixture of refrigerant is evaporated through heat transfer from the refrigerated space. Process 1-2: vapor refrigerant is compressed to a relatively high temperature and pressure requiring work input. Process 2-3: vapor refrigerant condenses to liquid through heat transfer to the cooler surroundings. Process 3-4: liquid refrigerant expands to the evaporator pressure. Two-phase liquid-vapor mixture

Vapor-Compression Refrigeration Cycle Engineering model: Each component is analyzed as a control volume at steady state. Dry compression is presumed: the refrigerant is a vapor. The compressor operates adiabatically. The refrigerant expanding through the valve undergoes a throttling process. Kinetic and potential energy changes are ignored.

Vapor-Compression Refrigeration Cycle  Operations in Idealized VCRC: Evaporator: Isobaric Heat Absorption Compressor: Isentropic compression Condenser: Isobaric Heat Rejection Expansion valve: Iso-enthalpic throttling

Vapor-Compression Refrigeration Cycle VCRC in Domestic Refrigerator and Air Conditioner:

Vapor-Compression Refrigeration Cycle  Operations in Idealized VCRC: TH All processes of the cycle shown in figure are internally reversible except for the throttling process. Despite the inclusion of this irreversible process, the cycle is commonly referred to as the ideal vapor-compression cycle. TH TC TC

Vapor-Compression Refrigeration Cycle  Operations in Idealized VCRC: TH TC

Vapor-Compression Refrigeration Cycle  p-h diagram for Water :

Vapor-Compression Refrigeration Cycle Applying SFEE Evaporator Compressor Assuming adiabatic compression Condenser Expansion valve Assuming a throttling process

Vapor-Compression Refrigeration Cycle Performance parameters Coefficient Of Performance (COP): Maximum theoretical COP: Refrigerating Effect / Refrigeration Capacity:

Vapor-Compression Refrigeration Cycle Performance parameters Refrigerating Effect: Ton of Refrigeration = 3.5 kJ/sec 1 ton of refrigeration relates that rate of heat removal from the refrigerated space by the cycle which is able to freeze a ton (2000 lb) of water at 0°C to ice at 0°C in 24 hours. Tonne of Refrigeration = 14000 kJ/hr = 3.86 kJ/sec

Vapor-Compression Refrigeration Cycle Example: Refrigerant 134a is the working fluid in an ideal vapor-compression refrigeration cycle where saturated vapor enters the compressor at -100C and saturated liquid leaves the condenser at 260C. The mass flow rate of the refrigerant is 0.08 kg/s. Determine the compressor power, in kW, the refrigeration capacity, in tons, the coefficient of performance, and the coefficient of performance of a Carnot refrigeration cycle operating between warm and cold regions at 16 and 00C, respectively.

Actual Vapor-Compression Cycle

Actual Vapor-Compression Cycle

Actual Vapor-Compression Cycle Introducing Suction Line Heat Exchanger

Example: 02 A vapour compression cycle works on Freon-12 refrigerant with condensation temperature of 40ºC and evaporator temperature of – 20ºC. Refrigeration effect of 2.86 ton is desired from the cycle. The compressor runs with 1200 rpm and has clearance volume of 2%. Considering compression index of 1.13 and following data determine. (a) the COP (b) the piston displacement in the reciprocating compressor used for compression. Properties of Freon - 12

Specific heat of liquid refrigerant = 1.055 kJ/kg K Example: 03 A commercial refrigerator operates with R-12 between 1.2368 bar and 13.672 bar. The vapour is dry and saturated at the compressor inlet. Assuming isentropic compression, determine the theoretical COP of the plant. The isentropic discharge temperature is 64.860C. If the actual COP of the plant is 80% of the theoretical, calculate the power required to run the compressor to obtain a refrigeration capacity of 1.5 TR. If the required sub cooled through 100 C after condensation, calculate the power required. The properties of R-12 are given below: Specific heat of liquid refrigerant = 1.055 kJ/kg K Sat Temp (0C) Saturation pressure(bar) Enthalpy(kJ/kg) Entropy(kJ/kgK) Liquid Vapour -25 1.237 13.33 176.48 0.0552 0.7126 55 13.672 90.28 207.95 0.3197 0.6774

Halocarbon compounds: Halocarbon group includes refrigerants containing one or more of three halogens such as chlorine, fluorine and bromine. These refrigerants are traded in market under the brand names of Freon, Genetron, Isotron and Arctron. Halocarbons, specially chlorine containing halocarbons (chloro fluoro carbons, CFCs) were commonly used from about 1940 to 1990s in most of vapour compression systems. But due to disastrous effect of chlorine in refrigerant upon the earth’s protective ozone layer the efforts are being made to replace the use of CFCs by the class of refrigerants having hydrogen in place of chlorine. Such new ecofriendly refrigerants are called hydrofluoro carbons (HFCs) for example R–134a (CF3 CH2F) can replace R-12 (CCl2F2).

Halocarbon compounds: REFRIGERANT Halocarbon compounds: Inorganic compounds: Name Chemical name Chemical formula R11 Trichloro mono fluoro-methane CCl3F R12 Dichloro difluoro-methane CCl2F2 R22 Monochloro difluoro-methane CHClF2 R134a Tetra Fluoro ethane CF3 CH2F

REFRIGERANT DESIRED PROPERTIES OF REFRIGERANTS (i) Low Boiling temperature at atmospheric conditions. (ii) Low freezing temperature at evaporator pressure. (iii) Higher Critical temperature than the condenser temperature. (iv) Large latent heat at evaporator temperature (v) Small specific volume at compressor inlet. (vi) Large Specific heat of refrigerant in vapour form. (vii) Thermal conductivity of refrigerant should be high. (viii) Viscosity of refrigerants should be small (ix) Refrigerant should be chemically inert and non toxic. (x) Refrigerant should be non-flammable. (xi) Refrigerant may have pleasant distinct odour so as to know about its leakage. (xii) Refrigerant should be readily available at lesser price.

DESIRED PROPERTIES OF REFRIGERANTS (i) Boiling temperature of refrigerant should be quite low at atmospheric conditions for effective refrigeration. For refrigerants having higher boiling temperatures at atmospheric conditions the compressor is run at higher vacuum. (ii) For an ideal refrigerant the freezing temperature of refrigerant should be quite low so as to prevent its freezing at evaporator temperature. Freezing point temperature should be less than evaporator temperature. For example, refrigerant R–22 has freezing point of – 160ºC and normally most of refrigerants have freezing point below – 30ºC. (iii) Critical temperature of the ideal refrigerant should be higher than the condenser temperature for the ease of condensation. (iv) Refrigerant should have large latent heat at evaporator temperature as this shall increase the refrigerating capacity per kg of refrigerant. (v) Refrigerant should have small specific volume at inlet to compressor as this reduces compressor size for same refrigeration capacity. (vi) Specific heat of refrigerant in liquid form should be small and it should be large for refrigerant in vapour form, since these increase the refrigerating capacity per kg of refrigerant. (vii) Thermal conductivity of refrigerant should be high. (viii) Viscosity of refrigerants should be small for the ease of better heat transfer and small pumping work requirement. (ix) Refrigerant should be chemically inert and non toxic. (x) Refrigerant should be non-flammable, non explosive and do not have any harmful effect upon coming in contact with material stored in refrigeration space. (xi) Refrigerant may have pleasant distinct odour so as to know about its leakage. (xii) Refrigerant should be readily available at lesser price.

Selecting Refrigerants Refrigerant selection is based on several factors: Performance: provides adequate cooling capacity cost-effectively. Safety: avoids hazards (i.e., toxicity). Environmental impact: minimizes harm to stratospheric ozone layer and reduces negative impact to global climate change.

Example: 04 Find the compressor power required per TR. If actual COP is 85% of the ideal, find compressor input for 3 ton capacity. If actual COP is 75% of the ideal, find capacity of cycle for 4kW compressor rating. Find the effect of superheating on refrigeration effect & COP.

A vapour compression cycle works on Freon-12 refrigerant with condensation temperature of 40ºC and evaporator temperature of – 20ºC. Refrigeration effect of 2.86 ton is desired from the cycle. The compressor runs with 1200 rpm and has clearance volume of 2%. Considering compression index of 1.13 and following data determine. (a) the COP (b) the piston displacement in the reciprocating compressor used for compression. Properties of Freon - 12

Refrigerant Types and Characteristics Global Warming Potential (GWP) is a simplified index that estimates the potential future influence on global warming associated with different gases when released to the atmosphere.

Refrigerant Types and Characteristics Chlorofluorocarbons (CFCs) and Hydrochlorofluorocarbons (HCFCs) are early synthetic refrigerants each containing chlorine. Because of the adverse effect of chlorine on Earth’s stratospheric ozone layer, use of these refrigerants is regulated by international agreement. Hydrofluorocarbons (HFCs) and HFC blends are chlorine-free refrigerants. Blends combine two or more HFCs. While these chlorine-free refrigerants do not contribute to ozone depletion, with the exception of R-1234yf, they have high GWP levels. Natural refrigerants are nonsynthetic, naturally occurring substances which serve as refrigerants. These include carbon dioxide, ammonia, and hydrocarbons. These refrigerants feature low GWP values; still, concerns have been raised over the toxicity of NH3 and the safety of the hydrocarbons.