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Home , Freon, Latent heat

Principles of Food and Bioprocess Engineering (FS 231) Foods are refrigerated to slow down the reactions that cause spoilage. On an average, reaction rates are reduced by half when the is reduced by 10 /C. When comes in contact with a product, ice melts, thereby absorbing the latent of fusion / (334.94 kJ/kg at 1 atm, 0 C). When dry ice ( CO2) is exposed to atmospheric conditions, it sublimates, thereby absorbing the of sublimation (620 kJ/kg at 1 atm, -78.5 /C).The drawback of using ice or dry ice as the means of refrigeration is that they cannot be re-used.

R-717 or (NH3) was a commonly used . It is not commonly used these days because of its toxicity. Its point is -33.3 /C at atmospheric . To raise its to 0 /C, the pressure must be increased to 430.43 kPa. Its point is -77.8 /C at atmospheric pressure. Its latent heat of is 1,314.2 kJ/kg at -15 /C. Pressure- Tables for R-717 are on pages 604 & 605 of textbook; Pressure-Enthalpy diagram is on page 807 of textbook.

R-12 or 12 (CCl22F ) was another commonly used refrigerant. It is not commonly used these days since it causes depletion of the ozone layer. Its boiling point is -29.8 /C at atmospheric pressure. To raise its boiling point to 0 /C, the pressure must be increased to 308.61 kPa. Its freezing point is -157.8 /C at atmospheric pressure. Its latent heat of vaporization is 161.7 kJ/kg at -15 /C. Pressure-Enthalpy Tables for R-12 are on pages 800 & 801 of textbook; Pressure- Enthalpy diagrams are on pages 799 & 803 of textbook.

R-134a or 1,1,1,2-Tetrafluoroethane (CF32CH F) is a refrigerant that is being used nowadays since it does not have the problems associated with ammonia or R-12. Its boiling point is -26.16 /C at atmospheric pressure. To raise it boiling point to 0 /C, the pressure must be increased to 292.769 kPa. Its freezing point is -96.6 /C at atmospheric pressure. Its latent heat of vaporization is 209.5 kJ/kg. Pressure-Enthalpy Tables for R-134a are on pages 808 - 810 of textbook and Pressure- Enthalpy diagrams are on pages 811 & 812 of textbook.

The designation of a refrigerant derived from a hydrocarbon CmnH FpClq is: R(m-1)(n+1)(p)

Some of the other commonly used refrigerants are R-11 (CCl33F), R-13 (CClF ), R-14 (CF4), R-22

(CHClF2), R-30 (CH22Cl ), R-113 (C23Cl F3), R-114, R-115, R-116, R-123, R-404A, R-408A, R-

409A, R-500, R-502, and R-744 (CO2). Freon systems operate at a lower pressure difference within the system and hence maintenance problems are minimized. When operating at an temperature of -15 /C and a condenser temperature of 30 /C, the pressure of the refrigerant at the evaporator and condenser are 236.5 kPa and 1,166.5 kPa respectively for Ammonia; 182.7 kPa and 744.6 kPa respectively for Freon; 164.0 kPa and 770.13 kPa for R-134a. Ammonia is toxic and flammable unlike Freon. Steel tubes are used for ammonia systems and copper tubes are used for Freon systems. The advantage of using ammonia is that it has a much higher latent heat of vaporization. 1 ton of refrigerant = required to melt 1 ton (2,000 lb) of ice in 1 day = 3516.8 Watts. Criteria for selecting a refrigerant: High latent heat of vaporization, low condensing pressure, low freezing temperature, high critical temperature, low vaporization temperature, low toxicity, low flammability, low corrosiveness, high chemical stability, easy detection of leaks, low cost, low environmental impact, good miscibility with oil, easy separability from . Compression Refrigeration Cycle

Evaporation : Constant pressure process – ( + Vapor) to (Vapor) Compression : Constant process – (Vapor) to (Vapor) : Constant pressure process – (Vapor) to (Liquid) Expansion : Constant enthalpy process – (Liquid) to (Liquid + Vapor) ( -- no heat crosses the boundary) Evaporator: In the evaporator, the liquid refrigerant vaporizes to gaseous state. Change of requires latent heat, which is extracted from the surroundings. Direct expansion allow the refrigerant to vaporize inside the evaporator coils which are in direct contact with the object being cooled. Indirect expansion evaporators involve the use of a medium (water/brine) which is cooled by vaporization of the refrigerant and then pumped to the object being cooled. Indirect expansion requires additional equipment (pump) and is used when cooling is required in several locations. In a practical refrigeration cycle, the refrigerant entering the is in superheated state since any liquid in the compressor could result in the liquid refrigerant being trapped in the head of the cylinder by the rising piston, which may damage the valves or cylinder head. Also, droplets of liquid refrigerant may wash away the lubricating oil from the walls of the cylinder, thus accelerating wear. Common types of evaporators are plate evaporator (coil brazed onto a plate) and flooded evaporator (coil is filled only with the liquid refrigerant). Compressor: The refrigerant enters the compressor in vapor state at low pressure and temperature. The compressor raises the temperature (well above ambient temperature to promote to surroundings) and pressure of the refrigerant. The 3 common types of are rotary (low capacity, +ve displacement), reciprocating (intermediate capacity ~ 100 tonnes, +ve displacement), and centrifugal (high capacity). When there is more than a 10 to 1 change in pressure, booster compressors are used in addition to the primary compressor. Condenser: The condenser transfers heat from the refrigerant to another medium (air/water). By rejecting heat, the gaseous refrigerant condenses to liquid. The 3 major types of condensers are -- air-cooled, water-cooled, and evaporative. A receiver (part of condenser or separate) is usually used in conjunction with a condenser. When the system is shut off, the liquid refrigerant is stored in the receiver. A small degree of sub-cooling of refrigerant is done in the condenser to reduce the amount of vapor formed in the expansion valve. It is desirable to have only very little vapor at the exit of the expansion valve since the vapor part of the refrigerant does not serve any purpose in cooling a product as it goes through the evaporator. The formation of also causes bubbling and impedes flow of the refrigerant through the expansion valve. Expansion valve: The expansion valve is a metering device that controls the flow of refrigerant to the evaporator. It also serves the function of expanding the liquid from the condenser pressure to the evaporator pressure. The refrigerant cools as it passes through the expansion valve. This partial conversion of the liquid refrigerant to as it passes through the expansion valve is called flashing. The 5 main types of metering devices are -- manually operated expansion valve, automatic low-side float valve, automatic high-side float valve, automatic expansion valve, and thermostatic expansion valve (capillary tube -- smaller unit, throttle valve -- larger unit). Vapor-Compression Refrigeration System

The above pressure-enthalpy diagram depicts the system operating under “ideal” (a-b-d-e-a loop) and “non-ideal” or “real” (a’-b’-d’-e’-a’ loop) conditions. Constant temperature lines are horizontal within the dome, vertical in the sub-cooled region, and curved downwards in the superheated region. For both ideal and non-ideal conditions, the following equations are valid:

Compressor: done on refrigerant by compressor = Qw3 = (H - H2)

Condenser: released by refrigerant in condenser = Qc3 = (H - H1)

Evaporator: Energy absorbed by refrigerant in evaporator = Qe2 = (H - H1) = Rate

Note: Qewc + Q = Q

For non-ideal conditions: Degree of in evaporator = Difference in at points a’ and a Degree of sub-cooling in condenser = Difference in temperatures at points d and d’