SNS COLLEGE OF ENGINEERING Coimbatore-107 Subject: Thermal Engineering Refrigeration – applications and types BY SIVAKUMAR P AP/MECH

Refrigeration Refrigeration is a process of cooling by the transfer of heat. Heat is a form of energy and is indestructible so if heat is removed from a space or substance to cool it to a temperature below that of its surroundings, the heat removed must be discarded to some substance at a higher temperature where it is of no consequence. Since heat will not flow freely from a body at a low temperature to another at a higher temperature, it is necessary to expend mechanical work, heat, or electrical energy from an external source to achieve it. Refrigeration thus depends on thermodynamics, heat transfer and fluid flow for its practical achievement.

The refrigerant, if first stored as a liquid under pressure, then allowed to flow at reduced pressure through an evaporator coil in the closed system, will withdraw heat from its surroundings during the evaporation stage. The heat so absorbed is removed from the refrigerated area when the vapour returns to that portion of the refrigeration equipment designed to cool down and compress it again to the liquid state for reuse. The two main refrigeration systems in commercial use are the absorption system and the vapour compression system. Most marine refrigerating plants are of the vapour compression type.

Refrigerating effect The amount of heat absorbed by each unit mass of refrigerant as it flows through an evaporator is known as the refrigerating effect, and is equal to the difference between the enthalpy of the vapour leaving the evaporator and the enthalpy of the liquid at the flow control. Thus, for the system shown in Fig 3, refrigerating effect,

Pressure-enthalpy (P-H) or Mollier diagram Line A to B represents the change from high to low pressure, or expansion process Line B to B’ represents the amount of liquid ‘flashed-off’ in the expansion valve cooling the remaining liquid. Line B to C represents the evaporation process at constant saturation temperature and pressure in the evaporator. At point C the refrigerant is a dry saturated vapour. Line C to C’ represents the superheat absorbed by the dry saturated vapour Line C’ to D represents the compression process. Line D to E represents the superheat given up by the vapour in the condenser. At point E the refrigerant is a dry saturated vapour. Line E to F represents the condensation process at constant saturation temperature and pressure. At point F the refrigerant is a saturated liquid. Line F to A represents the sub cooling of the condensed liquid

Refrigerating capacity The rate at which a system will absorb heat from the refrigerated space or substance is known as the refrigerating capacity, and is expressed as, refrigerating capacity, where m = mass flow of refrigerant through the evaporator (kg/s). To achieve a specified refrigerating capacity of 15OkW, say, the required mass flow rate is

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.

Types Vapour compression Refrigeration system Vapour Absorption Refrigeration system

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