Refrigeration systems
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Refrigeration systems

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Description of refrigeration systems

Description of refrigeration systems

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Refrigeration systems Refrigeration systems Presentation Transcript

  • Refrigeration Systems Elza Marisa P. de Figueiredo quissala@quissala.com
  • Fundamentals • Refrigeration describes the process of removing hear from spaces, objects or materials • Refrigeration means an artificial way of lowering the temperature
  • Mechanical refrigeration • Mechanical system or apparatus that transfers heat from one substance to another
  • Some questions • Why the refrigerant changes from liquid to vapor? • Why it changes from vapor to liquid? • What happens in terms of heat because of these changes of state?
  • Mechanical refrigeration (cont.)
  • Mechanical refrigeration (cont.) • Compressors (generally) use motor-driven rotating impellers to generate gas pressure. – • Gaseous refrigerant enters the compressor at low pressure and temperature and exits at high pressure and temperature. Inside condenser coils gaseous refrigerant condenses to liquid state. – – To facilitate phase change, the condenser dissipates heat energy to ambient air or water. High pressure refrigerant exits at lower temperature.
  • Mechanical refrigeration (cont.) • An expansion valve controls the flow of high pressure liquid refrigerant to the evaporator. – As refrigerant passes through the expansion valve it is further cooled by the Joule Thompson effect, the scientific principle that the temperature of a stream is reduced when forced through a narrow nozzle and allowed to expand.
  • Mechanical refrigeration (cont.) • Inside the evaporator, liquid refrigerant vaporizes into a gaseous state. – Vaporization requires heat energy, which is extracted from the industrial process load (food items to be cooled). – The refrigerant is returned to the compressor to repeat the cycle.
  • How to produce a refrigeration effect? • The material to be cooled needs to be exposed to a colder object or environment • The heat will flow in its natural direction: – From the warmer material to the colder material
  • Main Parts of a Refrigeration System
  • Main Parts of a Refrigeration System (cont.)
  • Expansion Valve • It regulates the amount of refrigerant to the cooling coil – The amount depends on the temperature of the space being cooled • The control bulb controls the opening and closing of the expansion valve – The substance varies
  • Expansion Valve (cont.) • The expansion and contraction transmit a pressure to the diaphragm – Move the diaphagm downward, opening the valve and allowing more refrigerant to enter the cooling coil
  • Expansion Valve (cont.) • When the temperature at the control bulb falls, the pressure above the diaphragm decreases and the valve tends to be close • The temperature near the evaporator outlet controls the operation of the expansion valve
  • Evaporator • It consists of a coil of cooper • The liquid enters the tubing at a reduced pressure and, therefore, with a lower boiling point. • As the refrigerant passes through the evaporator, the heat flowing to the coil from the surrounding air causes the rest of the liquid refrigerant to boil and vaporize.
  • Evaporator (cont.) • Refrigerant temperature in an evaporator is directly related to refrigerant pressure. • After the refrigerant has absorbed its latent heat of vaporization (that is, after it is entirely vaporized), the refrigerant continues to absorb heat until it becomes superheated
  • Evaporator (cont.) • The amount of superheat is determined by the amount of liquid refrigerant admitted to the evaporator. • This, in turn, is controlled by the spring adjustment of the expansion valve. – It increases the efficiency of the plant and evaporates all of the liquid. • This prevents liquid carry-over into the compressor (flooding back).
  • Evaporator (cont.) • Excessive circulation of the lubricating oil will cause the evaporator temperature to increase. • The main cause of slugging is improperly adjusted thermal expansion valve.
  • Compressor • It is essentially a pump. – It is used to pump heat uphill from the cold side to the hot side of the system.
  • Compressor (cont.) • The heat absorbed by the refrigerant in the evaporator must be removed before the refrigerant can again absorb latent heat. – The only way the vaporized refrigerant can be made to give up the latent heat of vaporization that it absorbed in the evaporator is by cooling and condensing it.
  • Compressor (cont.) • Because of the relatively high temperature of the available cooling medium, the only way to make the vapor condense is to compress it.
  • Compressor (cont.) • When we raise the pressure, we also raise the temperature. • The compressor also keeps the refrigerant circulating and maintains the required pressure difference between the high-pressure and lowpressure sides of the system.
  • Compressor (cont.) • Compressors used in refrigeration systems may be lubricated either by splash lubrication or by pressure lubrication. – Refrigeration compressors require a lubricant with a low pour point, and low wax content to keep any oil leaving the compressor from congealing in the evaporator.
  • Compressor (cont.) • The sudden reduction of pressure occurring within the crankcase of a refrigeration compressor during starting causes the release of refrigerant from the oil/refrigerant mixture. • Foaming of the oil in a refrigeration compressor crankcase is caused by refrigerant boiling out of the lube oil.
  • Compressor (cont.) • The oil in the sump of a secured refrigeration compressor is heated to reduce absorption of refrigerant by the oil. • Excessive oil foaming in the crankcase of a refrigeration compressor at start up can cause compressor damage from improper lubrication.
  • Compressor (cont.) • The oil level in a refrigeration compressor, the most accurate reading is obtained immediately after shutdown following a prolonged period of operation. • The refrigerant has had time to separate from the oil.
  • Compressor (cont.) • Refrigerant entering the compressor should be superheated vapor. Its possible to have liquid refrigerant returned to the suction side of a compressor due to a faulty or improperly adjusted expansion valve.
  • Compressor (cont.) • A beam valve is frequently used in refrigeration compressor discharge valves, and is designed to pass liquid slugs.
  • Capacity Control System • Most compressors are equipped with an oil-pressure-operated automatic capacity control system. • This system unloads or cuts cylinders out of operation following decreases in the refrigerant load requirements of the plant. • A cylinder is unloaded by a mechanism that holds the suction valve open so that no gas can be compressed.
  • Shaft Seals • Where the crankshaft extends through the crankcase, a leakproof seal must be maintained to prevent the refrigerant and oil from escaping and also to prevent air from entering the crankcase when the pressure in the crankcase is lower than the surrounding atmospheric pressure. • This is accomplished by crankshaft seal assemblies.
  • Shaft Seals (cont.)
  • Condenser • The compressor discharges the high-pressure, superheated refrigerant vapor to the condenser, where it flows around the tubes through which water is being pumped. • As the vapor gives up its superheat (sensible heat) to the seawater, the temperature of the vapor drops to the condensing point.
  • Condenser (cont.) • The refrigerant, now in liquid form, is sub-cooled slightly below its condensing point. – This is done at the existing pressure to ensure that it will not flash into vapor.
  • A water-cooled condenser
  • Condenser (cont.) • Circulating water is obtained through a branch connection from the fire main or by means of an individual pump taking suction from the sea.
  • Receiver • It acts as a temporary storage space and surge tank for the liquid refrigerant. • It also serves as a vapor seal to keep vapor out of the liquid line to the expansion valve. • A pressure drop in the liquid line of a refrigeration system may cause the liquid refrigerant to flash to gas.
  • Receiver (cont.)
  • Thank you !