A refrigeration compressor is used in a multi-box refrigeration system, having six of its eight cylinders controlled for variable load conditions. Four Types of Compressors. Back pressure valves are usually fitted at warmer rooms where the temperature is set at 4°C to 5°C or higher for eg. E-mail. Some oil always gets carried over with the compressed refrigerant gas and must be removed. Unloader mechanism works by the release of oil pressure from compressor crankcase oil pump via a solenoid valve to the compressor unloader. The high-pressure cutout switch stops the compressor motor at a pressure of about 90% of the maximum working pressure of the system. A mix of vapor and liquid refrigerant entering into compressor suction may cause severe damage to the compressor. The separation of oil is mechanical by slowing down and changing the direction of the gas/oil stream mixture. When fitted to compressor discharge lines, they are set to stop the compressor if the discharge temperature is too high. Therefore to avoid such damage, internal and external motor protection devices shuts off the motor power supply in case of any fault. The motor rotates the crankshaft, which then pushes the pistons. Expansion valve helps to maintain an appropriate amount of refrigerant in the high-pressure side and low-pressure side of the system. View Answer. The rate of heat absorption by the evaporator differs from different cargoes carried and the outside air temperature. Thermostatic expansion valve acts as a regulator where the refrigerant is metered from the high-pressure side to the low-pressure side of the system. This means that it is always necessary to control the compressor capacity so that it matches the actual heat load. For the reciprocating units, this is carried out by using unloader push pins to keep the suction valve lifted from their seats. Effective lubrication can be done For vapor-compression refrigeration systems, the pressure ratio of compressor sharply increases as the evaporating temperature decreases. The hermetic compressor has a direct drive with no coupling and no mechanical seal. Thermostats are also used to control the temperature in a refrigerated space by cycling the compressor ‘on and off ’ and by ‘opening and closing’ a solenoid valve in the liquid line. The refrigerating compressor crankcase has refrigerant under suction pressure. It reduces leakage of refrigerant 4. These refrigerants take on different forms as they change temperature and pressure. Modifying the conventional cycle configuration can offer a less complex and less costly alternative approach. The drawback of this system can be the evaporator unable to provide required superheat to the refrigerant entering at the suction of the compressor. R744 compressor refrigeration systems with two-phase ejectors as expansion devices have attracted much research activity in recent years because of the transcritical character of the refrigeration cycle. Back pressure valve may sometimes be fitted into the system to hold back a high evaporator pressure, where two to three evaporator outlet feeds into a common compressor suction line. Oh my, wow, there is a lot of detail about the components and safety devices on HVAC refrigeration system. One reason for this could be the a] system is low on refrigerant b] high pressure cut out is improperly adjusted c] discharge valves are leaking slightly d] discharge valves are leaking excessively. Differential lube oil pressure switch is used to protect against, too low oil pressure in forced lubrication systems. An indication of only liquid means the system is correctly working while the presence of any gas bubbles means the system is getting short of refrigerant. The coupling can allow some amount of flexibility during miss alignment of shafts. Commercial refrigeration compressors are divided into three main categories: If all of the reefer boxes are recently activated, what percentage of the total number of compressor cylinders will be unloaded? 16 Multipurpose refrigeration systems A refrigerator with a single compressor can provide refrigeration at several temperatures by throttling the refrigerant in stages. Cold refrigerant leaving the evaporator coil outlet can be used to under cool the warm liquid refrigerant leaving from the condenser outlet by using a heat exchanger as shown in the diagram known as suction to the liquid heat exchanger. Refrigerants, are chemical compounds that are alternately compressed and condensed into a liquid and then permitted to expand into a vapor or gas as they are pumped through the mechanical refrigeration system to cycle. Basic vapour compression refrigeration cycle consists of four major thermal processes; evaporation, compression, condensation and expansion .According to the requirement, vapour compression system is further modified for better performance and control. Couplings are used to connect large compressor shaft with the compressor motor shaft, a driving force in these large units are very high. The purpose of the compressor in the refrigeration cycle is to accept the low-pressure dry gas from the evaporator and raise its pressure to that of the condenser. View Answer. The oil separated from the refrigerant gets collected in the bottom of the separator and again returned to the crankcase or receiver through an automatic float valve. Here at stage two the temperature is lowered and then is passed to the stage three. The oil pressure must be greater than suction pressure for the oil to flow out of the bearings. The control is normally set to stop the compressor at a pressure corresponding to a saturation temperature of 5°C or 41°F below the lowest evaporating temperature. Nusselt number (NN) is given by. Purpose of … The working of Vapor Compression Refrigeration System can be completed under 4 processes and are as follows. A system of refrigeration with the direct evaporation can be either with the pump supply of refrigerant or without it. Compressor Selection Criteria Evaporator Operating Conditions Refrigerant Type Ambient Conditions Required Refrigeration Capacity Compression Ratio Flow Rate through the compressors of the designed system (Free Air Delivery, FAD) Compressor Cooling System Total Required Power Input 4. The electric motor is directly connected to the compressor with a single shaft avoiding the use of any coupling or mechanical seal and leaving no chance of refrigerant leaking into the atmosphere. The refrigerant and compressor oil inside the compressor housing is totally in contact with the motor rotor and stator windings. You perfectly explain each thing to me and I recommend this to be read. Hot gas bypass valves used in compressors do not have a capacity reduction device like compressor unloader. Volumetric efficiency of compressor increases 3. It creates back pressure on the evaporator coil and ensures that most of the liquid refrigerant is made available at lower temperature zones like meat or fish room. The solid desiccant is silica gel, activated alumina, zeolites, titanium dioxide whereas the commercial solid desiccant is activated carbons, metal oxides and specially developed porous metal hydrides. This acidic solution dissolves copper tubings and extracts copper from copper-based alloys like brass or bronze present in different parts of an air conditioning system. It is a constant pressure valve, balancing between the suction pressure against the pre-set spring force. Skematic of Compression Refrigeration System: EXPLANATION OF HOW IT WORKS/ IS USED: Refrigerant flows through the compressor, which raises the pressure of the refrigerant. Comment * Related Questions on Refrigeration and Air Conditioning. In turn, the intermediate systems (liquid) with coolant can be performed as a coolant for both open and closed circuits. After the … Lub oil pressure is set at 2 bar above the suction pressure. Unloading mechanism works by raising the suction valve at the open position so that the gas moves freely in and out through the valve without compression. Three types of elements are used to sense and relay temperature changes to the electrical contacts. A comparative study has been carried out of R-12, 22, 125, 134a, 152a, 218, 245, 500, 502, 507 and 717 as working fluids in a vapour-compression refrigeration system. How- ever, the complexity and cost of these improvements can block their application in the market. 3. When the coil gets energized, the diaphragm valve plate moves up into the open position and vice versa when the coil is de-energized. Other methods of capacity control include varying the compressor speed, and ‘hot gas bypass, which involves passing a proportion of the discharge gas from the compressor directly to the evaporator and bypassing the condenser. Vegetable storeroom or lobby. Therefore, to maintain their rigidity and minimize deflection, rotor profiles usually have a relatively small male rotor addendum in order to increase the female root diameter. Sometimes cargo/stores are freshly located in a warm climate, the cooling load on the system increases significantly. Next the refrigerant flows through the condenser, where it condenses from vapor form to liquid form, giving off heat in the process. Each stage operates on an ideal vapor-compression refrigeration cycle stopping and starting of the compressor to maintain the desired pressure and temperature. Refrigeration is the process of removing heat from a closed body or space enclosed so that its temperature is first lowered and then maintained at a required level which is below the temperature of surroundings. With a parallel compressor, no additional adjustments are necessary, which is a major advantage of this refrigeration cycle. The load controller senses the temperature and controls the capacity of the compressor by off-loading or cutting out one of the compressor unit. A general feature of screw compressors is that the pressure difference through them causes high rotor loads and this is especially the case for low-temperature refrigeration compressors, where these are large. D. NN = V²/t.cp . Thus both the saturation pressures of Pb and Po cancel out each other, therefore the degree of superheat of (Pb) is supposed to open the expansion valve to maintain 6° to 7° of superheat and ensuring no liquid enters the compressor suction. Helps in the lubrication of compressor piston, liner, and other moving metal parts. All these actions are in sequence. Compressors that start with non-equalized pressures require motors with a high starting torque (HST). Fusible plugs, which contain a metal alloy, melts when the temperature in the system corresponding to the MWP. Figure 5-6 shows the increase in refrigeration capacity (area under 4-7’) and decrease in compressor work (2-2’-6-5). Vapor-compression refrigeration systems exploit two fundamental properties of all fluids: (i) The boiling temperature varies with pressure, and (ii) a change in phase (liquid boiling to a vapor and condensing back to a liquid) is accompanied by the absorption or release of heat. 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