Bài giảng Kỹ thuật lạnh - Chapter 3 : Compressor - Nguyễn Duy Tuệ

stance over which this difference in density prevails, a certain minimum vertical distance should be provided between the liquid level in the thermosiphon receiver and the heat exchanger. Reference 11 recommends a 12/2015 120 minimum elevation difference of 1.8 m Chapter 3 : Compressor SREW COMPRESSOR Example: Design the thermosiphon oil-cooling system serving an ammonia screw compressor operating with an evaporating temperature of -20°C and a condensing temperature of 35°C. The full-load refrigerating capacity and power requirement at these conditions are 1025 kW (291.4 tons of refrigeration) and 342 kW (458.5 hp), respectively. 12/2015 121Chapter 3 : Compressor SREW COMPRESSOR Example: 12/2015 122Chapter 3 : Compressor SREW COMPRESSOR 12/2015 123Chapter 3 : Compressor SREW COMPRESSOR 12/2015 124Chapter 3 : Compressor SREW COMPRESSOR 7. Economizer circuit using a side port: The refrigerant in Cavity 5, for example, is at a pressure somewhere between suction and discharge. 12/2015 125Chapter 3 : Compressor SREW COMPRESSOR - Refrigerant can be supplied through this opening at an intermediate pressure, and the compressor continues the compression of all the refrigerant. - This opening, often called the side port, offers within one compressor some of the advantages of a multiple-compressor, two-stage installation - Manufacturers of screw compressors are usually able to choose the position of the side port so that the desired intermediate pressure can be provided. 12/2015 126Chapter 3 : Compressor SREW COMPRESSOR 12/2015 127Chapter 3 : Compressor SREW COMPRESSOR - Additional refrigeration capacity is provided, however, because the liquid flowing to the evaporators has been chilled and its enthalpy reduced. The power reqirement of the compressor will increase because of the additional gas to be compressed from the side-port pressure to the condensing pressure. 12/2015 128Chapter 3 : Compressor SREW COMPRESSOR Economizer cycle in its best operation is not quite as efficient as two stage Comparison of the coefficients of performance of a two-stage ammonia system with an economized single-stage compressor equipped with a flash-type subcooler. 12/2015 129Chapter 3 : Compressor SREW COMPRESSOR One reason for the inability of the economized system using a side port to attain the efficiency of a two-stage system is illustrated . This unrestrained expansion consitutes a thermodynamic loss. 12/2015 130Chapter 3 : Compressor SREW COMPRESSOR It can be inferred that the capacity of the system will increase. This increase occurs, because the enthalpy of liquid reaching the expansion valve is reduced, even though the volume flow rate at the inlet to the compressor remains unchanged. Due to the admission of additional gas during the compression process, the power requirement increases. 12/2015 131Chapter 3 : Compressor SREW COMPRESSOR 12/2015 132Chapter 3 : Compressor SREW COMPRESSOR - The economizer cycle is most effective when the compressor is operating at full refrigeration capacity. - With compressors equipped with slide valves for capacity control, the opening of the slide valve changes the pressure within the compressor at the side port. Because the start of compression is delayed, the pressure in the cavity is low when the side port is first uncovered. Thus, the pressure at the side port progressively drops as the slide valve opens. 12/2015 133Chapter 3 : Compressor SREW COMPRESSOR 12/2015 134Chapter 3 : Compressor SREW COMPRESSOR - Another potential application of the side port is to provide the suction for an intermediate- temperature evaporator. - Here again there are limitations imposed by the prospect of the drop in side-port pressure. In the food industry the intermediate-temperature evaporator is often serving spaces storing unfrozen food where the drop in evaporating temperatures much below freezing could damage products. A conclusion is that the side port offers attractive possibilities, but it also has limitations. 12/2015 135Chapter 3 : Compressor SCROLL COMPRESSOR Similar to the reciprocating compressor, the scroll compressor works on the principle of trapping the refrigerant vapor and compressing it by gradually shrinking the volume of the refrigerant. The scroll compressor uses two scroll configurations, mated face-to-face, to perform this compression process. The tips of the scrolls are fitted with seals that, along with a fine layer of oil, prevent the compressed refrigerant vapor from escaping through the mating surfaces. Note : Reference (page 8, [1]) 12/2015 136Chapter 3 : Compressor SCROLL COMPRESSOR The upper scroll, called the stationary scroll, contains a discharge port. The lower scroll, called the driven scroll, is connected to a motor by a shaft and bearing assembly. The refrigerant vapor enters through the outer edge of the scroll assembly and discharges through the port at the center of the stationary scroll. 12/2015 137Chapter 3 : Compressor SCROLL COMPRESSOR The center of the scroll journal bearing and the center of the motor shaft are offset. This offset imparts an orbiting motion to the driven scroll. Rotation of the motor shaft causes the scroll to orbit—not rotate—about the shaft center. 12/2015 138Chapter 3 : Compressor SCROLL COMPRESSOR This orbiting motion causes the mated scrolls to form pockets of refrigerant vapor. As the orbiting motion continues, the relative movement between the orbiting scroll and the stationary scroll causes the pockets to move toward the discharge port at the center of the assembly, gradually decreasing the refrigerant volume and increasing the pressure. 12/2015 139Chapter 3 : Compressor SCROLL COMPRESSOR Three revolutions of the motor shaft are required to complete the compression process. 12/2015 140Chapter 3 : Compressor SCROLL COMPRESSOR - During the first full revolution of the shaft, or the intake phase, the edges of the scrolls separate, allowing the refrigerant vapor to enter the space between the two scrolls. By the completion of first revolution, the edges of the scrolls meet again, forming two closed pockets of refrigerant. - During the second full revolution, or the compression phase, the volume of each pocket is progressively reduced, increasing the pressure of the trapped refrigerant vapor. Completion of the second revolution produces nearmaximum 12/2015 141 compression. Chapter 3 : Compressor SCROLL COMPRESSOR - During the third full revolution, or the discharge phase, the interior edges of the scrolls separate, releasing the compressed refrigerant through the discharge port. At the completion of the revolution, the volume of each pocket is reduced to zero, forcing the remaining refrigerant vapor out of the scrolls. - Notice that these three phases intake, compression, and discharge occur simultaneously in an ongoing sequence. While one pair of these pockets is being formed, another pair is being 12/2015 142 compressed and a third pair is being discharged. Chapter 3 : Compressor SCROLL COMPRESSOR In this example scroll compressor, refrigerant vapor enters through the suction opening. The refrigerant then passes through a gap in the motor, cooling the motor, before entering the compressor housing. The refrigerant vapor is drawn into the scroll assembly where it is compressed, discharged into the dome, and finally discharged out of the compressor through the discharge opening. In the air-conditioning industry, scroll compressors are widely used in heat pumps, rooftop units, split systems, self-contained units, and even small water 12/2015 143 chillers. Chapter 3 : Compressor SCROLL COMPRESSOR 12/2015 144Chapter 3 : Compressor SCROLL COMPRESSOR + Advantages of scroll compressors: Scroll type compressors are inherently more efficient compared to other types of compressors for many reasons: - The absence of pistons for gas compression enables scroll compressors to reach nearly 100% volumetric efficiency, leading to reduced energy costs. - Re-expansion losses, a typical feature of each piston stroke encountered in reciprocating models, are eliminated. 12/2015 145Chapter 3 : Compressor SCROLL COMPRESSOR 12/2015 146Chapter 3 : Compressor SCROLL COMPRESSOR - In addition, valve (ports) losses are eliminated, since suction and discharge valves (ports) do not exist. - Furthermore, due to the absence of several moving parts, scroll compressors are considerably quieter in operation compared to other types of compressors, like for example reciprocating type ones. - Their weight and footprint are considerably smaller compared to other bulkier types of compressors in use nowadays. 12/2015 147Chapter 3 : Compressor SCROLL COMPRESSOR - Gas pulsation is also minimised, if not eliminated and consequently, they can operate with less vibration. 12/2015 148Chapter 3 : Compressor SCROLL COMPRESSOR + Disadvantages of scroll compressors: - Being fully hermetic, perhaps the biggest disadvantage of scroll compressors is that they are generally not easily repairable. They cannot be disassembled for maintenance. - Many reciprocating compressors are tolerant on rotating in both directions. This is usually not the case for scroll compressors. 12/2015 149Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR - In the air-conditioning industry, helical-rotary compressors are most commonly used in water chillers ranging from 70 to 450 tons [200 to 1,500 kW]. - The centrifugal compressor uses the principle of dynamic compression, which involves converting energy from one form to another, to increase the pressure and temperature of the refrigerant. It converts kinetic energy (velocity) to static energy (pressure). The core component of a centrifugal compressor is the rotating impeller. 12/2015 150Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR - The center, or eye, of the impeller is fitted with blades that draw refrigerant vapor into radial passages that are internal to the impeller body. The rotation of the impeller causes the refrigerant vapor to accelerate within these passages, increasing its velocity and kinetic energy. 12/2015 151Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR - The accelerated refrigerant vapor leaves the impeller and enters the diffuser passages. These passages start out small and become larger as the refrigerant travels through them. As the size of the diffuser passage increases, the velocity, and therefore the kinetic energy, of the refrigerant decreases. The first law of thermodynamics states that energy is not destroyed—only converted from one form to another. Thus, the refrigerant’s kinetic energy (velocity) is converted to static energy (or static pressure). 12/2015 152Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR - Refrigerant, now at a higher pressure, collects in a larger space around the perimeter of the compressor called the volute. The volute also becomes larger as the refrigerant travels through it. Again, as the size of the volute increases, the kinetic energy is converted to static pressure. 12/2015 153Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR 12/2015 154Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR This chart plots the conversion of energy that takes place as the refrigerant passes through the centrifugal compressor. 12/2015 155Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR In the radial passages of the rotating impeller, the refrigerant vapor accelerates, increasing its velocity and kinetic energy. As the area increases in the diffuser passages, the velocity, and therefore the kinetic energy, of the refrigerant decreases. This reduction in kinetic energy (velocity) is offset by an increase in the refrigerant’s static energy or static pressure. Finally, the high-pressure refrigerant collects in the volute around the perimeter of the compressor, where further energy conversion takes place. 12/2015 156Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR Centrifugal Chiller Máy nén ly tâmCánh chỉnh tải Dàn ngưng tụ Bộ điều khiển Bình bay hơi 12/2015 157Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR Following are the advantages and isadvantages of centrifugal compressors, over to the reciprocating compressors: +Advantages : - High reliability, eliminating the need for multiple compressors and installed standby capacity. - For the same operating conditions, machine prices are lower for high volume flow rates. - Less plot area for installation for a given flow rate. 12/2015 158Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR - Machine is small and light weight with respect to its flow rate capacity. - Installation costs are lower due to smaller size Low total maintenance costs - When a turbine is selected as a driver, the centrifugal compressor’s speed level allows direct drive (no gear unit), thereby minimizing equipment cost, reducing power requirements, and increasing unit reliability. - Flow control is simple, continuous, and efficient over a relatively wide flow range. 12/2015 159Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR - No lube (or seal) oil contamination of process gas. - Absence of any pressure pulsation above surge point. + Disadvantages: - Lower efficiency than most positive displacement types for the same flow rate and pressure ratio, especially for pressure ratios over 2. Due to recycle not efficient below the surge point - Very sensitive to changes in gas properties, especially molecular weight 12/2015 160Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR - Not effective for low molecular weight gases. The pressure ratio capability per stage is low, tending to require a large number of machine stages, hence mechanical complexity. 12/2015 161Chapter 3 : Compressor CENTRIFUGAL COMPRESSOR 12/2015 162Chapter 3 : Compressor