There are two types of rotary compressors applied: positive displacement rotary screw compressors and centrifugal compressors.
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Figure 1. Compression is obtained by direct volume reduction with pure rotary motion. At a point determined by the design volume ratio, the discharge port is uncovered and the gas is released to the condenser. Speed control can also be used to control capacity. The design of a centrifugal compressor for refrigeration duty originated with Dr.
Willis Carrier just after World War I. The centrifugal compressor raises the pressure of the gas by increasing its kinetic energy. The kinetic energy is converted to static pressure when the refrigerant gas leaves the compressor and expands into the condenser. The compressor and motor are sealed within a single casing and refrigerant gas is utilized to cool the motor windings during operation.
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The high-pressure gas is released into the condenser, where water absorbs the heat and the gas changes phase to liquid. However, for R, these pressure conditions are psig in the evaporator and 6. Compressors using R typically use large diameter impellers approximately 40 in. Compressors using RA typically use much smaller impellers about 5 in. The large wheel diameters required by R puts a design constraint on the compressor and, to reduce the diameter, they typically utilize two or three impellers in series or stages to produce an equivalent pressure increase.
Rotary screw compressor operation. Since the evaporator in positive-pressure chillers is maintained at a pressure well above atmospheric, any leaks in the refrigeration system will result in a loss of refrigerant and the effect of any leaks is quickly evidenced by low refrigerant levels in the chiller. However, any leaks associated with a negativepressure machine result in the introduction of atmospheric air consisting of noncondensable gases and water vapor into the chiller.
Refrigeration Machines 15 Noncondensable gases create two problems: 1. The compressor does work when compressing the noncondensable gases, but they offer no refrigerating effect. Moisture introduced with atmospheric air is a contaminant that can allow the formation of acids within the chiller that can cause serious damage to motor windings of hermetic motors and bearings.
To remove potential noncondensable gases and moisture from negativepressure chillers, these chillers are furnished with purge units. Additionally, to reduce the potential for leaks when chillers are off, the evaporator should be provided with an external heater to raise the refrigerant pressure to above atmospheric.
The energy requirement for a rotary compressor chiller at peak load is a function of 1 the required leaving chilled water temperature, and 2 the temperature of the available condenser water.
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As the leaving chilled water temperature is reduced, the energy requirement to the compressor increases, as summarized in Table 1. Similarly, as the condenser water temperature increases, the compressor requires more energy see Chap.
Thus, the designer can minimize the cooling energy input by utilizing a rotary compressor chiller selected to operate with the highest possible leaving chilled water temperature and the lowest possible condenser water temperature. Air-cooled reciprocating compressor water chillers have a peak load power requirement of 1. Thus, the peak load COP for these units will range from 3.
Typical rotary compressor water chillers with water-cooled condensing have a peak load power requirement of 0. The energy consumption by a rotary compressor chiller decreases as the imposed cooling load is reduced, as shown in Figure 1. The vast majority of electric-drive rotary compressor water chillers utilize a single compressor. Typical rotary compressor part load performance. A peak load rating of 0.
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Positive-pressure chillers tend to be smaller and lighter than negativepressure chillers, which can result in smaller chiller rooms and lighter structures. Engine-Drive Chillers Natural gas and propane fueled spark ignition engines have been applied to rotary compressor systems. Engine-drive chillers have been around for many years, but their application, most typically utilizing natural gas for fuel, has been limited by a number of factors: 1. Air quality regulations. Much higher maintenance requirements. Short engine life. Larger physical size.
Lack of integration between engine and refrigeration subsystems. Currently, the engines used for chillers are either spark-ignition engines based on automotive blocks, heads, and moving components below about ton capacity or sparkignition engines using diesel blocks and moving components for larger chillers. While the automotive-derivative engines are advertised to have a 20, hr useful life, the real life may be much shorter, requiring an engine replacement every 2 years or so. The diesel-derivative engines require an overhaul every 10 — 12, hr equivalent to a diesel truck traveling , miles at 50 mph.
Gas engine-drive chillers remain more expensive than electric-drive units and they have higher overall operating costs, including maintenance costs, see Table 1. However, engine-drive chillers may be used during peak cooling load periods to reduce seasonal peak electrical demand charges see Sec.
Refrigeration Machines 19 1. Condensing Medium The heat collected by the water chiller, along with the excess compressor heat, must be rejected to a heat sink. Directly or indirectly, ambient atmospheric air is typically used as this heat sink. For air-cooled chillers, the condenser consists of a refrigerant-to-air coil and one or more fans to circulate outdoor air over the coil. Thus, the ambient air temperature must be no greater than F. Water-cooled chillers typically use a cooling tower to reject condenser heat to the atmosphere and Chaps.
However, in some areas, principally in large cities and at some universities and hospital complexes, large steam distribution systems are available. In years past, this steam was often cheaper than electricity and was used to provide cooling, utilizing the absorption refrigeration cycle. This is generally no longer the case and little or no new absorption cooling is utilized except where a waste heat source is available, such as with cogeneration or some industrial processes, or where the use of absorption cooling during peak cooling load periods may allow a reduction in seasonal electric demand charges see Sec.
The absorption refrigeration cycle is relatively old technology. However, by World War I, the technology and use of reciprocating compressors had advanced to the point where interest in and development of absorption cooling essentially stagnated until the s. The evaporator consists of a heat exchanger, held at low pressure, with a separate refrigerant typically, water pump. The pump sprays the refrigerant over the tubes containing the chilled water, absorbs heat from the water, and evaporates as a lowpressure gas.
The absorber is at a lower pressure than the evaporator is because the concentrated absorbent solution typically lithium bromide exerts a molecular attraction for the refrigerant. The absorbent solution is sprayed into contact with the refrigerant vapor. Condensing of the refrigerant occurs because the heat is absorbed by absorbent. The absorbent, then, is cooled by condenser water. The absorbent now consists of a dilute solution, due to its having absorbed water vapor refrigerant. The dilute solution is pumped to the concentrator, where heat is applied to re-evaporate the refrigerant.
The concentrated solution of absorbent is then returned to the absorber. The refrigerant vapor goes to the condenser, where it is condensed by the condenser water. Leaks allow air to enter the refrigerant system, introducing noncondensable gases.
The solution in the bottom of the absorber is relatively quiet and these gases tend to collect at this point. These chillers are two-stage machines with a resulting in an overall COP of 1. Single stage steam absorption chiller schematic. Because absorption cooling has a COP of only 0. Other factors that must be considered for absorption chillers include the following: 1. Additionally, due to their height, mechanical equipment rooms must be 6 —10 ft taller than rooms housing electric-drive chillers.
Absorption chillers will weight at least twice as much the equivalent electric-drive chiller. While noise and vibration are real concerns for electric-drive chillers see Sec. Due to the potential for crystallization of the lithium bromide in the chiller if it becomes too cool, the condenser water temperature must be kept above 75 —F.
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An emergency power source may be required if lengthy power outages are common. Without power and heat input, the chiller begins to cool and the lithium bromide solution may crystallize. However, because an absorption chiller has a very small electrical load requirement usually less than 10 kW , a dedicated back-up generator is not a major element.
Even though the energy cost for the absorption chiller may be higher, the increased maintenance costs associated with engine-drive systems may make the absorption chiller more cost effective. Here a single chiller provides chilled water to the cooling coils utilizing a single chilled water pump.
With the single compressor system, failure of any component compressor, pump, or condenser will result in no cooling being available. In cases where cooling is critical to the facility computer centers, hospital, laboratories, pharmaceutical or textile manufacturing, etc. Thus, half of the total design range is produced by each chiller. Table 2. TABLE 2. However, with this piping arrangement, if one chiller is not in operation, chilled water from the operating chiller will mix blend with the return water passing through the nonoperating chiller, effectively raising the chilled water supply temperature to the system.