Refrigeration – Automatic control – Refrigeration producer
Reexamination Certificate
2000-05-23
2003-01-14
Tanner, Harry B. (Department: 3744)
Refrigeration
Automatic control
Refrigeration producer
C062S208000, C062S126000
Reexamination Certificate
active
06505475
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the fields of repair, maintenance and tuning of refrigeration systems, and more particularly for systems and methods for measuring, analyzing and improving the efficiency of refrigeration systems.
BACKGROUND OF THE INVENTION
In the field of refrigeration and chillers, the evaporator heat exchanger is a large structure, containing a plurality of parallel tubes, within a larger vessel comprising a shell, through which refrigerant flows, absorbing heat and evaporating. Outside the tubes, an aqueous medium, such as brine, circulates and is cooled, which is then pumped to the process region to be cooled. Such an evaporator may hold hundreds or thousands of gallons of aqueous medium with an even larger circulating volume.
Mechanical refrigeration systems are well known. Their applications include refrigeration, heat pumps, and air conditioners used both in vehicles and in buildings. The vast majority of mechanical refrigeration systems operate according to similar, well known principles, employing a closed-loop fluid circuit through which refrigerant flows, with a source of mechanical energy, typically a compressor, providing the motive forces.
Typical refrigerants are substances that have a boiling point below the desired cooling temperature, and therefore absorb heat from the environment while evaporating under operational conditions. Thus, the environment is cooled, while heat is transferred to another location where the latent heat of vaporization is shed. Refrigerants thus absorb heat via evaporation from one area and reject it via condensation into another area. In many types of systems, a desirable refrigerant provides an evaporator pressure as high as possible and, simultaneously, a condenser pressure as low as possible. High evaporator pressures imply high vapor densities, and thus a greater system heat transfer capacity for a given compressor. However, the efficiency at the higher pressures is lower, especially as the condenser pressure approaches the critical pressure of the refrigerant. It has generally been that the maximum efficiency of a theoretical vapor compression cycle is achieved by fluids with low vapor heat capacity, associated with fluids with simple molecular structure and low molecular weight.
Refrigerants must satisfy a number of other requirements as best as possible including: compatibility with compressor lubricants and the materials of construction of refrigerating equipment, toxicity, environmental effects, cost availability, and safety.
The fluid refrigerants commonly used today typically include halogenated and partially halogenated alkanes, including chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HFCFs), and less commonly hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs). A number of other refrigerants are known, including propane and fluorocarbon ethers. Some common refrigerants are identified as R11, R12, R22, R500, and R502, each refrigerant having characteristics that make them suitable for different types of applications.
A refrigeration system typically includes a compressor, which compresses gaseous refrigerant to a relatively high pressure, while simultaneously heating the gas, a condenser, which sheds the heat from the compressed gas, allowing it to condense into a liquid phase, and an evaporator, in which the liquefied refrigerant is again vaporized, withdrawing the heat of vaporization from a process. The compressor therefore provides the motive force for active heat pumping from the evaporator to the condenser. The compressor typically requires a lubricant, in order to provide extended life and permit operation with close mechanical tolerances. Normally, the gaseous refrigerant and liquid lubricant are separated by gravity, so that the condenser remains relatively oil free. However, over time, lubricating oil migrates out of the compressor and its lubricating oil recycling system into the condenser. Once in the condenser, the lubricating oil becomes mixed with the liquefied refrigerant and is carried to the evaporator. Since the evaporator evaporates the refrigerant, the lubricating oil accumulates at the bottom of the evaporator. The oil in the evaporator tends to bubble, and forms a film on the walls of the evaporator tubes. In some cases, such as fin tube evaporators, a small amount of oil enhances heat transfer and is therefore beneficial. In other cases, such as nucleation boiling evaporator tubes, the presence of oil, for example over 1%, results in reduced heat transfer. See, Schlager, L. M., Pate, M. B., and Berges, A. E. “A Comparison of 150 and 300SUS Oil Effects on Refrigerant Evaporation and Condensation in a Smooth Tube and Micro-fin Tube”, ASHRAE Trans. 1989, 95(1): 387-97: Thome, J. R. “Comprehensive Thermodynamic Approach to Modelling Refrigerant-Lubricating Oil Mixtures”, Intl. J. HVAC&R Research (ASHRAE) 1995, 110-126; Poz, M. Y., “Heat Exchanger Analysis for Nonazeotropic Refrigerant Mixtures”, ashrae Trans. 1994, 100(1)727-735 (Paper No 95-5-1).
Several mechanisms are available seeking to control lubricating oil buildup in the evaporator. One mechanism provides a shunt for a portion of mixed liquid refrigerant and oil in the evaporator to the compressor, wherein the oil is subject to the normal recycling mechanisms. This shunt, however, may be inefficient and is difficult to control. Further, it is difficult to achieve and maintain low oil concentrations using this method.
It is also known to periodically purge the system, recycling the refrigerant with purified refrigerant and cleaning the system. This technique, however, generally permits rather large variance in system efficiency or relatively high maintenance costs. Further, this technique generally does not acknowledge that there is an optimum level of oil in the evaporator and, for example, the condenser. Thus, typical maintenance seeks to produce a “clean” system, subject to incremental changes after servicing.
It is thus known that the buildup of large quantities of refrigerant oil within an evaporator, which passes in small amounts from the compressor to the condenser as a gas, and which leaves the condenser and passes to the evaporator as a liquid, will reduce efficiency of the system, and further, it is known to provide in-line devices which continuously remove refrigerant oil from the refrigerant entering the evaporator. These devices include so-called oil edductors.
The inefficiency of these continuous removal devices is typically as a result of the bypassing of the evaporator by a portion of the refrigerant, and potentially a heat source to vaporize or partially distill the refrigerant to separate the oil. Therefore, only a small proportion of the refrigerant leaving the condenser may be subjected to this process, resulting in poor control of oil level in the evaporator and efficiency loss.
It is also known to reclaim and recycle refrigerant from a refrigeration system to separate oil and provide clean refrigerant. This process is typically performed manually and requires system shutdown.
Systems are available for measuring the efficiency of a chiller, i.e., a refrigeration system which cools water or a water solution, such as brine. In these systems, the efficiency is calculated based on Watt-hours of energy consumed (Volts×Amps×hours) per cooling unit, typically tons or British Thermal Unit (BTU) (the amount of energy required to change the temperature of one British ton of water 1° C.
Thus, a minimal measurement requires a clock, voltmeter, ammeter, and thermometers and flowmeters for the inlet and outlet water. Typically, further instruments are provided, including a chiller water pressure gage, gages for the pressure and temperature of evaporator and condenser. A data acquisition system is also typically provided to calculate the efficiency, in BTU/kWH.
The art, however, does not provide systems intended to measure the operating efficiency of commercial chillers, while permitting optimization of the system.
It is known that the charge conditions of a chiller may have a s
Schmidt Douglas
Zugibe Kevin
Hudson Technologies Inc.
Milde & Hoffberg LLP
Tanner Harry B.
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