Machinery Lubrication magazine published by Noria Corporation
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42 | November - December 2015 | www.machinerylubrication.com refrigerant, sending it downstream at a higher pressure. Frequently, reciprocating compressors are multiple-stage systems, which means that one cylinder's discharge will lead into the input side of the next cylinder. This allows for more compression than a single stage. These compressors have many lubricated parts, such as cylinders, valves and bearings. Rotary compressors normally use a set of screws or vanes to draw in the gas and compress it in the compression chamber. This could be compared to the function of a vane pump. Like reciprocating compres- sors, these systems have a variety of lubricated components, including gears, bearings, valves, etc. Centrifugal compressors utilize the rota- tional motion of the drive to rotate a series of impellers, which will provide the compres- sion action. These systems often are rotating at several thousand revolutions per minute. The lubricant must be thin enough to lubri- cate properly at these speeds but also thick enough to handle the heat and refrigerant contamination that can occur. With all of these compressor systems, the lubricant's base oil, additives and viscosity grade must be carefully selected. Compatibility with the refrigerant being compressed is perhaps the most important factor in choosing a base oil, as not all lubri- cants can handle this type of contamination. The additive package usually must have some anti-wear properties as well as demul- sibility in the event of moisture contamination. The viscosity is variable depending on the load, speed and tempera- ture at which the compressor will operate. Understanding Refrigeration Refrigeration has revolutionized many industries. Nearly every plant uses some sort of refrigeration, whether to help remove heat or simply for the comfort of the employees. How the cooling cycle works is quite simple. It involves the ideal gas law and how gases undergo a change in temperature when they are subjected to a change in pressure. The compressor acts as a pump to circulate the refrigerant. The refrigerant leaves the compressor as a high-pressure gas and moves into a condenser. Here, the gas is condensed into a liquid, which will then flow through a pipe until it reaches a metering device. This metering device is often referred to as a thermal expansion valve, piston or orifice. Basically, it necks down the opening in the line and causes a large pressure drop on the back side. As the pressure drops, so too does the refrigerant's temperature. Immediately after the metering device is the evaporator. This is where the heat transfer occurs. The air passing over the evaporator is warmer than desired. The heat in the air is absorbed by the refrigerant in the evaporator and then transported back to the condenser where it is removed. The compressor is what causes this movement. Perhaps you've heard the expression that air conditioners or refrigerators don't cool but actually move heat. This is exactly how the cycle works. Heat is moved from an area where it is not wanted to an area where it can be released. You can experience this effect on a hot summer day by walking to your outside air-conditioning unit (the condenser). The air coming out of the condenser's top will be hotter than the ambient air. Types of Refrigerants Refrigerants must be able to absorb and transfer heat. There are several types of refrig- erants, which are selected based on the desired temperature. Refrigerants must be able to readily change states from a liquid to a gas. This change of state is what allows for the sudden temperature drop after moving through the metering device. Depending on the refrigerant used, you can achieve very low temperature refrigeration or simply basic cooling capacity. Perhaps the most prominent types of refrig- erants are the hydrocarbon-based series. These are similar to what you would purchase for your house or car. They are often referred to by names such as R-22, R-134a, etc. Ammonia is another common refrigerant employed primarily in industrial facilities. It performs well and can achieve low temperatures for cooling or freezing. All told, there are dozens of different refrigerants composed of chlorofluo- rocarbons (CFC), hydrogen-containing CFC (HCFC), and hydrogen fluorine and carbon compounds (HFC), as well as combinations of each. Compressor Lubricants Lubricants perform several functions in a compressor system. Of course, they must be able to lubricate the machine. In some systems, the lubricant is required to act as a cooling fluid as well as a sealant. This is why it's important to select the proper lubricant for your compressor. When in doubt, check with the manufacturer about the correct oil for the system. Compressor lubricants are often a specialized blend of additives and base oils in order to provide the necessary lubricating properties while still being compatible with the refrigerant. Any incompatibility of the base oil and the refrigerant could have disastrous results for the equipment. IN THE TRENCHES The History of Refrigeration The refrigeration process dates back to the mid-1700s when experiments were conducted on how ice could be produced. The technology was perfected in the 1800s, and modern refrig- eration was born. This allowed for the cooling or freezing of liquids and foods, which enabled them to be preserved longer. With some adap- tation, these small refrigeration systems could be used to cool rooms in hospitals and homes to help the sick be more comfortable. This also had a huge impact on food and beverage companies, which previously had relied on tons of ice to provide the same cooling function.