NASHVILLE
JEFFERSONVILLE
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CHILLER UNIVERSITY |
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CHILLER DESIGNS
Just like one tool cannot perform every job, no one chiller can cool every heat load. Some chillers are designed to cool to very low temperatures while others are designed for only mid-range applications. Some designs can support very high flow rates of fluid while other designs may be designed for just a trickle of fluid. The same goes for ambient temperatures. Some chillers use a refrigerant that is made for higher ambient temperatures and some are better suited for colder environments. It is very important that all these things are taken into consideration when designing a chiller to suit a particular application. In addition to the information above, we must also consider the fluid being cooled. Di-ionized water or distilled water requires a slightly different approach that regular tap water. You see, DI and distilled water may actually cause some metals to break down when in contact with them. We all know how black iron pipe can rust when exposed to moisture. A similar reaction happens when DI or distilled water comes in contact with certain metals. The heat exchanger should be nickel-brazed with stainless steel piping carrying the water. This will eliminate the effects of the corrosion due to the DI or distilled water. Some applications require direct cooling of an oil or hydraulic fluid. By direct cooling, we mean there is no secondary device between the oil circuit and the refrigeration circuit. A secondary device would be a second heat exchanger in the system. A water chiller would pass its cold water through a second heat exchanger that has the oil flowing through it to generate the cooling effect. In the direct cooling method, the primary heat exchanger which has the cold refrigerant passing through it also has a cross flow of the oil to be cooled. When we do this direct cooling method for oil, we must take into consideration the viscosity of the fluid. For a five ton chiller for oil, we could not use the same heat exchanger as we would for water. The oil is to viscous or thick. The pressure drop through the heat exchanger would be quite large, thus slowing the flow rate of the oil. As the oil slows down it would be exposed to a longer period of time of having the cold refrigerant pass by. This would cool the oil even more and in being cooled, it would become thicker. You can see where this is becoming a problem. Now let's jump to the refrigeration side in this same example. Since the refrigeration is getting the oil colder, the refrigerant has less heat to remove and becomes colder too. Since the temperature in the heat exchanger is dropping, there is less heat to "boil" the liquid mist refrigerant back into a vapor. This will cause liquid refrigerant to flood back or return to the compressor. The cold liquid refrigerant can damage a compressor is a couple ways. As the temperature of the refrigerant drops, so does the pressure. With the lower pressure of refrigerant returning to the compressor, less lubricating oil returns to the compressor. This can lead to a compressor "locking up". It is similar to running an automobile engine with no oil in it. The pistons will seize in the cylinders and the motor will stop running. There is a tremendous amount of damage that can occur should the compressor be starved of its lubrication. With the drop in refrigeration pressure also comes a lack of return gas to the compressor. Yes, the gas is colder, but the volume is not present. This return gas to the compressor is what is used to cool the motor in the compressor. Without the return gas, the motor will overheat and possible break down. When the compressor overheats, the oil that is in the compressor may carbonize or break down, reducing its ability to lubricate. The liquid refrigerant can not be compressed by the compressor. So when the liquid enters the compressor, it is like a solid object. With a piston style compressor, it is like putting a rock in the cylinder. The rock would surely damage the cylinder as well as possibly break the piston or crankshaft. All this trouble was due to an improperly sized heat exchanger because someone didn't take into consideration the fluid being cooled.
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