The importance of peak-performing industrial compressors in a refrigerated food processing plant or cold storage warehouse cannot be understated. As one of the major components of the refrigeration system, inadequate compressor performance can affect food safety and product shelf life and significantly drive up operating costs.
During the refrigeration design process, facility owners and system engineers must make a series of decisions that will directly influence the reliability, efficiency and longevity of the industrial refrigeration system. In general, compressors require a lot of oil to work properly, serving functions like sealing the rotors, lubricating the bearings and cooling the discharge gas.
Failure to achieve the necessary temperature in the system's oil can lead to severe, long-term damage to the refrigeration equipment. Oil needs to cool for a compressor to work properly. When it does not, the ramifications can get expensive — in both repair expenses and downtime. The proper oil-cooling technique (paired with a regular maintenance schedule) can help extend a compressor’s lifespan by thousands of operating hours, but its importance is often glazed over.
Evaluating Oil-Cooling Options
Choosing the right oil-cooling technique depends on several factors, including the compressor size, cooling requirements, operating budget, and even a facility’s geographic location. Transitioning between techniques is no simple feat; reengineering and installing a more suitable system often demands a substantial investment, which can reach hundreds of thousands of dollars in design and construction expenses. Taking the necessary time to evaluate the best technique for a facility’s unique needs and conditions during the initial design phase is the most recommended approach to maximize a system’s lifespan.
In industrial refrigeration applications, three prevalent oil-cooling methods are employed, each possessing a distinct set of pros and cons.
- Liquid Injection Oil Cooling: Often used in ammonia refrigeration systems, this technique involves injecting refrigerant liquid directly into specific compressor porting based on operating conditions. The oil enters the discharge stream and flows directly into the oil separator. The liquid refrigerant absorbs heat from the oil, lowering its temperature, and subsequently returns to the compressor to sustain the cooling cycle. Liquid injection is quite common, often due to its cheaper upfront investment, but it can shorten the lifespan of the compressor at a much faster rate than other methods. It exposes bearings to liquid refrigerant, which can lead to compressor failure and operational problems if not monitored or adequately maintained. While controls for these systems have certainly improved through the years, when they do fail, systems produce large amounts of heat that can lock up compressors or — in extreme cases — melt the metal rotors.
- Thermosyphon Oil-Cooling: Relies on natural convection and gravity to flood the heat exchanger, effectively transferring heat to another fluid like refrigerant or water. This method provides a low-maintenance, intuitively designed and energy-efficient solution for managing compressor oil temperatures. Securing the precise orientation and placement of the heat exchanger and compressor stands as a vital imperative for maximizing thermosyphon efficiency, introducing a heightened level of complexity to the installation process. In addition, fluctuations in environmental conditions or fluid temperatures can directly affect the system's efficiency since its performance is closely tied to the temperature difference between the oil and secondary fluid. That said, a thermosyphon system can significantly reduce energy consumption and operating costs compared to pump systems. Since their configuration includes fewer moving parts, they also require less maintenance, are less prone to mechanical issues and can be simpler to troubleshoot.
- Glycol-Cooled Oil-Cooling: Also referred to as “water-cooled oil cooling,” it is a process that relies on a separate water/glycol circuit, often through a heat exchanger or oil cooler, to dissipate heat from the compressor's oil. This approach effectively controls oil temperature and is compatible with a wide range of applications and compressor sizes. The separation also helps mitigate the chances of refrigerant contamination. Facilities with access to a nearby body of water can also enjoy energy-efficiency benefits by implementing a glycol-cooled oil-cooling system. However, this method is not recommendable for facilities that lack easy access to large bodies of water, as they will need to source and treat their own supply.
Maintenance Oversights
Regular maintenance is crucial to ensuring a long-lasting and peak-performing refrigeration system. However, mindlessly checking boxes on a checklist can be as dangerous as not performing a maintenance check at all.
The following are common oversights during compressor inspection and repairs that can have negative consequences:
- Sampling oil immediately after starting a machine: If a compressor is running with compromised oil, it could corrupt the entire machine and prompt a replacement. Operators should sample oil to monitor its condition and note any harmful contaminants detected. However, immediately sampling the oil will tamper with results and provide a false reading from iron seeping into the oil when the faucet is opened. Let it run first — about 60 to 90 seconds — to ensure an accurate reading.
- Conducting vibration analyses in different locations: When conducting vibration analyses, a reading will be inaccurate if repeatedly measured at different locations on the compressor. Operators may place a sticker at the vibration points to ensure the accelerometer is always mounted in the same spot.
- Not following OEM instructions regarding bearing end play: Bearing end play must be carefully controlled for the shaft to rotate smoothly without excess axial movement — which could cause damage to the bearings, seals or other machine components — and speed up the need for a replacement. When it comes to bearing endplay and parts, one size does not fit all and, in the long run, failing to follow original equipment manufacturer (OEM) instructions can result in false readings.
- Failing to consider all factors in compressor alignment: Adequate alignment in an industrial refrigeration compressor prevents excessive wear and tear on the system’s components. Evaluating dial indicator sag and dial indicator hysteresis is vital when inspecting the alignment of rotary components. If a coupling is inadvertently rotated upside down, gravity may lead to sagging. It is essential to consider any resulting discrepancies when interpreting readings.
- Failing to factor hysteresis in dial indicator measurements: Dial indicator hysteresis is the difference in readings from a dial indicator when it is moved from one direction to the opposite direction, often caused by its mechanical design. Although a dial indicator is mechanical, the needle could still shift even when it is fully clamped. The effect of hysteresis can lead to inaccurate measurements if not considered appropriately.
For best results, maintenance inspections must be frequent and comprehensive.
