10 cooler ideas for refrigeration system efficiency

Sept. 17, 2007
Just as plant refrigeration processes are demanding more energy, plant operations professionals are pushing to reduce overall energy consumption in their manufacturing operations. Why? Because indications are that the future cost of energy will continue to rise. Follow these 10 ideas to improve your refrigeration system efficiency.

Refrigeration is one of the more energy-intensive technologies used in domestic manufacturing operations – primarily food plants. From an energy perspective, the trend in many food plants is toward processes that require more, not less, refrigeration, thereby escalating overall energy requirements. For example, food companies are responding to consumer demand by increasing the supply of frozen ready-to-cook entrees and sides. These products often require quick-freezing unit operations to maintain high product quality. Quick freezing unit operations are among the most energy-intensive in food plants, which increases the total energy required to produce a unit of finished product.

Just as plant refrigeration processes are demanding more energy, plant operations professionals are pushing to reduce overall energy consumption in their manufacturing operations. Why? Because indications are that the future cost of energy will continue to rise. So it’s increasingly important for plants to improve operational efficiency to maintain a competitive marketplace advantage. These trends put pressure on plant operations staff to identify effective measures to level or reduce the energy consumption in manufacturing.

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Strategies for improving the efficiency of industrial refrigeration systems are available for all to adopt. Seemingly a simple order, improving energy efficiency in these systems is difficult for a number of reasons:

  • Industrial refrigeration systems are custom-engineered to meet a plant’s specific process needs. This makes prescriptive energy conservation measures difficult to identify because each system is “one-of-a-kind.”
  • Food production facilities often require continuous refrigeration. Energy efficiency improvement measures that require shutdown are difficult to implement.
  • Because refrigeration uptime is crucial, plants’ operations staff are often risk-averse concerning any changes that might undermine their ability to provide “cold” to the production floor.
    Consider these 10 ideas aimed at improving industrial refrigeration system efficiency. Each is proven to work. You just need to evaluate them for implementation in your plant’s context.

1. Floating head pressure control
Many plants operate their refrigeration systems with higher than necessary head (condensing) pressures. Although the ability to reduce a system’s head pressure is limited by ambient conditions, many plants can operate with considerably lower minimum head pressures. If your ammonia-based refrigeration system’s head pressure never falls below 125 psig, you might have an opportunity to improve system efficiency. A useful guideline says you can expect the efficiency of your system’s compressors to improve by 1.3% for each degree F in lower saturated condensing temperature (1°F is about 3 psig for ammonia).

2. Raise suction pressure/temperature
If your plant uses evaporator pressure regulators on all of its loads, it might make sense to raise your system’s suction pressure setpoint. You can expect your system’s compressor capacity to improve by 2.5% for each degree F increase in saturated suction temperature. Efficiency increases depend on the starting point of your suction pressure increase, but improvements in the range of 2% for each degree F increase in saturated suction temperature are possible.

3. Variable-frequency drives for evaporator fans
Because most evaporators don’t operate at their design load 100% of the time, their capacity needs to be varied to meet instantaneous thermal loads. Evaporator efficiency at part-load conditions can be improved in most systems by using variable-frequency drives (VFD) on evaporator fans. The savings attributable to this technology depends on a number of factors including system suction pressure, evaporator part-load ratio, evaporator fan type, and face velocity of air over the evaporator coil.

4. Variable-speed compressors
Approach variable–speed refrigeration compressors with caution. Compressors in these systems tend to be driven by a large-frame motor, which makes the VFD option expensive. At most, consider having only one variable-speed compressor per suction pressure level in the plant. Use the VFD compressor as a trim machine for capacity modulation.

5. Variable-speed condenser fans
In many cases, VFD condenser fans can yield operating costs savings of 2% to 3%, depending on a number of factors including: relationship of heat rejection capacity available to that required, minimum head-pressure setpoint, condenser fan type, and others. If you pursue a VFD project for condenser fans, install VFDs on every condenser fan and modulate their capacity equally to maximize energy savings and avoid liquid management problems on the system’s high side.

6. Heat recovery at oil coolers
It’s possible to recover heat from the discharge gas on high-stage compressors. However, a more effective option is to recover heat from oil-cooling heat exchangers on screw compressor packages. The heat available from oil cooling heat exchangers is available in reasonable quantities and at a higher temperature when compared to the heat available for recovery from the discharge gas stream.

7. Compressor sequencing and control
Controls are required to match compressor capacity to system demand. The most widely used compressor technology in industrial refrigeration systems is the screw compressor. Unfortunately, screw compressor efficiency decreases as it unloads in response to decreasing demand. For example, a typical screw compressor operating at -20°F suction and 90°F condensing will have a full-load efficiency of about 2.2 BHP/ton. When unloaded to its minimum capacity (10% in this case), the horsepower per ton requirement increases to about 8.8 BHP/ton. Review your sequence of operation and minimize the time intervals at which individual machines operate at part-load ratios less than 70%.

8. Improve defrost sequences
Air-cooling evaporators that operate at low temperatures will accumulate frost. As the coil ices up, its capacity decreases, which decreases system efficiency. Manske (2000) estimated that poor hot-gas defrost sequences and controls accounted for 13% of the electrical energy consumption in a cold storage warehouse. Establish a defrost sequence that avoids hot gas dwell times in excess of 15 minutes duration and defrost individual evaporators only on an as-needed basis rather than defrosting on the basis of a time-clock.

9. Convert from liquid-injected oil cooling to external cooling
Screw compressors require some means of oil cooling. Using high-pressure liquid refrigerant for oil cooling is common in a number of systems. Liquid-injection oil cooling conspires to reduce the system’s efficiency because it increases compressor power requirements and reduces capacity. Converting from liquid injection to external (thermosiphon or fluid-cooled) oil coolers can yield savings in the range of 3% to 10%.

10. Reduce parasitic loads
Look for opportunities to eliminate the heat leaks into your system. Attending to failed insulation, inadequate door seals, open doors and oversized conveyor openings are examples of easy fixes that reduce the heat gains that rob your system of both capacity and efficiency. Visual inspections and more sophisticated thermal imaging can pinpoint these hot spots. Find and fix them.

Of course, you’ll need to evaluate each of these suggestions in the context of an individual system. Table 1 rates the 10 opportunities qualitatively (low-medium-high) and by their capital cost (both new and retrofit), operational risk and overall value.

Get more information

In 2004, the Industrial Refrigeration Consortium (IRC) published the Industrial Refrigeration Energy Efficiency Guidebook that covers these and other energy-efficiency improvement strategies for industrial refrigeration systems. In addition, the IRC has published several energy-efficiency improvement-related articles in its quarterly newsletter, Cold Front. Past issues of the Cold Front are available for download at: www.irc.wisc.edu/?/newsletter. Seeking out, evaluating and implementing energy efficiency improvements at your plant isn’t only enjoyable, it’s rewarding.

Douglas T. Reindl, Ph.D., P.E., is professor and director of the Industrial Refrigeration Consortium at the University of Wisconsin-Madison. Contact him at [email protected] and (608) 262-6381.