Industrial Refrigeration Systems and Energy Use
System designers should be aware of the fine lines between inherent component efficiencies and the overall impact of those operational tendencies in promoting an efficient refrigeration system design.
Almost any time the topic of energy conservation is woven into a conversation, the discussion invariably seems to revolve around a new widget or a proprietary logic contained within a control system. Via their algorithms, controllers tend to mask the underlying behavior of the device they are supposed to be controlling and sometimes are viewed as “black boxes.”
Instead of being distracted by the mumbo jumbo, when considering energy conservation efforts, it is prudent to be aware of the inherent nature of the components being used, their limitations and the intended use of each particular component. For instance, heat exchangers operate under various parameters of heat transfer coefficients, surface area, surface enhancements (internal and external) and temperature differences. Various types of compressors can exhibit differing modes of partial-load performance via control of the swept volume of the compressor. Manipulation of the rotational speed for capacity control is an additional parameter that also can prove useful. Both of these can affect how the compressor uses energy in part-load conditions.
It soon becomes apparent that the individual operational components used in an industrial refrigeration system are essentially optimized by their respective manufacturers. Built-in design features such as variable volume ratio control in screw compressors, fan blade geometry, refrigerant pumps, control valves and finned tube coil configurations for heat exchangers are pre-determined.
Consequently, when designing a system and selecting specific equipment, the system designer should consider how all of the components work together in varying dynamic operating conditions. This is a topic not often addressed but it is a crucial one.
From an initial design perspective, the system designer is usually only considering how much capacity each component needs to provide for its intended duty. This selection process also is biased by the need for the lowest first installed cost. Secondly, this is also based upon the need for the refrigeration system to provide its maximum cooling capacity during the hottest and typically most humid weather conditions.
It might seem reasonable to assume a refrigeration system should operate efficiently if it is comprised of efficient parts. Likewise, it could be considered a successful installation if the system meets all of its primary objectives of providing the necessary cooling capacity during the times of peak need.
Unfortunately, the selection process of most components is a static exercise. Their application and selection typically are based on the peak cooling load in the hottest weather conditions. Refrigeration systems do not normally operate in such a narrow range of conditions in the real world.
Instead, if you distill the basic requirements for operation, you find the underlying issue is based on varying cooling loads and seasonal weather conditions. These define the basic operating parameters the system will need to meet.
With this expanded range of operational requirements placed on the refrigeration system, it behooves the owner to understand the impact of decisions made. A system is not inherently efficient if it is comprised of efficient components. In a similar consideration, attempting to control components to achieve some perceived form of efficiency will not provide the fullest benefit possible either.
Ideally, the goal of designing an industrial refrigeration system is to use the components that exhibit the energy-saving features and capabilities required to meet the full range of operating conditions. These include conditions such as the minimum and maximum cooling loads at their coincidental minimum and maximum weather conditions. It is also reasonable to state that these requirements should be considered during the periods when energy costs fluctuate.
The bottom line is this: you want a refrigeration system that produces the cooling capacity required to maintain the desired storage conditions at the lowest possible operating cost for 8,760 hours per year.
|Figure 1a illustrates a typical set of operating conditions the system could encounter. The necessary refrigerated temperature in the examples is 10°F (-12°C). However, it is also possible to use different saturation temperatures to improve cycle efficiency - 0°F (-17°C) in lieu of -5°F (-20°C) for the evaporating temperature and 90°F (32°C) in lieu of 95°F (35°C) for the condensing temperature - as shown by comparison between figure 1a and figure 1b. Figure 1c shows a significant reduction in apparent energy input cost. This benefit is gained by allowing the condensing temperature to fall when weather conditions permit this.|
Figure 1a illustrates a typical set of operating conditions the system could encounter. The necessary refrigerated temperature in the examples is 10°F (-12°C). This is determined by the process temperatures required. However, it is also possible to use different saturation temperatures to improve cycle efficiency - 0°F (-17°C) in lieu of -5°F (-20°C) for the evaporating temperature and 90°F (32°C) in lieu of 95°F (35°C) for the condensing temperature - as shown by comparison between figure 1a and figure 1b.
Another way to improve cycle efficiency is to have the refrigeration system configuration (single or multi-stage compression, or by utilizing economizer cycles) optimized via equipment selections so the system can operate within the full range of expected operating conditions.
Figure 1c shows a significant reduction in apparent energy input cost. This benefit is gained by allowing the condensing temperature to fall when weather conditions permit this.
The differences between figures 1a and 1b are due to an increased heat exchanger capacity for both the condensers and evaporators. This translates into a reduction in the thermal lift the compressor needs to generate. The benefit of the lower thermal lift is a lower apparent operating cost. By optimizing the system to fully use the colder weather, the thermal lift is again reduced with the corresponding reduction in the apparent energy costs as shown in figure 1c.
A similar situation arises when a reduced cooling capacity (part-load condition) is encountered. By using the full heat exchanger capability during a part-load condition, it is possible to reduce the discharge pressure and/or raise the evaporating pressure. This in turn also reduces the apparent thermal lift required.
Once an optimum system configuration is determined and the minimum required heat exchanger capacities are selected, it is prudent to select a suitable control system. The control system capabilities should extend to maximizing the savings via time of day use for on-peak and off-peak energy and demand rates for load control strategies during part-load or full-load conditions.
Secondly, the control loops can be tuned via the incorporation and control of variable frequency drives on the fan, pump and compressor motors.
The application of suitable control algorithms can be thought of as accentuating the part-load efficiencies of the components to meet the actual part-load operating conditions.
By realizing the importance of the interaction between components in dynamic operational conditions, the system designer can have a long-lasting impact while delivering world-class refrigeration systems.