Energy savings ranks as one of the most pressing considerations on the minds of almost every plant operator. Not only are there financial considerations but environmental ones as well. Though energy improvements can be incorporated into almost every section of a facility, a specific focus on cooling systems can have a profound impact.
Cooling systems often account for the bulk of energy consumption in a plant. Industrial chillers use approximately 20 percent of the total electrical power generated in North America. The U.S. Department of Energy estimates that up to 30 percent of energy associated with chillers is loss due to inefficiency. Fortunately, not all cooling systems have to consume such high levels of energy. Simply being conscious of energy considerations and incorporating them into the design can significantly reduce the energy consumption of a chiller system.
What is Chiller Efficiency?
Despite being a heavily discussed topic, no single, cohesive definition of “energy efficiency” can be used for all systems. Rather, the term must be defined contextually. With regard to chiller systems, “energy efficiency” refers to producing the greatest amount of cooling power (often measured in tons) with the lowest power usage (usually measured in kilowatts). Because chiller systems are complex entities with essentially two coexisting cycles — the refrigeration and the process —many factors can affect system efficiency.
One quick measurement of any chiller system’s efficiency is to check the refrigerant approach temperature (RAT). Though this measurement often is used in relation to comfort-cooling applications, it is applicable to industrial applications as well. The RAT is the difference between the exiting process fluid temperature and the saturated temperature of the refrigerant at the evaporator outlet. A high RAT means a greater temperature difference exists. Also, a machine with a high RAT likely has a design or operational problem causing poor efficiency.
Though calculating a high RAT may be easy with the right information, pinpointing the exact cause of the score may be more difficult. Knowing how design adjustments contribute to the issue will allow building of the most efficient chiller system overall.
Just as chiller efficiency is adversely affected by high ambient temperature, the same can be said of very low ambient temperatures.
Ambient System Considerations
Holistic system analysis is key to designing an efficient cooling system. With this approach, the process requirements as well as the general conditions surrounding the unit are taken into account. The best system designs match what the chiller’s required capabilities with the limits that may exist.
For instance, the ambient conditions should factor into the cooling system design. Air-cooled chillers must be installed away from excessively high ambient temperatures in order to reach maximum efficiency and heat rejection from the condenser. In these conditions, the refrigeration circuit operates with a favorable head pressure, and it achieves proper liquid subcooling. High head pressure results in a high compression ratio and increased power consumption by the compressor. Low (or no) liquid subcooling reduces the cooling capacity of the chiller. Both of these issues will reduce chiller efficiency.
In indoor installations, ensuring an adequate supply of fresh air will aid the chiller’s efficiency. Hot air discharged by the chiller’s condenser fan must be exhausted away from the unit to avoid recirculation and hot-air buildup. The chiller manufacturer can provide room ventilation guidelines based on the heat load imposed on the chiller and the energy consumption of the refrigeration compressor.
Ventilation usually is much less of an issue with an outdoor installation. To reduce the chiller’s exposure to direct sunlight and the hottest temperatures of the day, it is best to place the chiller on the north-facing side of the plant when possible.
Just as chiller efficiency is adversely affected by high ambient temperature, the same can be said of very low ambient temperatures. For operation in cold ambient conditions, the chiller should be equipped with wind baffles, variable-speed fans or other head-pressure controls. Unless these types of design elements are incorporated into the chiller construction, low ambient operation will cause low head pressure and low compression ratio. Both reduce cooling capacity and efficiency and could potentially cause a system shutdown.
For the best chiller performance, know what the manufacturer’s acceptable ambient air temperature range is. Match the conditions around the unit to these recommendations. In water-cooled units, every 1-degree increase in condenser water temperature decreases chiller efficiency by 1 to 2 percent. If forgotten about, ambient temperatures and their effect on the system could cause inefficiencies of up to 10 percent.
In addition to incorporating the surrounding conditions to design an efficient chiller system, the actual system load plays a roll. A designer needs to know the expected load for the system and should use that information when building the system. Chillers are most efficient when they stay close to their designed full load.
After determining the most pressing concerns for the system, the next step is to choose the chiller with features that best match these demands.
Choosing Chiller Features
After determining the most pressing concerns for the system, the next step is to choose a chiller with features that best match these demands. With many chiller brands and manufacturers to choose from, however, this process quickly can become complicated. Performing an accurate features and benefits analysis is one way to choose between options.
For instance, an integrated start/stop control in a chiller is key energy-saving feature. This control, if present, reacts to the incoming temperature of the process fluid. (It can be enabled during the chiller setup.) By choosing an optimal setpoint, the system anticipates when to turn on or off to best reach the setpoint temperature in an efficient fashion. This feature prevents a heavy heat load from hitting the unit and causing it to expend more energy.
This option is appropriate for chiller setups that can tolerate temperature variation. As a chiller starts, it immediately begins to cool the water. When the lower setpoint is reached, the chiller shuts off. The water heats up again and moves through the process. Once the water reaches the higher setpoint, the chiller will again click on and begin cooling. This constant cycling of the chiller saves energy because it prevents constant energy use.
A flow-rate monitor is another desirable chiller feature. The process flow rate should match the unit’s rating most of the time. Falling below the design expectation lowers overall efficiency. In the evaporator, flow rates below 50 percent of maximum can cause the chiller to cycle excessively, resulting in poor temperature control. Implementing flow alarms helps to alert users to a problem and prevents prolonged unnecessary efficiency loss.
Additional features that aid in maintenance such as sight glasses, automatic alerts or refrigerant monitors, should be incorporated if possible. Maintenance is important in keeping a system in proper working order. Any features that can make maintenance easier help keep up the system’s efficiency.
The Role of Care and Upkeep
Though a chiller system may incorporate all ambient conditions and have many important features, these alone will not be enough to create lasting efficiency. Rather, the system can only remain efficient with regular maintenance and proper care. Though the specifics of required maintenance will change based on the setup of a particular system, most systems have some common points that need attention.
One common maintenance area is the refrigerant. Both the quantity and quality of the refrigerant matter in keeping the system in good condition. Low refrigerant levels cause the compressor to work harder to operate the refrigeration cycle. If this area requires more energy, then the system on the whole cannot be as efficient. The most common cause of low refrigerant levels is a leak. Though leaks can be difficult to locate, most can be repaired by a certified refrigeration technician.
An undercharged system also can cause issues for the compressor. The low refrigerant level can result in the compressor running outside of its designed operating envelope, compromising its longevity.
Besides the loss of refrigerant, leaks also result in the loss of oil, which is essential for proper compressor lubrication. Traces of oil dripping from joints into the refrigerant tubing or other connection points are a sure sign of a refrigerant leak. Over time, even a small leak can deplete the oil in the system and cause a compressor mechanical failure. As a result, every preventive maintenance program should include evaluating the oil level. Depending on the system design, the compressor may have an oil sight glass in the sump. More sophisticated designs may include an oil level sensor connected to the chiller’s controller to automatically monitor the oil level on a continuous basis.
Additionally, regular cleaning is key. When refrigeration coils or fans become clogged with dirt and debris, they cannot discharge heat effectively from the system. The machine will begin to strain and will require more energy to complete its work than when the system was clean. Grime and buildup force the machine to work harder to provide the same outcome.
For instance, a ~ 0.025” (0.6 mm) layer of pollutant buildup on the finned coils is estimated to increase chiller power consumption by 20 percent, according to the Australian Department of Energy. Simple weekly checks can keep debris accumulation to a minimum. If the chiller is equipped with condenser filters, be sure to clean them on a regular basis as dictated by the cleanliness of the ambient air.
Never underestimate the power of good maintenance. Regular, systematic maintenance is key to keeping an industrial chiller system in peak operational condition. Small measures such as control calibrations and sensor adjustments can have big impacts if not performed regularly. The cleaner key chiller components (think heat exchangers) are, the more efficient the system will be. Whether performed in house or by a manufacturer-recommended company, regular machine upkeep is an important element to keeping your machine energy efficient.
Building an efficient chiller system certainly is not a quick or easy task. It requires true investigation into what a process needs and how to best deliver it. But with a combination of solid, dedicated system design, appropriate chiller features and a strong maintenance routine, it is possible to establish a long-lasting, efficient chiller system. PC