The evaporator is a crucial component of any industrial refrigeration system. Advances in materials and components over the last decade have allowed manufacturers to develop increasingly compact systems at more attractive prices. But buyer beware: A low-priced chiller system can cost substantially more over its life in operating and maintenance costs if it is not designed to provide optimum performance in your application. Three key areas to evaluate in any evaporator are efficiency, durability and serviceability.
1. EfficiencyOne indication of system efficiency is the evaporator approach temperature. Approach is the difference between the leaving liquid temperature and the evaporating refrigerant temperature. The greater the approach (in other words, the greater the difference between the two temperatures), the higher the rate of heat transfer in the evaporator. However, from the overall system point of view, the lower the approach (in other words, the smaller the difference between the two temperatures), the less energy is needed to provide equal refrigeration capacity.
Traditionally, water-cooled evaporators have been designed for 9 or 10°F (5 to 6°C) approaches. However, with today’s demands for improved system efficiency, coupled with the advancement in compact heat exchanger design, evaporators can be designed to as low as 4 or 5°F (2 to 3°C) approaches. In fact, flooded evaporator designs can achieve approaches as low as 2°F (1°C). Many OEMs are now designing their packages with these low approach evaporators.
While many systems perform well with “off the shelf” evaporators, others - particularly compact systems - might need to be customized to cope with the chiller’s operational demands. Often, to increase a chiller’s performance, the operator simply will increase the gallons per minute of water flow. However, the increased water velocity can create excessive turbulence, which can cause impingement or vibration damage within the evaporator and shorten its useful life. The ideal evaporator design should be tailored to achieve the maximum turbulent flow to optimize heat exchange while not exceeding safe fluid velocities or pressure drops. Work closely with the evaporator manufacturer to ensure that the system will operate as intended in your application.
When evaluating efficiency, you also should make sure that an appropriate fouling factor has been used to determine the performance ratings. Water-cooled evaporators are subject to the accumulation of contaminants, or “fouling,” on the water-side heat transfer surfaces. Fouling gradually increases thermal resistance and degrades the overall performance of the equipment. A system rated without a fouling factor might cost less up-front, but its performance will drop when fouling occurs during normal operation.
The best way to evaluate efficiency is to make sure the evaporator’s performance ratings are true and in accordance with Air-Conditioning and Refrigeration Institute (ARI) standards, which establish rating criteria and procedures for measuring and verifying product performance. These standards allow products to be rated on a uniform basis, which promotes understanding of products, ensures compatibility and allows for more informed buying decisions.
2. DurabilityThe evaporator should be constructed of durable, corrosion-resistant materials and should be sized properly for the application. It also should be resistant to freezing. Localized freeze-ups are the primary cause of failure in water-cooled evaporators. While such freeze-ups cannot always be avoided completely, some evaporators are less susceptible to this problem than others.
You also should look for a system that is resistant to clogging and premature failure from process fluid residuals. An evaporator that is not properly designed for the application can fail, leaving a chiller system down and unproductive. The downtime cost of a process chilling system can be substantial, outweighing any savings initially obtained by specifying a lower cost - but improperly designed - evaporator for the application.
3. ServiceabilityNo matter how durable the system, some maintenance is inevitable over the life of the equipment. The evaporator should be designed to be serviced easily. Look for removable components and replaceable parts to ensure easy service and cleaning.
In conclusion, today’s evaporators are available in an ultra-compact footprint with full-rated performance. For example, you can now get up to 190 tons of refrigeration capacity with a shell-and-tube evaporator with only a 3.5' tube length. Similar advances have been made in water-cooled condensers, allowing for compactness in refrigeration system design. But choose wisely. A high-performance system might cost more up-front than other designs, but it can pay dividends in long-term operating and maintenance savings, as well as optimized refrigeration system performance.