In a world full of cookie-cutter products, process chillers that are designed specifically to meet the needs of the processor and the installation site can be a great asset when dealing with conditions such as extreme ambient air temperatures. Designing a chiller to handle ambient air temperatures ranging from -40 to 130°F (-40 to 54°C) with modern refrigerants presents challenges. Chillers designed to meet these tricky temperatures, however, do exist.
Low Ambient Climates
If the packaged chiller or remote condenser is installed in the northern third of the United States or Canada, or if the ambient air temperature can reach -20°F (-29°C) or lower, low ambient control methods can be employed.
To handle low ambient air conditions of -30 or -40°F (-35 or -40°C), choose either the flood condenser method or the fan-speed control method.
Flood Condenser Method. Chillers designed for these applications flood the condenser with refrigerant to keep refrigerant pressure at 210 psi or higher. No liquid refrigerant is allowed to reach the receiver until the refrigerant pressure reaches 210 psi. Hot gas also is used in a bypass going directly to the receiver to help warm the liquid refrigerant. A receiver heater is used to maintain a minimum refrigerant pressure of 100 psi.
Fan-Speed Control Method. The fan-speed control method can be employed as an alternative to control head pressure in low ambient conditions. The head pressure will increase to a predetermined setting of roughly 225 psi before the condenser fans start to modulate on and off. The condensing temperature typically is less than 100°F (38°C). This method offers less control than the flood condenser method, but it does not require as much refrigerant.
Another feature of this type of design that helps with low ambient conditions is the chiller barrel. This storage method of cooling was invented about 100 years ago. At the heart of these chillers is a stainless tank with an immersed stainless steel evaporator coil. There is at least 3” of constantly flowing coolant around all sides of the coil. This creates a consistent thermal flywheel so the refrigerant experiences narrower swings in pressure. This allows more control.
FIGURE 1. The refrigerant circuit of a typical chiller with high ambient controls is shown.
High Ambient Environments
When the ambient air conditions are between 120 and 130°F (49 and 54°C), the chiller definitely has to work harder than in low ambient temperature systems. Refrigerant pressures are higher, and the compressor/condenser fans run more often.
Keep in mind that the condenser selection for ambient conditions above 100°F (38°C) is larger in capacity than the condenser selected for the same heat load when ambient conditions are below 100°F (38°C). This allows for more surface area and increased airflow to help reject heat.
A condenser is chosen with a built-in sub-cooler and, from there, the liquid line runs through a suction-line heat exchanger to bring the liquid refrigerant temperature down. The intention is to keep the liquid refrigerant temperature entering the expansion valves at 100°F (38°C) or lower. This makes the valves more efficient and the absorption in the evaporator more effective.
Quite often, chiller manufacturers also employ cylinder unloading based on pressure to help reduce the refrigerant pressure throughout the circuit. Multi-stage controllers and proper refrigerant selection are important ingredients for proper high ambient control. Figure 1 illustrates the refrigerant circuit of a typical chiller with high ambient controls.
Key characteristics can be used to design chillers capable of operating in some of the harshest environments. From the hottest places in the Middle East, to mountaintop installations at more than 14,000 feet above sea level, to locations in Alaska with low ambient conditions of -54°F (-48°C), it is possible to provide a chiller that meets the needs of the processor, the application and physical location.
Low Ambient Chiller Features
- Stainless tank with an immersed stainless steel evaporator coil for narrower swings in pressure and more control
- No risk of chiller barrel freezeup
- Ability to handle temperatures to -40°F (-40°C)
- HFO refrigerant eliminates use of ozone-depleting and high Global Warming Potential (GWP) refrigerant per the Montreal Protocol
- Outdoor installation
High Ambient Chiller Features
- Larger size condenser, often with subcooler
- Cylinder unloading
- Multi-stage controllers
- Ability to handle temperatures up to 130°F (54°C)
- HFO refrigerant eliminates use of ozone-depleting and high global warming potential (GWP) refrigerant per the Montreal Protocol
- Outdoor installation
Using Low and High Ambient Chillers in the Field
Outdoors is where the high ambient or low ambient chiller options are used most often. Common reasons why processors require their chillers to be installed outdoors include space constraints or noise concerns. Special engineering is needed when the chiller will be exposed to extreme high and low temperatures on a roof or on the ground.
Yet when it comes to delicate processes such as sensitive instrumentation or other industrial applications, or cooling medical equipment, the chiller still must deliver consistent cooling. In situations where the chillers must be located outdoors, paying particular attention to chiller design will ensure the cooling system can maintain the process temperatures needed as ambient fluctuates.
Case in Point: Nevada. Located in the Mojave Desert, Henderson, Nev., has one of the hottest climates in the United States. It was there that a processor needed equipment to be cooled, and the chiller had to be installed outdoors. Normally in Henderson, there are mild winters and extremely hot summers, but snow occasionally falls during the cold months. The chiller had to operate efficiently in temperatures from 41 to 101°F (5 to 38°C).
Working with a chiller supplier, the engineers selected a closed-loop chiller for the application. In this configuration, the chiller pump pulls water through the chiller storage cooling tank (which provides close temperature control) and pushes it into the industrial process application. It recirculates the coolant at constant pressure, and the flow is adjustable by a manual bypass valve. In addition to the high ambient option, the company’s specifying engineers also selected a high temperature interlock option, a safety feature for the process. With the interlock, when the chiller temperature exceeds the set high temperature, a warning alerts the operators of refrigeration failure.
Case in Point: Alaska. More than 3,400 miles north of Henderson in Palmer, Alaska, another processor reached out to a chiller manufacturer for help cooling their equipment. Palmer is located about 50 minutes northeast of Anchorage in a valley in between two glaciers. The climate is considered subarctic with intense winds and highs of 70°F (21°C) in the summer and lows of -20°F (-29°C) in the winter. For its low ambient application, the specifying engineers selected a closed-loop chiller. Like those in Henderson, the Alaskan team installed the high temperature interlock. In addtion, they asked for automatic switchover. With this feature, if a pump or compressor fails, the chiller automatically switches over to city water. PC