Cooling a room or process efficiently is the primary goal of any refrigeration system. The variety of equipment and refrigerants available today present plenty of cooling options but consider your facility and processing requirements (as well as the goals for your operation) before selecting the most appropriate system. Capacity requirements and application needs also affect the refrigeration systems available to you.
The first step in selecting a system is reviewing the refrigeration system application: is the chilling or freezing application for a room, the product, or both? A mechanical refrigeration system can be used for both room and product chilling and freezing, while a cryogenic refrigeration system is only applicable to product chilling and freezing.
Next, determine the approximate capacity needs of your refrigeration system. The number of rooms, size of the building, amount of product, temperature of product, and time requirements for chilling or freezing all affect the size of a refrigeration system.
The application of equipment that will cool each room or area requires attentive care. Cooler and freezer rooms are prone to condensation and frost if operating conditions and the surrounding environment are not considered. Evaporators with proper coil design and critical process air handling units can help mitigate these condensation issues. When selecting equipment, also consider the effect of moisture or steam that discharges from products or equipment, infiltration issues, and wet sanitation procedures.
Finally, understand the regulatory requirement for operating the refrigeration system. In general, the larger the system, the greater the regulatory impact.
Mechanical refrigeration systems consist of four basic components to extract heat from the building or product and transfer it to the building exterior: an evaporator, a compressor, a condenser, and a metering device. A mechanical refrigeration system may also be designed as a direct or indirect cooling system. A direct cooling system uses the refrigeration fluid (refrigerant) as the medium to cool the room or product. An indirect cooling system uses refrigerant to cool a separate fluid that acts as the medium for cooling the room or product.
Since all mechanical refrigeration systems consist of the same four basic components, selection depends on the type of refrigerant needed for the application. This is determined by the amount of cooling (BTUs per hour) needed and the medium being cooled (air, water, gas, liquid, product, etc.).
Traditionally used in meat processing plants, dairies and the brewery industry, ammonia refrigerant is best utilized when the cooling needs are large. As a rule of thumb, any cooling need over 200 tons (1 ton = 12,000 BTUs per hour) is a good candidate for an ammonia system. Also, ammonia is more efficient because its systems require less power to remove a unit of heat energy compared with systems using other refrigerants. This difference is amplified in low-temperature applications: the lower the refrigeration temperature, the greater the efficiency difference when using ammonia compared to other refrigerants, such as halocarbon.
Facilities with more than 10,000 lb of ammonia on-site are required to have a process safety management (PSM) plan in place. While following standard industry practices and performing a hazard analysis is heavily advised, some processors may want to limit the amount of ammonia in a refrigeration system to reduce the paperwork requirements of a full-scale PSM plan.
To achieve this goal, one option is the modular design of a low-charge ammonia system. These units include the primary equipment mounted atop a structural steel base. They can occupy a small area within a facility or on the rooftop.
Conventional ammonia refrigeration systems have charge-to-capacity ratios between 30 and 50 lb of ammonia per ton of refrigeration capacity. Smaller low-charge ammonia systems offer ratios between 3 and 6 lb of ammonia per ton of refrigeration capacity. Use of low-charge ammonia refrigeration systems may be able to reduce both safety and regulatory concerns altogether in their modular design. Again, a hazard review is still heavily advised prior to installation, but the use of low-charge ammonia refrigeration reduces the time spent reviewing a full-scale process hazard analysis for larger ammonia systems.
Halocarbon refrigeration systems are typically applied in two basic configurations: unitary and rack-mounted. For refrigeration capacity less than 200 tons, a halocarbon (i.e., Freon) system is typically the more affordable option when compared with ammonia. Halocarbon is also odorless and can displace oxygen if a leak forms, so oxygen sensors are required in key locations throughout certain applications.
Unitary systems, also referred to as packaged systems, have the compressor and condenser built inside a metal cabinet. These systems are typically used for room cooling and have an evaporator inside the room connected to a compressor/condenser located outside. The evaporator has circulation fans that blow air through its coils, thus cooling the air in the room. Evaporators can be placed in processing rooms, storage rooms, or blast chiller and freezer boxes.
Rack-mounted systems combine multiple small compressors fastened to a single rack. The compressors are mounted together so they may share a common piping system. The suction side of each compressor are all connected to a single suction pipe. The discharge of each compressor is also connected to a single discharge pipe. These common pipes are routed through the facility and connected to multiple evaporators. The compressor rack is typically located inside the facility, and a remote air-cooled condenser is placed on the facility’s roof or exterior to displace heat to the outside environment. When compared with unitary systems, the advantage of rack-mounted systems is they can better adapt to a diversity in the cooling load by cycling multiple compressors in the rack – unitary systems are not as flexible with only two or three compressors. Also, since the rack system compressors function as one system, the total amount of compressor capacity can typically be reduced as well. If a single compressor needs maintenance or repair, the entire system does not need to be shut down.
Chiller Systems are indirect systems that use either water or a water-glycol mixture as the heat transfer medium. The refrigerant (typically halocarbon) is utilized to chill the water or water-glycol solution in a heat exchanger. The chilled water or glycol solution is then circulated (pumped) to the locations where it is used to remove the heat from the environment.
These systems can be applied in numerous places in a food plant. Chiller systems are used for chilling bakery ingredient water, vegetable processing water, die heads in packaging equipment, milk storage tanks, cook/chill systems, barrels for extruders, and HVAC system for rooms, to name a few. Whenever a water-glycol solution is used, the mixture is typically 50-50, and the glycol must be food grade (propylene glycol).
Most chiller system applications utilize halocarbon as the refrigerant. However, one method for reducing the amount of ammonia on-site without getting rid of it altogether is to use an ammonia-glycol heat exchanger. With this application, the ammonia use is limited to the area or room where the compressors, vessels, and other equipment are located. Thus, the total amount of ammonia in the system is kept to a minimum because it is not circulated throughout the plant. Rather, the chilled glycol is circulated throughout the plant.
Cryogenic systems directly impinge food products with a cryogenic liquid (either liquid nitrogen or carbon dioxide) to create the cooling effect. These systems are only applicable for chilling and freezing products through direct contact with the cryogenic liquid (A cryogenic system cannot be used to refrigerate a room.) The cryogenic liquid is also a consumable product, and the spent vapor must be exhausted from the product chilling or freezing chamber. The spent vapor cannot enter the room because it has the potential to cause asphyxiation by displacing oxygen. Cryogenic systems are typically used instead of mechanical freezing when:
Capital costs are being kept to a minimum, or
Cryogenic systems are typically used in chiller or freezing equipment that utilizes a linear tunnel or spiral belt. CO2 is sprayed directly down onto the product as it is conveyed on a belt. The nozzles are usually positioned along the entire travel path of the product in the cooling chamber. When the CO2 is discharged from the spray nozzles, a portion of the CO2 changes into a solid, and the remainder into vapor. The solid forms as snow in the cooling chamber and adheres to the product Then, the snow vaporizes (sublimates) and draws heat out of the product. Most of the cooling effect occurs from this sublimation of the solid (snow) CO2, and the remainder of the cooling effect is from the cold vapor portion of the nozzle discharge.