In recent years, interest in utilizing secondary refrigerants in large industrial refrigeration systems has been increasing. This is due in part to a desire to reduce the quantity of primary refrigerant and to limit the primary refrigerant charge to the refrigeration equipment room. In many installations, the primary refrigerant is ammonia; however, the primary refrigerant also may be other synthetic refrigerants. Often, facilities are interested in reducing ammonia inventory due to increased regulation and scrutiny of ammonia refrigeration systems. Insurance companies, governmental bodies and other authorities having jurisdiction may look more favorably on systems with lower quantities of ammonia refrigerant.
This article will describe the benefits and disadvantages of using two indirect systems for industrial refrigeration:
- Cascade refrigeration systems, where the central (primary) refrigeration system evaporator serves as the condenser for a secondary refrigerant.
- Closed systems, where the central (primary) refrigeration system removes heat from a circulating heat transfer fluid such as glycol.
We will first look at the potential benefits secondary refrigerants can provide and then look at the costs associated with those secondary refrigerants.
Benefit of Using Secondary Refrigerants
The benefits of using secondary refrigerants for industrial refrigeration include having a small primary refrigerant charge, increased safety by limiting the primary refrigerant to the engine room, a secondary pipe system that can sometimes be smaller, and the ability to use alternate insulations.
Small Primary Refrigerant Charge. By eliminating the use of primary charge outside of the refrigeration equipment room, the total quantity of primary refrigerant can be reduced.
Primary refrigerants routinely cost more per pound than a secondary refrigerant, so using a secondary refrigerant can result in a lower first cost of a system. A primary refrigerant such as R507, and even ammonia, is purchased at a higher cost than carbon dioxide (CO2) and most glycols. R507 is more prone to leaking through seals, flanges and other parts of the system than other refrigerants. The cost of replacing a smaller charge of primary refrigerant, should a loss of refrigerant occur, will be substantially less than for a direct system with primary refrigerant throughout the facility.
Using a smaller primary refrigerant charge also may reduce the size of the refrigerant receiver and pumpdown requirement in a system. In addition to cost savings due to a reduced primary refrigerant charge, the initial capital cost of refrigeration vessels will also be reduced.
Finally, for refrigerants such as ammonia that have inventory thresholds which trigger additional regulatory compliance, it may be possible to keep a primary refrigerant charge under the threshold when limiting the primary refrigeration system to the refrigeration equipment room.
Ability to Contain and Detect Leaks Provides Safety Benefits. Often, it is more difficult to detect leaks in synthetic refrigerants such as R507. Keeping the primary charge contained in the engine room reduces the potential of a harmful release. Also, the leak may be detected earlier and the effects of the leak mitigated with an exhaust system. Having a lower primary refrigerant charge reduces the total potential amount of refrigerant that can leak from a system.
A lower-risk refrigerant may be used as the secondary refrigerant. Busy docks and active processing spaces have higher potentials for refrigerant piping to be damaged. If a secondary refrigerant such as a propylene glycol leaks into the refrigerated area, there is less of a chance for injury to personnel or damage to equipment and loss of product. For secondary systems that are not considered hazardous, safety and monitoring systems may be reduced in coverage area.
Smaller Secondary System Pipe. In large process and refrigerated storage facilities, carbon dioxide utilized as a secondary refrigerant may allow a design engineer to use smaller suction pipe. The reduction in pipe size lowers the initial capital cost of the project by reducing the cost for the pipe and also the cost to insulate the pipe. Smaller pipe sizes reduce the risk of injury in handling during installation and repairs and can reduce the shipping, handling and rigging costs.
Potential to Use Less Expensive Piping. Secondary refrigerant systems using glycols and water can use materials of construction that are less expensive to procure and install. The lower operating pressures of these refrigerants allow the use of reduced-wall-thickness steel pipe and less expensive pipe classifications for lower pressure ratings. Materials that are compatible with glycol and water such as polyvinyl chloride (PVC) may be used in place of more expensive steel pipe for installation throughout a facility. PVC costs less to purchase, and it can be installed more quickly without requiring certified pipe welders and fitters.
Alternate Insulation Options. Secondary refrigerants that use nonferrous pipe or PVC can use insulations such as elastomeric materials. It generally takes less time to install elastomeric insulation, and the insulation may be jacketed with PVC or a mastic to provide additional protective covering. Elastomeric insulation can be easily repaired and, because it is used on nonferrous materials, vapor-barrier integrity is not as important as in other insulated pipe systems.
Costs of Using Secondary Refrigerants
While using a cascade or closed refrigeration system with primary and secondary refrigerant loops provides many process benefits, drawbacks do exist. Among them are increased operating costs due to reduced energy efficiency and increased system complexity.
Energy Efficiency. In most cases, secondary refrigerants add to the operating cost of a system. Additional electrical usage in secondary refrigerant systems appears in several sub-systems.
- For glycol, water and other brine-type systems, pumps are required to circulate the secondary refrigerant to the evaporators or process heat exchangers. The pump power requirements add to the overall operating cost of the system.
- To transfer heat from the secondary refrigerant system to the primary refrigerant system, a heat exchanger is used. The primary refrigerant system must operate at a temperature below the secondary refrigerant for heat transfer to occur. Heat exchanger selection can be optimized to balance initial costs and operating costs.
In an indirect system, the primary refrigeration system will always require more power to operate than in a direct refrigeration system. The difference between the primary refrigerant temperature and secondary refrigerant temperature is dependent upon the heat exchanger selection. A larger heat exchanger will reduce the difference in temperature between the primary and secondary refrigerant, and a smaller heat exchanger will increase the temperature difference. As the temperature of the primary refrigerant decreases, the required compressor power increases. Smaller heat exchangers also may increase the pressure drop through the secondary refrigerant side, which will require additional pump power.
It should be noted that in some cases where the right combination of operating temperatures and primary/secondary refrigerants are utilized, a cascade system may be more energy efficient than a direct refrigeration system; however, in the majority of installations, this is not the case.
System Complexity. Indirect refrigeration systems have added complexity in design and operation. Interfacing the primary and secondary refrigerant through a common heat exchanger and controlling the primary refrigerant system capacity to match the secondary refrigerant requirements increase the complexity of the refrigeration control system.
Defrost controls can get complicated in glycol and carbon dioxide systems that operate below freezing. For glycol systems, electric defrost or a warm glycol circuit is required to defrost evaporator coils. In carbon dioxide systems, warm glycol, electric defrost heaters or high-pressure hot-gas defrost may be used in the evaporators for defrost. These defrost methods can add additional electrical requirements and require additional distribution piping, valves and auxiliary equipment. Carbon-dioxide-based piping and fixtures must be designed and rated for high operating and static pressures.
In conclusion, each industrial refrigeration system application is unique and, therefore, each proposed installation should be evaluated to determine the best refrigeration system to meet the specific site requirements. A system that utilizes a secondary refrigerant either as an indirect system or a cascade refrigeration system may be a good option when all design factors are considered. The use of first-cost and operations-and-maintenance costs must be considered over the full lifecycle of the facility to determine the value of these systems. Finally, the considerations of non-financial parameters will need to be quantified to make the best decision for a given facility.