In the first part of this two-part article series (“The Regulatory Environment and the Importance of Reducing Charge in Cold Storage,” January/February 2016, p. 8 or www.process-cooling.com/Chapp-part1), I looked at the regulatory environment in the United States and the importance of reducing the charge of ammonia used in cold storage facilities and other refrigeration systems. The reduction of ammonia charge, as the article noted, currently is being achieved through one of four approaches:
- Eliminating all but the most essential ammonia charge.
- Maximizing the heat transfer from the ammonia residing within the system to the air or secondary fluid.
- Substituting other heat transfer fluids for ammonia wherever practical and economically viable.
- Rejecting the “more is better” philosophy on refrigeration when it comes to ammonia charge.
Pump-recirculated liquid systems carry high ammonia charge and are common in a majority of facilities. This article will examine a number of viable alternatives that can replace them.
Pumped-Recirculated Liquid Systems
The basis of this comparison is a conventional two-stage, pump-recirculated liquid system (figure 1 ). The system was installed in a 112,000-ft2 building with a 20’ high dock and the following load profile:
- Freezer: 283 tons of refrigeration (TR) at -10°F (-23°C).
- Blast Cells: 6 at 65 TR at -30°F (-34°C).
- Dock: 80 TR at 45°F (7°C).
The total refrigeration load for the facility is estimated at 753 TR.
The basic components of the system are:
- Booster compressor.
- High stage compressor.
- Evaporative condenser.
- High pressure receiver.
- Low temperature recirculator.
- Medium temperature recirculator.
- Freezer evaporators.
- Dock evaporators.
Ammonia Refrigerant Charge. The estimated ammonia charge for this design is 15,147 lb, or 20.1 lb of ammonia per ton of refrigeration. The industry seems to agree that, for any system to be considered low charge, it must contain no more than 50 percent of the charge of a conventional pump-recirculated liquid system. Said another way, a low charge ammonia refrigeration system must not contain more than 10 lb of ammonia for every ton of refrigeration.
Energy Consumption. Energy consumption could be rated at design loads and design horsepower. In reality, however, facilities rarely operate in this manner. The majority of the time, the refrigeration system is operating under part-load conditions or is in defrost mode. While this is a much more realistic way to assess energy efficiency, it also is much more difficult to capture the numbers and equally as difficult to compare one system to another.
For the purposes of this study, we have focused on the first approach — design loads and design horsepower — despite its inherit flaws. Based on previous work done by VaCon on a similar system, the baseline energy consumption is estimated at 2.5 kW/TR.
Web Extra: Packaged Systems
In addition to pumped recirculated refrigeration systems, CO2/NH3 cascade refrigeration systems and pumped volatile brine refrigeration systems, packaged ammonia refrigeration systems can be used for a low charge ammonia refrigeration system. Visit www.process-cooling.com to read this web-only extra.
The article will examine two viable alternative systems that can replace pumped recirculated liquid systems, which carry high ammonia charge and are common in a majority of facilities.
Installed Costs. Installed costs can vary significantly by region of the country. A canvas of a large number of contractors throughout the country showed that costs varied but, in general, they did not vary significantly for similar sized systems. It is fairly safe to say that the installed costs ranged from $6,000 to $8,000 per ton of refrigeration. For the purposes of this article, a realistic reference value for a pump-recirculated liquid system is $7,000 per ton of refrigeration.
Maintenance Costs. Especially as the world begins to evolve toward new technology, maintenance costs are important factors to consider when selecting a system. What is important to understand for the purposes of this article, however, is how these costs vary as a result of the type of system installed. As I review different systems, I will only assess maintenance costs on a relative basis in comparison to the pumped-recirculated liquid systems.
Carbon dioxide (CO2) systems have roots in industrial refrigeration as far back as the 19th century. Until recently, however, the thermophysical characteristics of carbon dioxide have made it difficult to utilize CO2 as a working refrigerant in industrial refrigeration systems in a practical and cost-effective manner. Modern advances in materials, manufacturing, equipment designs and controls, however, have made it a highly viable alternative to many of the common refrigerants and heat transfer fluids. At least one major cold storage company is using carbon dioxide/ammonia (CO2/NH3) systems extensively throughout its organization.
Two approaches are used today for subcritical CO2/NH3 industrial refrigeration systems:
The most common approach is a CO2/NH3 cascade refrigeration system.
- The less common approach is a pumped volatile brine (PVB) CO2/NH3 refrigeration system.
- In the interest of space, both systems will be discussed together, noting the differences between the two.
The two systems have some fundamental characteristics in common:
- Both systems operate in what is known as the subcritical region of two-phase CO2. The practical temperature range is -67 to 32°F (-55 to 0°C).
- The heat sink for the refrigerated spaces is accomplished with a second refrigerant. In the case of these two systems, that is ammonia. The heat exchanger used to accomplish this function is called a cascade heat exchanger. It serves as the ammonia evaporator and the CO2 condenser. Of special significance is the fact that the ammonia charge is dramatically reduced. At the same time — in almost all cases — the ammonia is confined to the machine room.
- Defrost is typically electric; however, hot gas defrost or glycol defrost also is used in these applications.
Both CO2/NH3 cascade refrigeration systems and pumped volatile brine (PVB) refrigeration systems have some special requirements for CO2.
- The evaporators — in particular, the low temperature evaporators — should be kept as oil free and water free as possible. This often calls for the use of an oil still and anhydrator.
- Another special requirement of systems with CO2 is consideration of stand-still conditions. Stand-still conditions occur when there is a system power loss, and they increase the potential for higher pressure in the system as a result of the CO2 liquid warming and evaporating. Of course, the system is equipped with safety-relief valves, but the objective is always to keep the refrigerant in the system. The most common means of preventing this from occurring is to use a small generator-driven condensing unit.
In addition, CO2 offers some advantages as a refrigerant:
- As a general rule, most of the CO2 is evaporated in both systems; however, the heat exchangers are not considered direct-expansion evaporators. While it should be noted that several manufacturers have introduced evaporators with lower recirculation rates for pumped-recirculated liquid systems, CO2 evaporators are generally close to a recirculation of one.
- Another advantage of CO2 as a refrigerant is its low temperature drop for every 1 psi of parasitic pressure drop in the system. This has the potential for some significant energy savings, especially in facilities in which parasitic losses are hard to prevent.
Both CO2/NH3 cascade refrigeration systems and pumped volatile brine (PVB) CO2/NH3 refrigeration systems are similar to a two-stage pumped-recirculated system, with one notable exception. In the PVB system, there is no booster compressor. However, any time a second heat transfer fluid is introduced into a system, the mechanical integrity of the cascade heat exchanger becomes highly significant. Cross-contamination of heat transfer fluids can have an extremely detrimental impact on the refrigeration system.
Finally, while CO2 may seem to be a benign substance, it must be remembered that it is heavier than air and can prove to be deadly if leaked into a confined space. CO2 gas detectors are essential in the system design.
Comparison of CO2 Systems
As shown in figures 2 and 3, both systems look similar. The ammonia side is, essentially, identical although there are small changes that can be made to optimize the performance of each. The fundamental difference between the CO2/NH3 system and PVB system is the absence of a compressor on the latter.
There are two primary benefits associated with the pumped volatile brine approach:
- Oil is no longer a concern. (There is no need for oil because there is no compressor.)
- Pump power is significantly lower than compressor power. This opens the door for potential energy savings.
Several different approaches are used in controlling evaporator temperatures. The most effective approach generally is chosen based upon the target freezer temperatures as well as the ratio of freezer load to cooler load.
Ammonia Refrigerant Charge. Most of the information available at the time of writing was provided by U.S. Cold Storage and represents the average for the facilities in operation. The information suggests a value of about 8 lb per ton of refrigeration. Theoretical values, however, suggest that the charge can be as low as 2 lb per ton of refrigeration. It is likely that the practical value is somewhere between 4 and 6 lb per ton of refrigeration.
For the purposes of this article series, a conservative value of 6 lb per ton of refrigeration was assumed for both the cascade and pumped volatile brine systems.
Energy Consumption. As noted previously, until actual operating data becomes available for a large sample population, energy consumption will be based upon design loads and design power consumption using a condensing temperature of 95°F (35°C).
For the purposes of this early work, there is little evidence to suggest much of a difference between the pump-recirculated liquid system and the cascade CO2/NH3 system. The PVB system, however, suggests a decrease in energy consumption based on the fact that there is no booster compressor. Because hard data is not available at this time, a conservative approach to this variable is to maintain a value of 2.5 kW/TR while recognizing that the actual numbers are likely to be much lower.
Installed Costs. A number of good arguments can be made to suggest that CO2/NH3 hybrid systems are less expensive to install than pump-recirculated liquid systems. One of the highly beneficial characteristics of carbon dioxide when compared to ammonia is the reduced pipe diameter for CO2 vapor. (Liquid lines for CO2 typically are larger than those for ammonia but still fall into the small diameter piping category.) Because vapor lines account for some of the larger diameter piping and longer runs of piping in any facility, the installed costs of vapor-carrying pipe can be significant. Smaller diameters translate to savings in:
- Piping (if the same materials are used for both systems).
- Welding (if the same weld procedures are used for both systems).
- Painting (if applicable).
Early information from U.S. Cold Storage suggests this to be the case; however, information obtained from three large U.S. refrigeration contractors that have installed CO2/NH3 cascade systems suggests otherwise. There are a number of possible explanations to this discrepancy but, for the purpose of remaining conservative with respect to new technology, the value used in this study was 5 percent higher than that of a pump-recirculated liquid, or $7,400 per ton of refrigeration.
Maintenance Costs. Other than the fact that half of this system uses carbon dioxide as the refrigerant, there is not a great deal of difference between the CO2/NH3 cascade system or the PVB system and a conventional pump-recirculated liquid system.
Of course, CO2 has a few challenges that must be carefully controlled. The two areas of special importance in the CO2 system are the filter driers and the oil-management system. A facility with a sound maintenance plan and a reliable maintenance staff should find no problems in meeting the requirements with little increase in maintenance cost. The conclusion here is that, while maintenance costs may be slightly higher than for a pump-recirculated liquid system, they will have a minimal impact. PC
Terry L. Chapp, P.E., is the national business development manager with Baltimore-based Danfoss. The manufacturer of industrial refrigeration technologies can be reached at 410-931-8250 or visit the website at www.danfoss.us.
This article is based on a white paper developed under the International Association of Refrigerated Warehouses (IARW) and the International Association for Cold Storage Construction (IACSC) Refrigeration & Energy Committee. For information about IARW or IACSC, call 703-373-4300. To read the full whitepaper, please visit http://bit.ly/AmmoniaPaper.