Using Ammonia as a Refrigerant
Ammonia has excellent cooling capacity, light weight and built-in leak detection (thanks to its odor). From the late 1800s when mechanical refrigeration first became available, ammonia was the refrigerant of choice, and after almost 120 years, it still is.
Ammonia (R-717) is used for large-capacity refrigeration equipment where compressor durability, economy of operation and plant safety are of importance. The majority of ammonia systems are found in the food refrigeration industries at holding temperatures above 30oF (-1oC), frozen storage and blast freezing to -50oF (-46oC).
Ammonia refrigeration also is found in gas processing, refineries and chemical plants. It is used in the pharmaceutical industry for freeze drying and process cooling. Other applications include ice rink freezing, concrete dam cooling and subsurface soil cooling. A small number of low temperature cascade systems use ammonia on the high side of the cascade with final temperatures between -60 and -150oF (-51 to -101oC).
Ammonia plants are large and typically require quantities of R-717 ranging from 3,000 to 500,000 lb. The equipment requires power in the range of 100 to 16,000 brake horsepower (BHP) and is usually field-erected as a one-of-a-kind system specifically designed to meet customer requirements. System components are normally selected from those provided by a relatively small group of manufacturers who work within the ammonia industry.
Operating conditions vary with the product being handled. Plants that do rapid product cooling and freezing generally operate at at least three temperature levels:
Approximately -50oF (-46oC) for freezing.
Approximately -20oF (-29oC) for holding freezers.
Approximately 30 to 35oF (-1 to 2oC) for produce storage, docks and cutting rooms.
In large plants, the refrigeration system may be spread out over several buildings with refrigeration piping used to move the refrigerant liquid and vapor throughout the buildings to and from the machine room. In such systems, pipe sizes of 12" and 14" are not uncommon.
Quick-frozen foods are handled on large, circular conveyors that spiral up from the work floor to a second-floor mezzanine area. It usually takes a half-hour for food entering the spiral freezer to reach the top. At this point, the product has been cooled to the final required temperature specified by the customer. The product throughput ranges from 4,000 to 10,000 lb/hr. The capacity of a spiral freezer depends on a number of variables, including specific food product, packaging, evaporating temperature in the freezer, number of ramps, belt speed and final temperature required.
After quick freezing, the product may be further packaged and transferred to a holding freezer for shipment. In some cases, the product may not be fully frozen and will go into a blast freezer room at approximately -40oF (-40oC) for a period of time sufficient to bring the product to the required temperature.
Another distinguishing characteristic of ammonia refrigeration systems is that they use evaporative condensing. This type of condensing provides the lowest possible system condensing pressure and temperature of any of the three types of condensing: air-cooled, water-cooled and evaporative cooled.
Air-cooled condensers operate solely on the dry bulb temperature of the air and condense approximately 25oF (14oC) above the air temperature. A 100oF (38oC) day results in a condensing temperature of 125oF (52oC). At this condensing temperature, ammonia will be at 293 psig. This is impractical for ammonia usage, primarily because of the high horsepower penalty associated with the high pressure.
Water-cooled condensers were common prior to the 1950s. In the case of water-cooled condensers, a cooling tower was used to cool water to approximately 80 to 85oF (26 to 29oC). This water was then used in a shell-and-tube condenser to condense the refrigerant. With the 85oF (29oC) water, it was typical to obtain 100 to 105oF (38 to 41oC) saturated liquid ammonia. This is acceptable but still above what is a practical saturation temperature. This system had an expensive shell-and-tube heat exchanger that required periodic service.
The evaporative-cooled condenser puts the two above components together in one unit containing a number of relatively thin serpentine steel coils, through which the ammonia vapor is passed for condensing. The unit also contains a sump of water that is pumped to the top of the unit and sprayed over the coils. Fans are used to either draw or blow air across the coils. In this process, some of the water evaporates from the coil surface and cools the refrigerant to the saturation temperature, causing it to condense. The temperature at which the water evaporates depends upon the wet bulb (dewpoint) temperature of the air and is not affected by the higher dry bulb temperature.
Evaporative condensers can provide refrigerant-condensing temperatures to within 10 to 15oF (5 to 8oC) of the local wet bulb temperature. This ensures that, with proper sizing of the evaporative condenser, the ammonia saturated temperature can be about 90oF (32oC) and certainly lower than 95oF (35oC) anywhere in the United States, even in the South where maximum wet bulb temperatures can be as high as 81oF (21oC). At 95oF condensing, the ammonia pressure is 181 psig, more than 100 psi less than what an air-cooled could provide on a 100oF day.
The evaporative condenser can be seen to provide the lowest possible condensing pressures when considering the common cooling source as the local atmosphere. Obviously, there will be special cases where a supply of cool water is available such as that from a lake or the sea in marine applications.
Types of Systems
Usually, the preferred method of supplying liquid refrigerant to the various evaporators throughout the plant is by pumping it at the saturated evaporator temperature from a low temperature receiver (recirculation unit) to the evaporators. Pressure from the pumps moves the refrigerant over any distance and variable heights without problems. The quantity of refrigerant supplied to each coil is several times the amount actually vaporized in the evaporator. The return liquid and vapor go back to the recirculation units, where the vapor is separated for return to the compressors. The liquid overfeed provides complete wetting of the coil surface with the boiling refrigerant for the best heat transfer rate.
These systems generally do not use direct expansion (DX) coils, so the condensing temperature does not need to be maintained for proper liquid feed. This means that the condensing temperature of the system can be greatly reduced when wet bulb temperatures drop during cooler seasons. For systems with hundreds or thousands of horsepower, reduced condensing pressures equate to reduced horsepower requirements and economical operation.
Codes and Regulations
Several refrigeration codes control the application of ammonia refrigeration systems. ASHRAE 15 Safety Standards for Refrigeration Systems is the code that covers all refrigeration and air-conditioning systems, and it has general recommendations. ANSI/IIAR2 Equipment, Design, and Installation of Ammonia Mechanical Refrigerating Systems is a code designed specifically for ammonia refrigeration systems and deals with the intricacies of ammonia and its proper handling.
In addition, there are two government regulations concerned with safety: OSHA PSM Regulation 29 CFR part 1910.119. EFF. 5/26/92 and EPA RMP Regulation 40 CFR part 68. EFF. 3/96. Both of these regulations contain 13 elements of concern that provide assurance that the plant is operated in a safe manner with proper maintenance, control of service, documentation and incident reporting. They are applicable to plants with more than 10,000 lb of ammonia.