Ammonia refrigeration systems are a common choice in many process cooling applications, including food processing, beverage handling, chillers and storage facilities. But designers must consider several factors when selecting an insulation system for the refrigeration system. For example, what will the ambient relative humidity be? Where will the pipes be located? If they will be outdoors, what are the expected maximum, minimum and average wind speeds? What is the pipe size? What is the desired system life? What assumptions have been made about the jacketing type, insulation thickness, number of layers of insulation, expansion joint spacing, installation techniques and aesthetics?
When selecting any insulation system, the designer must think about how to combat the three factors that can compromise any refrigeration system: heat gain, moisture and water vapor. Protecting the cold surfaces inherent in ammonia refrigeration systems is paramount, as they will naturally attract water vapor from the warmer and moister surroundings, potentially leading to corrosion. Excessive pipe corrosion can even lead to pipe failure and release of the contents into the environment - a critical concern, given the hazards and regulations associated with an ammonia leak.
One alternative for insulating ammonia lines and storage vessels is an insulation system consisting of extruded polystyrene in conjunction with polyvinylidene chloride (PVDC) vapor-retarding film and tape.
Keep in mind that extruded polystyrene is different from expanded polystyrene, the beaded material often found in cups, coolers and other items. The porous, open bead structure of expanded polystyrene allows water to pass through the material (figure 1). As expanded polystyrene absorbs water, its thermal performance is degraded.
In contrast, the nonporous, closed-cell structure of extruded polystyrene absorbs almost no water (figure 1). This characteristic means that extruded polystyrene’s thermal performance will remain high, and it will continue insulating despite the high moisture conditions in cold-service applications.
Benefits of Extruded PolystyreneThe main component of this high-performance insulation system is extruded polystyrene, a rigid thermoplastic foam produced in a continuous extrusion process. Extruded polystyrene is effective for insulating low-temperature systems as cold as -297°F (-183°C). The foam’s closed-cell, uniform structure, along with the naturally water-repellent nature of the polystyrene resin, provide moisture resistance. This characteristic allows the insulation to maintain its performance over time in moist environments like those in which ammonia refrigeration systems typically are used. The foam is odorless, nonfibrous, nondusting and nonirritating. Using extruded polystyrene in an ammonia refrigeration system provides a number of benefits such as good thermal conductivity, water resistance and condensation control.
Thermal Conductivity. An insulation’s effectiveness is measured by its thermal conductivity, or k-factor. The lower the k-factor, the more effective the insulation. Extruded polystyrene has a low k-factor that proves its effectiveness at preventing heat transfer from warm surroundings to the cold pipe or vessel, helping to reduce energy costs.
Water Resistance. Water vapor permeability is a measure of how quickly water vapor diffuses through a material. Water absorption measures how much water a material will take up when placed in direct contact with liquid water. To prevent icing, dripping water, pipe corrosion, loss of insulating ability and physical destruction of the insulation system, water and water-vapor infiltration must be minimized. Extruded polystyrene has water-vapor permeability and water absorption performance levels that provide backup protection in the event the vapor-retarding barrier is compromised.
Condensation Control. To minimize condensation, the outside surface of the insulation system must be maintained at a temperature above the dewpoint of the surrounding air. This control is best accomplished through the use of insulation that starts with and maintains a low k-factor. Extruded polystyrene provides a high performance level in preventing moisture condensation and the ensuing water problems.
Other benefits include:
- Good dimensional stability.
- High compressive strength.
- Design flexibility. It can be fabricated into various shapes for
and equipment insulation,
and it can be reused in many
- Ease of installation.
- Low friability.
- A clean surface.
- No known nutrients for mold and mildew.
ability to withstand frequent washdowns.
- Low permeation (less than 0.02 perms).
- Resistance to a range of environmental chemicals.
- Compatibility with all common insulation types.
- Compatibility with most current vapor-retarding film installation
- Uniform heat shrinkability for tight seals.
- No known nutrients for mold and mildew growth.
- Compliance with U.S. Department of Agriculture and U.S. Food and Drug
- An aesthetically pleasing white color.
In insulation for ammonia refrigeration systems, both traditional all-service jacket (a paper-backed aluminum foil wrap) and PVDC film and tape can be used as the vapor-retarding barrier. PVDC film is not known to be a nutrient source for mold and fungus growth, while the cellulose in paper-backed all-service jacket, if penetrated, can provide nutrients for fungal growth.
In conclusion, with its high latent heat, chemical stability, wide temperature range and low cost, it is easy to see why ammonia is a widely used refrigerant fluid. But, to extract the most benefit out of an ammonia refrigeration system, it is important to make an informed choice when selecting an insulation system. The insulation has to shield the system from water and water-vapor intrusion - a difficult chore for systems often located in harsh environments like rooftops and washdown areas subject to wide temperature ranges and high humidity.
With the ability to meet all of the stringent requirements found in ammonia refrigeration applications, insulation systems consisting of extruded polystyrene and PVDC offer an effective solution to provide long-lasting, superior service.
Sidebar: Get More InfoContact the following organizations for more information on why they recommend using closed-cell insulation such as extruded polystyrene in ammonia refrigeration systems:
- International Institute of Ammonia Refrigeration (IIAR),
- American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE), www.ashrae.org