Please Pass the Salt
Some common problems you might find with glycol heat transfer fluids include frequent testing to ensure proper pH is maintained, formation of rust deposits, maintenance of water-to-concentrate ratio, and instability of the base fluid. This condition is accelerated by the use of tap water as that portion of the water/concentrate mix. Deionized water or water that meets specific quality requirements should be used. Figure 1 depicts a schematic of a typical cold loop system.
Replacing conventional pressurized refrigerants with an organic salt as the heat transfer fluid in a secondary loop heat transfer design offers several advantages. Secondary loop systems allow the high pressure refrigeration component -- for example, ammonia -- to be centralized and isolated, minimizing refrigerant charge. Given the significant regulatory barriers associated with high volume ammonia usage, the use of an organic salt for a secondary refrigerant is a cost-effective and safe enabler for use in secondary loop systems.
Historically, calcium chloride systems have been used in low temperature applications. However, they are rather labor intensive, and regular maintenance is required to prevent corrosion. pH changes, which occur frequently with calcium chloride systems, must be monitored closely, and premixing is necessary before adding to the brine solution. This must be done with care to ensure proper mixture without separation when the warm brine is added to the cold brine solution.
The main advantage is that calcium chloride has a lower viscosity at very low temperatures, allowing for the use of smaller circulating pumps. This advantage is overcome by the stability of organic salt and the ability of a properly formulated product to maintain pH -- and to do so without the formation of corrosion in the system. Organic salt formulations may be used safely at lower temperatures without the frequent addition of the water/salt mixture. At higher operating temperatures, where glycols have been used traditionally, the organic salt continues to have lower viscosity, allowing pump requirements to be minimized (figure 2).
Mixtures of organic salt may be used for the various temperatures of system requirements. Slush points should be selected slightly lower than lowest temperature expected in system operation to compensate for possible unexpected drop in temperatures and heat gain in transmission piping (figure 3).
Construction MaterialsMaterials that typically are used in heat transfer systems can be used in applications of this specific product. Steel, cast iron, brass, copper and bronze are acceptable materials of construction. Hard solder and plastics are acceptable, too. The use of galvanized steel is not recommended due to its zinc content. Galvanized fittings in steel bosses also will result in corrosion possibilities. Zinc, in the presence of this product, can react with the inhibitor additives contained in the blend and will cause the zinc to precipitate out and deplete the inhibitor package. This precipitation can act as a catalyst, causing localized corrosion.
It is recommended that bypass filters be installed in the system. These filters will remove precipitates that can cause corrosion. Heat transfer fluids generally are not recommended for use in systems that contain dissimilar metals due to galvanic corrosion.
Elastomers that are compatible with organic salt formulations include:
- Butyl rubber (IR).
- Polyethylene soft, hard (L/HDPE).
- Ethylene propylene-diene-rubber (EPDM).
- Polyethylene vulcanized (VPE).
- Epoxide resins (EP).
- Polypropylene (PP).
The interior building construction members may be used to run piping under a controlled temperature environment, thereby controlling heat loss or gain through transmission lines. This may be an advantage when building in sites where zoning regulations prohibit the installation of piping on the outside of a building. Plastic piping must conform to temperature requirements set by the manufacturer of the material.
When using heat transfer fluids, a less hazardous working environment is provided for workers inside the production area due to the lower pressures and non-volatile fluids. The particular formula referred to is NSF-approved for use in food processing.
Retrofitting Old Systems. When other heat transfer fluids have been used, corrosion may have built up in the system. When using the organic salt formulation referred to in this article, the additives used not only prevent corrosion from forming but also will remove scale that has developed in the system. For this reason, a bypass filter should be used to remove the scale. Bypass filters may be kept in the system permanently, if desired.