System Water Chemistry Guidelines for Packaged & Portable Chillers
The properties of water make it ideal for heat transfer applications. It is safe, non-flammable, non- poisonous, easy to handle, widely available, and inexpensive in most industrialized areas.
When using water as a heat transfer fluid it is important to keep it within certain chemistry limits to avoid unwanted side effects. Water is a “universal solvent” because it can dissolve many solid substances and absorb gases. As a result, water can cause the corrosion of metals used in a cooling system. Often water is in an open system (exposed to air) and when the water evaporates, the dissolved minerals remain in the process fluid. When the concentration exceeds the solubility of some minerals, scale forms. The life-giving properties of water can also encourage biological growth that can foul heat transfer surfaces.
To avoid the unwanted side effects associated with water cooling, proper chemical treatment and preventive maintenance is required for continuous plant productivity.
Unwanted Side Effects of Improper Water Quality
- Corrosion
- Scale
- Fouling
- Biological Contamination
Cooling Water Chemistry Properties
- Electrical Conductivity
- pH
- Alkalinity
- Total Hardness
- Dissolved Gases
Chillers at their simplest have two main heat exchangers: one that absorbs the heat from the process (evaporator) and one that removes the heat from the chiller (condenser). All Thermal Care chillers use stainless steel brazed plate evaporators. Air-cooled chillers use air to remove heat from the chiller; however, water- cooled chillers use either a tube-in-tube or shell-in-tube condenser which has copper refrigerant tubes and a steel shell. These, as are all heat exchangers, are susceptible to fouling of heat transfer surfaces due to scale or debris. Fouling of these surfaces reduces the heat-transfer surface area while increasing the fluid velocities and pressure drop through the heat exchanger. All of these effects reduce the heat transfer and affect the efficiency of the chiller.
The complex nature of water chemistry requires a specialist to evaluate and implement appropriate sensing, measurement and treatment needed for satisfactory performance and life.The recommendations of the specialist may include filtration, monitoring, treatment and control devices. With the ever-changing regulations on water usage and treatment chemicals, the information is usually up to date when a specialist in the industry is involved. The following table shows the list of water characteristics and quality limitations.
Fill Water Chemistry Requirements
| Water Characteristic | Quality Limitation |
|---|---|
| Alkalinity (HCO3-) | 70-300 ppm |
| Aluminum (Al) | Less than 0.2 ppm |
| Ammonium (NH3) | Less than 2 ppm |
| Chlorides (Cl-) | Less than 300 ppm |
| Electrical Conductivity | 10-500μS/cm |
| Free (aggressive) Carbon Dioxide (CO2)† | Less than 5 ppm |
| Free Chlorine(Cl2) | Less than 1 ppm |
| HCO3-/SO42- | Greater than 1.0 |
| Hydrogen Sulfide (H2S) | Less than 0.05 ppm |
| Iron (Fe) | Less than 0.2 ppm |
| Manganese (Mn) | Less than 0.1 ppm |
| Nitrate (NO3) | Less than 100 ppm |
| pH | 7.5-9.0 |
| Sulfate (SO42-) | Less than 70 ppm |
| Total Hardness (dH)k | 4.0-8.5 |
† Dissolved carbon dioxide calculation is from the pH and total alkalinity values shown below or measured on the site using a test kit. Dissolved Carbon Dioxide, PPM = TA x 2 [(6.3-pH)/0.3] where TA = Total Alkalinity, PPM as CaCO3
Recommended Glycol Solutions
| Chilled Water Temperature | Percent Glycol by Volume |
|---|---|
| 50°F (10°C) | not required |
| 45°F (7.2°C) | 5% |
| 40°F (4.4°C) | 10% |
| 35°F (1.7°C) | 15% |
| 30°F (-1.1°C) | 20% |
| 25°F (-3.9°C) | 25% |
| 20°F (-6.7°C) | 30% |
CAUTION: When your application requires the use of glycol, use industrial grade glycol specifically designed for heat transfer systems and equipment. Never use glycol designed for automotive applications. Automotive glycols typically have additives engineered to benefit the materials and conditions found in an automotive engine; however, these additives can gel and foul heat exchange surfaces and result in loss of performance or even failure of the chiller. In addition, these additives can react with the materials of the pump shaft seals resulting in leaks or premature pump failures.
WARNING: Ethylene Glycol is flammable at higher temperatures in a vapor state. Carefully handle this material and keep away from open flames or other possible ignition sources.
