Protecting Your Vacuum Furnace Investment with the Right Equipment
Water is one of the least expensive and most readily available thermal heat conductors on Earth, and it’s critical for the protection and operation of your heat-treating equipment. Selecting the right water-cooling system, understanding the impurities in water, and maintaining your water-cooling system properly can prevent costly damage to your vacuum furnace.
Water and System Characteristics
There are several different configurations available for external cooling systems. As long as the water and system meet the below specifications for thermal energy transfer, it should work as intended, while preventing functionality issues with the furnace and water system.
Standard Water Characteristics
- Total Hardness as CaCO3 <100 ppm
- Total dissolved solids <200ppm
- Conductivity – 300 micromhos per centimeter
- pH – 7.0 to 8.0
- Suspended solids <10 ppm
*Water characteristics should be tested prior to equipment installation
- Maximum inlet temperature – 85 °F (29 °C)
- Minimum inlet temperature – above the dew point to avoid condensation
- Flow as specified on the equipment installation instructions
- Pressure between 40 and 90 psi*
- Supply and drain piping so that at rated flow, a 30 psi drop is available across equipment.*
Which external cooling system is right for you?
Some things to consider when choosing the right water-cooling system are:
- What is the budget?
- What size tank and heat exchanger will be needed? Or how much water will be needed per quench cycle?
- Will the system need to be designed to handle different quench cycle applications based on materials processed?
- Will the system need to accommodate multiple furnaces?
- Have you evaluated both your current and future water system needs?
- What are the local environmental laws?
- What are the local options for water supply?
- Does your facility already have a water-cooling system?
Water-cooling System Configurations
Water-cooling systems can be either an open or closed loop. In an open system, the coolant water is exposed to air at sight drains or a cooling tower, whereas a closed system does not have environmental exposure. The combination open/closed system is the most common type used in heat-treatment facilities because of its efficient design that minimizes waste and reduces water treatment costs. Below are the four main types of water-cooling systems to consider, each with pros and cons. Which one you choose will be based on the needs of your business and facility location.
Once Through System
In a once through system, the water is passed through the vacuum furnace equipment once and then is discarded or reused in other processes. This system has the least capital expenditure to set up, but it requires access to a large amount of clean city or well water. This can be cost prohibitive, causes more waste, and can cause environmental concerns. The water in this system is not usually treated for water hardness which can create build up over time and more need for chemical flushing and descaling.
Open Loop Recirculating System
The open loop recirculating system pumps water continuously through an open cooling tower or sump tank. A large amount of heat is removed through latent heat vaporization. The only water lost during this process is water that is evaporated. This makes this system less costly than a once through system. However, as the water evaporates, impurities tend to concentrate in the water solution, and debris can be picked up from the air. The concentration of impurities can be controlled by properly regulating the amount of water that is bled off the system and finding a balance that works within running specifications. This system needs to be monitored frequently as environmental conditions can change the amount of impurities in water over time.
Closed Loop Recirculating System
Closed-loop recirculating systems with water to air or water to water heat exchangers use very little water and have minimal treatment costs. These systems are limited to cooler climates as the ambient air temperature needs to be low enough to cool the water in the system. This type of system has very little water discharge, but scale and corrosion problems can still exist depending on the quality of water used. It might be beneficial to use distilled or reverse osmosis (RO) water to reduce scale and corrosion. If it is necessary to add anti-freeze or corrosion inhibitors to the recirculating water, ethylene glycol and low conductivity inhibitors are recommended (depending on location).
Open/Closed Loop System
The open/closed loop system is the most common type of water-cooling system in facilities with vacuum furnaces. This system combines an open system with a cooling tower, spray pond, evaporative condenser, or sump and a closed system for the furnace. The closed system uses a heat exchanger to transfer the heat generated by the furnace to the open system. The efficient design allows water to be added and treated without affecting the water/ethylene glycol mixture in the closed-loop side. Because less water is needed, treatment costs are lower, and maintenance is easier. Scale and corrosion might still be a problem though if the water used in the closed system has impurities.
Before selecting a system, Ipsen recommends consulting your plant engineer or water treatment expert for possible water sources and analysis information; this ensures the proper equipment is selected and an effective water treatment program is put in place.
Water Impurities and Water-Cooling Systems
As a universal solvent, water comes with many impurities including gases and organic or inorganic solids. These impurities can damage your water system if adequate precautions are not taken.
- Hardness: Water dissolves magnesium and calcium salts in the soil to create hard water. As hard water moves through the water system it can deposit these salts and create build up. A high concentration of these salts can also increase the conductivity of water.
- Suspended Solids: Solids (i.e. sand) are not actually dissolved in water. Due to the turbulent movement of water, the solids can become suspended and then deposited in the water system. This can cause clogging and reduce heat transfer. Suspended solids can be removed through filtration.
- Acidity: Water can contain a large amount of free mineral acid. This acid will corrode parts in the water system over time. Treatment with an alkaline agent may be necessary.
- Alkalinity: Alkalinity is water’s capacity to resist changes in pH by neutralizing acid. If water has a high alkalinity, it can cause scale to form. Treatment with an acid may be necessary.
- Slime and Algae: Slime and algae can form in open systems. Slime and algae are small biological materials (algae, fungi and bacteria) that form on surfaces that frequently come in to contact with water. The growth of organisms in piping creates differential oxygen concentration cells that may result in serious pitting of metal surfaces. Chemical treatment with chlorine is usually recommended.
- Dissolved Oxygen: Oxygen is usually the main contributor of severe corrosion. All water that has come in to contact with the atmosphere contains dissolved oxygen. Closed systems limit the amount of air exposure that water has, but open systems can have issues with too much oxygen. Direct de-aeration can be accomplished mechanically or by adding chemicals such as sodium sulfate. Generally, the most economical solution is the addition of corrosion inhibitors.
- Corrosion: Corrosion is caused by the transfer of electrons from the cooling water to metal parts, causing pitting and oxidation. Dissolved gasses like oxygen or carbon dioxide can accelerate this process. Corrosion inhibitors are usually recommended, the most common being chromate salts.
Maintenance Tips for Your Water-Cooling System
- Look for patches on the outside of the chamber. This could indicate a past hard water build up treatment which could be needed again.
- Monitor the inlet and exit water temperature on a weekly basis to make sure the water-cooling system is within the specified temperatures.
- Check each coolant path weekly.
- Perform a twice-yearly complete system flush.
- Check for other rust spots as this may indicate a slow leak in the water jacket.
- Check if the vacuum furnace vessel is too hot. Most vacuum furnaces are meant to operate with a temperature at or below 140 degrees on the external wall. Anything above 140 degrees indicates an issue with water flow.
- If you notice cooling water has decreased as it exits the system, it may be due to a buildup of rust and scale. You can use a chemical flush to clean out the buildup.
- If your heat exchanger is installed outdoors, it is important to winterize your water system. You can add a concentration of ethylene/propylene glycol to protect against freezing temperatures.