It is essential for a vacuum furnace to function correctly: expensive downtime is unacceptable regardless of which industry you are in. So I am offering you 5 tips for preventing faults caused by the process water in the cooling system or the use of unsuitable equipment. Lastly, to conclude, I’m also going to tell you what happens during an emergency situation...
1. Why process water makes all the difference
Corrosion, deposits, microbiological growth can have devastating effects on the cooling system. The process water must maintain specific purity standards, similar to those required for a boiler. For example, it must not be corrosive, it must not be demineralized, it must not contain dissolved oxygen and it must not contain fine sand in suspension to avoid clogging the cooling circuits, etc.
The best management of the cooling system requires checking a small number of important chemical, physical and microbiological parameters. The analyses needed for these checks are very simple and do not require special preparation or equipment which is expensive or hard to find.
2. Open or closed circuit? Keep an eye on the oxygen!
A suitable cooling system ensures maximum thermal efficiency. However, for efficient and problem-free operation it is essential to maintain the required values in the process water to prevent dangerous enrichment with dissolved oxygen. Unfortunately this occurs when using an open-circuit cooling system (such as a direct exchange cooling tower) with return to the tank in free fall: the water is cooled by direct exchange with the air, bringing with it a host of problems. Such as? For example, the creation of a favorable environment for bacterial growth resulting in less efficient heat transfer due to the formation of silt or bacterial flora.
For optimum heat exchange and management, closed-circuit cooling systems are preferable because they prevent the water coming into contact with the environment (and the oxygen). How? The liquid, contained in a special tank (of a size compatible with the features of the furnace) is protected on the surface by a layer of inert gas (nitrogen) which prevents the oxygen in the air from dissolving in or mixing with the water.
3. What is the right temperature of process water?
The water in the process cycle promotes rapid heat dissipation. But be careful, because if the water temperature is too high, especially during the shutdown phase for cooling and the hardening of the load, this can damage the vacuum furnace.
Inside the furnace, a gas-water heat exchanger exchanges heat with the load and the parts of the furnace itself (such as the heat chamber and resistor), heating the water circulating in the exchanger. This heating must not exceed the temperature threshold (approx. 60° C) at which the salts contained and dissolved in the water will separate and attach themselves to the metal, creating solid deposits in a layer inside the heat exchanger itself or in the various circuits, reducing the sections for water flow.
Filling the various circuits with excessively hot water can create structural distortions in the parts of the vacuum furnace most subject to heat stress. For example, the current loops in the copper conductor can reach melting point and cause water to fill the vessel, destroying the graphite part in the insulation and the resistor.
For the vacuum furnace to meet the considerable need for water it must have sufficient capacity (tub or tank) to quickly transfer heat from the furnace and from the load. The capacity of the tank determines the size of the system for cooling the water contained in it. Of course, the bigger the tank, the smaller the water cooling system it contains. When there are numerous furnaces, the size of the tank is calculated based on averaged values for behavior in the respective heat cycles.
The water entering the furnace must be below 25° C. This temperature has the advantage of serving all parts of the furnace, including the diffusion pump. When the water temperature reaches a greater value (up to 30° C) the circulation of the cooling water must be made independent from the diffusion pump using a chiller (cooling system using a refrigerated circuit) dedicated exclusively to this circuit.
4. Protect the cooling system from external cold
If the cooling system is located in extremely cold environments, the water tank can be equipped with a heater (electrical resistance). To avoid localized damage to parts of the system, when the vacuum furnace is turned off the circulation pumps must remain switched on.
If the cooling system is located in an area which is not particularly cold, antifreeze (glycol) can be mixed with the water to prevent the formation of ice in the winter. But pay attention, as the glycol must not be corrosive to steel (this also applies to the problem of disposal).
Let’s take a look now on the fifth tip to ensure significant energy and cost savings, with advantages for the environment and your finances! Lastly, to conclude, I’m going to tell you what happens during a power failure of the cooling system...
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5. Consider the cost of water cooling
Water can be cooled with closed-loop cooling towers or air coolers, with large chillers or using water-water heat exchangers, where a source of cold water is readily available (for example water from a lake or river). Nowadays, in particular, there is a cooling system called the closed circuit adiabatic system (international patent - manufacturer Alfa Laval Abatigo ABT); this is an advanced air cooler system, assisted by evaporated water, which allows considerable electricity and water savings while ensuring maximum respect for the environment. How does the closed circuit adiabatic system work? The adiabatic cooling system lowers the temperature of the air (air-water exchange) by increasing relative humidity, i.e., saturating the air with evaporated water to reach the wet-bulb temperature.
The cost of operating a cooling system which uses multiple water pumps and serves multiple furnaces can be high, especially if water delivery is not restricted during system shutdowns for short term maintenance or loading-unloading cycles which do not require any cooling.
What happens during a power failure of the system
The vacuum furnace must be cooled at all times during the process cycle. In the event of a power outage of the cooling system, 3 operations can be activated to keep the vacuum furnace safe:
- open a valve which is normally closed (which opens when the power supply is interrupted) from a pipe connected to the municipal water supply and, at the same time, close the valves for less important circuits in order to concentrate the flow on parts with high heat load;
- in the absence of an external water supply, use a pump which collects water directly from the tank, powered by the gas from the nitrogen tank (always connected to the furnace);
- use an electric power generator which activates the tank pumps that send water to the furnace.
Like all systems, the cooling system of your vacuum furnace also requires small adjustments to ensure efficient operation over time. And what about you? Are you constantly monitoring the cooling system to avoid damage to your vacuum furnace?
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