Proper Refractory Dry Out Is Key To Optimum Service Life
There have been significant improvements in castable and plastic refractories over the past 35 years through the selection of better raw materials, improved particle sizing, additives, improved bonding materials and improved manufacturing processes and placement methods. Noteworthy in particular are the advancements in low-, ultra-low- and no-cement castables. There also is more awareness by vendors and owners alike of the importance of proper dry out or heat up, especially using the new breed of castables.
Because monolithic refractory products have become more technically sophisticated, their application often requires a turnkey project, which includes the supply of materials, installation and dry out in one package. It is therefore imperative that all parties work closely together to ensure a proper dry out, which ultimately maximizes the performance of the refractory lining in the application for which it was designed.
Refractory terms and concepts
Some definitions of terms and concepts used within the industry will help to understand refractory basics.
Curing is the period following placement of the refractory material. A hydraulic bond is formed after placement of a conventional castable, usually within 24 hours or less. The environment-temperatures during installation and curing in particular-has a profound influence on the ability to drive off the chemically combined water.
A ceramic bond is the mechanical strength developed by a heat treatment that causes the cohesion of adjacent particles, usually by fusion. Recently, some practitioners have suggested that it is necessary to heat the refractory to a temperature between 850 and 1000C (1560 and 1830F) to form a ceramic bond. Some modulus of rupture (MOR) curves have shown minor increases in this range due to some phase formations, probably feldspatic. You also need to consider the actual temperatures achieved for the bulk of the lining. While the hot face reaches these temperatures, the internal temperatures are not anywhere near that required to form a ceramic bond.
Dry out is the removal of sufficient quantities of moisture from the hot face to warrant it safe to either start the main burners or to start the process at a later time. It is purely and simply a dry out. While the term "heat cure" is being used, nothing is being cured.
Bake out is a term that crept into the refractories vocabulary recently and had been accepted as valid. Elevating the temperature of the refractory lining-particularly plastics-causes the formation of a chemical bond.
Heat-up means continuing either of the initial phases mentioned above so the unit can be put into operation.
Heat-up should be encouraged whenever possible. From a purely ceramic or engineering standpoint, a cool down can be more detrimental to the lining than the heat-up. It is not recommended to cool the lining to inspect for cracks, because it is well known that cracks are created in the lining after heating and cooling the lining due to tensile stresses on the lining. Aluminum melters in the U.S. and elsewhere are abandoning the practice of cooling aluminum furnaces down for inspection after heat up.
A contract dry out should be considered if permanent burners do not have the turndown ratio (the maximum-to-minimum input fuel rates for a given amount of air) for low-temperature control and uniformity; if the process itself or heat from another process cannot be controlled at low temperatures (this also includes companies having processes that have unacceptable emissions during the dry out process at low temperatures); and if the installed life of uncured plastics, particularly phosbonded plastics, is running out and the permanent burners or process is not ready to provide heat.
Preparation for a dry out should take into account the following items, which have nothing to do with the actual dry out, but can improve the outcome of the dry out process:
- The temperature at which the material is placed (installed) is of utmost importance; pay attention to the refractory supplier's recommended procedure.
- Plan adequate access that permits proper placement of the temporary dry-out burners into the unit so that a proper dry out and heat-up can be accomplished. Ignoring these items often requires a compromise by placing burners in locations that do not permit good circulation of the heated medium used, which sacrifices temperature uniformity across the entire surface of the refractory lining.
- Forms (particularly wooden forms) must be removed before dry out. Leaving forms in to hold the refractory in place until an elevated temperature is reached is not a good practice because most forms are tightly constructed and tend to cover or insulate the hot face during the initial critical stage of the dry out at low temperatures. Also, the wood or tube material typically ignites at a temperature around 230C (445F) creating high, uncontrolled temperatures occasionally in excess of 500C (930F), which results in spalling of castables and/or the collapsing of plastic linings.
- Although a considerable amount of water is evaporated through the hot face, most is driven to the cold face and condenses at some point near the shell. It usually collects at the bottom of the furnace or vessel wall. Weep, or drain, holes should be provided where possible at the lowest points of the vessel or furnace. Many years of monitoring weep holes placed above these low points indicates that they are relatively ineffective, and, therefore, drilling holes all over a furnace or vessel is not recommended. In pressure vessels having extremely thick refractory linings, cutting weep holes in the steelwork often is not permitted. However, the steam always finds its way out and the dry out is completed without any detrimental effect to the refractory lining.
- The best time to install embedded thermocouples is during refractory installation when knowledge of interface temperatures of multiple linings is needed to study profiles to better understand refractory deterioration, or to obtain accurate cold-face temperatures on refractory-covered water tubes in boilers.
Information on the physical configuration or shape of the furnace or process unit, ancillary portions, such as incoming feed lines or exhaust gas ducts, and access points or openings (anything over 6 in., or 150 mm) is important in preparing for contract dry out. It also is important to consider areas and accessibility outside the man way for the burner, combustion air fan and gas train. Arriving at a job site and having to install a burner in a man way at a height of 15 ft (4.5 m) without a platform or scaffolding is difficult and unsafe without proper planning and coordination.
Exhaust considerations often are overlooked. Exhausting at the highest elevation is preferred. It could be necessary to control the exhaust with a temporary damper plate. In a situation where a stack exists, the damper should be operable, or it might be necessary to use a burner to activate the stack draft. To prevent potential safety hazards, exhaust points should be removed from areas where personnel might have to perform other duties.
There are many fuels that can be used for dry out, but the most convenient, safe and economical fuel is natural gas. Fuels such as propane require pre-site planning because they pose additional safety considerations, and extra manning may be required to ensure safe and efficient operation. Cooling air or water must be available for protection of unlined areas during the dry out.
Temporary bulkheads are required to isolate unlined areas from the rest of the unit or to divert heat to areas inaccessible for burner placement.
Mass flow dictates the number and placement of burners for dry out, which also influences controllability, temperature uniformity and improved film coefficient of heat transfer, which improves the rate at which heat is transmitted into the lining. Reynolds Law deals with the fluid friction between boundary layers and how they affect each other. Heat exchangers operate based on this principle, and it also applies to refractories.
Consideration should be given to putting the bottom of the vessel or furnace under positive pressure, or at least neutral, to flood the area with hot air and prevent the ingress of cold air. Thermocouples should be placed in the expected hottest areas near the burners and at the coldest exhaust points to ensure that the temperature differential across the lining will be at a minimum. Temperatures specified in dry-out schedules should be the temperatures of the hot gases in contact with the hot face and not the refractory itself.
Air is the medium providing the heat and subsequently should be the item to be controlled and measured. The installation method of a thermocouple onto or into the refractory can introduce as much as a 50C (90F) error. For example, a 25-mm (1 in.) thick dense, high thermal conductivity material, such as that used extensively in the petrochemical industry, will produce considerably different results versus a thick, lightweight castable. If specific temperatures are to be achieved at an interface, at the shell, or any other area, a thermocouple should be located at those locations.
Choosing a dry-out schedule
The best source for a dry-out schedule is the refractory material manufacturer, who has done considerable product research on and has field experience with the material. Dry-out schedule variations are based on differences in lining permeability, density and thickness. Free water does not cause dry-out problems, but instead, research shows that a higher water-to-cement ratio facilitates drying and heat-up. However, it also increases porosity and decreases strength. It is the chemically combined water that comes off en masse at temperatures between 200 and 320C (390 and 610F) that creates problems during dry out.
Visible steam and the interpretation of pressure steam are items of continual discussion in the industry. The author is convinced after years of observing steaming of refractories during dry out that it is not visible steam that creates the problem, as it has made it to the surface and is relatively free to escape. Whisks of steam emanating from the refractory are evidence that the dry out is achieving the desired result of driving the moisture out of the refractory lining. It is the invisible steam that ultimately leads to problems by way of explosive spalling as the result of being entrapped within the refractory lining. Pressure steam can lead to problems although it very seldom is observed. It is the stream of steam in which you cannot hold your hand at a distance of about 4 in. (100 mm) for more than about 10 seconds.
The optimum temperature used for dry out should be that which can sufficiently set the plastic, or with castable, can remove the moisture from the hot face to a safe level. Taking the unit to its operating temperature may be costly from a time, fuel or preparation standpoint, and not having cooling air or water in specific areas or some air on the permanent burners can be disastrous. It may be at this point, when an installed threshold thermocouple reaches equilibrium after a few hours hold, that the unit is considered sufficiently dry to safely proceed into operation using the permanent combustion equipment or process heat.
Very little research has been conducted in the area of refractory dry out. Knowledge is gained from internal studies and practical field experience (such as Hotwork's 15,000+ dry-out projects). Refractory manufacturers have done an excellent job in developing new products with the necessary quality control in place. Today, refractory additives are measured in ounces/ton compared with pounds/ton previously. Refractory installers now have to deal with refractory products that are more sensitive to proper water content, mix time and placement methods, and have done an excellent job of adapting to such requirements. In addition, there is more awareness of the importance of a proper dry out. The combined efforts of the manufacturer, installer and the dry-out contractor result in a final product that will perform to its design capability and meet or exceed the expectations of the end user.