17 Oct Challenges and key considerations of refractory dry out
Refractory materials through technology have become more sophisticated which has increased its use in comparison to brick lining of various components such as kilns incinerators, furnaces, and systems regularly subjected to extreme heat or abrasion, bringing with it the need for proper carefully controlled dry out, which ultimately maximizes the performance of the refractory lining in which it is designed for.
The new breed of castable and plastic refractory materials have improved over the years because of better raw materials available and again through technology, bonding materials, and manufacturing processes have seen significant improvements in particularly to low and no cement castables.
The refractory material and the mixing of water are also fully controlled by detailed measuring applications which are carefully calculated prior to the mixing, and these improvements have produced a longer lasting life of the refractory in operation.
The final improvement over the past number of years and before the equipment is put into service to maximize the performance and extend the life of the refractory lining, is to ensure a correct and carefully controlled dry out is carried as per the dry out procedure and its detailed heating schedule designed for various type of refractory materials.
This controlled process of heat application is essential so that the water from the lining system is removed slowly without causing damage to the refractory lining after which time the equipment can be put into service with the knowledge that it is now safe to be heated with the process burners/heating system.
The actual refractory dry out procedure, heating rates, hold points are always agreed upon between the owner, material supplier/designer, refractory installer and last but not least, the dry out subcontractor in order that all parties have signed off their agreement prior to the commencement of the dry out operation.
The requirement for a controlled dry out has increased because of the new types of refractory materials being used and before a system/unit becomes operational it is advisable the removal of free and combined water is carried out with qualified personnel and combustion equipment that provides the accurate controlled rates of heating in line with the dry out schedule.
High velocity gas burners are now the optimum combustion systems utilised in the dry out process, which have high turn down ratios (50:1) and can control rates of rise as low as 2 to 5 degrees per hour which will ensure the slow boiling off of the water and in turn eliminate the pockets of high pressure steaming being produced that could cause spalling of the refractory.
To avoid any misunderstanding the following 5 items define and identify the terminology which applies to refractory dry out applications:
Prior to the commencement of the dry out an ambient temperature air curing is normally mandatory. The water and the refractory material react at ambient temperature to form a strong bond which normally will occur within a 24 hour period after the installation is completed.
The heating of the refractory carried out under the guide line of the dry out schedule to safely remove free and combined water from the refractory after the curing period.
To carry out dry out phase and then continue the controlled heating up to operating temperature, at which time the process burner/burners a take over and the unit is put into operation.
Excess water that is remaining in the refractory pores after the installation is completed and will boil off easily during the early stages of the dry out operation at lower temperatures as unrestricted free water.
Combined water in the refractory in the hydrate phase that forms a bonded structure and unlike free water does not boil off at lower temperatures (100°C). The combined water is released during the increase in temperature at various temperature stages of the dry out schedule.
It is unfortunate but due to the cost of engaging a qualified refractory dry out contractor the client sometimes believes it is cheaper to use their process burner systems which do not have the necessary turn down ratios and more often than not they find themselves heating in uncontrolled conditions, especially from ambient to 600°C temperature range and then the danger is high pressure steam is rapidly produced and the chance of refractory spalling is unavoidable. This of course defeats the object because costly repairs of the refractory spalling area have to be carried out and the unit is out of service longer than planned. In most of these instances the client learns a costly lesson and then engages a qualified dry out contractor to perform all their future dry out requirements.
To ensure a long life for the system, and to guarantee optimal performance for the unit it is therefore essential that the refractory dry out is carried out by a qualified contractor, using high-velocity gas burner combustion systems.
The high velocity burners with high excess air and high turn down ratios are used and offer the best optimum control at both low and high temperatures
This technique produces an internal positive pressure by firing through nominated man ways, and nozzle locations or furnace doors, whilst manually controlling the exhaust through other nominated nozzles, man ways or exhaust stacks.
The number of burners to be used is dependant on vessel, furnace size, manway/nozzle locations, and peak temperature requirements in line with the dry out schedule
Burner nozzles are carefully located in such a manner that there is no flame impingement on any part of the refractory lining
Refractory dry outs using high-velocity air burners are powerful heating units and together with separate 15 kW combustion fans are high air and heat out put systems ranging from 1.5 thorough to 10 GJ out put per hour but the turn down ratio of gas to air ensures the ideal controlling for ambient to high temperature ranges.
Attempting to complete a refractory dry out without adequate experience, suitable equipment, or without a detailed refractory dry out schedule as supplied by the refractory manufacturer and or the refractory installation contractor can lead to damage to systems and costly repairs for the business.
With the above in mind Heat Tech offers a qualified refractory dry out service with many years of experience. We take pride in providing an efficient and timely service, and have available a full range of high velocity combustion systems (Both gas or oil fired).
We value all your enquiries and would be only too pleased in providing a personalised refractory dry out service that will suit your exact requirements, and therefore please don’t hesitate in contacting Heat Tech Australia whom will be only too pleased to assist.