North American TwinBed ® II Regenerator Bed - Maintenance and Cleaning
Sheet 4343-4
CAUSES OF BED PLUGGING The TwinBed heat storage bed is subject to plugging similarly to a filter when used on furnaces with dirty flue gases. The plugging matter can be suspended particulate or vapors that condense while passing through the lower temperature zones of the regenerator bed.
DETERMINING TIME BETWEEN BED CLEANINGS Since cleaning necessity varies significantly between furnaces, the effect of plugging on the performance of the burner should be tracked. This program of data gathering, storing, and analysis can be as simple or involved as one desires. The main idea is to develop data to compare the burner performance with a "clean" bed to that of a "dirty" bed. This information can be used to establish the time to reach an unacceptable reduction of production rate and/or furnace efficiency. With the system operating with clean beds and normalized to all flow rates, temperatures, and pressures, the following data can be recorded for each burner: with the burner at a given firing rate (air flow), measure pressure drop across the bed; at the same firing rate, measure bed temperature and pressure drop when exhausting. The data can be recorded periodically to establish the rate of plugging and schedule a bed inspection/ cleaning. BED REMOVAL Note: The amount of time to clean each regenerator varies with the material handling equipment, familiarity of the personnel with the cleaning procedure, and amount of plugging material accumulated in the bed prior to cleaning. WARNING: Personnel must wear safety clothing including the following: face shield, dust respirator, steel toe shoes, heat Insulating gloves, heat resistant or leather clothing.
There are two stages of plugging:
Stage 1 : Particulate is evenly distributed in a layer on or within the bed. Combustion air flow is reduced due to the increased pressure drop, but minimal heat transfer loss occurs. Stage 2 : If plugging material is allowed to build up, combustion air may fluidize portions of the bed, causing channeling of the flow (little flow in some areas and high flow in others). In this condition, the heat recovery is reduced and exhaust temperature will rise. Aluminum oxide (particulate) and fluxing salts (condensed vapor) are the typical contaminants in aluminum melters . The frequency of bed cleaning varies with each application. Two to four weeks is typical for fluxing aluminum melters and anywhere from three months to 12 months for non-fluxing melters depending on type of scrap charged, and level of scrap contamination. How quickly a bed will plug is affected by furnace operating procedures, production rate, quality of the charge, furnace cleaning practice, quantity of salt used, and the amount of salt allowed into the main hearth chamber. Certain furnace operations can abnormally accelerate plugging. Alloying, furnace cleaning, and batch flux charging can release excessive volumes of particulate and vapor. If care is not taken, beds can plug within hours. Plugging can be minimized by operating the burners in "direct fire" or "stop cycle" during and for a few minutes after these operations. The dirty flue gases will then exit the flue rather than through the burner beds. In steel reheating furnaces the contaminants are iron oxide particles. Cleaning usually is required after the first nine months of operation and every six months thereafter. Glass melting has both particulate (batch carryover) and sublimed/condensed volatiles. Batch carryover sits on top, volatiles collect according to vertical temperature profile, just like aluminum furnace fluxes.
WARNING: Bed material must be cooled to safe temperature before opening burner.
Place burner(s) to be cleaned in the direct fire mode to cool the bed media. Other burner(s) can continue firing to heat the furnace while a burner is turned off to clean the regenerator. Before cooling, the temperature gradient across the heat transfer media will range from furnace temperature at top to ambient temperature at bottom. Establish a maximum working temperature that personnel can work with, then adjust the direct fire (cool down) time accordingly.
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