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Examining cleaning room requirements, part 1.

Examining Cleaning Room Requirements, Part 1

The object of this series of articles is to provide information to permit examination of cleaning room requirements and determine if the foundry's present shotblast machines are adequately equipped to perform the tasks reliably at a reasonable cost.

Four basic measurements can be used to determine if a particular machine is operating efficiently. They are: * present cleaning cycle time vs. cleaning cycle time when equipment was new; * 300 hr life for vanes/blades, impellers, impeller cases/cages; * 1000 hr life for bottom end liners; * 0.33 lb of steel shot per wheel horse power per hour.

This series focuses on efficient, low cost operation of all types of blast equipment, concentrating on the most important components: the airwash separator, abrasives, centrifugal wheels and wheel performance.

Airwash Separators

Blast cleaning mixes sand, dust, core rods, nails, scrap and broken abrasive particles with good abrasive. An effective means of separation is required to remove these costly contaminants from the good abrasive in order that it may be reused. Airwash separators are used in abrasive blast equipment to perform this function.

The basic components of a good airwash separator are a scalping or screening device to remove coarse material, a distributing arrangement to form an abrasive curtain and an airwash section to remove dust, sand and very fine or broken abrasive.

Scalping drums located above the airwash separator are the most commonly used method for removing large foreign material. However, oscillating conveyors with scalping decks are used when huge quantities of tramp material are mixed with the abrasive.

If good abrasive is being discharged with the scrap metal, chances are that the holes in the screen are blocked, preventing the abrasive from falling through the screen to the airwash. Carry-out also can be caused by a change in the rotational speed of the drum. Check to make sure that it is turning at the specified speed, typically 28-35 rpm.

The abrasive/sand mixture falling from the separator lip passes through the airwash zone. High velocity air passing through the abrasive carries dust and sand particles into a collection hopper for discharge to a refuse container. It is absolutely necessary that a proper volume and velocity of air through the separator be maintained. It is recommended that 38 cfm of air be available per in. width of the airwash section.

A good airwash separator system should be capable of removing contaminants without discharging good abrasive to the refuse container. Also, it should ensure that only good, usable abrasive reaches the storage hopper. Therefore, it is critical that the abrasive curtain span the entire width of the airwash. Even a small gap will allow the majority of the air to go around rather than through the falling abrasive, resulting in contaminated abrasive reaching the storage hopper.

An airwash separator is sized to handle a fixed volume. The size of the separator installed on the equipment initially was determined by calculating the amount of horsepower required to meet production requirements (abrasive flow) and maximum sand flow rate.

The amount of sand flowing through the airwash separator has a greater effect on the capacity of the separator than will a change in the amount of shot. This is precisely where most foundries experience problems--that is, under-estimating the sand flow rate.

Sand retained in the shot is the single biggest contributor to short vane life and premature failure to other parts. The best method to obtain clean shot in the storage hopper is to regulate the volume of sand and shot flowing through the airwash section without exceeding the capacity of the separator. A surge hopper, for example, can hold excess material until the blast cycle is completed.

An excellent means of assuring an abrasive curtain across the entire width of the separator is by pneumatically controlling the separator metering gate through the use of a probe. The metering gate would be closed until the sand and shot were distributed across the separator, at which time the probe would sense the load and open the gate.

Another means of removing sand from the abrasive is a recirculation cycle. One or two metering gates would be strategically located in the storage hopper. The gates would allow abrasive to flow to the elevator at the end of each blast cycle, bypassing the centrifugal wheel and going directly to the airwash separator.

An inexpensive means of increasing the sand handling capacity of the separator is to increase the speed of the centrifugal wheel, thereby decreasing the abrasive flow. Because the energy imparted by the shot to the castings increases as the wheel velocity increases, in most instances it is possible to reduce the shot size used to compensate for the reduced flow (more pellets at a higher velocity).

When a batch barrel is used, a pre-blast tumble cycle will remove a large percentage of the sand before abrasive flows from the centrifugal wheel.
COPYRIGHT 1989 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Briere, Thomas A.
Publication:Modern Casting
Date:Jul 1, 1989
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