Cleaning mechanical

Mechanical cleaning involves the physical application of a force to clean a surface. Different types of force can be used in combination to achieve the required results. Depending on the type of applied force (manual, motive, or vibratory), mechanical cleaning can be classified as manual, abrasive, or ultrasonic. Each of these is discussed below. 

Manual cleaning is one of the most hazardous, labor intensive, inefficient, and expensive methods for cleaning process facilities. It is limited to surfaces that are relatively easy to reach. Chemical cleaning should be used to remove unwanted materials and contaminants in crevices, difficult-to-reach surfaces, and remote comers. 

With this method, an abrasive material is applied by a fluid at high velocity against the surface to be cleaned. Depending on the fluid used, a system is classified as dry or wet. 

Harder, less friable materials clean faster and are more economical than softer, friable abrasives. However, hard materials produce more cutting and gouging. If surface depletion due to cutting and gouging is unacceptable, a softer material must be used, at the expense of speed. 

Dry blast cleaning is a relatively simple system that uses compressed air to provide the motive force to the abrasive material. Air pressure for dry systems 

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When fouling occurs, even mechanical cleaning does not remove all traces of the bio film. Previously fouled and cleaned surfaces are more rapidly colonized than new surfaces. Residual biofilm materials promote colonization and reduce the lag time before significant fouling reappears. 

Mechanically cleaned racks allow smaller clear openings because head loss does not become so high. Mechanical cleaning can be intermittent or continuous. Intermittent cleaning is cycled by float-operated switches controlled by a float in the influent channel. 

Table 2. Mechanical Cleaning of U.S. Coal by Equipment Type...

Heat Recovery Equipment. Factors that limit heat recovery appHcations are corrosion, fouling, safety, and cost of heat-exchange surface. Most heat interchange utilizes sheU and tube-type units because of the mgged constmction, ease of mechanical cleaning, and ease of fabrication in a variety of materials. However, there is a rich assortment of other heat exchangers. Examples found in chemical plants in special appHcations include the foUowing. 

I Normal triangular pitch does not permit mechanical cleaning. With a wide triangular pitch, which is equal to 2 (tube diameter plus cleaning lane)/V, mechanical cleaning is possible on removahle bundles. This wide spacing is infrequently used. 

Mechanical cleaning of the inside of the tubes was described by John [Chem. Eng., 66, I87-I92 (Dec. 14, 1959)]. Rods and conventional mechanic tube cleaners cannot pass from one end of the U tube to the other. Power-driven tube cleaners, which can clean both the straight legs of the tubes and the bends, are available. 

Tubes are most commonly arranged on an equilateral triangular pitch. Tubes are arranged on a square pitch primarily for mechanical cleaning purposes in removable-bundle exchangers. 

Process-side cleaning. Either chemical or mechanical cleaning on the inside of the tubes can readily be accomplished. 

Mild scaling solutions requiring mechanical cleaning, since tubes are short and large in diameter... 

Mechanical cleaning includes scrubbing, scraping, brushing, mechanical shocking, and ultrasonic procedures. Scrubbing with a bristle brush and a mild abrasive is the most widely used of these methods the others are used principally as supplements to remove heavily encrusted corrosion products before scrubbing. Care should be used to avoid the removal of sound metal. 

The condenser was mechanically cleaned every 6 months. Heavy deposition due to silt, sand, and mud plagued this system. 

In the factory processes the sticklac is first passed through crushing rollers and sieved. The lac passes through the sieve but retains the bulk of the woody matter. The sieved lac is then washed by a stream of water and dried by a current of hot air. A second mechanical cleaning process removes small sticks which have not been removed in the earlier roller process. The product, seedlac, now contains 3-8% of impurities. 

As with other removable-bundle exchangers, the inside of the shell, and to the outside of the tubes. However, unlike the straight-tube exchanger, whose tube internals can be mechanically cleaned, there is no way to physically access the U-bend region inside each tube, so chemical methods are required for tubeside maintenance. 

The spiral-plate exchanger usually should not be used when a hard deposit forms during operation, because the spacer studs prevent such deposits from being easily removed by mechanical cleaning. When, as for some pressures, spacer studs can be omitted, this limitation is not present. 

Mechanical Cleaning Methods Possible Not possible Not possible... 

In Figure 3-5e the square-pitch layout has been rotated 45, yet it is es,sentially the same as Figure 3-5a. In Figure 3-5d a mechanically clean-able modification of triangular pitch is shown. If the tubes are spread wide enough, it is possible to allow the cleaning lanes indicated. 

In-Line Square Pitch. Popular for conditions requiring low-pressure drop and/or cleaning lanes for mechanical cleaning of outside of tubes coefficient lower than triangular pitch. 

Diamond Square Pitch. Popular arrangement for reasonable low-pressure drop (not as low as in-line square), mechanical cleaning requirements, and better coefficient than in-line square pitch. 

Bends make mechanical cleaning of tube interiors difficult. Also, only a few outer bends can be replaced, so retubing usually involves replacement of all tubes. 

NOTE The view that deposition is an indicator of more widespread problems is also common to cooling water systems. As a result of the complex nature of deposits, their removal by acid or mechanical cleaning of boiler or cooling systems is not always as simple a process as may be portrayed. 

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