Suction system

The fluid dehvery in an air-spray system can be pressure or suction fed. In a pressure-fed system, the fluid is brought to the atomizer under positive pressure generated with an external pump, a gas pressure over the coating material in a tank, or an elevation head. In a suction system, the annular flow of air around the fluid tip generates sufficient vacuum to aspirate the coating material from a container through a fluid tube and into the air stream. In this case, the paint supply is normally located in a small cup attached to the spray device to keep the elevation differential and frictional pressure drop in the fluid-supply tube small. 

Mechanical seal in suction system under vacuum is leaking air into system, causing pump curve to drop. 

When the suction system is under vacuum, the spare pump has difficulty getting into system. 

Turbulence Design Design, Inappropriate suction system (vessel, piping, connections, fittings) design. 

The suction system piping should be kept as simple as reasonably possible and adequately sized. Usually the suction pipe should be larger than the pump suction nozzle. 

The second major job of the suction system is to maintain the pressure above the vapor pressure at all points. 

NPSH calculations might have to be modified if there are significant amounts of dissolved gas in the pump suction liquid. See Suction System NPSH Available" in this handbook for calculations when dissolved gas does not need to be considered. In that case the suction liquid s vapor pressure is a term in the equation. With dissolved gases, the gas saturation pressure is often much higher than the liquid s vapor pressure. 

Figure 3-41. Typical suction systems. (Adapted by permission,...

Cavitation of a centrifugal pump, or any pump, develops when there is insufficient NPSH for the liquid to flow into the inlet of the pump, allowing flashing or bubble formation in the suction system and entrance to the pump. Each pump design or family of dimensional features related to the inlet and impeller eye area and entrance pattern requires a specific minimum value of NPSH to operate satisfactorily without flashing, cavitating, and loss of suction flowt... 

For design of a new system, it is recommended that the results of the summation above be multiplied by 2 or 3 to establish the jet system inert (noncondensables) capacity, and add to this the non-condensed process vapors that are released into the jet suction system. 

Figure 32.51 illustrates how system NPSH or NPSH-available is calculated for the usual suction systems outlined. For a centrifugal pump, the basic NPSH is calculated from ... 

If the suction level is above the pump there is usually no problem in priming (that is, ensuring that the pump suction system is full of liquid) unless it is pumping a volatile and vapor locks when stationary. In many... 

Figure 3-41. Typical suction systems. (Adapted by permission, Carter, R. and Karassik, R.P.-477. Worthington Corp.)...

Centrifugal pumps, 181 Discharge systems, 187 Example calculation, 186 Flow friction losses, 185. 186 Friction losses, pipe, see Chapter 2 Friction, 188 Pressure head, 184—186 Static head, 184-186 Suction head, 184, 185 Suction lift, 184, 185 Suction systems, 186 Hvdroclones, 265—267 Application system, 267 Ignition, flammable mixtures, 493 Impellers, centrifugal, reducing diameter, 203 Impellers,... 

Measures to reduce radon concentrations have been studied in an old house in which the radon decay-product concentration initially exceeded 0.3 Working Level (WL). Some of the measures were only partially successful. Installation of a concrete floor, designed to prevent ingress of radon in soil gas, reduced the radon decay-product concentration below 0.1 WL, but radon continued to enter the house through pores in an internal wall of primitive construction that descended to the foundations. Radon flow was driven by the small pressure difference between indoor air and soil gas. An under-floor suction system effected a satisfactory remedy and maintained the concentration of radon decay products below 0.03 WL. 

The fans were switched on at time E, causing a further decrease in concentration. Low concentrations were maintained for 4 days, at the end of which only one of the fans was kept in operation (office system F to G, sitting room system G to H). The increase in the concentration of radon decay-products after switching off one or both fans is evident. After time I, the radon decay-product concentration decreased, but less rapidly than in the first trial and the final value was not as low. However, a trend to still lower values was apparent when the exercise was concluded on day 19. Measurements of the concentration of radon in the exhausts of the two suction systems were made on three days and the results are given in Table II. 

The radon production rates for the dwelling, whilst the under-floor suction systems were operating varied over the range 4.7 to 18 Bq m"3 h"1, confirming the effectiveness of the system. 

Whilst the under-floor suction systems were operated, further measurements were made of the radon concentration in the pores of the wall dividing the office area from the scullery. On day 7 (Figure 11), when the concentration of the radon decay-products was low (approximately 7 mWL), the concentration of radon near the base of the wall was about 8800 Bq m 3. This had risen to more than 13000 Bq m 3 when measurements were made over-night between days 10 and 11. 

The internal walls between the scullery and the office and between the kitchen and the sitting room descended below the base of the concrete floor to the sub-soil. Clearly, the effectiveness of the suction systems in reducing the soil gas concentration at the base of the walls was limited because of the distance from the suction points. 

Suction on cavities created under the concrete floor with low-power fans was highly successful in reducing the radon decay-product concentrltions below the reference level. Such under-floor suction systems are relatively easy to instal and cheap to operate. They appear to hold considerable promise. 

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