Entry losses

When dealing with water treatment applications you carmot avoid pipe flow calculations. We have a pipeline in which the throughput capacity of 500 Liter/sec. The flow is split into two pipelines and the inside diamter of the pipe is 350 mm. The length of the pipeline is 55 m. The entry loss is 0.70 and the exit loss is 1.00. There are two 45° bends and two 90° bends in the lines, (a) Determine the flow per pipe (b) determine the line velocity (c) determine the resulting hydraulic loss in meters. 

The calculation of the pressure drop for a chosen exhaust depends on the calculation method (Chapter 9). Pressure drop is usually calculated as the product of a hood entry loss factor, and the dynamic pressure in the connecting duct, p,/. The is expressed a.s p v-/l, where p is the air density and 1/ IS the air velocity in the duct. Some common hood entry loss factors are given in Table 10.4. 

TABLE 10.4 Entry Loss Factors for Flangod or Unflanged Round, Square, and Rectangular Tapered Openings... 

Exhaust shape Hood entry loss factor... 

The pressure loss associated with this type of exhaust opening is the sum of two pressure losses. The slot hood is usually thought of as a sharp-edged orifice and the duct entry (from the slot plenum) is a flanged opening. The rec ommended hood entry loss is given by Eq. (10.56) - ... 

Compound hood An extraction hood that has two or more points of appreciable entry loss. 

Discharge or entry loss of a louver The reduction in airflow caused by a louver. The discharge loss coefficient is equal to the actual airflow rate divided by the theoretical airflow rate at given pressure difference across the louver. If tested with the airflow in the reverse direction, the coefficient becomes the entry loss coefficient. 

Entry loss The loss of energy in the moving stream that takes place as a fluid enters an opening. 

Hood entry loss The pressure drop (energy loss) that occurs due to turbulence at the entrance to the extraction system. 

To size the fan it is necessary to know the total air volume and the pressures in the system. These are calculated from the losses in the system on the longest or index leg, and begin with the hood. The hood entry loss can be expressed as 0.6 of the velocity head and is accurate enough for first estimates. The losses are then calculated on the velocities in the ducts. Each change of direction means a small loss in each length of duct. Added to the pressure drop loss across the collector and the outlet losses, these give the total static pressure required in the system. 

Entry losses (at maximum design velocity) are calculated from the equation ... 

Example Use of gradually increasing diameter of gas ducts at entry of heat exchanger can reduce entry losses as compared to the loss when a small-diameter duct is directly connected to the heat exchanger (due to the formation of vortices). 

Entry loss = 0.45 velocity pressure for tapered takeoff 0.65 velocity pressure for straight takeoff. 

NOTE If grinding wheels are used for disc grinding purposes, hoods must conform to structural strength and materials as described in 9.1. Entry loss = 0.45 velocity pressure for tapered takeoff. 

An inlet vane control system (Fig. 29) comprises a static guide-vane unit which is installed in front of the fan s impeller and whose radial vanes can be swivelled by means of a control device so as to vary the inlet air flow conditions. These vanes deflect the inflowing air in the direction of rotation or in the opposite direction. As a result of this preliminary guidance, the entry losses are substantially less than those associated with ordinary damper control. 

A further saving in power consumption can be achieved by the use of an inlet nozzle, which reduces the entry losses, besides providing a convenient means of measuring the rate of flow delivered by the fan. See Fig. 34. 

CW = the pressure losses due to conduits and fittings between the tank (A) and the pump, including entry loss... 

Entry Loss The loss in static pressure caused by air flowing into a duct or hood. It is usually expressed in inches of water gauge. 

The number velocity K is calculated for both the short and long ends based on entry loss, exit loss, and frictional loss in the tube head. 

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