Fan failures

Increased capacity in the event of fan failure, since the natural draft stack effect is much greater with induced draft. 

For inlet process fluids above 350°F, forced draft design should be used otherwise, fan failure could subject the fan blades and bearings to excessive temperatures. 

Low natural draft capability on fan failure due to small stack effect. 

Air Fin Exchanger Failure - Loss of air fm exchanger capacity may result from fan failure or inadvertent louver closure. 

In Europe, the gas safety controls must meet the requirements of CEN standards, including flame failure devices, solenoid control valve, pilot con trols, ignition and governor. Overheat-type thermostats and either a pressure switch or an airflow-proving device are fitted to ensure that the burner will cut off in the event of no air flowing through the heater, such as occurs with fan failure. 

With a fan-furnace heating system, once the temperature is reached, the fan is controlled to start automatically. Should fan failure occur, provision should be made to damp down the furnace automatically to avoid overheating s causing tube damage. 

Table 62.3 Generator fan failure data and hazard calculations...

For each failure time, calculate the corresponding cumulative hazard value, which is the sum of its hazard value and the hazard values of all preceding failure times. This calculation is done recursively by simple addition. For example, for the generator fan failure at... 

Plot each failure time vertically against its corresponding cumulative hazard value on the horizontal axis of the hazard paper. This was done for the 12 generator fan failures and is shown in Figure 62.7. 

Figures 62.8, 62.9, 62.10 show the data for generator fan failure plotted on exponential, normal and log normal hazard paper respectively. The exponential plot is a reasonably straight line which indicates that the failure rate is relatively constant over the range of the data. It should be noted that the reason the probability scale on the exponential hazard plot is crossed out is because that is not the proper way to plot data. (This will be discussed later.) The normal plot is curved concave upward which...

Figure 62.8 Normal hazard plot of generator fan failure data...

Figure 62.9 Generator fan failure data plotted on log normal hazard data...

The line the data supports on a hazard plot determines engineering information relating to the distribution of time to failure. Fan failure data and simulated data are illustrated here to explain how the information is obtained. The methods provide estimates of distribution parameters, percentiles, and probabilities of failure. The methods that give estimates of distribution parameters differ slightly from the hazard paper of one theoretical distribution to another and are given separately for each distribution. The methods that give estimates of distribution percentiles and failure probabilities are the same for all papers and are given first. 

Suppose, for example, that an estimate based on a Wei-bull fit to the fan data is desired of the fifth percentile of the distribution of time to fan failure. Enter the Weibull plot. Figure 62.6, on the probability scale at the chosen percentage point, 5 per cent. Go vertically down to the fitted line and then horizontally to the time scale where the estimate of the percentile is read and is 14,000 hours. 

An estimate of the probability of failure before some chosen specific time is obtained by the following. Suppose that an estimate is desired of the probability of fan failure before 100,000 hours, based on a Weibull fit to the fan data. Enter the Weibull plot on the vertical time scale at the chosen time, 100,000 hours. Go horizontally to the fitted line and then up to the probability scale where the estimate of the probability of failure is read and is 38 per cent. In other words, an estimated 38 per cent of the fans will fail before they run for 100,000 hours. 

Another environmental effect that should not be overlooked is the amount of airflow through the fan. To remain at an appropriate operating temperature the fan requires sufficient airflow to remove fan motor heat. Fan motor capacitor failure will cause the motor to operate at a lower speed and efficiency, especially after the motor has been shut off by the occupant or electrical power interruption. Operating the fan in either of these modes will lead to higher radon levels in the living space and invites early fan failure. 

Mech. exhaust fan failure 2. Same as //I above. I B 1 None... 

Upon fan failure, increased capacity compared to forced-draft coolers, since the airflow is designed for natural draft... 

Upon fan failure, very poor natural draft capability... 

There should be no failure of a supply air fan, return air fan, exhaust air fan or dust extract system fan. Failure can cause a system imbalance, resulting in a pressure cascade malfunction with a resultant airflow reversal. 

With air coolers, louver closure is considered a total failure (10). Upon fan failure, or a fan drive (e.g., power or steam) failure, a credit is often taken for natural convection effects. This credit is usually 20 to 30 percent of the normal duty of induced-draft condensers. Forced-draft condensers have a considerably weaker chimney effect, and the credit taken is usually 10 to 15 percent of their normal duty. The above natural draft credit may not apply if a fire occurs near the cooler. 

Motor-control circuits have been designed to provide sequential startup of the main HCF ventilation fans in a manner that ensures that the zone differential pressure hierarchy is maintained. Interlocks are employed to shut down the necessary fans to avoid adverse pressure differentials in the event of fan failures. 

A TSR requirement to verify that Zone 1 and Zone 2A ventilation exhaust HEPA and charcoal filters are in-service vwll be implemented to assure that exhaust gases are being filtered when the HCF ventilation system is in operation. A TSR requirement to verify the ventilation system fan sequencing interlock is operable vvnil be implemented to ensure that proper building airflow patterns are maintained in the event of exhaust fan failures. The ventilation system exhaust ducting provides only an inherent passive safety function (i.e., confinement) and no specific TSR controls are required to ensure continued performance of this function. 

A ventilation system fan sequencing interlock ensures that Zone 1 confinement boundary integrity is not compromised by either exhaust fan faiiures or operational errors. This interlock automatically starts the backup exhaust fan if the operating exhaust fan fails, thereby maintaining the desired directional airflow control across ventilation exhaust system boundaries. Thus, no operator action is required to maintain Zone 1-to-Zone 2A differential pressure in the event of exhaust fan failures. 

The primary function allocated to the machine side of this interface involves prevention of adverse pressure differentials. This function is performed by the ventilation system fan sequencing interlock, which establishes a hierarchy of fan operation to ensure that hot exhaust ducting does not get pressurized in the event of exhaust fan failures or mis-operation. An interlock automatically starts the backup fan upon loss of an operating fan. in addition, the interlock automatically shuts down all upstream fans if both of the Zone 1, Zone 2A, or stack exhaust fans fail. The interlock also allows only a sequenced startup of ventilation system fans to prevent pressurization of the hot exhaust ducting because of operator failure to adhere to the required fan startup sequence. 

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