Fan Discharge Pressure

As the exterior fouling on the tubes worsens, the portion of the screen, through which the air flows backward increases. As the dirt accumulates on the underside of the tubes, the portion of the screen through which the air is drawn upward decreases. Even though the airflow blown through the bundle is decreasing, the total airflow delivered by the fan is constant. 

Fan operation is indicated on a performance curve, as shown in Fig. 14.3. The head developed by the fan is equivalent to 5 or 10 in of water. 

As the fan airflow is pretty constant, the fan s head is also constant. Another way of stating this is to say that, as a tube bundle fouls, the resistance to airflow increases. This reduces the airflow through the bundle, but the pressure loss of the airflow through the tube bundle does not change. 

If the head developed and the flow produced by a fan are both constant, then the power needed to run the fan must also be constant. Why Because the power needed to spin a fan is proportional to the produced flow and the produced head. 

To prove this to yourself, find the electric circuit breaker for a fan s motor. The amp (amperage) meter on the circuit breaker will have a black needle and a red needle. The black needle indicates the actual current, or amp load. The red needle is the amperage load that will trip the motor, as a result of overamping. Over time, as the tube bundle fouls and airflow through the bundle is restricted, the black needle never moves. 

An induced-draft fan (see Fig. 17.1) is a different story. As the tube bundle fouls, 

Naturally, there is no reverse airflow on an induced-draft fan. That can occur only in a forced-draft fan. Reverse airflow can be observed with a forced-draft fan, by seeing which portions of the screen, shown in Fig. 17.2, will not allow a dollar bill to stick to the underside of the screen. 

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The fan discharge pressure remains constant, regardless of the fin tube fouling. 

Combustion calculations show that an oil-fired watertube boiler requires 200,000 lb/h (25.2 kg/s) for air of combustion at maximum load. Select forced- and induced-draft fans for this boiler if the average temperature of the inlet air is 75°F (297 K) and the average temperature of the combustion gas leaving the air heater is 350°F (450 K) with an ambient barometric pressure of 29.9 inHg. Pressure losses on the air-inlet side are, in inFLO air heater, 1.5 air supply ducts, 0.75 boiler windbox, 1.75 burners, 1.25. Draft losses in the boiler and related equipment are, in inH20 furnace pressure, 0.20 boiler, 3.0 superheater, 1.0 economizer, 1.50 air heater, 2.00 uptake ducts and dampers, 1.25. Determine the fan discharge pressure and horsepower input. The boiler burns 18,000 lb/h (2.27 kg/s) of oil at full load. 

Compute the induced-draft fan discharge pressure. Find the sum of the draft losses from the burner outlet to the induced-draft inlet. These losses are, for this boiler ... 

In an air-flow system that has been operating normally, the system resistances gradually increase, and as the air flow drops the fan discharge pressure rises, eventually reaching its maximum. 

Fans are classified according to the discharge pressure. Reprinted per written permission from the Air Movement and Control Association International, Inc., the AMCA Standard 99-1401-66 from Standards Handbook 99-56 1986, the total static pressure classification for operating limits for central station units is as follows ... 

Figure 12-126. Evase duct attachment on fan discharge to improve the regain of static pressure. Fans can be purchased with or without the evase section. (Used by permission Bui. 331, 1992. The New York Blower Co. For more information, contact the company at www.nyb.com.)...

For the Ian existing in Example 12-15, what will he the conditions if the static pressure must he changed from 2 to 3.5 in. due to a change in the system pressure level into which the fan discharges The piping system, air density, and Ian size remain unchanged. What will he the new operating conditions for this fan ... 

Although the analysis of parallel operation indicates that a fan may not operate satisfactorily, often it actually will operate, but under modified conditions. The effect of a slight difference in the individual fan ductwork can be enough to allow operation, or sometimes a change in damper setting will allow operation. Usually in such situations, efficiency will be reduced with a higher horsepower consumption. If the fans discharge toward each other in such a way as to affect each other s operation, the fans may actually operate at a reduced pressure, somewhere between the static and total pressure curves. ... 

Step 19. The fan approximate discharge pressure Pforce (inches of water) is next calculated applying DPAT, DR, and ACFM. DR which was used to calculate ACFM in Eq. (5.51), has been interpolated from Table 5.5 at the fan elevation and at the ambient temperature of air. 

Optimized fan station controls include automatic fan cycling and a damper position to discharge pressure cascade loop to keep the most open user supply damper at 90% opening. 

The role of PIC-09 is to provide overpressure protection at the fan discharge. When the pressure limit set on PIC-09 is reached, it takes over control from PIC-10 and protects the ductwork from being damaged. 

Note The static pressure is normally used in most fan-selection procedures because this is the pressure value used in computing pressure and draft losses in boilers, economizers, air heaters, and ducts. In any fan system, the total air pressure equals static pressure + velocity pressure. However, the velocity pressure at the fan discharge is not considered in draft calculations unless there are factors requiring its evaluation. These requirements are generally related to pressure losses in the fan-control devices. 

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