Impulse Fan Systems

An impulse fan system (IFS) is a device for moving air by directing a high-velocity jet of air in the direction that air movement is desired. It is suitable for producing longitudinal air movements in tunnels since it allows free movement of trains past the fan system. (A conventional low-velocity fan would require the tunnel to be blocked at the point of the fan, for example by a door, to prevent the air discharged by the fan from recirculating into its intake. For this reason the SES restricts the location of impulse fan systems to line segments, while conventional supply/exhaust fans can only be located in ventilation shafts.)

In modeling an impulse fan “system” the SES considers only characteristics of the high-velocity air jet and its effect on air pressure and flow in the tunnel in which it is located. While an axial or centrifugal fan is usually incorporated within an impulse fan assembly, the SES does not model the pressure versus flow characteristics of the fan itself, nor the transient pressures created by passing trains blocking the IFS intake or discharge.

Impulse Fan System Input

The Number of Impulse Fan Types is entered on Form 1E. Each Impulse Fan Type is described by completing one copy of Form 7C. (IFS information should not be confused with the Number of Fan Types on Form 1D and their description on Forms 7A and 7B.) Impulse fans are located in line segments,

4-47 Line Segment Impulse Fan

Type Type

9 IFS Type 1

10 IFS Type 2

11 IFS Type 3

12 IFS Type 4

13 IFS Type 5

14 IFS Type 6

Input Form 7C is used to describe each Impulse Fan Type. The first Form 7C entered describes IFS Type No. 1, the second Form 7C describes IFS Type No. 2, etc.

The Impulse Fan Flow Rate is the rate of air discharged from the IFS nozzle in cubic feet per minute. The Impulse Fan Nozzle Discharge Velocity is the velocity of the air discharged from the IFS nozzle, and is measured at the face of the nozzle. The Impulse Fan Pressure Efficiency is a

dimensionless number which accounts for energy losses due to turbulence and other effects associated with the IFS discharge jet. This number may range from 0.0 (all energy lost) to 1.0 (no energy losses). See Section 7E of Appendix B of the Programmer's Manual for the equation used to describe an IFS in the SES. Experimental tests have shown that a pressure efficiency of 0.90 can be obtained with a discharge velocity of 5900 fpm in a single-track tunnel with an aerodynamically well-designed nozzle and discharge arrangement, using a discharge angle of 30° from the longitudinal axis of the tunnel. For small changes in the discharge angle, the Impulse Fan Pressure Efficiency can be adjusted by the ratio of the cosine of the new angle to the cosine of 30°.

The impulse fan begins operation when the simulation time is greater than the Time at Which the Impulse Fan is Switched On. The impulse fan stops operation when the simulation time is greater than or equal to the Time at Which the Impulse Fan is Switched Off. A “Run-up” and “Run-down” attenuation curve is not used for impulse fans. An impulse fan will remain inactive for the entire simulation if 0.0 is entered for both the time it is “switched on” and “switched off.”

REFERENCES

1. Donsky, Benjamin, “Complete Pump Characteristics and the Effects of Specific Speeds on Hydraulic Transients,” Journal of Basic Engineering, Dec., 1961.

2. “Fan Engineering,” Buffalo Forge Company, 8th Edition, 1983.

3. ASHRAE Handbook and Product Directory, 1977 Fundamentals.

4. “Mark's Standard Handbook for Mechanical Engineers,” McGraw-Hill, 8th Edition et al.

5. I.E. Idel’chik, “Handbook of Hydraulic Resistance,” 3rd Edition 1994.

6. Associated Engineers Report No. UMTA-DC-MTD-7-71-7. “Preliminary Steady-State Subway Aerodynamic Analysis (Incompressible).” Prepared by Graduate Aeronautical Laboratories/California Institute of Technology for United States Department of Transportation.

5-1 5. THERMODYNAMIC PHENOMENA

The temperature and humidity at any point in an underground rapid transit system are influenced by the movement of air carrying sensible and latent heat through the system and by the sources and sinks which add and remove heat at various locations in the system. The temperature and humidity of subway system air are also affected by the temperature conditions above ground and by the temperature of the system walls and surrounding deep heat sink. The predominant source of sensible heat in an operating subway system results from the acceleration and braking cycles of the train. Sensible and latent heat are also rejected from vehicle air conditioners, passengers, and ancillary sources. Heat is removed from the system through the expulsion of heated air from ventilation shafts and by heat conduction across the tunnel walls into the surrounding underground heat sink. Heat may also be added or removed by mechanical means such as heating or cooling equipment.

As noted in the previous section, the geometrical partitioning required for the computation of aerodynamic parameters is accomplished by dividing the subway station, tunnels, and ventilation shafts into a number of segments each of which has statistically uniform cross-sectional area, perimeter, and wall thermal properties. This basic geometrical partitioning is also the basis for the calculation of the system temperature and humidity. However, since temperature and humidity values may vary along the length of subway segments for which the aerodynamic values are statistically uniform, these segments are partitioned into smaller geometrical entities called “subsegments,” as indicated in Figure 5.1. Each

subsegment of a segment has an independently computed temperature and humidity, making it possible to reflect small-scale variations in subway air sensible and latent heat.

The computations of temperature and humidity are carried out automatically in the program using airflow information computed in the aerodynamic portion of the program and train heat release information computed in the train performance portion of the program. The user must specify other sources of heat addition (or removal) which are not computed automatically from the train performance simulation. The user must also specify certain thermodynamic starting conditions within the subway in order for the simulation to begin. These include subsegment air temperatures and wall surface temperatures, and temperature at the system boundaries (e.g., outside air at portals or temperature at interfaces with contiguous portions of the underground system not included in the simulation).

In addition to the mandatory requirement that the user supply values for the initial conditions and boundary conditions, the program also allows the user the option of specifying heat rates for any steady-state and/or unsteady-steady-state heat and humidity sources and sinks. Station lighting would be an example of a steady-state heat source. The program also includes the optional capability to evaluate the effects of evaporation on the latent and sensible heat content of the system air. Finally, as part of the computational sequence, the program includes the added optional capability of a procedure for determining the impact of a trackway exhaust system on the station temperature.

TYPICAL VENTILATION

SHAFT SUBSEGMENT

TYPICAL VENTILATION

SHAFT SECTION VENTILATION

SHAFT V1

VENTILATION SHAFT V2

VENTILATION SHAFT V3

JUNCTION PORTAL

TYPICAL LINE SUBSEGMENT

TYPICAL LINE SEGMENT

TYPICAL LINE SECTION

TYPICAL STATION

(DIVISION FOR TEMPERATURE AND HUMIDITY SUBPROGRAM COMPUTATIONS)

5-3 As part of the designer-oriented features of the program, the user has the option of allowing the program to provide air-conditioning or heating load estimates. These estimates provide information regarding the rate of heat removal or addition necessary to maintain user-specified average temperature and humidity design conditions for specified areas of the system.

The program also accounts for buoyant effects of airflows in tunnel, station, and ventilation shaft segments caused by differences in air density due to varying elevations and temperatures throughout the system. This feature of the program operates during a fire simulation as explained in detail in Chapter 9.

Another feature of the program, designed to permit a more accurate thermodynamic simulation of the mixing of airflows at the junctions of system sections (i.e., at the nodes), allows the user the option of specifying the degree of mixing which can occur among airflows in the system which are allowed to communicate at the system intersections. This feature permits the user to specify whether complete thermodynamic mixing is expected to occur at a node or whether the mixing is expected to be only partial.

The airflows and temperature/humidity information computed by the program for each of the sections which can communicate at a given node are used by the program to compute automatically the degree of temperature/humidity exchange which can occur at the node.

Finally, the program provides the user with the option of evaluating the effects of long-term changes in tunnel wall temperature upon the environmental conditions in an operating system. This capability is made possible through a special procedure which allows the user to implement an ancillary analytical program for estimating future values of tunnel wall temperature together with the short-term computations of system air temperatures provided by the main program.