Trackway Exhaust (Input Form 1G)

A trackway exhaust system is designed to prevent the heat rejected by the train from entering the station environment. This is intended to reduce the temperature in stations that are cooled by means of fans and train-produced piston ventilation or to reduce the cooling load requirement of mechanical cooling systems.

The major sources of heat on a train are the acceleration and deceleration resistor grids, traction motors and, if the train is equipped with an air conditioning system, the air conditioning condenser.

Depending on vehicle design, this heat is usually discharged into the surrounding air either beneath or above the car, or both. The trackway exhaust system attempts to capture this heated air, preventing it from mixing with the air in the station. An exhaust system operates by sweeping the air from beneath or above the train and discharging it outside of the station. The system may comprise either exhaust or a

5-13 trackway exhaust system does not affect the airflows within the system. If a balanced supply and exhaust system is used, there is no net change in airflow within the system, and only heat is removed. If an exhaust-only system is used, there is a net removal of air from the station in addition to the removal of heat. The removal of heat is simulated by using the trackway exhaust feature of the SES program, and the removal of air from the station must be simulated by the addition of one or more ventilation shafts

containing fans serving the station. The fans which are located in these shafts should be chosen with operating characteristics such that the average airflow rate being removed from the station through these additional ventilation shafts is equal to the rate of air removal at which the trackway exhaust system is designed to operate.

The trackway exhaust system only captures train heat when it is released within station segments.

Trackway exhaust systems are usually designed to serve only the platform area of stations, and while approaches to the station may be physically located within the station structure, they should not be designated station segments unless they are equipped with a trackway exhaust system. The trackway exhaust system does not operate if a train passes through the station in constant speed mode (see Chapter 8 - Train Performance) as would usually be the case if there is no scheduled stop in the station for the route on which the train is operating.

The amount of heat captured by the trackway exhaust system when it is operating³ is expressed as a percentage of the heat rejected from the train. This is referred to hereafter as the percent of heat captured. The percent of heat captured varies with train speed. The program is set up so that a different value of percent of heat captured can be used for three different stages of train activity:

A. Train is stopped. The value of percent of heat captured is applied to the train heat released in “station” (Type 2) line segments.

B. Train is moving at a speed less than or equal to the “maximum train speed at which the trackway exhaust system operates.” This value of percent of heat captured is applied to the portion of the train that is within “station” line segments as the train coasts or brakes into the station, or accelerates out of the station.

C. Train is moving at a speed greater than the “maximum train speed at which the trackway exhaust system operates.”

The values of percent of heat captured for activities A and B are input on form 1G. The program automatically uses zero as the value for activity C.

The percent of heat captured also varies with the type and location of the vehicle heat source. The program allows for flexibility in this area by using a different percent of heat captured for the two following types of vehicle heat sources:

3 Trackway exhaust system “operation” as used herein refers strictly to heat extraction by the system. “Operation” does not connote the aerodynamic behavior of the trackway exhaust mechanical system (e.g. on-off operation of the fans).

A. Train propulsion/braking system heat. This heat results from acceleration losses from either acceleration resistor grids or chopper losses, plus electrical heating of the traction motors, heat from work done against friction from mechanical rolling resistance and curve resistance, and braking heat (both deceleration resistor grid heat from dynamic braking and heat from friction brakes). These sources produce sensible heat only.

B. Train auxiliaries and passenger heat. This is the heat load from on-board auxiliary equipment (lights, air compressor, miscellaneous electrical equipment and, if present, vehicle air conditioning) and metabolic heat from passengers if the vehicle is not air conditioned. These heat loads from the vehicle are entered on form 9C and vary with the number of passengers on board the train. NOTE: The trackway exhaust system captures a percentage of only the sensible portion of this heat; the trackway exhaust system has no effect on the latent portion of this heat load.

The division of the vehicle heat sources into two categories better accomodates vehicle designs in which one category of heat is discharged beneath the vehicle, and the other category is discharged above the vehicle. In such a situation, two different trackway exhaust systems may be provided (underplatform exhaust and overtrack exhaust). Further flexibility is provided by the ability to simulate three different trackway exhaust system configurations: 1) a trackway exhaust system that captures only train propulsion/braking system heat, 2) a trackway exhaust system that captures only train auxiliaries and passenger heat, and 3) a trackway exhaust system that captures both types of heat. This is explained in further detail in section 3.3 of this manual.

The percent of heat captured for each of the two vehicle heat sources discussed above must be entered for both train activities A and B. Thus a total of four entries of percent of heat captured are entered on Form 1G.

A summary of the trackway exhaust percent of heat captured is shown in Table 5.1.

Table 5.1 Trackway Exhaust Percent of Heat Captured

TRAIN HEAT SOURCE

Activity Speed Propulsion/Braking Auxiliaries/Passengers

A Uv=0 P1 P3

B 0<Uv<Umax P2 P4

5-15 where: Uv is the vehicle (train) speed

Umax is the maximum train speed at which the trackway exhaust system operates P1, P2, P3, P4 are heat capture percentages

The trackway exhaust system simulation operates in the following manner: the train will usually approach the station and begin braking at a speed which is above the Maximum Train Speed at which the Trackway Exhaust System Operates. As the train slows to a speed which is below this maximum speed, the trackway exhaust system applies the activity “B” percent of heat captured for the propulsion/braking heat and sensible auxiliaries/passenger heat for the portion of the train which is inside the station. When the train comes to a stop, the activity “A” percent of heat captured is applied for the propulsion/braking heat and sensible auxiliaries/passenger heat for the portion of the train which is inside the station. The activity “B” percent of heat captured is again used to reduce both types of vehicle heat rejection for the portion of the train which is inside the station while the train is accelerating out of the station. This continues until the train speed becomes greater than the Maximum Train Speed at which the Trackway Exhaust System Operates, above which the train heat rejection assumes its normal value.