Requirements Of Twin Tower Heatless Regenerative Type Air Dryers For Diesel & Electric Locomotives - Online Article


The presence of moisture in pneumatic system of locomotives causes malfunction in of electro pneumatic and pneumatic devices .A1thoug automatic drain valves have been provided in locomotives hut the problem of moisture carry over along with compressed air has not been overcomes .It has therefore, been considered necessary to provide air dryers on locomotives so that air supplied to the electro pneumatic and pneumatic equipment is clean and dry and does not cause any condensation of the moisture in the system.


The technical requirements cover the design inspection testing and installation of the air dryers for the use on electric and diesel electric locomotives. It shall be the responsibility of the supplier to work out suitable design of the air dryer to meet the technical requirements indicated in this specification.

General Conditions

Service Conditions

The equipment shall be capable of working satisfactorily under the service conditions indicated below:


Mean sea level to an altitude of one thousand meters above mean sea level.


0 degree to 55 degree Centigrade. The temperature inside locomotive may reach as high as 70 degree centigrade.

Operating Voltage

The electro pneumatic or electrical devices shall be suitable for 72 volts dc on diesel locomotive and 110 volts dc for electric locomotives. The voltage variation on diesel may he between 48 to 90 volts and on electric locomotives it may be between 85 to 130 volts dc. Supplier will include protection against sudden variation in voltage like surge in air dryer design itself.

Relative Humidity

UPTO 100%

In-Let Temprature

The temperature of compressed air at the inlet of air dryer may be as high as 60 degree.

Vibrations & Shocks

The equipment should be capable to withstand, without damage, the vibrations and shocks normally encountered during service. The conditions are indicated below:

  • Maximum Vertical Acceleration = 1.Og
  • Maximum Longitudinal Acceleration = 3.Og
  • Maximum Transverse Acceleration = O.5g ("g" being acceleration due to gravity)

Details of Vibration, Shock & Bump Tests

The assembly is secured in a suitable position to a machine producing vibrations of sinusoidal forms with adjustable amplitude and frequency and is then subjected to test given below. The assembly tinder test shall be able to withstand successfully the electrical tests and in particular, the dielectric tests applicable to the equipment concerned.

  • Determination of resonant frequencies
  • Tests with sustained vibrations
  • Tests for stimulating the efect of shunting shocks

Other Conditions

Equipment shall be capable of operating efficiently in spite of dirt, dust, mist, torrential rain, heavy sand or stone storms and presence of oil vapours and radiant heat etc. to which the rolling stock is normally exposed in service.

Technical Requirements

Air dryer should be of heatless twin tower regenerative type. It should be of light weight. The system should be suitable for continuous operation.

Operating Pressure

The air dryer should be Capable to work at nominal main reservoir pressure between 8 -10 kg./cm for both diesel and electric locomotives. At times pressure may go as high as 11 kg/cm.

Air Dryer Capacity

Diesel Locomotives

On diesel locomotives compressor capacity varies from 2400 lit/mm at 400 rpm to 6000 lit/mm at 1000 rpm at 10 kg/cm2.Howev it should be capable to work with air flow variation from 2400 it/min to 6000 lit/mm Normal duty cycle of compressor for air dryer design purpose, be taken as the design for continuous air flow during train working except for a period of 30 minutes or so at highest compressor capacity, during initial charging of the train.

Electronic Locomotives

On electric locomotives compressor capacity varies from 1000 lit/rn in to 2000 lit/mm at 10 kg/cm2 for passenger locomotive and 2000 lit/mm to 3000 lit/mm for freight locomotives. Design air flow for air dryer should he 1500 lit/mm for passenger locomotives and 2000 lit/mm for freight locomotives. However, it should be capable to work with air flow variation from 1000 to 2000 lit/mm on passengers locomotives and from 2000 to 3000 lit/mm on freight locomotives. Normal duty cycle of compressor for air dryer design purpose, be taken as the design for continuous air flow during train working except for a period of 30 minutes or so at highest compressor capacity, during initial charging of train.

Dew Point Depression

On design air flow, air dryer should be capable to give a dew point depression of atleast 30 degree centigrade for all conditions of ambient temperature as given in service conditions. In the worst case of air flow and ambient temperature, air dryer should be capable to give dew point depression of 15 degree centigrade. The performance of air dryer in terms of dew point depression as given above, must confirm to specification for at least 4 years. Dew point depression = A-B A = Inlet Dew Point Temperature Actual temperature at inlet to Air Dryer, at actual pressure, assuming 100% saturation of compressed air. B= Outlet Dew Point Temperature At outlet to Air Dryer, use dew point meter to measure dew point temperature at actual pressure.

Purge Loss & Depressurization Loss

Purge and depressurisation loss in terms of percentage of inlet air flow required for regeneration should not be more than 20% at design capacity.

Pressure Loss

The pressure drop across the air dryer shall not be more than 3% of inlet pressure. The actual pressure drop for the unit offered must be indicated. HUMIDITY INDICATOR Air dryer should be provided with humidity indicator which indicates the quality of outgoing air whether it is dry or wet. The colour code used for indication should be preferably as under:

  • Dry air -blue colour
  • Wet air - white colour


The desiccant to be used should be available indigenously in India. The life of desiccant shall not be less than four years. Desiccant should not disintegrate during use under operating conditions prevalent in the locomotives. Provision of compactor to maintain constant mechanical pressure on the desiccant packed in the Vessels may he provided so that the desiccant remains compact and tight. The type of compactor and its detail shall be furnished in the offer. The material basically used as desiccant is activated alumina. The specification of the activated alumina to be used is described below.


There shall be following two grades of the material:

  • Grade 1: For high pressure air drying and
  • Grade 2: For general purposes. The material shall consist of activated alumina in spherical or pellet form.

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