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Trans ACNR RMPU (Roof Mounted Packaged AC Unit) for LBH Coaches of India Railways

How Air Conditioning Works in an Indian Railways LHB Coach: The RMPU Explained

LHB coach air conditioning is delivered by two roof mounted package units, known as RMPUs, fitted above the false ceiling at each end of the coach. Each unit is rated at a minimum of 7.0 tonnes of refrigeration, giving 14 TR for the whole coach. Both units are governed by a single microprocessor controller, which reads the temperature of the air returning from the saloon and decides how many compressors to run.

That is the short answer. The longer answer is a good deal more interesting, because an air conditioner that lives on the roof of a train travelling at 160 kilometres per hour through a Rajasthan summer has to solve problems that a household air conditioner never encounters. This article walks through the whole system, from the physics of the refrigeration cycle to the control logic that decides when a compressor starts. It is written for rail enthusiasts, engineering students and curious passengers, so the plain-language explanation comes first and the specification detail follows in a later section.

Quick Answers

What does RMPU stand for?

Roof Mounted Package Unit. It is a complete air conditioning plant built into a single weatherproof box that sits in a recess in the coach roof.

Two, one at each end, with one microprocessor controller governing both.

Not less than 7.0 TR per unit under the specified dry summer test condition, which works out to roughly 24.6 kilowatts of heat removal per unit, or about 49 kilowatts for the coach.

R-407C, in a vapour compression system.

Yes. Each unit carries a 6.0 kW electric heater bank for winter operation.

The design condition is 20°C to 25°C inside during summer at 40 to 60 per cent relative humidity, and 17°C to 21°C inside during winter.

What an RMPU Is, and Why an LHB Coach Carries Two

A roof mounted package unit is exactly what the name suggests. Instead of scattering the compressor, the condenser, the cooling coil and the fans around the underframe of the coach, the entire plant is assembled into one self-supporting box in the factory, charged with refrigerant, tested, and then lowered into a recess in the coach roof by an overhead crane.

The unit does not form part of the coach structure. It has its own frame, and it connects to the coach through two flexible fabric bellows: one for the air being drawn back out of the saloon, and one for the conditioned air being pushed into the supply duct. The whole assembly weighs no more than 700 kilograms, and the specification requires it to be installed above the false ceiling with only the condenser portion left exposed to the outside air.

Two Units, One Controller

An LHB air conditioned coach carries two of these units, one towards each end. The reason is straightforward: a single unit large enough to cool a 23.5 metre coach would be heavy, difficult to lift, and would leave the coach with no cooling at all if it failed. Two units share the load and share the risk.

Both units answer to one microprocessor controller unit. The controller is the part of the system that turns a box of machinery into an air conditioning system. It reads six temperature sensors and two humidity sensors across the coach, averages the readings from both ends, and then decides second by second what should be running.

A Short History: How the LHB Coach Arrived in India

The letters LHB stand for Linke Hofmann Busch, a rolling stock builder based at Salzgitter in Germany. The firm was taken over by Alstom in 1998, briefly traded as Alstom LHB GmbH, and its Salzgitter operation now forms part of Alstom Transport Deutschland GmbH.

Indian Railways signed two contracts with the company in October 1995: one for transfer of technology, and one for the supply of the first two rakes, amounting to 24 coaches. Those 24 comprised 19 air conditioned chair cars, two executive class chair cars and three generator cum brake vans. They arrived and were commissioned in 2001. Rail Coach Factory, Kapurthala took up domestic manufacture, and the Comptroller and Auditor General of India records that the first LHB coach was introduced on the Indian Railways network in December 2003.

The design differs from the older Integral Coach Factory pattern in several ways that matter to the air conditioning engineer:

  • Length. The LHB shell measures 23,540 mm over body, against 21,337 mm for the older design. That is roughly two additional metres of saloon to cool.
  • Capacity. An AC three tier LHB coach seats 72 passengers, against 64 in the older design. More passengers means more body heat, more moisture and more fresh air demand.
  • Weight. LHB coaches weigh approximately 10 per cent less than the corresponding conventional coaches.
  • Speed. The coaches are designed for 160 km/h and have been tested at 180 km/h, with potential for operation up to 200 km/h.

 

In March 2018 the Ministry of Railways announced that the production units of Indian Railways would manufacture only LHB coaches from 2018-19 onwards. The Press Information Bureau reported in December 2025 that more than 42,600 LHB coaches had been manufactured between 2014 and 2025. As of February 2026, the Ministry placed the coaching fleet at approximately 82,200 vehicles, of which about 25,000 are air conditioned.

How LHB Coach Air Conditioning Cools: The Refrigeration Cycle in Plain Language

Before following the air through the coach, it helps to be clear about one point that confuses many people. An air conditioner does not create cold. It moves heat from one place to another, in the same way that a bucket moves water. The RMPU picks up heat from inside the coach and drops it outside on the roof. Everything else is plumbing.

The tool that does the moving is the refrigerant, a fluid chosen because it boils at a conveniently low temperature. In an LHB RMPU that fluid is R-407C. The cycle has four steps.

Step One: Compression

The compressor draws in cool, low pressure refrigerant vapour and squeezes it. Squeezing a gas raises both its pressure and its temperature, so the vapour leaves the compressor hot. The LHB RMPU uses hermetically sealed scroll compressors, in which two spiral scrolls, one fixed and one orbiting, trap pockets of gas and progressively squeeze them towards the centre. There are two compressors in every unit, and therefore four in every coach.

Step Two: Condensation

The hot vapour now passes into the condenser coil, which sits in the exposed part of the unit on the roof. Axial fans blow outside air across the coil. Because the refrigerant is hotter than the outside air, heat flows out of the refrigerant and into the atmosphere, and the vapour condenses into a warm liquid. This is why the condenser section must be left open to the sky, and why the air blowing out of the top of an LHB coach roof on a summer afternoon is noticeably warm.

Step Three: Expansion

The warm liquid then passes through a thermostatic expansion valve. The valve is a controlled restriction. On the far side of it the pressure collapses, and the liquid becomes very cold. The expansion valve carries a sensing bulb clamped to the suction pipe leaving the cooling coil, so it can feel how much refrigerant the coil actually needs and meter the flow accordingly.

Step Four: Evaporation

The cold liquid enters the evaporator coil, also called the cooling coil, which sits in the enclosed part of the unit. A blower draws the coach air across this coil. Heat flows out of the air and into the refrigerant, which boils back into a vapour. The air leaves colder and drier, the vapour returns to the compressor, and the cycle begins again.

Two points are worth pausing on. First, the air is drawn across the cooling coil rather than blown across it, an arrangement the specification requires so that the blower sits downstream and cannot spray condensate into the supply duct. Second, moisture in the coach air condenses on the cold coil and drips into stainless steel trays, which drain to the outside through pipes with traps. That is the water you sometimes see dripping from an air conditioned coach at a platform. It is not a leak. It is the passengers’ own breath and perspiration, collected and thrown away.

Follow the Air: One Complete Circuit Through the Coach

Now trace the path that a single parcel of air takes.

1. It leaves the saloon through the return air grille

Air that has already circulated through the saloon is drawn back through return air grilles and up through openings in the coach roof into the underside of the RMPU. Two temperature sensors sit just behind the return air grille, because the temperature of the air coming back from the saloon is the single most important measurement the whole system makes.

2. Fresh air joins the stream

A coach full of sleeping passengers consumes oxygen and produces carbon dioxide and moisture, so a sealed recirculating system would become stale within an hour. Outside air is therefore drawn in through grilles in the upper part of the lateral coach wall, passed through fresh air filters, and mixed with the return air. The specification requires 0.35 cubic metres per minute of fresh air per person, which is 21 cubic metres per hour per person, and the fresh air quantity test requires a measured delivery of not less than 26 cubic metres per minute for the unit.

3. The mixed air is filtered

Each unit carries two fresh air filters and two mixed or return air filters. The mixed air filter covers the entire face of the cooling coil, so that no unfiltered air can slip past the edge. The fresh air filters are serviced from the side wall of the coach by opening the intake cassette, and the mixed air filters are serviced from the top of the unit. Blocked filters are among the causes of poor cooling named most often in Indian Railways maintenance material, for the simple reason that a blocked filter starves the coil of air.

4. It crosses the cooling coil

The blower draws the mixed air across the evaporator coil. The coil is built from copper tubes with aluminium fins, pre-coated and required to survive a 1,000 hour salt fog test to ASTM B-117, because coaches spend a good part of their lives in coastal, salt-laden air.

5. It travels down the supply duct

The conditioned air leaves through a single supply opening, passes through a flexible bellows into the coach duct, and is distributed along the saloon through perforated deflectors. The blower is designed to deliver against 20 millimetres water gauge of static head, which is the resistance offered by the duct and the diffusers along the length of the coach.

And then the parcel of air circulates through the saloon, warms up, picks up moisture, and returns to the grille to begin the journey again.

Inside the Box: What an LHB RMPU Contains

LHB coach air conditioning is built to a shopping list rather than left to the supplier’s judgement. The specification sets out exactly what goes into every unit, and the table below lists the principal items and the quantity fitted per RMPU.

Component

Quantity per RMPU

Function

Hermetically sealed scroll compressor

2

Raises refrigerant pressure and temperature

Condenser coil unit

2

Rejects heat to the outside air

Condenser motor and fan

2 each

Draws outside air across the condenser

Evaporator (cooling) coil unit

2

Absorbs heat from the coach air

Blower motor

1

Drives both supply air fans

Blower fans

2

Move conditioned air into the supply duct

Thermostatic expansion valve

2

Meters refrigerant into the cooling coil

Filter drier and sight glass

2 each

Removes moisture; allows visual check of refrigerant

Liquid receiver

2

Holds the liquid refrigerant charge

Electric heater bank

1 set of 6.0 kW

Warms the coach in winter

Fresh air filters

2

Cleans incoming outside air

Mixed / return air filters

2

Cleans the air entering the cooling coil

NTC temperature sensors

3

Return air, supply air and fresh air measurement

Hygrostat (30–100% RH)

1

Measures relative humidity for dehumidification

Stainless steel drip trays

2

Collects and drains condensate

Anti-vibration mounting pads

4

Isolates the unit from the coach structure

The counting convention is worth noting, because it explains a good deal of the control logic. Every RMPU contains two entirely separate refrigeration circuits, each with its own compressor, condenser and cooling coil. A coach therefore has four independent refrigeration circuits. If one fails, three remain.

What "7.0 TR" Actually Means

TR stands for tonnes of refrigeration, a unit that dates from the days when cooling was measured against the ice it replaced. One tonne of refrigeration is the rate of heat removal needed to freeze one short ton of water into ice in 24 hours. In modern units, one TR is approximately 3.517 kilowatts.

So a 7.0 TR unit removes roughly 24.6 kilowatts of heat, and a coach with two such units has about 49 kilowatts of installed cooling. Spread across a 72-berth AC three tier coach, that is a little under 0.7 kilowatts per passenger, which must cover body heat, solar gain through the roof and windows, heat conducted through the shell, lighting, and the energy needed to cool and dry the incoming fresh air.

The specification is precise about the conditions under which that 7.0 TR must be achieved. The cooling capacity test is conducted with the outside chamber held at 50°C dry bulb and 25°C wet bulb, and the inside chamber at 25°C dry bulb and 16°C wet bulb, with the supply air static head set at 20 mm water gauge. Under those conditions the unit must deliver not less than 7.0 TR, and each individual refrigerant circuit must deliver not less than 3.5 TR. The power drawn by the unit must not exceed 16.5 kVA.

There is a second capacity requirement that shows how the design anticipates failure. The unit must be able to work with only one condenser fan running, and the cooling capacity obtained in that condition must not be less than 75 per cent of the rated capacity.

The Conditions the Unit Must Survive

An LHB RMPU is not asked to perform in a laboratory. The specification lists the service conditions the equipment must withstand, and the list explains a great deal about why the unit is built the way it is.

Condition

Requirement

Ambient temperature

−4°C to 57°C

Average ambient

35°C

Maximum temperature inside coach standing in the sun

70°C

Train speed

200 km/h

Relative humidity

Up to 100 per cent

Altitude

Up to 1,200 m above sea level

Atmosphere

Desert terrain and dust content up to 1.6 mg per cubic metre

Annual rainfall

Between 1,750 mm and 6,250 mm in certain areas

Coastal exposure

Humid, salt-laden and corrosive atmosphere

Shock and vibration

3.0 g maximum in vertical, lateral and longitudinal directions

Noise inside the coach

Not more than 60 dB, to UIC 553

Read that table again and the design choices fall into place. The 1.6 mg per cubic metre dust figure is why the filters are large and serviceable from outside. The salt-laden coastal requirement is why the coils are pre-coated and salt fog tested. The 70°C figure for a coach standing in the sun is why the unit must complete a high temperature start-up test, in which the condenser room is held at 57°C and the unit is run for one hour without any protective device tripping.

How the Coach Decides When to Cool

This is the part that most passengers never see, and it is where an LHB coach differs most sharply from a window air conditioner.

The seven-position temperature switch

Temperature is selected on a stepped rotary switch on the switch panel. The seven positions carry paired cooling and heating set points.

Switch position

Cooling set point

Heating set point

1

20.0°C

17.0°C

2

20.5°C

17.5°C

3

21.2°C

18.2°C

4

21.9°C

18.9°C

5

22.6°C

19.6°C

6

24.3°C

20.3°C

7

25.0°C

21.0°C

The staging logic

The controller does not simply switch everything on when the coach is warm. It compares the return air temperature with the selected set point and stages the plant in three bands:

  • Return air below the set point. Only the blower and the exhaust fan run. No compressor starts. The coach is already at temperature, so the system merely circulates and ventilates.
  • Return air 1°C to 2°C above the set point. The condenser fans and one compressor run. Half the capacity is enough.
  • Return air more than 2°C above the set point. The condenser fans and both compressors run, and continue until the return air temperature comes down.

 

Several timing rules protect the machinery. The first compressor starts 30 seconds after the air conditioning switch is turned on, and the second follows five seconds later, so that four compressors across a coach never draw their starting current at the same instant. No compressor may cycle more than ten times an hour, and every compressor is held on for at least 30 seconds and off for at least 30 seconds regardless of what the temperature is doing. The condenser fans switch off within 30 seconds of both compressors stopping.

There is also a neat piece of pressure-based logic on the condenser side. Only one condenser fan runs while the working pressure stays below 275 psig. If the pressure rises above that threshold, which is what happens on a hot afternoon when the condenser is struggling to reject heat, the second condenser fan is brought in.

Dehumidification

Comfort is not only a matter of temperature. Air at 24°C and 80 per cent relative humidity feels unpleasant in a way that air at 24°C and 50 per cent does not. The specification requires the design to hold relative humidity inside the compartment to a maximum of 60 per cent under all circumstances.

The controller manages this through a hygrostat mounted at the return air grille. If relative humidity rises above 60 per cent while the temperature is already below the cooling set point, the system enters dehumidification: one compressor and the heater run together, while the second compressor and one condenser fan stop. This sounds contradictory, but it is not. The cooling coil wrings moisture out of the air, and the heater then warms that dried air back up so that the coach does not become uncomfortably cold. The heater switches off after three minutes and stays off for five, after which the humidity is measured again and the cycle repeats if necessary.

Winter heating

Each unit carries a 6.0 kW electric heater bank in the evaporator section. Heating is not staged in the way cooling is; instead the controller varies the proportion of each minute for which the heater is energised:

  • Up to 1°C below set point: heater on for 5 seconds, off for 55 seconds.
  • More than 1°C and up to 3°C below set point: on for 30 seconds, off for 30 seconds.
  • More than 3°C below set point: on for 55 seconds, off for 5 seconds.

 

The heaters carry three independent layers of protection against overheating. An overheat protection switch acts through the controller at 65°C of supply air temperature. If that fails and the temperature reaches 85°C, the controller opens the heater contactor directly. If both fail, a fusible ESTI cartridge, a sealed glass bulb filled with an inert fluid, bursts at 130°C and disconnects the supply permanently. The unit also shuts down altogether if supply air temperature falls below 5°C or rises above 85°C.

What happens when something fails

The controller is designed to degrade in stages rather than stop.

 

  • If one condenser motor trips, the second condenser fan motor starts immediately.
  • If one return air sensor fails, the reading is taken from the healthy sensor at the other end of the coach. If both fail, the controller falls back on the fresh air sensor. If all temperature sensors fail, the controller runs the blower only and flags the fault on the display.
  • If a compressor trips three times within one hour on high or low pressure, the controller locks that compressor out and displays a fault, rather than allowing it to cycle itself to destruction.
  • If the refrigeration circuit fails completely, or the controller itself malfunctions, the blower and exhaust fan continue to run for as long as supply voltage is available, so the coach is at least ventilated.


Every one of these events is recorded. The controller stores at least 2,000 faults on a first-in, first-out basis, together with roughly seven days of operating events sampled every two minutes, and the whole log can be downloaded over USB for analysis in a spreadsheet. When a coach comes in for attention, the maintenance staff do not have to guess what happened at three in the morning outside Itarsi. They can read it.

Why the Roof, and Not the Underframe?

LHB coach air conditioning did not always live on the roof. Older air conditioned coaches carried their plant under the floor, with long refrigerant pipe runs to coils elsewhere in the vehicle. Indian Railways training material sets out the reasoning behind the move to a roof mounted package, and the arguments are worth summarising because they are all consequences of the same design decision.

 

  • Shorter pipe runs: A package unit contains its entire refrigerant circuit within one box, so there is far less pipework to leak and far less capacity lost along the way.
  • Smaller refrigerant charge: Training material puts the charge at roughly 3 kg per circuit, against about 8 kg for the older under-slung arrangement.
  • Exposure: Equipment on the roof is out of reach of flood water, cattle run-over and flying ballast.
  • Replacement time: Because the unit arrives pre-charged, pre-tested and connected through plug-in couplers, training material puts installation at about four hours per coach rather than four days.
  • Cleaner intake: Fresh air is taken from the roof line rather than from beneath the coach.

 

The trade-off is that maintenance now happens on the roof, under the overhead line. This is why the specification insists that every serviceable item is reachable from the top of the coach with a ladder, that service covers latch positively so that they cannot open in service, and that the fresh air filters can be changed from the side wall without climbing on to the roof at all.

Frequently Asked Questions

What does RMPU stand for?

RMPU stands for Roof Mounted Package Unit. It is a complete air conditioning plant, including compressors, condensers, cooling coils, blower, filters and heater, assembled into a single weatherproof box that is fitted into a recess in the coach roof.

Two RMPUs, one towards each end of the coach, governed by a single microprocessor controller. Between them they contain four compressors, four condensers and four cooling coils.

Not less than 7.0 TR per RMPU under the specified dry summer test condition, giving approximately 14 TR, or about 49 kilowatts, for the complete coach. Each of the four refrigeration circuits must deliver not less than 3.5 TR.

The design condition is 20°C to 25°C inside during summer at 40 to 60 per cent relative humidity, and 17°C to 21°C during winter. The actual set point is chosen on a seven-position rotary switch, with cooling set points from 20.0°C to 25.0°C and heating set points from 17.0°C to 21.0°C.

R-407C. The December 2011 revision of RDSO/PE/SPEC/AC/0061-2005 made R-407C the only permitted refrigerant for this application.

Yes. Each RMPU carries a 6.0 kW electric heater bank in the evaporator section, with three independent overheat protections acting at 65°C, 85°C and 130°C. The newer specification moves towards reverse cycle heat pump operation instead.

It is condensate. Moisture from the passengers’ breath and perspiration condenses on the cold evaporator coil, collects in stainless steel drip trays, and drains to the outside through pipes fitted with traps.

The specification requires 0.35 cubic metres per minute of fresh air per person, which is 21 cubic metres per hour per person. The unit is tested to deliver not less than 26 cubic metres per minute of fresh air.

Mounting the unit in the roof keeps the entire refrigerant circuit inside one box, shortens the pipe runs, keeps the machinery clear of flood water and flying ballast, and allows a complete unit to be exchanged rather than repaired in place. The condenser section is left exposed so that it can reject heat to the atmosphere.

If the refrigeration circuit or the controller fails, the blower and exhaust fan continue to run for as long as supply voltage is available, so the coach is ventilated even when it cannot be cooled. Every fault is recorded in the controller memory for the maintenance depot to read.