Coach air conditioning on Indian Railways centres on the RMPU (Roof Mounted Package Unit) — a self-contained AC unit combining compressor, condenser, evaporator, and blower in one roof-mounted package, fitted across all self-generating (SG), End-On-Generation (EOG), and Head-On-Generation (HOG) coach types.
Psychrometric Fundamentals
- Wet Bulb Temperature (WBT) — the temperature a thermometer bulb wrapped in a wet wick reads, accounting for evaporative cooling.
- Dew Point (DP) — the temperature at which air becomes saturated and moisture begins to condense.
- Relative Humidity (% RH) — the ratio of actual moisture content in air to the maximum it could hold at that temperature.
Refrigerants
R22 has historically been the standard refrigerant on RMPUs, but R407c is now used as a drop-in substitute — an R22 refrigerant blend with an approximate 7K temperature glide, meaning it evaporates/condenses over a range of temperatures rather than at one fixed point, unlike a pure single-component refrigerant.
RMPU Technical Specifications
- Compressor: hermetically sealed scroll type, 12.5 HP.
- Condenser motor: 1 HP / 0.75 HP. Blower motor: 1.5 HP.
- WRA (Water Retention Assembly) monoblock pump: 0.5 HP at 415V, or 1.5 HP at 110V DC motor type.
- Refrigerant charge: R22 or R407c, 3 kg per RMPU.
- Alternator (SG-fitted coaches): 25kW, 110V, 3-phase, brushless type, totally enclosed, reversible shunt-wound.
- Battery: 800/1100 Ah, 110V DC, LMLA/VRLA type. Battery charging transformer-cum-rectifier: 200A.
Memory Aid — Efficient vs Inefficient RMPU Working
Inefficient RMPU working means more running hours to achieve the same cooling — placing avoidable burden on the battery and alternator. Efficient working means fewer running hours for the same cooling — directly reducing that burden. This is why RMPU efficiency checks matter for the whole electrical system, not just passenger comfort.
Coach Type Classification (AC Stock)
| Coach Type | Code | Seats/Berths | Tare Weight (tons) |
|---|---|---|---|
| AC First Class Sleeper (EOG) | LWFAC | 24 | 43.34 |
| AC First Class Sleeper (SG) | LWGFAC | 24 | 47.50 |
| AC Two Tier Sleeper (EOG) | LWACCW | 52 | 44.50 |
| AC Two Tier Sleeper (SG) | LWGACCW | 52 | 48.40 |
| AC Three Tier Sleeper (EOG) | LWACCN | 72 | 45.60 |
| AC Three Tier Sleeper (SG) | LWGACCN | 72 | 50.60 |
| AC Double Decker Chair Car (EOG) | LWCZDAC | 120 | 49.53 |
| AC Hot Buffet Car (EOG) | LWCBAC | 18 | 40.80 |
| First AC + AC Two Tier (Composite) | LFCWAC | 10+28 | 42.50 |
| AC Chair Car / 2nd | LSCZAC | 78 | 40.40 |
| AC Chair Car / First Class | LFCZAC | 56 | 40.40 |
| Generator cum Luggage & Brake Van | LWLRRM | 5 (4 crew + 1 guard) | 53.00 |
Exam Focus — SG vs EOG Weight Difference
Notice that every SG (Self-Generating) variant is heavier than its EOG counterpart with the same seating (e.g. LWGFAC at 47.50 tons vs LWFAC at 43.34 tons) — because an SG coach carries its own alternator, ERRU, and larger battery onboard, whereas an EOG coach draws power externally from the power car and needs less onboard generation equipment.