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24 July 2026

📖 GTU BME IC engine testing, couplings-clutch-brake, gears/shaft

Ch. 5 & 6 ⚙️ Air Compressors, Ch. 6 ❄️ Vapour Absorption Refrigeration Cycle, Ch. 9 🔥 Steam Properties & Calorimeters, Ch. 10 💧 Pumps – Classification & Working

📘 Student Sathidar BME · GTU Important Concepts
📖 Ch. 5 · 6 · 9 · 10

Basic Mechanical Engineering

Compressors · Refrigeration · Steam & Calorimeters · Pumps — GTU exam focus

Ch. 5 & 6

⚙️ Air Compressors

🏷️ Classification of Air Compressors

By principle of operation Positive displacement · Dynamic (Centrifugal, Axial)
By delivery pressure Low (< 1.1 bar) · Medium (1.1–2.5) · High (> 2.5)
By number of stages Single stage (≤ 10 bar) · Multi stage (> 10 bar)
By piston action Single acting · Double acting
By cooling method Air cooled · Water cooled
By power drive Electric motor · I.C. engine · Gas turbine

📊 Multi‑staging in Compressors

  • Lower power requirement – power needed is less than a single‑stage compressor for the same pressure ratio.
  • Higher volumetric efficiency – volumetric efficiency improves as pressure ratio per stage is reduced.
  • Reduced leakage – pressure and temperature range in each stage is smaller, so leakage losses are lower.
  • Optimised design – high‑pressure cylinders are built to withstand high pressure, while low‑pressure cylinders handle lower pressures.

🌀 Roots Blower

Positive displacement blower with two meshing lobes. The output volume is delivered at suction pressure; no internal compression occurs. The lobes trap gas and move it from inlet to discharge, creating a pressure rise at the outlet.

🚀 Centrifugal Air Compressor – Working

When power is applied, the impeller rotates. A vacuum is created at the eye of the impeller, drawing air in. The rotating blades impart kinetic energy to the air, throwing it outward at high velocity. The air then passes through a diffuser where velocity is converted to pressure.

Ch. 6

❄️ Vapour Absorption Refrigeration Cycle

🔄 Cycle Components & Working

1. Compressor Draws low‑pressure, low‑temperature vapour from the evaporator and compresses it to high pressure & temperature, then delivers it to the condenser. (Process 1–2 on p‑h & T‑s diagrams)
2. Condenser Coils where high‑pressure refrigerant rejects latent heat to cooling medium (air or water), condensing to liquid. (Process 2–3)
3. Expansion device Capillary tube or throttle valve. Liquid refrigerant at high pressure expands to low pressure & temperature. (Process 3–4)
4. Evaporator Low‑pressure liquid absorbs heat from the space to be cooled, evaporating into vapour, which is then drawn back to the compressor.
💡 Key point: The cycle uses heat energy (instead of mechanical work) to drive the refrigeration effect, making it suitable where waste heat is available.
Ch. 9

🔥 Steam Properties & Calorimeters

🌡️ Important Steam Properties

Enthalpy of formation Heat content associated with forming 1 mole (or unit mass) of a substance from its constituent elements under standard conditions.
Enthalpy of evaporation Latent heat of vaporization – heat required to convert 1 kg of water at boiling point into 1 kg of steam without temperature change.
Heat of superheat Energy added to saturated steam to raise its temperature above saturation temperature at constant pressure.

🏭 Uses of Steam

Power generationDrives turbines in thermal power plants.
HeatingResidential, commercial, industrial via radiators & HVAC.
Industrial processesPetroleum refining, chemical, food processing.
Steam enginesHistorically powered locomotives, ships, machinery.
SterilisationAutoclaves for medical equipment & lab instruments.
HumidificationMaintains moisture levels in HVAC systems.
CookingSteaming vegetables, fish, large‑scale food production.
CleaningSteam cleaners for sanitising surfaces in food service & healthcare.

🧪 Separating Calorimeter

Construction: Wet steam enters through an inlet into a large chamber where it expands and slows down. Separated water collects at the bottom and drains out through a condensate outlet. The drier steam exits from the top.

Working:

  1. Wet steam enters the calorimeter.
  2. Steam expands and slows down in the chamber – moisture separates.
  3. Separated water is drained and collected in a measuring vessel.
  4. Remaining dry steam exits.
📐 Dryness fraction (x):
x = ms ms + mw

where ms = mass of dry steam, mw = mass of water condensate.

Dryness fraction formula
x = ms ms + mw

📉 Throttling Calorimeter – Derivation & Formula

Construction: Wet steam enters through an inlet, passes through a small throttling orifice (or valve), expands into a chamber where it becomes superheated. A thermometer measures the superheated temperature, and a pressure gauge records the pressure before throttling.

Working:

  1. Wet steam enters the calorimeter.
  2. Steam passes through the throttling orifice – rapid pressure drop.
  3. If dryness fraction is high, steam becomes superheated after expansion.
  4. Temperature of superheated steam is measured.
  5. Pressure before throttling is recorded.
📐 Derivation of dryness fraction from throttling
Before throttling (wet steam at pressure P1):
h1 = hf + x · hfg

where hf = saturated liquid enthalpy, hfg = latent heat, x = dryness fraction.

After throttling (superheated steam at pressure P2, temperature T2):
h2 = hg + Cp (T2 − Tsat)

where hg = saturated vapour enthalpy at P1, Tsat = saturation temperature at P1, Cp = specific heat of superheated steam.

Since throttling is isenthalpic: h1 = h2
hf + x · hfg = hg + Cp (T2 − Tsat)
Solving for x:
x = hg + Cp (T2 − Tsat) − hf hfg
Throttling calorimeter – dryness fraction (final)
x = hg + Cp (T2 − Tsat) − hf hfg
Ch. 10

💧 Pumps – Classification & Working

🏷️ Definition & Classification

Pump: A mechanical device that moves fluids (liquids or gases) by increasing pressure, forcing fluid from one location to another.

Dynamic (Kinetic) Pumps Add energy continuously; convert mechanical to kinetic energy.
Centrifugal (radial, axial, mixed flow) · Regenerative
Positive Displacement Pumps Move fluid by trapping a fixed volume and forcing it out.
Reciprocating (piston, diaphragm) · Rotary (gear, vane, screw, lobe)

↕️ Single‑acting Reciprocating Pump

Construction: Cylinder, piston with connecting rod & crank, suction pipe with suction valve, delivery pipe with delivery valve, foot valve, crank shaft.

Working – two strokes:

1. Suction stroke Piston moves away from cylinder head → pressure drops → suction valve opens → fluid enters cylinder. Delivery valve remains closed.
2. Delivery stroke Piston moves toward cylinder head → pressure rises → suction valve closes → delivery valve opens → fluid is forced out.

🔄 Centrifugal Pump – Construction, Working & Priming

Construction: Volute casing, impeller (rotating disk with vanes), shaft, suction pipe, delivery pipe, flanges, bearings, mechanical seal / gland packing.

Working:

  1. Suction: Impeller rotates → low pressure at eye → fluid pushed in from suction pipe.
  2. Acceleration: Vanes impart kinetic energy to fluid, throwing it outward at high velocity.
  3. Discharge: Fluid enters volute casing, slows down, pressure rises, and exits through delivery pipe.
⚠️ Priming: Before starting, the pump casing must be filled with liquid. If air is present, the impeller cannot create sufficient suction. Priming removes air and ensures proper operation.

⚙️ Vane Type Rotary Pump

Construction: Casing, rotor (slotted, off‑centered), vanes (sliding blades), inlet (suction) port, outlet (discharge) port, shaft with bearings.

Working phases:

  1. Suction phase: Rotor turns → volume between vanes increases on suction side → low pressure draws fluid in.
  2. Trapping phase: Fluid trapped between adjacent vanes and casing wall; vanes move fluid around.
  3. Discharge phase: Volume on discharge side decreases → vanes retract → fluid is compressed and forced out through outlet port.

📋 Pump Selection – Applications

Filtered water Centrifugal pump – clean, low viscosity, continuous flow.
Crude oil Gear pump or screw pump – positive displacement for viscous fluids.
Refined petroleum & sediments Progressive cavity pump – handles suspended particles without clogging.
Water with mud Diaphragm pump or slurry pump – handles abrasive & solids‑laden fluids.
✅ All important GTU topics from Chapters 5, 6, 9 & 10 covered.