About this previous year paper
REFRIGERATION & AIR CONDITIONING (EME604)
Section-wise Solved Answers & Notes
SECTION – A (10 × 2 = 20 Marks)
Short, direct answers
(a) Refrigeration effect & unit of refrigeration
Refrigeration effect: Amount of heat removed from a space or substance.
Unit: Ton of Refrigeration (TR)
1 TR=210 kJ/min=3.5 kW1\ \text{TR} = 210\ \text{kJ/min} = 3.5\ \text{kW}1 TR=210 kJ/min=3.5 kW
(b) Boot-strap cycle of air refrigeration system
It is an improved air refrigeration cycle having an additional heat exchanger and compressor, used in aircraft refrigeration to achieve lower temperatures and higher COP.
(c) Open vs Closed air refrigeration system
| Open system | Closed system |
|---|---|
| Air exhausted to atmosphere | Air recirculated |
| Used in aircraft | Used in laboratory systems |
| Simple design | Better efficiency |
(d) Dry Air Rated Temperature (DART)
It is the equivalent dry-bulb temperature that gives the same cooling effect as moist air under actual conditions.
(e) Actual vs theoretical vapour compression cycle
Actual cycle differs due to: Pressure drops
Superheating of vapour Sub-cooling of liquid
Compressor inefficiency These reduce COP compared to theoretical cycle.
(f) Capillary tube operation
A capillary tube is a throttling device which reduces pressure of refrigerant from condenser to evaporator by frictional resistance.
(g) Psychrometric process
A psychrometric process involves changes in air properties such as DBT, WBT, humidity and enthalpy.
Examples: • Heating
• Cooling • Humidification
• Dehumidification
(h) Modified comfort chart
It shows human comfort zone considering DBT, RH, air velocity, clothing and activity level.
(Neat sketch must be drawn in exam)
(i) Heat gain through ducts Q=UA(T1−T2)Q = UA(T_1 - T_2)Q=UA(T1−T2)
Where U = overall heat transfer coefficient, A = area.
(j) Factors in load estimation (comfort AC)
• Heat gain through walls, roof • Occupants
• Lighting • Equipment
• Infiltration & ventilation
SECTION – B (Any 5 × 10 = 50 Marks)
(a) Open cycle air refrigeration system (Numerical – outline)
Steps: Draw P-V and T-S diagrams
Calculate COP
COP=Refrigeration effectWork doneCOP = \frac{Refrigeration\ effect}{Work\ done}COP=Work doneRefrigeration effect Calculate air circulation rate
Find piston displacement (Use ideal gas relations and given index 1.4)
(b) Vapour compression system using R-12 Given data used to determine:
Refrigerating effect Refrigerating load (TR)
COP
Key relation: QH=m(h2−h3)Q_H = m(h_2 - h_3)QH=m(h2−h3)
(c) Aqua-ammonia absorption system
Important relations: msxs=mwxwm_s x_s = m_w x_wmsxs=mwxw
Where: msm_sms = strong solution flow rate
mwm_wmw = weak solution flow rate Used in large-capacity plants.
(d) Heating & humidification of air Use psychrometric chart to determine:
Heat added Moisture added
Sensible Heat Factor (SHF)
SHF=Sensible heatTotal heatSHF = \frac{Sensible\ heat}{Total\ heat}SHF=Total heatSensible heat
(e) Cooling & dehumidification process
Steps: Determine inlet and outlet air properties
Find moisture removed Calculate cooling coil capacity
Used in comfort air-conditioning systems.
(f) Practical vapour absorption refrigeration system
Main components: • Generator
• Condenser • Evaporator
• Absorber • Pump
Working: Heat replaces compressor work.
(g) Definitions
• Dew point temperature: Temperature at which condensation begins
• Specific humidity: Mass of water vapour per kg of dry air
• Relative humidity: Ratio of actual to saturated vapour pressure
• Degree of saturation: Ratio of actual to saturated humidity
(h) Ideal refrigerant & secondary refrigerants
Ideal refrigerant properties: • High latent heat
• Low toxicity • Non-flammable
• Easy availability
Secondary refrigerants:
Brine, glycol – used to transfer cooling effect.
SECTION – C (Any 2 × 15 = 30 Marks)
Q3. Multi-stage VCR vs Cascade system
Multi-stage system • Compression in stages
• Intercooling used • High efficiency
Cascade system
• Two different refrigerants • Used for very low temperatures
Advantages over simple VCR: • Lower power consumption
• Higher COP • Reduced compressor work
Q4. Bell-Coleman cycle (Numerical – outline)
Steps: Calculate isentropic temperatures
Use efficiencies of compressor & expander Calculate work input
Convert cooling load (TR) to kW Used in aircraft refrigeration.
Q5.
(a) Effect on COP of VCR system • Increase condenser pressure → COP decreases
• Decrease evaporator pressure → COP decreases • Sub-cooling → COP increases
(b) Flash chamber
Used to: • Reduce flash gas
• Improve refrigeration effect • Increase COP
Explained using P-h chart and schematic diagram.