THEORY EXAMINATION (SEM–VI) 2016-17 REFRIGERATION & AIR CONDITIONING

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

(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_wms​xs​=mw​xw​

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.