REFRIGERATION AND AIR CONDITIONING – KME601
Section-wise Important Questions & Ready Answers
SECTION A
(Attempt all – 2 marks each)
(a) Refrigerating Effect and COP
Refrigerating effect is the quantity of heat removed from the refrigerated space in order to maintain it at a temperature lower than its surroundings. Coefficient of Performance (COP) is the ratio of refrigerating effect to the work input required to produce that effect.
(b) COP Range of Refrigerator and Heat Pump
The COP of a refrigerator is always greater than unity and generally ranges between 2 and 6. The COP of a heat pump is higher than that of a refrigerator since it includes both the refrigerating effect and the work input.
(c) Higher COP of Vapour Compression Cycle
The COP of a vapour compression refrigeration cycle is higher than that of an air refrigeration cycle because less work input is required for the same refrigerating effect and losses are comparatively lower.
(d) Sub-Cooling and Superheating in VCRS
Sub-cooling refers to cooling the liquid refrigerant below its saturation temperature before expansion, increasing refrigerating effect. Superheating is heating the vapour refrigerant above saturation temperature before compression, ensuring dry compression.
(e) VAR System Component Equivalent to Compressor
In a vapour absorption refrigeration system, the combined action of the absorber, pump, and generator performs the same function as the compressor in a vapour compression system.
(f) Compressor Preferred with Refrigerant R-113
Centrifugal compressors are generally preferred with refrigerant R-113 due to its low pressure and high volumetric flow rate.
(g) Apparatus Dew Point (ADP)
ADP is the effective surface temperature of a cooling coil at which air leaves the coil after cooling and dehumidification.
(h) Sensible Heat Factor (SHF) and By-Pass Factor (BPF)
SHF is the ratio of sensible heat to total heat in an air-conditioning process. BPF represents the fraction of air that passes through the coil without being cooled to the coil surface temperature.
(i) Hermetically Sealed Compressor
A hermetically sealed compressor is one in which the compressor and motor are enclosed in a single airtight casing, preventing refrigerant leakage.
(j) Cooling Tower
A cooling tower is a heat rejection device that removes waste heat from water by cooling it through evaporation and heat exchange with air.
SECTION B
(Attempt any three – 10 marks each)
1. Carnot Refrigerator, Heat Pump, and Heat Engine
A Carnot cycle represents the ideal maximum efficiency. When operated as a refrigerator or heat pump, COP depends only on source and sink temperatures. As a heat engine, it converts heat into work with maximum theoretical efficiency.
2. Actual vs Ideal Vapour Compression Cycle
The actual VCR cycle differs due to pressure drops, superheating, sub-cooling, and irreversibilities. These losses reduce COP compared to the ideal cycle. The actual cycle is represented on p-h and T-s diagrams.
3. Absorption Refrigeration System (NH₃-H₂O)
In absorption systems, heat is used instead of mechanical work. The maximum COP is governed by operating temperatures. The mass of steam required is calculated based on refrigeration load and actual COP.
4. Winter Air-Conditioning Process
In winter air conditioning, air is preheated, humidified, and reheated to achieve desired conditions. Psychrometric charts are used to represent the processes and determine air washer efficiency.
5. Refrigeration for Food Preservation
Refrigeration slows down bacterial growth and enzymatic reactions, preserving food quality. It helps retain nutritional value, freshness, and shelf life.
SECTION C
Q3(a) Classification of Refrigeration Systems
Refrigeration systems are classified into air refrigeration, vapour compression, vapour absorption, steam jet, and special refrigeration systems. Aircraft systems such as bootstrap systems provide higher efficiency and cooling capacity.
Q3(b) COP of Bell-Coleman Cycle
The COP of an air refrigeration system is derived using temperature and pressure relations during compression and expansion processes.
Q4(a) Ammonia VCR System (Numerical)
Using given dryness fractions and work input, COP and compressor volume flow rate are calculated based on refrigeration effect and mass flow rate.
Q4(b) COP of Multiple Evaporator System
The COP expression for a system with multiple evaporators depends on total refrigeration effect and compressor work.
Q5(a) Compressor Function in VCR and VAR Systems
In VCR systems, the compressor raises refrigerant pressure. In VAR systems, this function is achieved by absorber, pump, and generator combination.
Q5(b) Physical vs Thermodynamic Properties of Refrigerants
Physical properties include density and viscosity, while thermodynamic properties include latent heat and saturation temperature. Thermodynamic properties are more critical for performance evaluation.
Q6(a) Cooling Coil and Dehumidification (Numerical)
Mass of water removed and cooling coil capacity are determined using psychrometric relations and airflow rate.
Q6(b) Sensible and Latent Heat Calculation
Sensible and latent heat loads are computed based on temperature and humidity differences, and SHF is evaluated.
Q7(a) Frictional and Dynamic Losses in Ducts
Frictional losses occur due to surface roughness, while dynamic losses arise from bends and fittings. Pressure drop is calculated using standard duct flow equations.
Q7(b) Steam Jet and Magnetic Refrigeration Systems
Steam jet refrigeration uses ejectors and waste heat, while magnetic refrigeration relies on magnetocaloric effect for cooling.
