(SEM VI) THEORY EXAMINATION 2021-22 REFRIGERATION AND AIR CONDITIONING

REFRIGERATION AND AIR CONDITIONING (KME601)

Section-wise Detailed Answers – B.Tech Semester VI

SECTION A

(Attempt all questions – brief but conceptually complete answers)

Q1(a) COP of reversed Carnot cycle

A reversed Carnot cycle operating between two temperature limits represents the maximum possible efficiency of a refrigeration system. When the machine operates as a refrigerator, the Coefficient of Performance is defined as the ratio of refrigeration effect to work input. It depends only on the absolute temperatures of the cold and hot reservoirs. When the same machine operates as a heat pump, the COP is defined as the ratio of heat delivered to work input. Since the heat pump supplies both the refrigeration effect and the work input as useful output, its COP is always higher than that of the refrigerator operating between the same temperatures.

Q1(b) Advantages of multistage vapor compression system

Multistage vapor compression systems offer several advantages over single-stage systems, especially when operating over large temperature differences. By dividing compression into multiple stages, the work required for compression is reduced, improving overall efficiency. These systems reduce discharge temperature, minimize compressor wear, improve volumetric efficiency, and allow better lubrication. They are particularly suitable for low-temperature refrigeration applications.

Q1(c) Comparison of aircraft air cooling systems using DART and Mach number

Aircraft cooling systems are compared based on parameters such as Dry Air Rated Temperature (DART) and Mach number. At low Mach numbers, simple air cycle systems are effective. As Mach number increases, regenerative and bootstrap systems perform better due to higher ram pressure and temperature rise. Bootstrap systems are preferred at high speeds because they provide higher cooling capacity and efficiency by utilizing additional compression stages.

Q1(d) Relative humidity and specific humidity

Relative humidity is the ratio of actual mass of water vapor present in air to the maximum amount of water vapor air can hold at the same temperature, expressed as a percentage. Specific humidity is the mass of water vapor present per unit mass of dry air. These parameters define moisture content of air and are fundamental in psychrometric analysis.

Q1(e) Factors affecting human comfort and comfort chart

Human comfort depends on air temperature, relative humidity, air velocity, metabolic rate, and clothing insulation. The comfort chart graphically represents the zone where most people feel thermally comfortable. It helps HVAC engineers design air conditioning systems that maintain indoor conditions within acceptable comfort limits.

Q1(f) Refrigerants R-12 and R-729 and future refrigerants

R-12 is Dichlorodifluoromethane, a CFC refrigerant, while R-729 refers to air used as a refrigerant in air cycle systems. Due to environmental concerns, future refrigerants include R-134a, R-410A, R-407C, ammonia, carbon dioxide, and hydrocarbons, which have lower ozone depletion and global warming potential.

Q1(g) Principle of thermo-electric refrigeration

Thermo-electric refrigeration works on the Peltier effect, where heat is absorbed or rejected at the junction of two dissimilar conductors when electric current passes through them. This system has no moving parts, produces no noise, and is suitable for small-capacity cooling applications.

Q1(h) Ozone layer depletion and prevention

Ozone layer depletion occurs due to release of chlorine and bromine compounds from CFCs and halons into the atmosphere. These compounds break down ozone molecules, reducing protection against ultraviolet radiation. Prevention methods include phasing out CFCs, adopting eco-friendly refrigerants, and enforcing international agreements like the Montreal Protocol.

Q1(i) Dry ice and its preparation

Dry ice is solid carbon dioxide. It is prepared by compressing and cooling CO₂ gas, then allowing it to expand rapidly to form solid pellets or blocks. Dry ice sublimates directly into gas and is used for refrigeration, food preservation, and transportation of medical products.

Q1(j) Regenerative air cooling system

A regenerative air cooling system uses a heat exchanger to pre-cool compressed air by exchanging heat with the cold air leaving the expander. This improves system efficiency by reducing compressor work and increasing refrigeration effect.

SECTION B

(Attempt any three – descriptive answers)

Q2(a) Psychrometric definitions

Dew point temperature is the temperature at which air becomes saturated when cooled at constant pressure. Adiabatic saturation temperature is the temperature achieved when air is cooled by evaporating water into it under adiabatic conditions. Sensible Heat Factor is the ratio of sensible heat to total heat. Apparatus dew point represents the effective surface temperature of cooling coil. Dry bulb temperature is the temperature measured by a standard thermometer exposed to air.

Q2(b) Cooling load components of air-conditioning plant

Cooling load consists of heat gains from external sources such as solar radiation, walls, roofs, windows, and internal sources such as occupants, lighting, equipment, infiltration, and ventilation. Accurate estimation of cooling load ensures proper sizing of air conditioning systems and energy efficiency.

Q2(c) Cascade refrigeration system

Cascade refrigeration system uses two or more refrigeration cycles operating at different temperature ranges. The condenser of the low-temperature cycle is cooled by the evaporator of the high-temperature cycle. This arrangement is suitable for very low temperatures. The optimum intermediate temperature is the geometric mean of upper and lower temperature limits, which minimizes work input.

Q2(d) Lithium bromide water absorption system

In a lithium bromide-water absorption system, water acts as refrigerant and lithium bromide as absorbent. Refrigeration is produced by evaporation of water under vacuum. The system uses heat energy instead of mechanical work, making it suitable for waste-heat utilization and large air-conditioning applications.

Q2(e) Boot-strap air evaporative cooling system

Boot-strap system is an advanced aircraft cooling system that uses two compressors and a heat exchanger. Air is compressed, cooled, recompressed, and then expanded to produce refrigeration. It offers high efficiency at high Mach numbers and is widely used in modern aircraft.

SECTION C

(Attempt any one part – numerical and descriptive)

Q3(a) Multi-evaporator vapor compression system

In a single compressor system with multiple evaporators operating at the same temperature, the total refrigeration capacity is the sum of individual evaporator loads. Using refrigerant properties of R-12, enthalpy values at evaporator exit, condenser exit, and compressor inlet are obtained from tables. Compressor work is calculated using isentropic compression assumption. COP is then determined as the ratio of total refrigeration effect to compressor work.

Q3(b) Heating and humidification of air

In air conditioning with heating and steam humidification, outdoor air properties are determined using psychrometric relations. Heat supplied in the heating section raises dry bulb temperature at constant humidity ratio. Steam injection increases humidity ratio and enthalpy. Energy balance equations are applied to calculate heat supply rate and mass flow rate of steam.

Q4(a) Bell-Coleman air refrigeration cycle

In Bell-Coleman cycle, air is compressed, cooled at constant pressure, expanded, and then used for cooling. Using given isentropic efficiencies, actual temperatures after compression and expansion are determined. Refrigeration capacity, compressor power, and COP are calculated using air-standard relations.

Q5(a) Effect of sub-cooling and superheating

Sub-cooling increases refrigeration effect and COP by reducing throttling losses. Superheating ensures dry compression and increases refrigeration effect slightly but may increase compressor work. Proper control of both improves system performance.

Q6(b) Application of refrigeration in food storage

Refrigeration slows down microbial growth, enzyme activity, and chemical reactions in food. Cold storage preserves fruits, vegetables, meat, dairy products, and pharmaceuticals, reducing wastage and extending shelf life.

Q7(a) Evaporative cooling and adiabatic mixing

Evaporative cooling reduces air temperature by evaporating water into air at constant enthalpy. Adiabatic mixing involves mixing two air streams without heat exchange. Both processes are represented on the psychrometric chart and are widely used in HVAC systems.