Refrigeration And Air Conditioning — Part 1 MCQs with Explanations

Nusselt number (N N ) is the ratio of convective to conductive heat transfer at a boundary in a fluid. A Nusselt number of value one represents heat transfer by pure conduction. Nusselt number (N N ) is given by $${{\text{N}}_{\text{N}}} = \frac{{{\text{h}}l}}{{\text{k}}}$$

In case of sensible heating of air, the coil efficiency is given by 1 - B.P.F.

It is very important that the refrigerant has a low boiling point, that is it should have the ability to turn into gas easily when it absorbs heat. If it has a high boiling point, the compressor will have to create too much of a vacuum in order to lower the pressure to make vaporization happen.

Physical Properties of Refrigerants - Low Condensing Pressure - High Evaporator Pressure - High Critical Pressure - High Vapor Density - High Dielectric strength - High Latent Heat of Vaporization - High Heat Transfer Coefficient

The operating pressure for refrigerating units using R-12 as a refrigerant is 8 bar

Cooling water is required for Condenser, absorber and separator (rectifier) in ammonia absorption plant

The coefficient of performance of a domestic refrigerator is less as compared to a domestic air-conditioner.

In aqua ammonia and Lithium bromide water absorption refrigeration systems, the refrigerants are respectively Ammonia and water

In order to cool and dehumidify a stream of moist air, it must be passed over the coil at a temperature Which is lower than the dew point temperature of the incoming stream

During the heating and dehumidification process dry bulb temperature of the air increases while its dew point and wet bulb temperature reduces.

During the heating and dehumidification process dry bulb temperature of the air increases while its dew point and wet bulb temperature reduces.

In a domestic vapour compression refrigerator, the refrigerant commonly used is R-12.

In refrigerators, the temperature difference between the evaporating refrigerant and the medium being cooled should be As low as possible (3 to 11°C)

The minimum temperature to which water can be cooled in a cooling tower is Wet bulb temperature of air

The processes are used for scientific, industrial and commercial purposes. Many gases can be put into a liquid state at normal atmospheric pressure by simple cooling; a few, such as carbon dioxide, require pressurization as well. At ambient pressure the boiling point of liquefied helium is 4.22 K

The value of C.O.P. in vapour compression cycle is usually always more than unity.

The process in which the moisture or water vapor or the humidity is removed from the air keeping its dry bulb (DB) temperature constant is called as the dehumidification process The specific humidity during cooling and dehumidification Decreases

The suction pipe diameter of refrigerating unit compressor in comparison to delivery side is Bigger

In a vapour compression refrigeration system, a throttle valve is used in place of an expander because The positive work in isentropic expansion of liquid is very small

In a refrigeration system, heat absorbed in comparison to heat rejected is less

The optimum effective temperature for human comfort is Lower in winter than in summer

In the domestic refrigerators the evaporators are commonly known as the freezers since the ice is made in these compartments. In case of the window and split air conditioners and other air conditioning systems where the evaporator is directly used for cooling the room air, it is called as the cooling coil.

The refrigerant vaporizes from the heat it absorbs heat in the evaporator. Flow control device (expansion valve): This controls the flow of the liquid refrigerant into the evaporator. Control devices usually are thermostatic, meaning that they are responsive to the temperature of the refrigerant.

Option A: Carnot cycle The Carnot cycle is an ideal thermodynamic cycle that gives the maximum possible efficiency. It consists of two isothermal and two adiabatic processes. It is used only as a standard of comparison and cannot be practically implemented because isothermal compression and expansion are very difficult to achieve. Option B: Reversed Rankine cycle The vapour compression refrigeration system follows the processes of compression, condensation, expansion (throttling), and evaporation. These are the exact processes of the Reversed Rankine cycle. It is a practical and achievable cycle, which is why almost all refrigerators and air conditioners work on this principle. Option C: Reversed Carnot cycle The Reversed Carnot cycle is the theoretical model of refrigeration with the highest possible COP (Coefficient of Performance). However, it cannot be applied in real systems because achieving isothermal heat addition and rejection along with isentropic compression/expansion is practically impossible. So, it serves as an "ideal reference" but not the real working cycle. Conclusion: While the Reversed Carnot cycle is the ideal and most efficient refrigeration cycle, it is not feasible for practical use. The Vapour Compression Refrigeration system actually works on the Reversed Rankine cycle , which is why the correct answer is Option B: Reversed Rankine cycle .

Refrigerant with a lower latent heat of vaporization will absorb less energy during evaporation than those with higher hfg. The temperature for a given pressure at which the gaseous refrigerant begins to condense into a liquid.

A refrigeration cycle operates between condenser temperature of +27°C and evaporator temperature of -23°C. The Carnot coefficient of performance of cycle will be 5.

The bypass factor (B.P.F.) in case of sensible heating of air is $$\frac{{{\text{t}}{{\text{d}}_3} - {\text{t}}{{\text{d}}_2}}}{{{\text{t}}{{\text{d}}_3} - {\text{t}}{{\text{d}}_1}}}$$ Where, td 1 = Dry bulb temperature of air entering the heating coil, td 2 = Dry bulb temperature of air leaving the heating coil and td 3 = Dry bulb temperature of heating coil

The general rule for rating refrigeration systems (excepting for CO 2 system) is to approximate following h.p. per ton of refrigeration 1 to 2 h.p. per ton of refrigeration.

The freezing point of R-12 is -157.7°C

The coefficient of performance (C.O.P.) of a refrigerator working as a heat pump is given by (C.O.P.) P = (C.O.P.) R + 1

During a refrigeration cycle, heat is rejected by the refrigerant in a Condenser

Freon group of refrigerants are Nontoxic and non-inflammable

The Freon group of refrigerants are Halocarbon refrigerants

One ton of the refrigeration is The refrigeration effect to freeze 1 ton of water at 0°C into ice at 0°C in 24 hours

The power per tonne of refrigeration is $$\frac{{3.5}}{{{\text{C}}{\text{.O}}{\text{.P}}{\text{.}}}}$$

An important characteristic of absorption system of refrigeration is quiet operation

The evaporator used in household refrigerators is Frosting evaporator

Chaperon equation is a relation between temperature, pressure, specific volume and enthalpy. The Clausius-Clapeyron equation used to estimate the vapor pressure at another temperature, if the vapor pressure is known at some temperature and if the enthalpy of vaporization is known.

If the evaporator temperature of a plant is lowered, keeping the condenser temperature constant, the h.p. of compressor required will be more.

In vapour compression cycle, the condition of refrigerant is saturated liquid After passing through the condenser