Tag: refrigerators and heat pumps

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Questions Related to refrigerators and heat pumps

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

What is the function of the evaporator in a refrigerator?

  1. The evaporator supplies the heat to the substance, which is to be cooled.

  2. The evaporator absorbs the heat from the atmosphere.

  3. The evaporator absorbs the heat from the substance, which is to be cooled.

  4. Both a and b
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Evaporator is an important component together with other major components in a refrigeration system such as compressor, condenser and expansion device. The reason for refrigeration is to remove heat from air, water or other substance.

It is here that the liquid refrigerant is expanded and evaporated. It acts as a heat exchanger that transfers heat from the substance being cooled to a boiling temperature.
Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

Which of the following is not the component of heat pump?

  1. Condenser

  2. Compressor

  3. Cooler

  4. Expansion valve
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

A heat pump is a device that transfers heat energy from a source of heat to what is called a "heat sink". Heat pumps move thermal energy in the opposite direction of spontaneous heat transfer, by absorbing heat from a cold space and releasing it to a warmer one. A heat pump uses a small amount of external power to accomplish the work of transferring energy from the heat source to the heat sink,there are four main components of heat pump: compressor, condenser, expansion valve and evaporator

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

What is most commonly used as the refrigerant  in heat pumps?

  1. Chlorofluorohydrocarbons

  2. Fluorine gas

  3. Hydrogen gas

  4. Carbon gas
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A refrigerant is generally a fluid , which undergoes state change , from liquid to gas and back again . Chlorofluorohydrocarbons are most commonly used  as refrigerant in heat pumps . 

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

An ideal refrigerator operates according to the reverse Carnot cycle and transmits heat from a cold source with water at a temperature of $27^{\circ}C$ to a boiler with water at a temperature of $100^{ \circ  }C$. What amount of water must be frozen in the cooler to convert 1 kg of water into vapor in the boiler ?

  1. 4.94 kg

  2. 3.24 kg

  3. 5.63 kg

  4. 2.12 kg
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The coefficient of performance (COP) for a Carnot refrigerator is T_cold / (T_hot - T_cold). With T_cold = 300K and T_hot = 373K, COP = 300 / 73 = 4.11. Heat removed from cold source (Q_cold) = COP * Work. Heat added to hot source (Q_hot) = Q_cold + Work = Work * (COP + 1). To convert 1 kg of water to vapor requires 2260 kJ. Using the ratio of heat transfer, the mass of ice frozen is calculated based on the latent heat of fusion (334 kJ/kg).

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

A reversible engine operates between temperatures 900 K & $T _2$($T _2$ < 900 K), & another reversible engine between $T _2$ & 400 K ($T _2$ > 400 K) in series. What is the value of $T _2$ if work outputs of both the engines are equal?

  1. 600K

  2. 625K

  3. 650K

  4. 675K
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Output work  $W = \Delta T$
Thus for equal work output, temperature difference should be equal.
$900-T _2=T _2-400$
Or  $2T _2 = 1300$ 
$\implies$ $T _2=650 \ K$
Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

A carnot cycle is having maximum efficiency because

  1. it comprises of two adiabatic process which requires no heat in its execution.

  2. it comprises of two isothermal process.

  3. its every process is reversible

  4. none of the above
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Carnot cycle consists of 4 process. All of them are ideal and practically not possible.

1.Isothermal heat addition- Nothing can be more efficient than this heating since no finite temperature difference exists between heat source and receiver. Hence it is a reversible process and most efficient,

2.Isentropic(reversible adiabatic) expansion- Most efficient expansion, as no heat loss is taking place, full energy is utilised only for expansion and thereby doing work.

3.Isothermal heat rejection- Most efficient heat rejection to a heat sink at same temperature.

4.Isentropic compression- Compressed(without friction) with no heat lost to surrounding. Power input is used only for increasing the pressure and temperature.

In this cycle every process is reversible and hence it is most efficent.
When the cycle is completed input energy and power is used only for the purpose it was intended. Output was completely used for intended purpose
Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

A series combination of two Carnots engines operate between the temperatures of $180^0C$ and $20^0C$. If the engines produce equal amount of work,then what is the intermediate temperature(In $^0C$)?

  1. 80

  2. 90

  3. 100

  4. 110
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation


A series combination of two Carnot engines operate between the temperatures of $180^0C$and $20^0C$.If the engines produce equal amount of work
The intermediate temperature in series combination is given by 
$T _i=\dfrac{T _1+T _2}{2}=\dfrac{180+20}{2}=100^oC$
Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

What is the function of refrigerants in heat pumps?

  1. Refrigerants supply heat to the atmosphere

  2. Refrigerants evaporate heat and cool the room

  3. Refrigerant carries heat from the atmosphere to the room,which is to be heated

  4. None of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

A refrigerant is a substance or mixture, usually a fluid, used in a heat pump and refrigeration cycle. In most cycles it undergoes phase transitions from a liquid to a gas and back again.A heat pump is a device that transfers heat energy from a source of heat to what is called a "heat sink". Heat pumps move thermal energy in the opposite direction of spontaneous heat transfer, by absorbing heat from a cold space and releasing it to a warmer one. A heat pump uses a small amount of external power to accomplish the work of transferring energy from the heat source to the heat sink.


the function of refrigerants in heat pumps refrigerant carries heat from the atmosphere to the room,which is to be heated

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

In a cyclic heat engine operating between a source temperature of $600^0C$ and a sink temperature of $20^0 C$, the least rate of heat rejection per kW net output of the engine is,

  1. 0.505kW

  2. 0.490kW

  3. 0.333kW

  4. none of the above
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$\therefore Heat\quad absorption=\dfrac { { T } _{ 1 }-{ T } _{ 2 } }{ { T } _{ 1 } } $


$=\dfrac { 873-293 }{ 873 } =0.664$


$\therefore Heat\quad Rejected=1-heat\quad absorbed$
$=1-0.664$
$\therefore Heat\quad rejected=0.335$

Hence the heat of rejection per kW is $0.335$