What is the focus of EWING's Construction?
Click to see answer
Determination of suction temperature for optimum Coefficient of Performance (COP).
Click to see question
What is the focus of EWING's Construction?
Determination of suction temperature for optimum Coefficient of Performance (COP).
What is the primary function of a refrigerator?
To maintain a body at a temperature lower than the temperature of the surroundings.
What is a characteristic of the Saturated Reversed Carnot Cycle?
It requires 2 compressors, leading to high initial cost and maintenance.
Why can't an engine or turbine be used in the Saturated Reversed Carnot Cycle?
Because expansion occurs in the wet domain.
How does a refrigerator operate?
It operates in a cycle.
What is a key characteristic of the processes in the Carnot cycle?
All processes are reversible.
Where is the expansion valve placed in an actual vapor compression system?
Very close to the evaporator.
What does A2 represent in the context of the saturation lines on the T-s diagram?
A2 will be zero if the saturated liquid line is vertical.
What is the Coefficient of Performance (COP) of the Saturated Reversed Carnot Cycle?
It is the same as that of the Reversed Carnot Cycle.
What does Area A1 represent on a T-S diagram?
The area of the superheating horn.
What happens when a reversible process is reversed?
All energy transfers are reversed in direction but remain the same in magnitude.
What does the Carnot cycle represent in a steady flow system?
It represents the net work done and heat transfer in the cycle.
Why is the expansion valve placed close to the evaporator?
To prevent any pressure loss.
What happens to heat transfer if the saturated liquid line is vertical?
Heat transferred to increase the temperature of liquid along the liquid saturation line will be zero.
What is Area A2 called on a T-S diagram?
The area of throttling loss.
What are heat pumps and refrigerators considered in thermodynamics?
They are reversed heat engines.
What is the equation for the net work done in the Carnot cycle?
W_net = Q1 - Q2.
What causes pressure reduction in the evaporator?
Frictional effects and bends in the evaporator.
What is the implication of a vertical saturated liquid line on specific heat?
It implies that the liquid specific heat should be small for minimum A2.
What does the Carnot efficiency formula represent?
The efficiency of a Carnot cycle, defined as η = 1 - (T2/T1).
What type of cycle is the Carnot Cycle?
It is a reversible cycle.
What is the tube connecting the condenser to the expansion valve called?
The liquid line.
What does Area A3 represent on a T-S diagram?
The area of loss in refrigeration effect.
In the Carnot cycle, what does Q1 represent?
The heat absorbed from the hot reservoir.
Under what condition will A1 be zero?
A1 will be zero if the saturated vapor line is vertical or has a positive slope.
What is the relationship between total input heat and net output work in a Carnot cycle?
Net output work is equal to total input heat minus the heat rejected, W_net = Q_in - Q_out.
How does the specific volume of saturated vapor change with evaporator temperature/pressure?
It increases with the decrease in evaporator temperature/pressure.
What happens during the reversible isothermal process (1-2) of the Carnot Cycle?
Heat Q1 enters the system at temperature t1 reversibly.
What is the relationship between liquid temperature at the condenser exit and surrounding temperature?
Liquid temperature at the condenser exit is higher than the surrounding temperature, leading to heat rejection.
What is the relationship between mass flow rate and swept volume in a vapor compression cycle?
The mass flow rate of the refrigerant is related to the swept volume (Vs) and the specific volume at the inlet of the compressor.
What is the relationship between the degree of superheat and the degree of subcool in this context?
The degree of superheat is always greater than the degree of subcool.
What does the area 12''34'1 represent in the Reversed Carnot Cycle?
Net work or net heat.
In the Carnot cycle, what does Q2 represent?
The heat rejected to the cold reservoir.
What happens to the specific isentropic compression work when the evaporator pressure decreases or the condenser pressure increases?
The specific isentropic compression work increases.
What happens to the evaporator temperature when the compressor is switched on?
It starts decreasing from room temperature to the designed temperature.
What is the primary function of a Single Stage Saturation (SSS) Cycle?
To compress refrigerant vapor and facilitate heat transfer.
How does the Refrigeration Effect (RE) change with a decrease in evaporator temperature at a fixed condenser temperature?
RE decreases due to the positive slope of the saturated vapor line.
What is the significance of using a pure substance in a Carnot cycle?
It allows for clear definitions of phase changes and thermodynamic properties.
What happens to vapor density as pressure decreases?
Vapor density decreases drastically.
What replaces the diathermic wall during the reversible adiabatic process (2-3)?
The diathermic wall is replaced with an adiabatic wall.
Why is subcooling considered useful in the vapor compression cycle?
It helps in improving the efficiency of the system by reducing the pressure loss.
What type of gas is used in the Reversed Carnot Cycle?
Ideal Gas.
What is the formula for the refrigeration effect in a vapor compression cycle?
Refrigeration Effect = (h1 - h4) kW.
Is the enthalpy of subcooled liquid available in refrigeration tables?
No, it is not available.
What is the tube connecting the evaporator and compressor called?
The suction line.
What is the significance of considering a constant pressure line in the alternate approach?
It helps in analyzing work representation on the T-S diagram.
How does the divergence of the constant entropy line on the p-H diagram affect enthalpy?
The difference between enthalpy increases.
What is the effect of pressure drop in a vapor compression system?
It is equivalent to a drop in evaporator temperature.
What is the significance of the Carnot cycle in thermodynamics?
It sets the maximum efficiency limit for heat engines.
What occurs during the pull down period in a vapor compression system?
The evaporator is at room temperature and the compressor starts to decrease the evaporator temperature.
What is the main component that compresses the refrigerant in a Vapor Compression Cycle?
The compressor.
What happens to the Refrigeration Effect (RE) when the condenser temperature increases at a fixed evaporator temperature?
RE decreases.
What does the symbol 'Q' represent in the context of the Carnot cycle?
It represents heat transfer in the cycle.
How does the mass flow rate change with decreasing pressure?
The mass flow rate decreases.
What is done by the system during the adiabatic process (2-3)?
Work WE is done by the system adiabatically and reversibly at the expense of internal energy.
What is done to reduce pressure loss in the liquid line?
The tube length is shortened.
What processes are involved in the Reversed Carnot Cycle?
Isentropic and Isothermal processes.
What is the specific refrigeration effect?
The difference between h1 and h4 in kJ/kg.
What type of process occurs during the transition from state 3 to state 4 in the Carnot Cycle?
Reversible isothermal process.
How is the actual refrigeration effect calculated?
Actual Refrigeration Effect = (h1 - h4) - (h3 - h2) kW.
What happens to the saturated liquid at pressure P3'' when compressed to P_c?
It reaches state 3'.
What is the steady flow energy equation used for in a reversible process?
It describes the energy changes per unit mass in a steady flow system.
What process shifts the state from 1 to 1' in a vapor compression system?
Superheating.
How much does a pressure drop of 0.1 bar affect the evaporator temperature for R22?
It results in a 1.1 K drop.
What type of substance does the Carnot cycle typically involve?
A pure substance.
Which refrigerant shows more divergence of isentropic lines?
Ammonia (NH3).
What can happen if the motor is selected based on rated capacity during the pull off period?
It may burn down.
What happens to the refrigerant in the evaporator of a Vapor Compression Cycle?
It absorbs heat and evaporates.
What is the relationship between the loss in Refrigeration Effect (RE) and the specific heat of the liquid refrigerant?
The loss is proportional to the specific heat of the liquid refrigerant.
How does the Coefficient of Performance (COP) of a Reversed Carnot Cycle change with temperature?
COP increases with increasing the lower temperature or decreasing the higher temperature.
What does 'W_net' signify in the Carnot cycle equations?
It signifies the net work output of the cycle.
What is the primary function of a heat pump?
To maintain a body at a temperature higher than the surroundings.
What is the primary function of an actual vapor compression system?
To transfer heat from a low-temperature reservoir to a high-temperature reservoir.
What do natural occurring substances like H2O, NH3, and CO2 have in common regarding their vapor and liquid lines?
They have symmetrical saturated vapor and liquid lines, resulting in large areas A1 and A2.
What is the effect of decreasing evaporator temperature on refrigeration capacity?
Refrigeration capacity decreases drastically.
What state does the refrigerant reach after subcooling in the additional exchanger?
State 3a.
What is the relationship between work and internal energy in the Carnot Cycle?
W = U2 - U1 + Q1 for process 1-2 and W = U3 - U2 for process 2-3.
What happens to heat Q2 during the transition from state 3 to state 4?
Heat Q2 leaves the system at temperature t2 reversibly.
Does the specific refrigeration effect depend on the actual mass flow rate?
No, it does not depend on the actual mass flow rate.
What does the volumetric refrigeration effect represent?
Volumetric Refrigeration Effect = (ṁ * (h1 - h4)) kJ/h.
What is a significant characteristic of the Reversed Carnot Cycle regarding compression and expansion?
There is no issue of wet compression or wet expansion.
What is assumed about the liquid in this context?
The liquid is assumed to be incompressible.
What happens to the entropy change (ds) when dQ = 0?
ds = 0, indicating an adiabatic process.
What causes further superheating from state 1' to 1a'?
Imperfect insulation of the suction line.
If the evaporator temperature is -10 ℃ and the pressure drop is 0.1 bar, what is the effective evaporator temperature?
-11.1 ℃.
What is the relationship between evaporator pressure and specific volume?
Decreasing the evaporator pressure increases specific volume.
What is a solution to prevent the motor from burning down during the pull off period?
Reduce the mass flow rate to the compressor using a hand valve.
What is the role of the condenser in a Single Stage Saturation Cycle?
To release heat and condense the refrigerant back into liquid.
What is the relationship between Refrigeration Effect (RE) and the latent heat of vaporization?
RE is proportional to the latent heat of vaporization.
Which temperature has a stronger effect on the change in COP of a Reversed Carnot Cycle?
The lower temperature has a stronger effect than the higher temperature.
What is the volumic refrigeration effect of R 12?
What is the desired effect in refrigerators called?
Cooling effect or cooling load, measured in Ton of refrigeration (TR).
What does the 'Ten Point Cycle' refer to in vapor compression systems?
A detailed representation of the thermodynamic processes in the vapor compression cycle.
Which refrigerants have small areas of the superheating horn?
R12 and R134a.
What factors affect mass flow rate in a vapor compression cycle?
Volumetric efficiency of the compressor, condenser temperature, and evaporator temperature.
What state does the refrigerant reach in the liquid line due to heat gain?
State 3b.
What are the condenser and evaporator temperatures for the refrigerant properties discussed?
Condenser temperature is 30°C and evaporator temperature is -15°C.
For which type of refrigerants is the specific refrigeration effect more significant?
For lighter molecules like NH3 and water.
What type of wall is used during the transition from state 4 to state 1 in the Carnot Cycle?
Adiabatic wall.
Is the Reversed Carnot Cycle practical?
No, it is not a practical cycle.
How is enthalpy treated in relation to temperature?
Enthalpy is a function of temperature only.
What does the property relation imply in the context of adiabatic work?
It relates changes in work, heat, and entropy.
What is the main difference in appearance between SSS and RCC?
The appearance of the superheating horn (area A1) in SSS is due to dispensing with the isothermal compressor in the RC cycle.
What leads to pressure drop in the suction line?
Frictional resistance and bends in the tubes.
What is the effect of choosing a larger diameter pipe in a vapor compression system?
It leads to lower vapor velocity and reduces pressure loss.
What is the Mean Effective Pressure for a reciprocating compressor?
Compressor work divided by the swept flow rate.
What is the adiabatic discharge temperature?
The temperature after compression, determined by the ratio of specific heats.
What is the significance of the expansion valve in the Vapor Compression Cycle?
It reduces the pressure of the refrigerant before it enters the evaporator.
For which refrigerant is the latent heat of vaporization considered very large?
Ammonia (NH3).
Why is superheating of the refrigerant leaving the evaporator desirable?
To avoid wet compression.
What is the relationship between input work and desired COP in a Reversed Carnot Cycle?
Desired COP is calculated as the ratio of heat input to work input.
What are the four main processes in a Vapor Compression Cycle (VCC)?
Compression, Condensation, Expansion, and Evaporation.
What is the volumic refrigeration effect of R 22?
What is one ton of refrigeration equivalent to?
The heat transfer rate required to produce 1 US ton (900 Kg) of ice at 0°C from water at 0°C in 24 hours.
What is the tube connecting the compressor to the condenser called?
Discharge line.
What are the main components of an actual vapor compression system?
Compressor, condenser, expansion valve, and evaporator.
How does mass flow rate change with increasing condenser temperature?
It decreases.
How does the area A1 of R22 compare to that of R12?
R22 has a larger area A1 than R12.
What are the values of T1 and T4 for NH3?
T1 = T4 = Te = -15°C.
What happens if the specific refrigeration effect is higher?
Less mass flow rate of the refrigerant is required for the same cooling capacity.
What occurs during the reversible adiabatic process from state 4 to state 1?
Work W_P is done upon the system adiabatically and reversibly, increasing the internal energy.
How many compressors and expanders does the Reversed Carnot Cycle require?
Two compressors and two expanders.
What is the significance of integrating the steady flow energy equation?
It allows for the calculation of work done in a reversible process.
What is the significance of pump work in this scenario?
Pump work is neglected as it is very small.
What process replaces irreversible expansion in the SSS cycle?
Throttling replaces irreversible expansion.
Why is sufficient vapor velocity important in a vapor compression system?
To ensure that lubricating oil picked up by the vapors can reach back to the compressor.
What happens to pressure from state 1a' to 1''?
There is a pressure drop due to friction and bends.
What is the formula for specific volumic compression work (Wv)?
Wv = (h2 - h1) / (v2 - v1).
What does the net work equal according to the 1st law of thermodynamics on a T-S diagram?
The area enclosed by the cycle.
What is the specific heat ratio for ammonia (NH3)?
1.3.
What is the state of the refrigerant after it exits the compressor in a Single Stage Saturation Cycle?
High-pressure vapor.
What can cause wet compression in a refrigerant system?
Heat transfer in the tube from the surroundings even after insulation.
What is the formula for calculating the desired COP in a Reversed Carnot Cycle?
COP = Q_input / W_input.
What is the formula for calculating work done during the compression process?
W = h1 - h2.
Why does R 11 require a centrifugal compressor?
Due to low value of h1 - h4 and high value of v1, leading to high swept volume.
What is the formula for the Coefficient of Performance (COP) in refrigeration?
COP = Q_desired / W_input.
What is the vapor temperature at the compressor exit compared to the surrounding temperature?
It is more than the surrounding temperature.
How does the compressor function in a vapor compression system?
It increases the pressure of the refrigerant vapor, raising its temperature.
What is the characteristic of ethane derivatives like CCl3CF3 regarding their saturated vapor line?
They have positive slopes of the saturated vapor line, so compression usually starts from a superheated state.
What is the relationship between mass flow rate and evaporator temperature?
Mass flow rate decreases with the decrease in evaporator temperature.
What is the pressure at the evaporator (Pe) for NH3?
Pe = 2.36.
What does the product of mass flow rate and specific volume at state 1 determine?
The required swept volume flow rate and the physical size of the compressor.
How many reversible adiabatic and isothermal processes constitute a Carnot Cycle?
Two reversible adiabatic and two reversible isothermal processes.
How are constant pressure lines in the subcooled region treated for practical purposes?
They are considered to be coincident with the saturated liquid lines.
What is the effect of throttling on work output?
During throttling, no work is obtained.
What does the equation W = -∫(v dp) represent?
It represents the work done during a reversible process in a steady flow system.
What occurs at the suction valve of the compressor?
Further drop in pressure, reaching state 1'''
Why does the compressor require cooling when using ammonia?
Because of the high adiabatic discharge temperature.
Why can't net work or heat be simply found by the area enclosed by the cycle during a throttling process?
Because throttling is an irreversible process.
What is the primary function of the vapor compression cycle?
To transfer heat from a low-temperature reservoir to a high-temperature reservoir.
What happens to volumic compression work (Wv) as evaporator pressure decreases?
Initially increases, reaches a maximum, then decreases.
What is the typical cycle used in refrigeration and air conditioning systems?
The Vapor Compression Cycle.
How does superheating affect the refrigeration effect (RE)?
It increases the RE.
How is heat transferred during the condensation process represented?
Q = h2 - h3.
How does the cooling capacity of R 22 compare to R 12 when using the same compressor?
R 22 provides more cooling capacity than R 12.
How is the COP for a heat pump calculated?
COP = Q_H / W_input, where Q_H is the heat delivered.
Why does actual compression require more work than isentropic compression?
Due to friction and finite heat transfer to the surroundings, causing entropy to not be constant.
Why is heat rejection at the compressor exit beneficial?
It helps reduce the load on the condenser.
What role does the condenser play in the vapor compression cycle?
It removes heat from the refrigerant, causing it to condense into a liquid.
Why is the constant temperature line not exactly vertical on the p-h diagram?
Because the enthalpy at points 3' and 3'' is not the same, although it is shown as the same in the figure.
What is the temperature (Tc) and pressure (Pc) at the condenser for NH3?
Tc = 30°C and Pc = 11.66.
Why is NH3 preferred in refrigeration systems?
Because it has a higher specific refrigeration effect, requiring lower mass flow rate for the same cooling capacity.
What is the relationship between work W, internal energy U, and heat Q in the Carnot Cycle?
W = U4 - U1 - Q1.
What is the comparison based on for the Reversed Brayton cycle and the Reversed Carnot cycle?
The same compression ratio.
How does isotropic expansion differ from throttling?
Isotropic expansion provides available work, while throttling does not.
What is neglected when integrating the steady flow energy equation?
Kinetic energy (KE) and potential energy (PE).
What is assumed about the line 2-a-3-0 in the context of compressor work?
It is a constant pressure line.
Why can't ammonia be used in hermetic sealed compressors?
Due to the need for cooling of the compressor by water.
What is the impact of superheating on the area of the superheating horn?
It increases the area of the superheating horn, which can increase compression work.
What is the formula for work done during the expansion process?
W = h3 - h4.
What are the main components of a vapor compression cycle?
Compressor, condenser, expansion valve, and evaporator.
What is the relationship between h1 - h4 and mass flow rate for refrigerants?
A low value of h1 - h4 requires a high mass flow rate.
What does the equation Q_input = Q_effect + W_input represent?
The relationship between heat transfer and work input in refrigeration systems.
What is done to reduce pressure loss in the discharge line?
The tube length is shortened.
What happens in the expansion valve of a vapor compression system?
The refrigerant expands, dropping in pressure and temperature.
What is the significance of the difference in enthalpy at points 3' and 3'' on the p-h diagram?
The difference is negligible.
What is the enthalpy (h1) for NH3?
h1 = h1g = 1443.9 kJ/kg.
What is the typical isentropic efficiency for hermetically sealed compressors?
Typically 0.5.
What does the net work required in the Reversed Brayton cycle represent on the Pv diagram?
The area 12'34'1.
What is the impact of the SSS cycle on refrigeration effect?
In the SSS cycle, the refrigeration effect is reduced, represented as area A3, known as loss of refrigeration effect.
What is the relationship between areas A3 and A2 for the constant pressure line?
A3 must equal A2.
How does superheating affect the Coefficient of Performance (COP) with different refrigerants?
With R12, R134a, and R22, COP may increase, but with NH3, COP decreases due to a large increase in the area of the superheating horn.
How is the heat absorbed during the evaporation process calculated?
Q = h4 - h1.
What role does the compressor play in the vapor compression cycle?
It compresses the refrigerant, raising its pressure and temperature.
What does the swept volume (Vs) depend on for a refrigerant?
It depends on mass flow rate (m) and specific volume (v1).
What is the pressure at the compressor cylinder discharge?
2 inches.
What is the purpose of the evaporator in a vapor compression system?
To absorb heat from the environment, causing the refrigerant to evaporate.
What is a major issue with the Reversed Carnot Cycle using pure substances?
Wet compression.
What is the enthalpy (h3) and (h4) for NH3?
h3 = h4 = h3f = 341.8 kJ/kg.
Why is isothermal compression adopted as standard for air compressors?
Because it requires less work by maintaining temperature while pressure increases.
What do areas A1 and A2 represent in the context of SSS and RC cycles?
Areas A1 and A2 account for the deviation of the SSS cycle from the RC cycle.
What area represents the net work required in the Reversed Carnot cycle on the Pv diagram?
The area 12341.
What does the equation W = Q1 - Q2 represent in the context of work?
Net work is equal to the heat input minus heat output.
What is the formula for the Coefficient of Performance (COP) in a VCC?
COP = Q_effective / W_input.
Why is subcooling of the refrigerant leaving the condenser required?
It increases the RE and ensures only liquid refrigerant enters the expansion device.
What happens in the condenser of the vapor compression cycle?
The refrigerant releases heat and condenses from gas to liquid.
What is the significance of COP in relation to different refrigerants?
COP is almost the same for all refrigerants despite differences in RE and work input.
What causes the pressure drop in the discharge valve?
The pressure drop in the discharge valve makes it at 2 inches.
What is the RPM of the compressor in the Reversed Carnot Cycle?
1440 RPM.
What is the difference in enthalpy (h1 - h4) for NH3?
h1 - h4 = 1102 kJ/kg.
