As it is prohibited to allow liquid refrigerant to enter into compressor because of non compressible nature of refrigerants which can cause serious damage to the compressor, it is necessary to fully evaporate the refrigerant before entering in to compressor. This can be done by several ways which are listed below:
Heat is exchanged between high pressure liquid refrigerant coming from condenser and Low pressure vapor refrigerant coming from evaporator in suction tube. Since there is no change of phase of refrigerant during heat transfer, so all heat is transferred as sensible heat.
Applying the first law of thermodynamics to this process, theoretically heat given by high pressure liquid refrigerant must be equal to heat gain by vapor in suction line in sensible way.
m_ref*cp_hpl*(T7-T8)=m_ref*cp_lpv*(T3-T2)
where: m_ref=mass flow of refrigerant
cp_hpl=specific heat of high pressure liquid refrigerant
T7=Temperature of liquid refrigerant in liquid line before heat exchanger
T8=Temperature of liquid refrigerant in liquid line after heat exchanger
cp_lpv=Specific heat of low pressure vapor refrigerant
T2= Temperature of vapor refrigerant in suction line before heat exchanger
T3=Temperature of vapor refrigerant in suction line after heat exchanger
degree of subcool=T7-T8
The process of liquid suction heat exchanger is shown in PH chart as below:
(assuming refrigerant leaving the evaporator in saturated vapor form).
The refrigeration cycle is 1-2-3-4-5-6,7-8,9-10,1
Without liquid suction heat exchanger, the process 2-3 and 7-8 will not the part of PH diagram.
In terms of enthalpy, the first law of thermodynamics is applied on liquid suction heat exchanger as:
h1'-h1=h3-h2
Refrigeration effect without liquid suction heat exchanger=h2-h1'
Refrigeration effect with liquid suction heat exchanger=h2-h1
if we see the change in refrigeration effect= (h2-h1) - (h2-h1')
which is always positive. hence refrigeration effect increase with the use of liquid suction heat exchanger, which overall increase the COP of the system. Also it ensure no liquid refrigerant will ever enter the compressor.
For more about PH chart please visit:
http://athermocreation.blogspot.in/2018/01/pressure-enthalpy-chart.html
3. Capillary suction heat exchanger:
This is most commonly type used heat exchanger specially in refrigerator. In this type of heat exchanger capillary is soldered with suction tube and both suction tube and capillary tube act as counter flow heat exchanger.
The vapor refrigerant from evaporator took heat from capillary tube and enter in super heat condition in compressor. This increase the net refrigeration effect and overall COP of the system.
The below figure will show the capillary suction heat exchanger with in a refrigeration cycle.
Heat given by capillary tube (sensible) is equal to heat gain by vapor going from evaporator to compressor in suction tube.
m_ref*cp_liq*(T2-T3)=m_ref*cp_vap*(T5-T4)
where, m_ref = mass flow rate of refrigerant
cp_liq = specific heat of liquid refrigerant
T2 = Temperature of refrigerant entering at capillary
T3 =Temperature of refrigerant at capillary outlet or evaporator inlet
cp_vap = specific heat of vapor refrigerant
T5= Suction temperature
T4= Temperature of vapor refrigerant at evaporator outlet.
Effect of cooling capacity and COP can be easily understand from below PH chart. After using this type of heat exchanger, The expansion process is no more constant enthalpy process, The amount of vapor at evaporator inlet is reduced and cooling capacity is increased.
The low temperature saturated vapor from evaporator outlet take heat from high temperature liquid flowing through capillary tube and temperature of vapor in suction line increased. The heat taken by vapor in suction line from refrigerant flowing through capillary is the increased in cooling capacity of refrigeration system. This is explained more by PH chart as below.
The original cycle is without capillary suction heat exchanger: 4-1'-2-3'-4
The refrigerated cycle with capillary suction heat exchanger: 5-1-2-3-4-5
The increase in refrigeration effect is as below:
m_ref*(h3'-h3)=Q_in
where h3= enthalpy of refrigerant at evaporator inlet with capillary suction heat exchanger.
h3'=enthalpy of refrigerant at evaporator inlet if capillary suction heat exchanger not used.
Q_in= increased in refrigeration effect.
By applying first law of thermodynamic,
m_ref*(h3'-h3)=m_ref*(h5-h4)
where h5=enthalpy of refrigerant at compressor inlet
h4=enthalpy of saturated vapor at evaporator outlet.
Net refrigeration effect if capillary suction heat exchanger is used= m_ref*(h4-h3)
Net refrigeration effect if capillary suction heat exchanger is not used=m_ref*(h4-h3')
For more about the PH chart, Please visit: http://athermocreation.blogspot.in/2018/01/pressure-enthalpy-chart.html
1. super heating of refrigerant at the outlet of refrigerant by using Thermal expansion valve. (used in air-conditioning).
2. By doing liquid suction heat exchanger (in domestic and commercial refrigerator).
3. By doing capillary suction heat exchanger. (In refrigerators)
Let's discussed these one by one.
Super heating of refrigerant in the evaporator itself:
The air conditioners are used to provide thermal comfort to human being or machines(like data server rooms) so they function well and give best results.
Thermal comfort: Thermal comfort is a state of mind when human body feel comfort to its surrounding (flow of air and temperature and relative humidity in the air).
Air conditioner are thermally balanced such that around 5 deg C super heated vapor leaves the evaporator and enter into the compressor. This is usually done by using thermal expansion valve with 5 deg C or more degree of super heat. If thermal expansion valve sensor(placed at outlet of evaporator), sense the temperature lower than the set, it will reduce the refrigerant flow rate or vice versa. So thermal expansion valve ensure vapor refrigerant will only enter in to the compressor.
Liquid - suction heat exchanger:
Refrigerant exiting the condenser is in pure liquid state(theoretically) but has some vapor (practically) due to pressure drop in the condenser tubing. For a refrigerator system, to increase the performance it is better to do sub cooling of refrigerant before entering the expansion valve. It is done by using a heat exchanger between suction line and liquid line.
Heat is exchanged between high pressure liquid refrigerant coming from condenser and Low pressure vapor refrigerant coming from evaporator in suction tube. Since there is no change of phase of refrigerant during heat transfer, so all heat is transferred as sensible heat.
Applying the first law of thermodynamics to this process, theoretically heat given by high pressure liquid refrigerant must be equal to heat gain by vapor in suction line in sensible way.
m_ref*cp_hpl*(T7-T8)=m_ref*cp_lpv*(T3-T2)
where: m_ref=mass flow of refrigerant
cp_hpl=specific heat of high pressure liquid refrigerant
T7=Temperature of liquid refrigerant in liquid line before heat exchanger
T8=Temperature of liquid refrigerant in liquid line after heat exchanger
cp_lpv=Specific heat of low pressure vapor refrigerant
T2= Temperature of vapor refrigerant in suction line before heat exchanger
T3=Temperature of vapor refrigerant in suction line after heat exchanger
degree of subcool=T7-T8
The process of liquid suction heat exchanger is shown in PH chart as below:
(assuming refrigerant leaving the evaporator in saturated vapor form).
The refrigeration cycle is 1-2-3-4-5-6,7-8,9-10,1
Without liquid suction heat exchanger, the process 2-3 and 7-8 will not the part of PH diagram.
In terms of enthalpy, the first law of thermodynamics is applied on liquid suction heat exchanger as:
h1'-h1=h3-h2
Refrigeration effect without liquid suction heat exchanger=h2-h1'
Refrigeration effect with liquid suction heat exchanger=h2-h1
if we see the change in refrigeration effect= (h2-h1) - (h2-h1')
which is always positive. hence refrigeration effect increase with the use of liquid suction heat exchanger, which overall increase the COP of the system. Also it ensure no liquid refrigerant will ever enter the compressor.
For more about PH chart please visit:
http://athermocreation.blogspot.in/2018/01/pressure-enthalpy-chart.html
3. Capillary suction heat exchanger:
This is most commonly type used heat exchanger specially in refrigerator. In this type of heat exchanger capillary is soldered with suction tube and both suction tube and capillary tube act as counter flow heat exchanger.
The vapor refrigerant from evaporator took heat from capillary tube and enter in super heat condition in compressor. This increase the net refrigeration effect and overall COP of the system.
The below figure will show the capillary suction heat exchanger with in a refrigeration cycle.
Heat given by capillary tube (sensible) is equal to heat gain by vapor going from evaporator to compressor in suction tube.
m_ref*cp_liq*(T2-T3)=m_ref*cp_vap*(T5-T4)
where, m_ref = mass flow rate of refrigerant
cp_liq = specific heat of liquid refrigerant
T2 = Temperature of refrigerant entering at capillary
T3 =Temperature of refrigerant at capillary outlet or evaporator inlet
cp_vap = specific heat of vapor refrigerant
T5= Suction temperature
T4= Temperature of vapor refrigerant at evaporator outlet.
Effect of cooling capacity and COP can be easily understand from below PH chart. After using this type of heat exchanger, The expansion process is no more constant enthalpy process, The amount of vapor at evaporator inlet is reduced and cooling capacity is increased.
The low temperature saturated vapor from evaporator outlet take heat from high temperature liquid flowing through capillary tube and temperature of vapor in suction line increased. The heat taken by vapor in suction line from refrigerant flowing through capillary is the increased in cooling capacity of refrigeration system. This is explained more by PH chart as below.
The original cycle is without capillary suction heat exchanger: 4-1'-2-3'-4
The refrigerated cycle with capillary suction heat exchanger: 5-1-2-3-4-5
The increase in refrigeration effect is as below:
m_ref*(h3'-h3)=Q_in
where h3= enthalpy of refrigerant at evaporator inlet with capillary suction heat exchanger.
h3'=enthalpy of refrigerant at evaporator inlet if capillary suction heat exchanger not used.
Q_in= increased in refrigeration effect.
By applying first law of thermodynamic,
m_ref*(h3'-h3)=m_ref*(h5-h4)
where h5=enthalpy of refrigerant at compressor inlet
h4=enthalpy of saturated vapor at evaporator outlet.
Net refrigeration effect if capillary suction heat exchanger is used= m_ref*(h4-h3)
Net refrigeration effect if capillary suction heat exchanger is not used=m_ref*(h4-h3')
For more about the PH chart, Please visit: http://athermocreation.blogspot.in/2018/01/pressure-enthalpy-chart.html
Comments
Post a Comment